motorgliders as towplanes

[Brad[_2_]]

I know this has come up before in RAS. But thought I would bring up
the subject again.

For a club looking at long term projections, which at some point will
include either sticking a "new" engine on a Pawnee or getting rid of
it, does it make sense to start evaluating getting a 2 place
motorglider to serve as a tug and also as a touring/training tool?

Can a MG tug pull a loaded 2 place Blanik on a standard day at SL,
from a 1800' grass strip? Or perhaps such a tug could be used to tow
the members single place ships, and the heavier/ 2-place ships stow
behind the clubs remaining pawnee?

We are blessed with 3 towplanes in our club, there are upcoming
factors that will/are causing us to look at several different
scenarios and am wondering if tossing a MG into the mix might be one
such solution.

Brad

[Dan[_6_]]

On Mar 8, 10:03*am, Brad > wrote:
> I know this has come up before in RAS. But thought I would bring up
> the subject again.
>
> For a club looking at long term projections, which at some point will
> include either sticking a "new" engine on a Pawnee or getting rid of
> it, does it make sense to start evaluating getting a 2 place
> motorglider to serve as a tug and also as a touring/training tool?
>
> Can a MG tug pull a loaded 2 place Blanik on a standard day at SL,
> from a 1800' grass strip? Or perhaps such a tug could be used to tow
> the members single place ships, and the heavier/ 2-place ships stow
> behind the clubs remaining pawnee?
>
> We are blessed with 3 towplanes in our club, there are upcoming
> factors that will/are causing us to look at several different
> scenarios and am wondering if tossing a MG into the mix might be one
> such solution.
>
> Brad

I recently flew a Lambada and was very impressed with it's take off
and climb performance. We didn't tow any gliders so I can't report on
that. I flew the 80 HP version. We were at sea level, two 200+lbs
guys and only 80 HP, takeoff and climb performance seemed to be better
than a Super Cub. The 80 HP version isn't used for towing but the 100
HP version is. It is reported to have about as good as a 150 HP Super
Cub performance in towing. The Lambada soared pretty well, I was
really impressed. The L/D is claimed to be about 30/1 and that seemed
about right while gliding between thermals. It's a well thought out
little machine.
Check out Urban Air USA. My demo ride was with the very sailplane
experienced pilot Jim Lee. Great guy and a great team.

good luck,
Dan

[Darryl Ramm]

On Mar 8, 10:03*am, Brad > wrote:
> I know this has come up before in RAS. But thought I would bring up
> the subject again.
>
> For a club looking at long term projections, which at some point will
> include either sticking a "new" engine on a Pawnee or getting rid of
> it, does it make sense to start evaluating getting a 2 place
> motorglider to serve as a tug and also as a touring/training tool?
>
> Can a MG tug pull a loaded 2 place Blanik on a standard day at SL,
> from a 1800' grass strip? Or perhaps such a tug could be used to tow
> the members single place ships, and the heavier/ 2-place ships stow
> behind the clubs remaining pawnee?
>
> We are blessed with 3 towplanes in our club, there are upcoming
> factors that will/are causing us to look at several different
> scenarios and am wondering if tossing a MG into the mix might be one
> such solution.
>
> Brad

You don't say what type of motorglider you are thinking of (seems even
sustainers count as motorgliders if you believe Soaring Magazine)

But having said that there is not one touring (i.e. non pylon style)
motorglider I can think of that I'd want to tow behind in any
sailplane, well certainly not a two place glider. A Kstana or
something with a big engine might be able to tow a light glider but it
is not going to touch a good Pawnee or simmilar and so what then you
are left with a not great tug and a not very good XC trainer (or XC
anything). But I don't understimate how the appeal of being flexible/
multi-purpose and doing something clever/different might just suck
people in...

As a self-launch motorglider owner in general I tend to think good XC
trainers do *not* have motors. Think Duo Discus or DG-1000S class two
seater. Motorgliders are compelx and expensive to operate and most
have more vices than a modern two-seater, so you are not going to let
newer pilots (the very ones you want to be encouraging to go XC) go
fly them solo etc. And if you only do dual in that glider and send
them solo in an a conventional glider you are sending the XC student/
mentoree a very bad signal.

Motorgliders can be great for some things, inluding orientatiion
flights, etc. but you can also do those in a two place power plane.

Darryl

[Craig Lowrie]

We spent quite some time looking at this. We currently have 2xPawnee 235's
and a Super Cub 180. We also have a Falke 2000 motorglider. Normally we
only need on tug, except on busy periods when we occationally need two.
The motorglider is underutilised, so the idea of a motorglider that can
tow seemed attractive.

To keep operations simple, we assumed that one of our Pawnees would be
servicable, so that the Motorglider could tow single seaters (which is
usually the cause of our peak demand), and therefore the utilisation of
the motorised fleet would increase and we would need one less aircraft.

Initially we evaliated the 100hp fixed pitch Rotax Falke. It did tow, but
seemed a bit marginal from our operation. Also it needed careful
management of the engine temperature to avoid long term issues.

Then we looked at the Grob 109 Turbo. This gives about 130hp for the
initial takeoff and was substantially better than the Falke whilst still
delivering economy which was more than two times better than the Pawnee. I
would say that it was the best we looked at and also better than the super
dimona, which we also studied briefly.

In the UK a G109T has been towing from a grass strip for almost two years
now, towing a variety of single and two seat gliders (including duo's
etc) and has very few problems, I understand.

The concept is certainly worth considering. In the meantime, we continue
to watch developments, as well as plans to re-engine Pawnees with 230hp
Diesel Engines.

Craig Lowrie, UK

At 17:28 08 March 2009, Darryl Ramm wrote:
>On Mar 8, 10:03=A0am, Brad wrote:
>> I know this has come up before in RAS. But thought I would bring up
>> the subject again.
>>
>> For a club looking at long term projections, which at some point will
>> include either sticking a "new" engine on a Pawnee or getting rid of
>> it, does it make sense to start evaluating getting a 2 place
>> motorglider to serve as a tug and also as a touring/training tool?
>>
>> Can a MG tug pull a loaded 2 place Blanik on a standard day at SL,
>> from a 1800' grass strip? Or perhaps such a tug could be used to tow
>> the members single place ships, and the heavier/ 2-place ships stow
>> behind the clubs remaining pawnee?
>>
>> We are blessed with 3 towplanes in our club, there are upcoming
>> factors that will/are causing us to look at several different
>> scenarios and am wondering if tossing a MG into the mix might be one
>> such solution.
>>
>> Brad
>
>You don't say what type of motorglider you are thinking of (seems even
>sustainers count as motorgliders if you believe Soaring Magazine)
>
>But having said that there is not one touring (i.e. non pylon style)
>motorglider I can think of that I'd want to tow behind in any
>sailplane, well certainly not a two place glider. A Kstana or
>something with a big engine might be able to tow a light glider but it
>is not going to touch a good Pawnee or simmilar and so what then you
>are left with a not great tug and a not very good XC trainer (or XC
>anything). But I don't understimate how the appeal of being flexible/
>multi-purpose and doing something clever/different might just suck
>people in...
>
>As a self-launch motorglider owner in general I tend to think good XC
>trainers do *not* have motors. Think Duo Discus or DG-1000S class two
>seater. Motorgliders are compelx and expensive to operate and most
>have more vices than a modern two-seater, so you are not going to let
>newer pilots (the very ones you want to be encouraging to go XC) go
>fly them solo etc. And if you only do dual in that glider and send
>them solo in an a conventional glider you are sending the XC student/
>mentoree a very bad signal.
>
>Motorgliders can be great for some things, inluding orientatiion
>flights, etc. but you can also do those in a two place power plane.
>
>Darryl
>

[[email protected]]

On Mar 8, 1:03*pm, Brad > wrote:
> I know this has come up before in RAS. But thought I would bring up
> the subject again.
>
> For a club looking at long term projections, which at some point will
> include either sticking a "new" engine on a Pawnee or getting rid of
> it, does it make sense to start evaluating getting a 2 place
> motorglider to serve as a tug and also as a touring/training tool?
>
> Can a MG tug pull a loaded 2 place Blanik on a standard day at SL,
> from a 1800' grass strip? Or perhaps such a tug could be used to tow
> the members single place ships, and the heavier/ 2-place ships stow
> behind the clubs remaining pawnee?
>
> We are blessed with 3 towplanes in our club, there are upcoming
> factors that will/are causing us to look at several different
> scenarios and am wondering if tossing a MG into the mix might be one
> such solution.
>
> Brad

I remember reading about a Pipistrel Sinus motorglider towing an LS8
to 2400 feet AGL in 6 minutes in Italy.............
I assume that a lighter gliders should climb even better!
There is also a video on Youtube showing a Lambada motorglider
towing a double seater metal glider.
The Sinus with an 80 HP Rotax
The Lambada with a 100 HP Rotax
Motorgliders will tow into the future!!!!!
Maybe at 3 gallons an hour car fuel.

[Robert Danewid]

In Sweden it is quite common to use Super Dimonas as tugs and we tow two
seaters with no problem. Climb rate is like a 150 hp PA-18. But the
ground run is a bit longer.

The big advantages are less fuel consumption and that that you can tow
with a gliding license only.

Robert
ASW 28-18E
RD


Brad skrev:
> I know this has come up before in RAS. But thought I would bring up
> the subject again.
>
> For a club looking at long term projections, which at some point will
> include either sticking a "new" engine on a Pawnee or getting rid of
> it, does it make sense to start evaluating getting a 2 place
> motorglider to serve as a tug and also as a touring/training tool?
>
> Can a MG tug pull a loaded 2 place Blanik on a standard day at SL,
> from a 1800' grass strip? Or perhaps such a tug could be used to tow
> the members single place ships, and the heavier/ 2-place ships stow
> behind the clubs remaining pawnee?
>
> We are blessed with 3 towplanes in our club, there are upcoming
> factors that will/are causing us to look at several different
> scenarios and am wondering if tossing a MG into the mix might be one
> such solution.
>
> Brad

[brianDG303[_2_]]

I'm struggling with a similar question, and it relates to rebuilds. I
guess different rebuilds cost very different amounts, and it seem that
a Pawnee rebuild is $35K and a little more. Could it be that a cheaper
to rebuild tow plane could be a better choice?

[Andreas Maurer]

On Sun, 8 Mar 2009 12:28:24 -0700 (PDT), wrote:


>I remember reading about a Pipistrel Sinus motorglider towing an LS8
>to 2400 feet AGL in 6 minutes in Italy.............
>I assume that a lighter gliders should climb even better!
>There is also a video on Youtube showing a Lambada motorglider
>towing a double seater metal glider.
>The Sinus with an 80 HP Rotax
>The Lambada with a 100 HP Rotax
>Motorgliders will tow into the future!!!!!
>Maybe at 3 gallons an hour car fuel.


Once airborne a 100hp tow plane (motorglider, ultralight) climbs well,
even with a heavy double seater in tow.

The limiting factor is initial acceleration - high density altitudes
and muddy gound might prevent a safe tow.
In my opinion a paved runway is necessary if you want safe tows even
at high density altitudes.

[Martin Gregorie[_4_]]

On Sun, 08 Mar 2009 14:29:37 -0700, brianDG303 wrote:

> I'm struggling with a similar question, and it relates to rebuilds. I
> guess different rebuilds cost very different amounts, and it seem that a
> Pawnee rebuild is $35K and a little more. Could it be that a cheaper to
> rebuild tow plane could be a better choice?

Seems to me that you need to total up several factors:
1) The cost of replacing/rebuilding the tow plane
2) The cost of insuring the replacement for the time you'll keep it
3) The total operating cost for the time you'll keep it, i.e. include
both overhauls/part replacement as well as fuel and oil.

Then calculate the total cost per annum of each course of action.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

[Darryl Ramm]

On Mar 8, 3:23*pm, Martin Gregorie
> wrote:
> On Sun, 08 Mar 2009 14:29:37 -0700, brianDG303 wrote:
> > I'm struggling with a similar question, and it relates to rebuilds. I
> > guess different rebuilds cost very different amounts, and it seem that a
> > Pawnee rebuild is $35K and a little more. Could it be that a cheaper to
> > rebuild tow plane could be a better choice?
>
> Seems to me that you need to total up several factors:
> 1) The cost of replacing/rebuilding the tow plane
> 2) The cost of insuring the replacement for the time you'll keep it
> 3) The total operating cost for the time you'll keep it, i.e. include
> * *both overhauls/part replacement as well as fuel and oil.
>
> Then calculate the total cost per annum of each course of action.
>
> --
> martin@ * | Martin Gregorie
> gregorie. | Essex, UK
> org * * * |

SO let's make it an even dozen.

Some other things that apply to all choices (including the Pawnee...)

4) who will maintain the tow plane, what experience on type/model do
they have and how many others doing similar things are available to
learn/get help from?
5) Vendor/product risk - a risk factor for chance the vendor will go
out of business or there will be a nasty AD etc. (yes nastier than the
spar mounts on Pawnee, and yes this risk applies to Pawnees etc. as
well)
6) Parts risks/costs - for some models where the vendor is no longer
making them, what is the risk parts won't be available, will be hard
to find or will be expensive to fabricate if at all possible? (And yes
this risk applies to Pawnees etc. as well, but is reduced somewhat by
their wide use)
7) Operational limitations - find out how whatever you are looking at
tows on the days you need to, on the surfaces, at the temperatures and
density altitudes etc. (e.g. more than a few motor gliders have
cooling issues when it gets hot outside).
8) The expected TBO of the engine - I'd be pretty cautious about
assuming anything based on published TBOs - in general and especially
if the motorglider is towing. I'd go talk to real owners who have been
doing something similar. I have a lot less confidence in the engines
in some of these motorgliders doing as well there say compared to a
Lycoming in a Pawnee - but that is just my bias, unconfirmed by
reality either way. Bear in mind that with towing you may be putting a
lot more time on the engine than others so you'll be the canary in the
coal mine.
9) The utility at all of the motorglider as a XC glider/training
glider (you may just maintain it as a tow plane).
10) Insurance costs, pilot experience requirements and other issues. I
have no idea there, but I'd be calling my insurance agent early on in
the process (like now). Especially if some of the desire is to get
glider pilots with a motorglider endorsement towing - I cringe a bit
at that unless those folks have some power experience and/or a lot of
time in the motorglider.
11) Experimental category issues (if it's an experimental motorglider)
in the USA - I'd expect you would run into issues towing with
motorglider with an contest and exhibition certification.
12) You need to know that a tow release system is available or what
one involve/will cost to put through as a 337 only (if the FSDO would
do that?) and what issues that involves. One of the appeals of the tow
setup on many Pawnee's is the retractable tow rope. I'm guessing there
may not be space to install that in many motorgliders.

Anyhow just more brain food.

Darryl

[rlovinggood]

On Mar 8, 1:03*pm, Brad > wrote:
> I know this has come up before in RAS. But thought I would bring up
> the subject again.
>
> For a club looking at long term projections, which at some point will
> include either sticking a "new" engine on a Pawnee or getting rid of
> it, does it make sense to start evaluating getting a 2 place
> motorglider to serve as a tug and also as a touring/training tool?
>
> Can a MG tug pull a loaded 2 place Blanik on a standard day at SL,
> from a 1800' grass strip? Or perhaps such a tug could be used to tow
> the members single place ships, and the heavier/ 2-place ships stow
> behind the clubs remaining pawnee?
>
> We are blessed with 3 towplanes in our club, there are upcoming
> factors that will/are causing us to look at several different
> scenarios and am wondering if tossing a MG into the mix might be one
> such solution.
>
> Brad

Brad,

Assuming we are talking about towing with MG in the US; can you get
insurance to do this?

Ray Lovinggood
Carrboro, NC, USA

[Jim White[_2_]]

Hi Craig.

I landed out at Sackville Farm in the 15s last year and was ear bashed for
about an hour by Tim on how good the Grob109T was for towing. Unfortunately
he was not able to give me a tow out as the undercarriage had been damaged
and it was in repair.

I think you need a very good surface to operate off and that the Grob (and
probably similar SLMGs) are too lightly built for anything more than
occasional tugging.

At BGC our tugs do c. 2000 cycles p.a. each. I doubt that a MG would stay
that pace.

Jim


At 18:00 08 March 2009, Craig Lowrie wrote:

[The Real Doctor]

On 8 Mar, 17:03, Brad > wrote:

> Can a MG tug pull a loaded 2 place Blanik on a standard day at SL,
> from a 1800' grass strip?

Mendip GC in the UK tows two seaters very successfully witha Rotax
Falke. As Andreas Maurer writes, initial acceleration is a little low
(a little? I damn nearly wet myself as the upwind wall approached) but
the climb feels like anything else.

Ian

[Andreas Maurer]

On 9 Mar 2009 08:30:04 GMT, Jim White > wrote:


>I think you need a very good surface to operate off and that the Grob (and
>probably similar SLMGs) are too lightly built for anything more than
>occasional tugging.

Their strength is completely sufficient for towing all dyearlong -
lots of clubs in Germany are using motorgliders as their primary tug
plane. The only problem is that they usually need a long grass or a
paved runway to be safe.



>At BGC our tugs do c. 2000 cycles p.a. each. I doubt that a MG would stay
>that pace.

No problem at all for a motorglider.:)

[[email protected]]

On Mar 8, 1:03*pm, Brad > wrote:
> Can a MG tug pull a loaded 2 place Blanik on a standard day at SL,
> from a 1800' grass strip? Or perhaps such a tug could be used to tow
> the members single place ships, and the heavier/ 2-place ships stow
> behind the clubs remaining pawnee?
> Brad

http://www.youtube.com/watch?v=5UYPyNgf4Q4

Lambada motorglider towing
UFM 13 Rotax 912 100hp

[bildan]

On Mar 9, 9:14*am, wrote:
> On Mar 8, 1:03*pm, Brad > wrote:
>
> > Can a MG tug pull a loaded 2 place Blanik on a standard day at SL,
> > from a 1800' grass strip? Or perhaps such a tug could be used to tow
> > the members single place ships, and the heavier/ 2-place ships stow
> > behind the clubs remaining pawnee?
> > Brad
>
> http://www.youtube.com/watch?v=5UYPyNgf4Q4
>
> Lambada motorglider towing
> UFM 13 Rotax 912 100hp

I watched a Lambada tow an LS-3 from Boulder (elev 5300) on a hot day
and it looked about the same as the SSB 180HP Supercub. The LS-3
pilot reported the same. I can't speak to the maintainability of the
Lambada but keep in mind that the Pawnee isn't sterling in that
department either.

[Surfer!]

In message >, Jim White
> writes
>Hi Craig.
>
>I landed out at Sackville Farm in the 15s last year and was ear bashed for
>about an hour by Tim on how good the Grob109T was for towing. Unfortunately
>he was not able to give me a tow out as the undercarriage had been damaged
>and it was in repair.
>
>I think you need a very good surface to operate off and that the Grob (and
>probably similar SLMGs) are too lightly built for anything more than
>occasional tugging.
>

I also landed there last year and got a ride with Tim in his Grob. He
also did a tow for another arrival from Shennington who wasn't feeling
too euphoric to fly back.

Apparently the same one visited my own club the previous year to
demonstrate how good it was a towing, and they gave it our heaviest
two-seater with one of our heaviest crew combos, and it got them off the
ground. I believe our aerotow strip is just shy of 800m, and it rises
slightly at the end they took off towards. So far as I know the Grob
performed admirably.

>At BGC our tugs do c. 2000 cycles p.a. each. I doubt that a MG would stay
>that pace.

--
Surfer!
Email to: ramwater at uk2 dot net

[Bruce]

wrote:
> On Mar 8, 1:03 pm, Brad > wrote:
>> Can a MG tug pull a loaded 2 place Blanik on a standard day at SL,
>> from a 1800' grass strip? Or perhaps such a tug could be used to tow
>> the members single place ships, and the heavier/ 2-place ships stow
>> behind the clubs remaining pawnee?
>> Brad
>
> http://www.youtube.com/watch?v=5UYPyNgf4Q4
>
> Lambada motorglider towing
> UFM 13 Rotax 912 100hp
>
Personally I would far rather be behind the 100hp Samba or Lambada with
my Std Cirrus - hot and high off grass the accelleration is slower, but
the propwash is limited and climb is better than the asthmatic 180hp
(must be really little ponies over there) super cub.

The wingloading on the Lambada is very similar and the tow combination
is nicely balanced. Behind a 235hp Rallye it is, by comparison
wonderful. Same time to altitude without all the - extreme speed just
off the deck waiting for the tug to leap skyward, then get to be like a
good martini and get all shaken up by the wake turbulence - then scream
around on the outside of all the thermals.

Give me a motorglider any time...

Bruce

[Derek Copeland[_2_]]

I have some experience of aerotowing behind motorgliders, most Motorfalkes
and Grob 109s. They are alright, but struggle a bit with two-seaters.
There have been times when I have beem towed so slowly (about 50knots is a
K13, two-up) that the glider felt as if it was on the point of stalling!
Remember that a glider has to produce more lift when climbing.

I believe that engine cooling is often a problem, and that M/Gs are less
tolerant to the glider getting out of position. At my club we are
expressly forbidden to low tow, or to carry out training exercises such as
'boxing the tow' when M/Gs are being used. Sorry but give me a 235hp
Pawnee (or a winch launch) anytime!

Derek C

At 15:30 09 March 2009, bildan wrote:
>On Mar 9, 9:14=A0am, wrote:
>> On Mar 8, 1:03=A0pm, Brad wrote:
>>
>> > Can a MG tug pull a loaded 2 place Blanik on a standard day at SL,
>> > from a 1800' grass strip? Or perhaps such a tug could be used to
tow
>> > the members single place ships, and the heavier/ 2-place ships stow
>> > behind the clubs remaining pawnee?
>> > Brad
>>
>> http://www.youtube.com/watch?v=3D5UYPyNgf4Q4
>>
>> Lambada motorglider towing
>> UFM 13 Rotax 912 100hp
>
>I watched a Lambada tow an LS-3 from Boulder (elev 5300) on a hot day
>and it looked about the same as the SSB 180HP Supercub. The LS-3
>pilot reported the same. I can't speak to the maintainability of the
>Lambada but keep in mind that the Pawnee isn't sterling in that
>department either.
>

[Mike the Strike]

I observed a 100HP Lambada equipped with 13m wings being used as a
towplane in the 2000 Worlds in South Africa. It seemed to perform
very well despite the moderate density altitude. They were launching
from an asphalt runway and the ground acceleration was slower than
most other towplanes, but once airborne no difference was notable.

The Lambada and similar modern motorgliders are very light (300kg) -
nearly half the weight of the older designs, such as the Grob 109 -
and perform very well.

I have flown a Grob 109 and it had enough trouble getting itself into
the air. The Lambada also handles a lot better than the older heavy
beasts and is a joy to fly.

Mike

[bildan]

On Mar 10, 9:56*am, Mike the Strike > wrote:
> I observed a 100HP Lambada equipped with 13m wings being used as a
> towplane in the 2000 Worlds in South Africa. *It seemed to perform
> very well despite the moderate density altitude. *They were launching
> from an asphalt runway and the ground acceleration was slower than
> most other towplanes, but once airborne no difference was notable.
>
> The Lambada and similar modern motorgliders are very light (300kg) -
> nearly half the weight of the older designs, such as the Grob 109 -
> and perform very well.
>
> I have flown a Grob 109 and it had enough trouble getting itself into
> the air. *The Lambada also handles a lot better than the older heavy
> beasts and is a joy to fly.
>
> Mike

What the Lambada's towing performance at less than 1320 pounds gross
suggests to me is that the US Sport Light Aircraft regulations offer
the possibility of a formidable tug.

The Lambada's aerodynamics are great but 80 - 100 HP is inadequate for
heavy 2-seaters and ballasted singles. 135Hp or so combined with a
propeller optimized for max thrust below 60 knots would make a huge
difference. Aerodynamics doesn't count for much below takeoff speed -
there it's all weight, engine and propeller.

[The Real Doctor]

On 10 Mar, 14:15, Derek Copeland > wrote:

> Remember that a glider has to produce more lift when climbing.

Why?

Ian

[Big Wings]

The G109B Turbo with constant speed prop. is a different beast altogether
with much more power and climbing capability.


> I have flown a Grob 109 and it had enough trouble getting itself into
> the air. =A0The Lambada also handles a lot better than the older heavy
> beasts and is a joy to fly.

[Derek Copeland[_2_]]

At 16:40 10 March 2009, The Real Doctor wrote:
>On 10 Mar, 14:15, Derek Copeland wrote:
>
>> Remember that a glider has to produce more lift when climbing.
>
>Why?
>
>Ian
>
>
The glider does not just get pulled up by the tow rope! The tug acts as a
remote power source so that the glider can climb through the air without
losing speed. The wings have to produce more lift to support the weight of
the glider, plus the vector of the weight and the climb angle.

Gliders that have stalling speeds below 40 knots in free flight often
start buffeting and feeling slightly out of control if they are flown much
below 50 knots on aerotow.

Derek Copeland

[Alan Garside]

Also see what the adverse yaw is like on tow due to the increased lift
being generated


At 18:15 10 March 2009, Derek Copeland wrote:
>At 16:40 10 March 2009, The Real Doctor wrote:
>>On 10 Mar, 14:15, Derek Copeland wrote:
>>
>>> Remember that a glider has to produce more lift when climbing.
>>
>>Why?
>>
>>Ian
>>
>>
>The glider does not just get pulled up by the tow rope! The tug acts as
a
>remote power source so that the glider can climb through the air without
>losing speed. The wings have to produce more lift to support the weight
of
>the glider, plus the vector of the weight and the climb angle.
>
>Gliders that have stalling speeds below 40 knots in free flight often
>start buffeting and feeling slightly out of control if they are flown
much
>below 50 knots on aerotow.
>
>Derek Copeland
>

[Bruce Hoult]

On Mar 11, 3:15*am, Derek Copeland > wrote:
> Remember that a glider has to produce more lift when climbing.

Er .. say what?

Any constant rate of climb (including flying level or constant
descent) requires exactly the same amount of upward force -- identical
to the weight of the aircraft.

In a powered aircraft flying level the weight and lift balance, and
the thrust and drag balance.

In a glider gliding, the lift from the wings is slightly less than the
weight (it is multiplied by the cosine of the glide angle), and the
balance of the upward force comes from drag (multiplied by the sine of
the glide angle).

In a glider being towed upwards, the lift from the wings is also less
than the weight (by the cosine of the climb angle), with the balance
of the upward force coming from the difference of the tow rope force
and the drag (multiplied by the sine of the climb angle).

If you're climbing at only a few hundred feet per minute while being
towed at 60 or 70 knots (6000 or 7000 fpm) then these angles are tiny
and the lift is essentially equal to the weight, but if a powerful
towplane could climb at, say, 45 degrees, then (far from having to
generate more lift than usual) your wings would only have to generate
lift equal to 70% of your weight.

[Derek Copeland[_2_]]

Bruce,

So can you explain why the stalling speed definitely seems to increase
during an aerotow? Either the wing must be flying at a greater angle of
attack, i.e. producing more lift for a given airspeed, or the wing loading
must increase in some way.

As I said before, gliders that will quite happy fly at 40 knots in free
flight seem to need at least 50 knots on aerotow, even in smooth air. If
you aerotow behind a slow, low powered tug such as a motorglider, it often
seems to be quite difficult to keep up with its rate of climb, even though
it is very low. If you pull back the stick enough to do this, the glider
will start to buffet and the controls become rather ineffective. Both
symptoms of being close to the stall I believe! This is staying high
enough to avoid the tug's slipstream BTW, which could also produce
similar effects. If you do drop into the slipstream, it is often very
difficult to climb out of it again.

I agree that the accepted theory of flight says that in steady flight, the
vector of lift plus thrust must equal weight plus drag. I suppose that if
you had a tug powerful enough to produce enough thrust to more than equal
it's own weight plus the weight of the glider, then you could go
vertically up without the wings producing any lift.

Discuss!

Derek C


At 02:34 11 March 2009, Bruce Hoult wrote:
>On Mar 11, 3:15=A0am, Derek Copeland wrote:
>> Remember that a glider has to produce more lift when climbing.

>
>Er .. say what?
>
>Any constant rate of climb (including flying level or constant
>descent) requires exactly the same amount of upward force -- identical
>to the weight of the aircraft.
>
>In a powered aircraft flying level the weight and lift balance, and
>the thrust and drag balance.
>
>In a glider gliding, the lift from the wings is slightly less than the
>weight (it is multiplied by the cosine of the glide angle), and the
>balance of the upward force comes from drag (multiplied by the sine of
>the glide angle).
>
>In a glider being towed upwards, the lift from the wings is also less
>than the weight (by the cosine of the climb angle), with the balance
>of the upward force coming from the difference of the tow rope force
>and the drag (multiplied by the sine of the climb angle).
>
>If you're climbing at only a few hundred feet per minute while being
>towed at 60 or 70 knots (6000 or 7000 fpm) then these angles are tiny
>and the lift is essentially equal to the weight, but if a powerful
>towplane could climb at, say, 45 degrees, then (far from having to
>generate more lift than usual) your wings would only have to generate
>lift equal to 70% of your weight.
>

[The Real Doctor]

On 10 Mar, 18:15, Derek Copeland > wrote:
> At 16:40 10 March 2009, The Real Doctor wrote:>On 10 Mar, 14:15, Derek Copeland *wrote:
>
> >> Remember that a glider has to produce more lift when climbing.
>
> >Why?

> The glider does not just get pulled up by the tow rope! The tug acts as a
> remote power source so that the glider can climb through the air without
> losing speed. The wings have to produce more lift to support the weight of
> the glider...

How big do you think that effect is?

Ian

[Bruce Hoult]

On Mar 11, 6:30*pm, Derek Copeland > wrote:
> Bruce,
>
> So can you explain why the stalling speed definitely seems to increase
> during an aerotow? Either the wing must be flying at a greater angle of
> attack, i.e. producing more lift for a given airspeed, or the wing loading
> must increase in some way.
>
> As I said before, gliders that will quite happy fly at 40 knots in free
> flight seem to need at least 50 knots on aerotow, even in smooth air.

I agree that this effect exists, and I have noticed it, even behind
fast powerful tugs. For example I usually fly a Janus, which is
recommended to tow with +6 flaps. You'd normally switch to zero flap
anywhere above about 50 knots and be thinking about -4 at normal
towing speed. But if you go to zero flap while towing it feels very
mushy.

I'm not convinced that the stall speed is *actually* increased. I've
never been game to find out.

The only thing I can think of is that even though you're not flying
through the prop wash, you're still in the air that the wings of the
towplane have imparted a downward velocity to. The downwardly flowing
air exists maybe 5m or so both above and below the path that the wing
took. I don't know how to calculate the actual downward velocity, or
whether it is significant, but it may amount to the glider effectively
flying through significant sink.


> I agree that the accepted theory of flight says that in steady flight, the
> vector of lift plus thrust must equal weight plus drag. I suppose that if
> you had a tug powerful enough to produce enough thrust to more than equal
> it's own weight plus the weight of the glider, then you could go
> vertically up without the wings producing any lift.

Right. Try towing with a Huey :-)

[Jim Beckman[_2_]]

At 09:25 11 March 2009, Bruce Hoult wrote:

>Right. Try towing with a Huey :-)

It's been done. Well, maybe not with a Huey, but some big military
glider. Richard Schreder used to tell hilarious stories about flying in
some sort of contest in South America, and being retrieved from an
outlanding by a military helicopter. He went to great lengths trying to
communicate to the chopper crew that he wanted only horizontal flight,
*no* vertical maneuvers. I guess the rope was long enough and the
retrieve worked out OK.

Schreder was a fantastically entertaining speaker at any kind of glider
convention or seminar. Plenty good enough to make up for the silly rug he
wore on his head.

Jim Beckman

[Wayne Paul]

"Jim Beckman" > wrote in message
...
> At 09:25 11 March 2009, Bruce Hoult wrote:
>
>>Right. Try towing with a Huey :-)
>
> It's been done. Well, maybe not with a Huey, but some big military
> glider. Richard Schreder used to tell hilarious stories about flying in
> some sort of contest in South America, and being retrieved from an
> outlanding by a military helicopter. He went to great lengths trying to
> communicate to the chopper crew that he wanted only horizontal flight,
> *no* vertical maneuvers. I guess the rope was long enough and the
> retrieve worked out OK.
>
> Schreder was a fantastically entertaining speaker at any kind of glider
> convention or seminar. Plenty good enough to make up for the silly rug he
> wore on his head.
>
> Jim Beckman
>

Here is the Bryan Times story about Schreder's trip to Chile.
http://www.soaridaho.com/Schreder/Schreder/Schreder_in_Chile.html

The helicopter retrieve story can be found in the book "10,000 Feet and
Climbing."

Wayne
HP-14 "6F"
http://www.soaridaho.com/

[Mike the Strike]

Some gliders definitely will fly slower in free flight than on aero-
tow. The Discus 2 is a good example of this. I have run out of
elevator authority several times on tows slower than I'd like and
ended up slipping into low tow position or losing control altogether
and releasing.

I believe this is related to the angle of incidence of the wing, which
requires the Discus to adopt a high nose-up attitude at slow speeds.
Under aero-tow, the rope pulls the nose down, which requires extra
lift from the tailplane. At slow tow speeds, the tailplane cannot
develop enough lift to maintain the proper angle of attack of the wing
because of this pull from the rope.

When dry, my Discus 2 stalls below 40 knots and will thermal happily
between 40 and 50. I cannot safely aero-tow at 50 knots and usually
call for at least 65.

This effect is most notable with high performance standard class ships
whose angle of incidence is more optimized for high-speed flight.
Flapped ships by and large do not suffer from this problem, nor do
most training ships or lower performance single-seaters.

We are not imagining this!

Mike

[[email protected][_2_]]

On Mar 11, 11:07*am, Mike the Strike > wrote:
> Some gliders definitely will fly slower in free flight than on aero-
> tow. *The Discus 2 is a good example of this. *I have run out of
> elevator authority several times on tows slower than I'd like and
> ended up slipping into low tow position or losing control altogether
> and releasing.
>
> I believe this is related to the angle of incidence of the wing, which
> requires the Discus to adopt a high nose-up attitude at slow speeds.
> Under aero-tow, the rope pulls the nose down, which requires extra
> lift from the tailplane. *At slow tow speeds, the tailplane cannot
> develop enough lift to maintain the proper angle of attack of the wing
> because of this pull from the rope.
>
> When dry, my Discus 2 stalls below 40 knots and will thermal happily
> between 40 and 50. *I cannot safely aero-tow at 50 knots and usually
> call for at least 65.
>
> This effect is most notable with high performance standard class ships
> whose angle of incidence is more optimized for high-speed flight.
> Flapped ships by and large do not suffer from this problem, nor do
> most training ships or lower performance single-seaters.
>
> We are not imagining this!
>
> Mike

If this is the case (and I think it is, combined with downwash
effects), then might low tow alleviate some of the low speed problems,
by changing the angle of the towrope at the glider end?

I agree that my LS6 sure feels "iffy" when the tow gets slow, even
though well above stall speed. I'll have to try to see if this effect
is as noticable at low tow next time I get a tow.

As far as vertical tows, I seem to remember they did a glider acro
routine at the NZ WGC that involved hanging a glider from it's nose
hook under a Hughes 500 in hover, then releasing for a tailslide to
start the routine. I recall the transition from normal tow to the
hover described as "interesting" in the glider (!!).

Kirk
66

[Derek Copeland[_2_]]

K13s, which by no stretch of the imagination can be called 'modern',
behave in exactly the same manner during a slow and low powered aerotow
behind a motorglider! They are not comfortable to fly much below 55 knots
on tow. With a conventional tug (Robin DR400/Piper Pawnee) we get about 60
to 65 knots, which gives you much better control in turbulent conditions.

When I fly flapped gliders I tend to use some amount of positive flap,
mostly to get a better view of the tug, but also because it feels more
controllable.

BTW we have been specifically warned against aerotowing in low tow as our
motogliders can run out of forward elevator if you do.

Derek Copeland


At 16:07 11 March 2009, Mike the Strike wrote:
>Some gliders definitely will fly slower in free flight than on aero-
>tow. The Discus 2 is a good example of this. I have run out of
>elevator authority several times on tows slower than I'd like and
>ended up slipping into low tow position or losing control altogether
>and releasing.
>
>I believe this is related to the angle of incidence of the wing, which
>requires the Discus to adopt a high nose-up attitude at slow speeds.
>Under aero-tow, the rope pulls the nose down, which requires extra
>lift from the tailplane. At slow tow speeds, the tailplane cannot
>develop enough lift to maintain the proper angle of attack of the wing
>because of this pull from the rope.
>
>When dry, my Discus 2 stalls below 40 knots and will thermal happily
>between 40 and 50. I cannot safely aero-tow at 50 knots and usually
>call for at least 65.
>
>This effect is most notable with high performance standard class ships
>whose angle of incidence is more optimized for high-speed flight.
>Flapped ships by and large do not suffer from this problem, nor do
>most training ships or lower performance single-seaters.
>
>We are not imagining this!
>
>Mike
>

[Ian[_2_]]

On Tue, 10 Mar 2009 08:56:47 -0700, Mike the Strike wrote:

> I observed a 100HP Lambada equipped with 13m wings being used as a
> towplane in the 2000 Worlds in South Africa. It seemed to perform very
> well despite the moderate density altitude. They were launching from an
> asphalt runway and the ground acceleration was slower than most other
> towplanes, but once airborne no difference was notable.

I think you mean the Samba, sibling of the Lambada. This has a a shorter
wing + extensions to bring it to 12m. These have been used by a number of
clubs in South Afria and a Samba was also used for a number of years at
Gariep Dam (towing everything up to ASH 25's).

Some of the feedback I heard from the Gariep operation:

- The short wings result in a bit more drag, which is a problem with
marginal tows. But their are no aileron extensions on the wing extensions
so in long wing configuration it lacks aileron authority for good control
in strong weather.

- That Samba had a manually adjusted variable pitch prop. The pilot spent
a lot of effort adjusting the prop during the take off run and the tow to
get the most out of it. They also tried an electric auto variable pitch
prop but the electric motor burnt out very quickly.

> The Lambada and similar modern motorgliders are very light (300kg) -
> nearly half the weight of the older designs

This can also be a problem if the glider gets out of position on tow. A/T
training might get quite uncomfortable.

I did some research into M/G tugging a few years ago and put together
some notes. You can read them at

http://www.zsd.co.za/ian/gliding/cgc_docs/mgtugs/mgtugs.html

and some feedback from tow tests that we did at our club.

http://www.zsd.co.za/ian/gliding/cgc_docs/mgtugs/towtests.html

We never bought the Samba mentioned in the 2nd article. In hindsight I
think we made the right choice as the airframe of the Samba is just too
light and fragile to survive getting "clubbed".

But later we bought a 2nd hand 80 HP Rotax Falke. We had it equipped with
a tow hook, and did a few tows with it. We have a long hard runway near
sea level. The 80HP was fine with single seaters but not safe with two
seaters so we stopped using it for towing. (But we do use it for
training.)

Now we have just up-graded it 100HP and fitted a tugging fixed pitch
prop. We are optimistic this will make it a useful tug. In a year's time
I might be able to give some more feedback.

The Falke is much heavier, more robust and easier to fly then the Samba.
It has already stood up well to a few years of club abuse. But the tow
performance is going to be less than that of the Samba.

(In the meantime we have no plans to sell our 180 HP Super Cub tow plane
and there is still lots of training work for the Motor Glider to do.)

Turbochargers and variable pitch props help make up for the lack of
displacement of the Rotax 4 stroke. But they both add complexity and
costs which might not work out well in a club environment. Perhaps one
day someone will persuade Jabiru to water cool their 120HP motor. That
should make the basis of a decent M/G tug.


Ian

[Mike the Strike]

You're right - it was the Samba, the Lambada's cousin. I witnessed
several launches there, including some ballasted ships and none of
them looked scary, although ground run was long.

I'm a great believer in towplane mass and power - my all time favorite
is the 600 HP AgCat.

Mike

[Bruce Hoult]

On Mar 12, 5:07*am, Mike the Strike > wrote:
> I believe this is related to the angle of incidence of the wing, which
> requires the Discus to adopt a high nose-up attitude at slow speeds.
> Under aero-tow, the rope pulls the nose down, which requires extra
> lift from the tailplane. *At slow tow speeds, the tailplane cannot
> develop enough lift to maintain the proper angle of attack of the wing
> because of this pull from the rope.

Running out of elevator in such a situation does not imply that the
wing is anywhere near stalling.

I had a similar situation when I first flew a PW-5 behind a powerful
Pawnee tug. I was keeping the wheels of the tug on the horizon, and
it was horrid. I had almost no viz of the tug and the stick needed to
be waaay back.

After a while I realized that we were climbing so steeply that I
should be much lower. I was looking at the top of the fuselage! I
dropped down to where the tailplane was lined up with the wing and
things got much much better -- and the tug was *way* above the
horizon.

[Bruce Hoult]

On Mar 12, 6:07*am, " >
wrote:
> As far as vertical tows, I seem to remember they did a glider acro
> routine at the NZ WGC that involved hanging a glider from it's nose
> hook under a Hughes 500 in hover, then releasing for a tailslide to
> start the routine. *I recall the transition from normal tow to the
> hover described as "interesting" in the glider (!!).

Yes, except it was a belly hook, with the glider hanging largely
upside down under the helicopter once it stalled (and stopped
oscillating).

You probably didn't see the practice run a day or two earlier. The
helicopter wasn't able to maintain height with the glider dangling
underneath it and called the glider to ask if it was planning to
release while there was still altitude. Bruce Drake in the glider
replied that he was going to have to do his seatbelts up tighter next
time because he was having trouble reaching the release! He did
eventually manage it.

[Mike Schumann]

I can't imagine why the stall speed would change on tow. The controls may
feel different because the tow rope is pulling on the nose, so any attempt
to turn or change the angle of attack will face an increased counter force,
but that's different than a change in the stall speed of the glider.

Mike Schumann

"Derek Copeland" > wrote in message
...
> Bruce,
>
> So can you explain why the stalling speed definitely seems to increase
> during an aerotow? Either the wing must be flying at a greater angle of
> attack, i.e. producing more lift for a given airspeed, or the wing loading
> must increase in some way.
>
> As I said before, gliders that will quite happy fly at 40 knots in free
> flight seem to need at least 50 knots on aerotow, even in smooth air. If
> you aerotow behind a slow, low powered tug such as a motorglider, it often
> seems to be quite difficult to keep up with its rate of climb, even though
> it is very low. If you pull back the stick enough to do this, the glider
> will start to buffet and the controls become rather ineffective. Both
> symptoms of being close to the stall I believe! This is staying high
> enough to avoid the tug's slipstream BTW, which could also produce
> similar effects. If you do drop into the slipstream, it is often very
> difficult to climb out of it again.
>
> I agree that the accepted theory of flight says that in steady flight, the
> vector of lift plus thrust must equal weight plus drag. I suppose that if
> you had a tug powerful enough to produce enough thrust to more than equal
> it's own weight plus the weight of the glider, then you could go
> vertically up without the wings producing any lift.
>
> Discuss!
>
> Derek C
>
>
> At 02:34 11 March 2009, Bruce Hoult wrote:
>>On Mar 11, 3:15=A0am, Derek Copeland wrote:
>>> Remember that a glider has to produce more lift when climbing.
>
>>
>>Er .. say what?
>>
>>Any constant rate of climb (including flying level or constant
>>descent) requires exactly the same amount of upward force -- identical
>>to the weight of the aircraft.
>>
>>In a powered aircraft flying level the weight and lift balance, and
>>the thrust and drag balance.
>>
>>In a glider gliding, the lift from the wings is slightly less than the
>>weight (it is multiplied by the cosine of the glide angle), and the
>>balance of the upward force comes from drag (multiplied by the sine of
>>the glide angle).
>>
>>In a glider being towed upwards, the lift from the wings is also less
>>than the weight (by the cosine of the climb angle), with the balance
>>of the upward force coming from the difference of the tow rope force
>>and the drag (multiplied by the sine of the climb angle).
>>
>>If you're climbing at only a few hundred feet per minute while being
>>towed at 60 or 70 knots (6000 or 7000 fpm) then these angles are tiny
>>and the lift is essentially equal to the weight, but if a powerful
>>towplane could climb at, say, 45 degrees, then (far from having to
>>generate more lift than usual) your wings would only have to generate
>>lift equal to 70% of your weight.
>>

[Mike the Strike]

On Mar 11, 6:33*pm, "Mike Schumann" <mike-nos...@traditions-
nospam.com> wrote:
> I can't imagine why the stall speed would change on tow. *The controls may
> feel different because the tow rope is pulling on the nose, so any attempt
> to turn or change the angle of attack will face an increased counter force,
> but that's different than a change in the stall speed of the glider.

Read my earlier post!

The tow rope in some gliders (especially those standard class racing
gliders with a shallow angle of incidence) acts to pull the nose down,
reducing the angle of attack of the wing and tailplane. The stall
speed depends not only on speed, but angle of attack - if you reduce
it by pulling down on the nose, lift will be reduced. As I mentioned
earlier, the Discus 2 runs out of elevator authority somewhere below
60 knots and descends into low tow, even though its free-flight stall
speed is less than 40 knots. It's not just a difference of feel - the
glider wallows and almost becomes uncontrollable.

Mike

[Peter Wyld[_2_]]

At 20:28 11 March 2009, Mike the Strike wrote:
>You're right - it was the Samba, the Lambada's cousin. I witnessed
>several launches there, including some ballasted ships and none of
>them looked scary, although ground run was long.
>
>I'm a great believer in towplane mass and power - my all time favorite
>is the 600 HP AgCat.
>
>Mike
>

Long ? About 3km available on the northerly run and 1.4 south!
The Samba didn't need anything like that. It is still being used for
towing at Gariep and other places.
Its launch rate was only a little less than the other tugs used (C182's
mostly). It might have been entertaining if he had been towing the Nimbus
3 with brakes open instead of a C182 (which was only just climbing!).

[Bruce]

Bruce wrote:
> wrote:
>> On Mar 8, 1:03 pm, Brad > wrote:
>>> Can a MG tug pull a loaded 2 place Blanik on a standard day at SL,
>>> from a 1800' grass strip? Or perhaps such a tug could be used to tow
>>> the members single place ships, and the heavier/ 2-place ships stow
>>> behind the clubs remaining pawnee?
>>> Brad
>>
>> http://www.youtube.com/watch?v=5UYPyNgf4Q4
>>
>> Lambada motorglider towing
>> UFM 13 Rotax 912 100hp
>>
> Personally I would far rather be behind the 100hp Samba or Lambada with
> my Std Cirrus - hot and high off grass the accelleration is slower, but
> the propwash is limited and climb is better than the asthmatic 180hp
> (must be really little ponies over there) super cub.
>
> The wingloading on the Lambada is very similar and the tow combination
> is nicely balanced. Behind a 235hp Rallye it is, by comparison
> wonderful. Same time to altitude without all the - extreme speed just
> off the deck waiting for the tug to leap skyward, then get to be like a
> good martini and get all shaken up by the wake turbulence - then scream
> around on the outside of all the thermals.
>
> Give me a motorglider any time...
>
> Bruce
In case anyone thought that the MG/UL has too little grunt.
Follow the link below.

The story behind it is - experienced pilot in getting current again on
Janus forgot the thing has a drogue chute.
On a check flight the Janus is towed up from an airfield (Orient) at
5100" MSL at around 20 centigrade.
As you can see the Samba accelerates slowly - then the drogue chute
deploys. (one bump too many )
Tuggie happens to be enormously experienced CFI - uses his head and
abuses the Rotax a little. Combination manages to make a circuit and the
Janus gets dropped over the threshold.
Samba engine did not over heat or suffer any apparent damage, although
it was kept in the time limited maximum power range for the entire circuit.
For information , low down there are very few options straight out on 36
that would not include reducing the Janus to kit form if he had been
dropped before getting back to the runway.

http://www.youtube.com/watch?v=4_w3ngf3-R8&feature=related

With a light single seater it beats the climb performance of the super cub.

[Mike the Strike]

On Mar 12, 1:09*pm, Bruce > wrote:
> Bruce wrote:
> > wrote:
> >> On Mar 8, 1:03 pm, Brad > wrote:
> >>> Can a MG tug pull a loaded 2 place Blanik on a standard day at SL,
> >>> from a 1800' grass strip? Or perhaps such a tug could be used to tow
> >>> the members single place ships, and the heavier/ 2-place ships stow
> >>> behind the clubs remaining pawnee?
> >>> Brad
>
> >>http://www.youtube.com/watch?v=5UYPyNgf4Q4
>
> >> Lambada motorglider towing
> >> UFM 13 Rotax 912 100hp
>
> > Personally I would far rather be behind the 100hp Samba or Lambada with
> > my Std Cirrus - hot and high off grass the accelleration is slower, but
> > the propwash is limited and climb is better than the asthmatic 180hp
> > (must be really little ponies over there) super cub.
>
> > The wingloading on the Lambada is very similar and the tow combination
> > is nicely balanced. Behind a 235hp Rallye it is, by comparison
> > wonderful. Same time to altitude without all the - extreme speed just
> > off the deck waiting for the tug to leap skyward, then get to be like a
> > good martini and get all shaken up by the wake turbulence - then scream
> > around on the outside of all the thermals.
>
> > Give me a motorglider any time...
>
> > Bruce
>
> In case anyone thought that the MG/UL has too little grunt.
> Follow the link below.
>
> The story behind it is - experienced pilot in getting current again on
> Janus forgot the thing has a drogue chute.
> On a check flight the Janus is towed up from an airfield (Orient) at
> 5100" MSL at around 20 centigrade.
> As you can see the Samba accelerates slowly - then the drogue chute
> deploys. (one bump too many )
> Tuggie happens to be enormously experienced CFI - uses his head and
> abuses the Rotax a little. Combination manages to make a circuit and the
> Janus gets dropped over the threshold.
> Samba engine did not over heat or suffer any apparent damage, although
> it was kept in the time limited maximum power range for the entire circuit.

[Tuno]

Mike I don't think the effect has anything to do with standard vs
flapped gliders. I've owned three flapped ships and this phenomenon
has perplexed me on each.

I had a tow in my ballasted ASG-29 yesterday and I'd told the tow
pilot to maintain minimum 80 mph. Several times he got down to 70 mph
(60 knots IAS) and I was fighting to maintain control of the glider,
at a speed that would be just a couple knots below best L/D were I in
free glide.

I've learned to ask for a speed that will keep the glider happy at
neutral flap setting (think standard class) and then lower the flaps
when the tug is flying to slow, and I have to do this at some point on
darn near every tow.

2NO

[Bruce Hoult]

On Mar 13, 9:09*am, Bruce > wrote:
> The story behind it is - experienced pilot in getting current again on
> Janus forgot the thing has a drogue chute.
> On a check flight the Janus is towed up from an airfield (Orient) at
> 5100" MSL at around 20 centigrade.
> As you can see the Samba accelerates slowly - then the drogue chute
> deploys. (one bump too many )
> Tuggie happens to be enormously experienced CFI - uses his head and
> abuses the Rotax a little. Combination manages to make a circuit and the
> Janus gets dropped over the threshold.

Nice.

We disable the parachute on our Janus. I'm not even sure why it has
one. The spoilers are about the same as a Duo, PLUS it has a landing
flap setting that is effective enough that you need a fairly steep
approach if you want to use half spoilers as well, PLUS it has one
heck of an effective slip which (unlike the parachute) can be
modulated or used multiple times as desired.

[Mike Schumann]

Running out of elevator authority is very different then stalling. A glider
stalls when the angle of attack increases past a critical point. Reducing
the angle of attack, increases your stall margin.

Mike Schumann

"Mike the Strike" > wrote in message
...
On Mar 11, 6:33 pm, "Mike Schumann" <mike-nos...@traditions-
nospam.com> wrote:
> I can't imagine why the stall speed would change on tow. The controls may
> feel different because the tow rope is pulling on the nose, so any attempt
> to turn or change the angle of attack will face an increased counter
> force,
> but that's different than a change in the stall speed of the glider.

Read my earlier post!

The tow rope in some gliders (especially those standard class racing
gliders with a shallow angle of incidence) acts to pull the nose down,
reducing the angle of attack of the wing and tailplane. The stall
speed depends not only on speed, but angle of attack - if you reduce
it by pulling down on the nose, lift will be reduced. As I mentioned
earlier, the Discus 2 runs out of elevator authority somewhere below
60 knots and descends into low tow, even though its free-flight stall
speed is less than 40 knots. It's not just a difference of feel - the
glider wallows and almost becomes uncontrollable.

Mike

[5Z]

On Mar 12, 4:02*pm, Tuno > wrote:
> Mike I don't think the effect has anything to do with standard vs
> flapped gliders. I've owned three flapped ships and this phenomenon
> has perplexed me on each.

Region 12 contest at Cal City maybe 20 years ago, I was flying my
ASW-20B. Fully ballasted, behind a Maule the takeoff and first 4-500'
were OK, then inexplicably the tow pilot decides to slow down. I
start asking, then screaming for more speed, but to no avail - the
IDIOT decided to fly with the radio off for some reason.

I forget what the indicated airspeed was, but it was a little above
stall, perhaps 50-55 knots. With the stick at the aft stop, the
glider still settled slowly into low tow and that's where I stayed up
to release altitude. The ailerons had almost no authority - the stick
felt as sloppy as it would on the ground with no air loads on the
ailerons.

My left hand was holding the release as I waited for indications of a
spin entry. Had I been lower when this started to happen, the only
safe option would have been to release and land in the desert scrub.
But I was comfortable enough with the ship's stall / spin
characteristics (exciting to say the least, but manageable) that I
felt it was safer to just hang on to 2K or a thermal - again I don't
recall that detail. But I do remember that it took a good 15 - 20
minutes of thermalling away from the other contestants before I
stopped shaking from both fear and anger.

-Tom

[Derek Copeland[_2_]]

At moderate climb angles, and as long as you are not totally dangling on
the end a bit or rope (as in a helicopter lift), the glider is essentially
still in free flight during an aerotow. To produce enough lift to climb,
the glider's wing must be at a higher angle of attack for a given
airspeed than it would be in normal gliding flight. Therefore the airspeed
at which the wing stalls must be higher.

Or looking at it another way, the wing has to support the normal weight of
the glider and its occupants, plus the component of the weight acting
backwards and downwards, so an effective increase in wing loading.

Quite a few pilots who have aerotowed behind motorgliders and other slow,
low powered, tugs have commented on how the glider feels uncomfortably
close to the stall, even when the airspeed is a least 10 knots above the
normal stalling speed, so I am reasonably sure that this is a real effect.


Derek Copeland


At 18:19 13 March 2009, Mike Schumann wrote:
>Running out of elevator authority is very different then stalling. A
>glider
>stalls when the angle of attack increases past a critical point.
Reducing
>
>the angle of attack, increases your stall margin.
>
>Mike Schumann
>
>"Mike the Strike" wrote in message
...
>On Mar 11, 6:33 pm, "Mike Schumann" wrote:
>> I can't imagine why the stall speed would change on tow. The controls
>may
>> feel different because the tow rope is pulling on the nose, so any
>attempt
>> to turn or change the angle of attack will face an increased counter
>> force,
>> but that's different than a change in the stall speed of the glider.
>
>Read my earlier post!
>
>The tow rope in some gliders (especially those standard class racing
>gliders with a shallow angle of incidence) acts to pull the nose down,
>reducing the angle of attack of the wing and tailplane. The stall
>speed depends not only on speed, but angle of attack - if you reduce
>it by pulling down on the nose, lift will be reduced. As I mentioned
>earlier, the Discus 2 runs out of elevator authority somewhere below
>60 knots and descends into low tow, even though its free-flight stall
>speed is less than 40 knots. It's not just a difference of feel - the
>glider wallows and almost becomes uncontrollable.
>
>Mike
>
>
>

[The Real Doctor]

On 13 Mar, 20:15, Derek Copeland > wrote:
> At moderate climb angles, and as long as you are not totally dangling on
> the end a bit or rope (as in a helicopter lift), the glider is essentially
> still in free flight during an aerotow. To produce enough lift to climb,
> the glider's wing must be at a higher angle of attack for a given
> airspeed than it would be in normal gliding flight.

How much extra lift do you think is required to climb?

> Or looking at it another way, the wing has to support the normal weight of
> the glider and its occupants, plus the component of the weight acting
> backwards and downwards, so an effective increase in wing loading.

I'm sorry, but that just doesn't make sense. Lift has to balance all
the other forces acting at right angles to the direction of flight.
You can't count the weight twice.

Ian

[The Real Doctor]

On 12 Mar, 22:29, Bruce Hoult > wrote:

> We disable the parachute on our Janus. *I'm not even sure why it has
> one.

I believe the answer is "as a fudge to meet the old JAR speed-limiting-
in-a-45-degree-dive requirement.Most drag chutes were never seriously
intended to be used for approach, or indeed to be used at all.

Do your insurers know, by the way, that you have disabled a flight
control?

Ian

[Z Goudie[_2_]]

At 21:55 13 March 2009, The Real Doctor wrote:
>How much extra lift do you think is required to climb?

Without going into the math (euphemism for I'm not sure how to!).

If you are in a free 60 knot glide sinking at 1.5 knots and you want to
climb at 4 knots you have to increase the angle of attack ie pitch up to a
more nose high position to achieve that.

That of course causes a corresponding increase in drag and the glider
starts to slow down immediately.

On tow the tug produces the necessary force to keep the glider flying at
the same speed while climbing but for a given IAS (ignoring any position
error) surely the angle of attack and thus the stall speed must be higher
than in free flight.

[[email protected]]

On Mar 13, 8:45*pm, Z Goudie > wrote:
> At 21:55 13 March 2009, The Real Doctor wrote:
>
> >How much extra lift do you think is required to climb?
>
> Without going into the math (euphemism for I'm not sure how to!).

If you are not sure how to do the math, how can you be sure that you
are correct ?

In fact, I am able to do the math ( practicing aero engineer) and you
are not correct. The difference between lift ( and thus angle of
attack ) in a STEADY descent and a STEADY climb is practically non
existent.

To do the math, you should draw out a diagram of the drag, lift,
weight and towline. The aircraft is climbing on a line that is an
angle "gamma". Draw the towline force on this line pulling the
aircraft forward and up. Draw the drag in the opposite direction.
Draw lift at a right angle to this line and finally draw weight
pulling straight down.

Total up these 4 forces and making them balance out in the up-down and
forward-back directions gives you the relationship that lift = cosine
( gamma ) * weight. The steeper you climb, then less the lift !!!

If you make different assumption of the direction of the towplane
force, then you would get a different result.


Todd Smith
3S

[Z Goudie[_2_]]

At 01:03 14 March 2009, wrote:
>>If you are not sure how to do the math, how can you be sure that you
>are correct ?
>
>In fact, I am able to do the math ( practicing aero engineer) and you
>are not correct. The difference between lift ( and thus angle of
>attack ) in a STEADY descent and a STEADY climb is practically non
>existent.

I should have known better than get into this discussion!

Yes you are of course correct in that the VERTICAL component of lift equal
to the gliders weight is exactly the same whether it is freely descending
at 60kts (at 1.5kts sink, 1:40 slope down) or being towed behind a tug at
60kts (at 10kts climb, 1:6 slope up).

There is however an increase in the angle between that Vertical component
required and the actual wing lift which is near enough at right angles to
the airflow.

This will require a higher wing loading (admittedly only a couple of
percent perhaps) to produce the vertical vector with a consequent related
change in stalling speed.

To small to be noticeable at reasonable towing speeds but if the tug gets
a bit slow and erratic then it will get interesting.

[Bruce Hoult]

On Mar 14, 10:57*am, The Real Doctor >
wrote:
> On 12 Mar, 22:29, Bruce Hoult > wrote:
>
> > We disable the parachute on our Janus. *I'm not even sure why it has
> > one.
>
> I believe the answer is "as a fudge to meet the old JAR speed-limiting-
> in-a-45-degree-dive requirement.Most drag chutes were never seriously
> intended to be used for approach, or indeed to be used at all.
>
> Do your insurers know, by the way, that you have disabled a flight
> control?

I have no idea. That would I imagine be a question for our chief
instructor. Personally I would not use it even if it was enabled, as
the effectiveness of the remaining controls is perfectly adequate and
exceeds that of more recent designs e.g. the Duo Discus.

[Jim White[_2_]]

As you all know I know nothing about aerodynamics but... surely you only
need to produce more lift to produce an acceleration. If you climb at a
steady 400ft / min at a constant speed you are not accelerating so you do
not need more lift to go up.

Surely the point of the tug is to overcome the drag that would otherwise
burn off potential energy and add energy into the system which the glider
converts into height.

I do not see why the wing has to produce more lift than in level flight at
the same speed. As the vertical component of the flight is now up rather
than down the aircraft must assume a more nose up attitude in order to
provide the same angle of attack at the leading edge giving the impression
that the angle of attack is increased leading those that know less than I
about aerodynamics (Delboy) the idea that the aircraft is producing more
lift.

Any aerodynamicists out there?

Jim



At 00:45 14 March 2009, Z Goudie wrote:
>At 21:55 13 March 2009, The Real Doctor wrote:
>>How much extra lift do you think is required to climb?
>
>Without going into the math (euphemism for I'm not sure how to!).
>
>If you are in a free 60 knot glide sinking at 1.5 knots and you want to
>climb at 4 knots you have to increase the angle of attack ie pitch up to
a
>more nose high position to achieve that.
>
>That of course causes a corresponding increase in drag and the glider
>starts to slow down immediately.
>
>On tow the tug produces the necessary force to keep the glider flying at
>the same speed while climbing but for a given IAS (ignoring any position
>error) surely the angle of attack and thus the stall speed must be
higher
>than in free flight.
>
>
>

[Bruce Hoult]

On Mar 14, 5:45*pm, Z Goudie > wrote:
> At 01:03 14 March 2009, wrote:
>
> >>If you are not sure how to do the math, how can you be sure that you
> >are correct ?
>
> >In fact, I am able to do the math ( practicing aero engineer) and you
> >are not correct. *The difference between lift ( and thus angle of
> >attack ) in a STEADY descent and a STEADY climb is practically non
> >existent.
>
> I should have known better than get into this discussion!

I can do the math also (note that I gave the same formula in an
earlier post as Todd just gave you).


> Yes you are of course correct in that the VERTICAL component of lift equal
> to the gliders weight is exactly the same whether it is freely descending
> at 60kts (at 1.5kts sink, 1:40 slope down) or being towed behind a tug at
> 60kts (at 10kts climb, 1:6 slope up).

That is not correct, and that is not what Todd said.

In a constant speed and constant angle climb or descent, the vertical
component of lift DECREASES the more the flight path departs from
horizontal.

The total force required to support the aircraft is what remains
constant. The difference is made up by the sum of the thrust (zero in
gliding flight of course) and the drag.


> There is however an increase in the angle between that Vertical component
> required and the actual wing lift which is near enough at right angles to
> the airflow.

Right.

Except it is not "near enough". The wing lift is BY DEFINITION
exactly and always at right angles to the airflow.


> This will require a higher wing loading (admittedly only a couple of
> percent perhaps) to produce the vertical vector with a consequent related
> change in stalling speed.

You are drawing the wrong triangle, with the right angle in the wrong
place. You would be correct if the glider was climbing while being
pulled by a horizontal rope. It isn't.

[Derek Copeland[_2_]]

Hi Jim,

We cross swords on yet another forum! I am only trying to explain and
understand WHY gliders seem to require more speed to fly in a steady and
controlled manner on aerotow than is necessary in free flight. There seems
to be a general agreement that this is a real effect, by everyone who has
ever had a slow aerotow! I don't believe that it is purely a control
issue, as it still seem to be the case even in smooth air. I have at least
an average gliding instructor's knowledge of aerodynamics and theory of
flight BTW, and I am aware that angle of attack only relates to the
relative airflow.

In steady and unaccelerated flight, the theory states that lift acting
upwards at 90 degrees to the wings chord line equals weight acting
vertically downwards due to gravity, and that thrust (in this case
provided by the tug via the rope) equals drag. There is a vector effect
when climbing or descending, which is overcome in powered aircraft by
increasing or reducing the power setting, i.e. thrust.

If anyone can come up with a convincing theory as to why this 'aerotow
effect' exists, I would like to know it as much as anybody.

Derek Copeland


At 08:15 14 March 2009, Jim White wrote:
>As you all know I know nothing about aerodynamics but... surely you only
>need to produce more lift to produce an acceleration. If you climb at a
>steady 400ft / min at a constant speed you are not accelerating so you
do
>not need more lift to go up.
>
>Surely the point of the tug is to overcome the drag that would otherwise
>burn off potential energy and add energy into the system which the
glider
>converts into height.
>
>I do not see why the wing has to produce more lift than in level flight
at
>the same speed. As the vertical component of the flight is now up rather
>than down the aircraft must assume a more nose up attitude in order to
>provide the same angle of attack at the leading edge giving the
impression
>that the angle of attack is increased leading those that know less than
I
>about aerodynamics (Delboy) the idea that the aircraft is producing more
>lift.
>
>Any aerodynamicists out there?
>
>Jim
>
>
>
>At 00:45 14 March 2009, Z Goudie wrote:
>>At 21:55 13 March 2009, The Real Doctor wrote:
>>>How much extra lift do you think is required to climb?
>>
>>Without going into the math (euphemism for I'm not sure how to!).
>>
>>If you are in a free 60 knot glide sinking at 1.5 knots and you want to
>>climb at 4 knots you have to increase the angle of attack ie pitch up
to
>a
>>more nose high position to achieve that.
>>
>>That of course causes a corresponding increase in drag and the glider
>>starts to slow down immediately.
>>
>>On tow the tug produces the necessary force to keep the glider flying
at
>>the same speed while climbing but for a given IAS (ignoring any
position
>>error) surely the angle of attack and thus the stall speed must be
>higher
>>than in free flight.
>>
>>
>>
>

[The Real Doctor]

On 14 Mar, 00:45, Z Goudie > wrote:
> At 21:55 13 March 2009, The Real Doctor wrote:
>
> >How much extra lift do you think is required to climb?
>
> Without going into the math (euphemism for I'm not sure how to!).
>
> If you are in a free 60 knot glide sinking at 1.5 knots and you want to
> climb at 4 knots you have to increase the angle of attack ie pitch up to a
> more nose high position to achieve that.

Not quite. Your flight path has move upwards, of course, but the
effect on angle of attck needed is more subtle.

> That of course causes a corresponding increase in drag and the glider
> starts to slow down immediately.
>
> On tow the tug produces the necessary force to keep the glider flying at
> the same speed while climbing but for a given IAS (ignoring any position
> error) surely the angle of attack and thus the stall speed must be higher
> than in free flight.

Only a little.

In free flight the vertical components of lift and drag both balance
the weight, and the horizontal component of lift balances the
horizontal component of drag. Since the drag is, for aerodynamic
reasons, much less than the left, a little horizontal lift component
balances a lot of horizontal drag component - in other words, the
glide path is quite shallow.

As a result, there is very little vertical component of drag. Roughly
speaking (and using the theta = sin theta = tan theta approximation,
if that means anything), if the glide angle is 40:1, only 1/40 of the
drag acts vertically, and since the drag is 1/40 of the lift in the
first place, the lift does about 1599/1600 of the holding up, with the
drag contributing the remaining 1/1600.

For all practical purposes, it's the lift which holds the glider up,
and "lift = weight" is a perfectly reasonable approximation for all
normal flight paths. The difference is actually 1 - cos(arctan(D/L))
which for 40:1 is 0.9997.

Including climbing.

So now we wanted to climb at 4kts, which is about 15:1. That's still a
very shallow angle, so the theta = sin theta = tan theta approximation
is still good. However, this time the horizontal component of lift now
acts backwards, and can't balance the horizontal component of drag. So
we need to pull the thing along. Hello tug. The simplest way of
looking at the tow force is as a combination of horizontal and
vertical forces. The precise ratio depends on lots of different
things: rope size, material and length, tow position and so on. It's
simplest just to think of it as a horizontal force and leave any
vertical component to modify the effective weight of the glider
slightly.

OK, so now we have a tow force. It has to oppose the horizontal
componet of the drag (which is a large proportion of a small force)
and the horizontal component of lift (which is a small component of a
large force). Overall, the two work out about the same, with the lift
component rather bigger in most cases.

Vertically the weight is now augmented by a downwards component of
drag, but this is still very small: if the wing is still working at
40:1 and the glide path is 15:1, the vertical component of lift has to
contribute 599/600 of the weight. The lift is slightly more tilted
than in free flight, but it makes hardly any difference - around 0.2%.

So in a 40:1 gliding descent, lift is 1599/1600 of weight, and in a
15:1 climb, lift is 599/600 of weight. The difference is 0.1% of
weight. If we want to climb at the same speed as we descended, that
certainly requires a change in angle of attack, but an absolutely
minuscule one. It won't make any difference to the L/D ratio, so my
assumption that it was 40:1 in the climb as well as the glide is
reasonable.

Summary: in a steady climb or glide, the lift needed from a glider's
wing is as equal to the weight as makes no difference and the angle of
attack needed is dependent on speed. Any differences in control
forces, feel and so on come from other factors: angle of the tow rope,
position of the tow hook, tug wing downwash and so on.

Ian

[The Real Doctor]

On 14 Mar, 08:06, Bruce Hoult > wrote:
> On Mar 14, 10:57*am, The Real Doctor >
> wrote:

> > Do your insurers know, by the way, that you have disabled a flight
> > control?
>
> I have no idea. *That would I imagine be a question for our chief
> instructor. *Personally I would not use it even if it was enabled, as
> the effectiveness of the remaining controls is perfectly adequate and
> exceeds that of more recent designs e.g. the Duo Discus.

I don't doubt it - the tail chute was never, as far as I know, fitted
to my club's Cirrus. But it would be an interesting problem in case of
an overshoot accident, wouldn't it?

Ian

[Bob Cook[_2_]]

Todd,


Nice! You must be the only guy out there who understands this stuff, the
forces acting on a glider in flight. (Other than me!)

Yes, analysis shows that in CLIMBING flight, lift must just be LESS that
it is in level flight. Same for descending (gliding flight) Lift is less
than it would be in level flight.

The difference between descending flight, level flight and climbing
flights is POWER.

In the case of a glider on towplane, the power (energy) comes from the
fuel powering the engine which in turn produces thrust at the propeller
which in turn produces a pulling force throught the rope to the glider.
We could call this "Thrust"

Excess power makes an aircraft climb!

If Thrust is greater than drag, the aircraft will climb. If Thrust is
equal to drag the aircraft will fly level. If thrust is less than drag,
(or nonexistant as in a glider in free flight), the aircraft will
descend.

As you said, lift does vary, but very little if climb or descent angles
are kept reasonable.

Drag and Thrust are the important variables. Gravity MUST remain
constant, and lift hardly varies worth considering.

Note that power, we should say energy, can be imparted to a glider in
several ways that will result in climbing flight. Of course by a tow
plane as mentioned above, but it could be energy from a THERMAL, RIDGE,
WAVE etc. These will all make a glider climb!

To beter understand how the lift gets less as the climb angle gets
greater, let's look at teh "extreme". Consider a glider attached by a
nose hook to a huge construction crane. The crane operator applies POWER
to the lifting cable and the glider is slowly lifted, vertically into the
air.

The glider has only two forces acting on it now, THRUST from the lifting
cable, and gravity. Thrust acting vertically upward, and gravity acting
vertically downward. In fact, these forces woud be equal, but oppposite
to each other. LIFT would necessarily be ZERO!

Cookie (From blairstown)









At 01:03 14 March 2009, wrote:
>On Mar 13, 8:45=A0pm, Z Goudie wrote:
>> At 21:55 13 March 2009, The Real Doctor wrote:
>>
>> >How much extra lift do you think is required to climb?
>>
>> Without going into the math (euphemism for I'm not sure how to!).
>
>If you are not sure how to do the math, how can you be sure that you
>are correct ?
>
>In fact, I am able to do the math ( practicing aero engineer) and you
>are not correct. The difference between lift ( and thus angle of
>attack ) in a STEADY descent and a STEADY climb is practically non
>existent.
>
>To do the math, you should draw out a diagram of the drag, lift,
>weight and towline. The aircraft is climbing on a line that is an
>angle "gamma". Draw the towline force on this line pulling the
>aircraft forward and up. Draw the drag in the opposite direction.
>Draw lift at a right angle to this line and finally draw weight
>pulling straight down.
>
>Total up these 4 forces and making them balance out in the up-down and
>forward-back directions gives you the relationship that lift =3D cosine
>( gamma ) * weight. The steeper you climb, then less the lift !!!
>
>If you make different assumption of the direction of the towplane
>force, then you would get a different result.
>
>
>Todd Smith
>3S
>
>
>

[Bob Cook[_2_]]

Todd,


Nice! You must be the only guy out there who understands this stuff, the
forces acting on a glider in flight. (Other than me!)

Yes, analysis shows that in CLIMBING flight, lift must just be LESS that
it is in level flight. Same for descending (gliding flight) Lift is less
than it would be in level flight.

The difference between descending flight, level flight and climbing
flights is POWER.

In the case of a glider on towplane, the power (energy) comes from the
fuel powering the engine which in turn produces thrust at the propeller
which in turn produces a pulling force throught the rope to the glider.
We could call this "Thrust"

Excess power makes an aircraft climb!

If Thrust is greater than drag, the aircraft will climb. If Thrust is
equal to drag the aircraft will fly level. If thrust is less than drag,
(or nonexistant as in a glider in free flight), the aircraft will
descend.

As you said, lift does vary, but very little if climb or descent angles
are kept reasonable.

Drag and Thrust are the important variables. Gravity MUST remain
constant, and lift hardly varies worth considering.

Note that power, we should say energy, can be imparted to a glider in
several ways that will result in climbing flight. Of course by a tow
plane as mentioned above, but it could be energy from a THERMAL, RIDGE,
WAVE etc. These will all make a glider climb!

To beter understand how the lift gets less as the climb angle gets
greater, let's look at teh "extreme". Consider a glider attached by a
nose hook to a huge construction crane. The crane operator applies POWER
to the lifting cable and the glider is slowly lifted, vertically into the
air.

The glider has only two forces acting on it now, THRUST from the lifting
cable, and gravity. Thrust acting vertically upward, and gravity acting
vertically downward. In fact, these forces woud be equal, but oppposite
to each other. LIFT would necessarily be ZERO!

Cookie (From blairstown)









At 01:03 14 March 2009, wrote:
>On Mar 13, 8:45=A0pm, Z Goudie wrote:
>> At 21:55 13 March 2009, The Real Doctor wrote:
>>
>> >How much extra lift do you think is required to climb?
>>
>> Without going into the math (euphemism for I'm not sure how to!).
>
>If you are not sure how to do the math, how can you be sure that you
>are correct ?
>
>In fact, I am able to do the math ( practicing aero engineer) and you
>are not correct. The difference between lift ( and thus angle of
>attack ) in a STEADY descent and a STEADY climb is practically non
>existent.
>
>To do the math, you should draw out a diagram of the drag, lift,
>weight and towline. The aircraft is climbing on a line that is an
>angle "gamma". Draw the towline force on this line pulling the
>aircraft forward and up. Draw the drag in the opposite direction.
>Draw lift at a right angle to this line and finally draw weight
>pulling straight down.
>
>Total up these 4 forces and making them balance out in the up-down and
>forward-back directions gives you the relationship that lift =3D cosine
>( gamma ) * weight. The steeper you climb, then less the lift !!!
>
>If you make different assumption of the direction of the towplane
>force, then you would get a different result.
>
>
>Todd Smith
>3S
>
>
>

[The Real Doctor]

On 14 Mar, 13:15, Bob Cook > wrote:

> To beter understand how the lift gets less as the climb angle gets
> greater, let's look at teh "extreme". Consider a glider attached by a
> nose hook to a huge construction crane. *The crane operator *applies POWER
> to the lifting cable and the glider is slowly lifted, vertically into the
> air.

Bad example, since tow planes pull - give or take a wee bit -
horizontally, regardless of climb angle.

Ian

[[email protected]]

On Mar 14, 9:43*am, The Real Doctor > wrote:

> Bad example, since tow planes pull - give or take a wee bit -
> horizontally, regardless of climb angle.
>
> Ian

Well, if I re-draw my force diagram so that the tow force is
horizontal. I get a new equation for lift :-)


lift = weight / ( cos( gamma) - sin(gamma) / LOD )

For a 10 degree climb ( thats about a 10knot climb rate at 60 knots
airspeed) and a LOD ( lift over drag ) of 30.

I get lift = 1.021 * weight.

That would increase stall speed by 1%.

So, while I completely agree that on tow a glider can feel much closer
to stall at an airspeed that is much higher than it's nominal stall
speed. I do not believe this is because it need's "more lift to climb
than glide". The math seems to show this is a dead end of
investigation.

More likely causes have already been proposed.
Elevator force needed to balance the pitching moment from the tow
rope.
Disturbed air behind the towplane.

Todd
3S

[[email protected]]

On Mar 14, 9:15*am, Bob Cook > wrote:
> Todd,
>
> Nice! *You must be *the only guy out there who understands this stuff, the
> forces acting on a glider in flight. *(Other than me!)


Hey Cookie, I hope to see you more this summer than last. If I can
afford the gas and tow fees :-)

Todd

[The Real Doctor]

On 14 Mar, 11:24, The Real Doctor > wrote:

> So in a 40:1 gliding descent, lift is 1599/1600 of weight, and in a
> 15:1 climb, lift is 599/600 of weight.

Correction for the purists ... "lift is *supporting* 1599/1600 ...
lift is *supporting* 599/600 ..."

Ian

[The Real Doctor]

On 14 Mar, 14:49, wrote:

> So, while I completely agree that on tow a glider can feel much closer
> to stall at an airspeed that is much higher than it's nominal stall
> speed. *I do not believe this is because it need's "more lift to climb
> than glide". *The math seems to show this is a dead end of
> investigation.
>
> More likely causes have already been proposed.
> * * Elevator force needed to balance the pitching moment from the tow
> rope.
> * * Disturbed air behind the towplane.

Agreed. My money is on the towplane wake.

Ian

[sisu1a]

> Agreed. My money is on the towplane wake.


I put my monies on the elevator authority/AoA ratio. We fly above
the wing wake (USA...) in most cases, in relatively clean air, but
sometimes in the clean air below it. Box the wake, it will tell you
where it is and where it isn't...

But typically glider's noses, on tow, are unnaturally high (and thus
AoA is higher...) for a given airspeed, in addition to being more
forcefully held there, both effects of course due to the rope's
pull. The elevator is the same size whether on tow or free flight
though, so the authority it can exert against the countering forces is
proportionately lower than in free flight...

The fix is the same regardless of why though- more speed... please!
(wings rocking in vain...)

-Paul

[The Real Doctor]

On 14 Mar, 16:53, The Real Doctor > wrote:
> On 14 Mar, 11:24, The Real Doctor > wrote:
>
> > So in a 40:1 gliding descent, lift is 1599/1600 of weight, and in a
> > 15:1 climb, lift is 599/600 of weight.
>
> Correction for the purists ... "lift is *supporting* 1599/1600 ...
> lift is *supporting* 599/600 ..."

Further correction

Oops. I forgot that drag acts down when climbing. I can't be bothered
doing the maths again, but I think it will lead to L = 1599/1600 W
gliding, 601/600 W climbing.

Ian

[The Real Doctor]

On 14 Mar, 17:24, sisu1a > wrote:
> > Agreed. My money is on the towplane wake.
>
> I put my monies on the elevator authority/AoA ratio. * *We fly above
> the wing wake (USA...) in most cases, in relatively clean air, but
> sometimes in the clean air below it. *Box the wake, it will tell you
> where it is and where it isn't...

That'll tell you where the expanding conical turbulent wake of the
prop is ... there's also the smooth effect of the vortex sheet shed
from the wing to consider. One useful interpretation of the induced
drag is the effect the vortex sheet (modelled as tow tip vortices) has
on the local angle of attack of the wing ... the same will apply to
anything behind the tug ... like a glider ...

Ian

[Derek Copeland[_2_]]

Most (sensible) people fly either above the tug's slipstream or below it.
In the first case the tug may be pulling the glider's nose down and in
the second case up. It doesn't seem to make a lot of difference to the
way the glider flies. There is a theory, mostly believed in by the good
folks of Oz, that low tow is slightly more stable. I can't somehow
imagine that the downwash from the tug has that much effect on a glider on
the end of a 150ft rope!

Derek Copeland

At 20:23 14 March 2009, The Real Doctor wrote:
>On 14 Mar, 17:24, sisu1a wrote:
>> > Agreed. My money is on the towplane wake.
>>
>> I put my monies on the elevator authority/AoA ratio. =A0 =A0We fly
above
>> the wing wake (USA...) in most cases, in relatively clean air, but
>> sometimes in the clean air below it. =A0Box the wake, it will tell you
>> where it is and where it isn't...
>
>That'll tell you where the expanding conical turbulent wake of the
>prop is ... there's also the smooth effect of the vortex sheet shed
>from the wing to consider. One useful interpretation of the induced
>drag is the effect the vortex sheet (modelled as tow tip vortices) has
>on the local angle of attack of the wing ... the same will apply to
>anything behind the tug ... like a glider ...
>
>Ian
>

[sisu1a]

> > I put my monies on the elevator authority/AoA ratio. * *We fly above
> > the wing wake (USA...) in most cases, in relatively clean air, but
> > sometimes in the clean air below it. *Box the wake, it will tell you
> > where it is and where it isn't...
>
> That'll tell you where the expanding conical turbulent wake of the
> prop is ... there's also the smooth effect of the vortex sheet shed
> from the wing to consider. One useful interpretation of the induced
> drag is the effect the vortex sheet (modelled as tow tip vortices) has
> on the local angle of attack of the wing ... the same will apply to
> anything behind the tug ... like a glider ...
>
> Ian

Not as I understand it, I am of the thought that the wake as we know
it IS the sheet of downwash from the wings, and the propwash is quite
insignificant compared with the disturbance from the wings, although
usually residing somewhere within this greater wake although can be
higher up too. If you are in a fabric ship, the propwash can sometimes
be detected sometimes by the pulsation on the skin like a drumbeat,
but in modern glass not so much. I've been wronger before, but my
money is still on the elevator authority/AoA ratio. ;)

-Paul

[Martin Gregorie[_4_]]

On Sat, 14 Mar 2009 13:23:37 -0700, The Real Doctor wrote:

> On 14 Mar, 17:24, sisu1a > wrote:
>> > Agreed. My money is on the towplane wake.
>>
>> I put my monies on the elevator authority/AoA ratio. Â* Â*We fly above
>> the wing wake (USA...) in most cases, in relatively clean air, but
>> sometimes in the clean air below it. Â*Box the wake, it will tell you
>> where it is and where it isn't...
>
> That'll tell you where the expanding conical turbulent wake of the prop
> is ... there's also the smooth effect of the vortex sheet shed from the
> wing to consider. One useful interpretation of the induced drag is the
> effect the vortex sheet (modelled as tow tip vortices) has on the local
> angle of attack of the wing ... the same will apply to anything behind
> the tug ... like a glider ...
>
As an approximation the downwash angle behind a wing is 1/3 of its AOA,
so that will add something like 2 to 3 degrees nose-up attitude to the
glider.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

[The Real Doctor]

On 14 Mar, 21:15, Derek Copeland > wrote:
> Most (sensible) people fly either above the tug's slipstream or below it.

That's the propellor wake. There is also a much larger region of
disturbed, but not turbulent, airflow caused by the action of the
wing. As a rough guide, there are significant effects in a cylinder
two wingspans in diameter (ie twice as wide as the wing) centred on
the tug's flight path and extending back to the point where lift
started being developed. In practice, of course, viscosity in the air
damps it out, but it will still be a significant effect at the other
end of the tow rope.

(The other end of the wing's wake is the starting vortex on the
runway. Nasty thing have happened to light aircraft which flew into a
starting vortex.)

> In the first case the tug may be pulling the glider's nose down and in
> the second case up. It doesn't seem to make a lot of difference to the
> way the glider flies. There is a theory, mostly believed in by the good
> folks of Oz, that low tow is slightly more stable.

It makes sense. In low tow, the pull of the towrope is upwards, so it
tends to pitch the glider nose up. If you sink a bit (with controls
held steady) the pitching up moment increases and the glider climbs
back to position. If you rise a bit the pitching up moment decreases
and the control forces you have been using to balance it pitch the
nose down and the glider falls back to position.

High tow is just the opposite: glider rises, pitching up moment
increases, glider rises, pitching up moment increases and so on until
either (a) the glider pilot does something about it or (b) the tug
pilot dies.

OK, it's not quite that simple, or impossible to control (clearly),
but a trimmed glider in low tow will generally be more stable than in
high tow.

> I can't somehow
> imagine that the downwash from the tug has that much effect on a glider on
> the end of a 150ft rope!

See above. The downwash from the wing of a 757 has a considerable
effect several miles from the aircraft.

Ian

[The Real Doctor]

On 14 Mar, 21:26, sisu1a > wrote:

> Not as I understand it, I am of the thought that the wake as we know
> it IS the sheet of downwash from the wings, and the propwash is quite
> insignificant compared with the disturbance from the wings, although
> usually residing somewhere within this greater wake although can be
> higher up too.

Hmm. All I need to do is find a tug pilot who is willing to switch the
engine off mid-tow and I can investigate this properly.

Ian

[Big Wings]

At 11:24 14 March 2009, The Real Doctor wrote:
> Summary: in a steady climb or glide, the lift needed from a
> glider's wing is as equal to the weight as makes no
> difference ...

Hmmm. In a glider with C of G in the correct place the tail plane will be
producing a down thrust, hence the wing will need to generate lift greater
than the weight of the glider when in more or less level flight. The more
back stick, the more down thrust and the harder the wing will need to work
to maintain level flight.

Now my Discus turbo stalls in free flight in the low 40s (kts). With the
engine out and the thrust line well above the fuselage, thus pushing the
nose down, the best climb speed in the POH is given as 49 to 54 kts
depending on weight. If I let if fall to about 45 kts it is still
perfectly controllable - just less efficient. However aerotowing on the
nose hook at that speed would not be a happy experience. I have never
done a slow tow on a belly hook so don't know if the symptoms are the
same.

What I have observed on slow tows, and has been reported by others in this
thread, is that the ailerons are ineffective, the glider tends to wallow,
the stick is a long way back and the nose high - even though the speed is
above the normal 1G stalling speed. These seem to be symptoms of an
accelerated stall or incipient spin - but the rope pulling ahead seems to
stop the glider rotating into a spin with the associated wing drop.

I wish I knew the answer, but if I have the symtoms of an accelerated
stall in more or less straight level flight above the 1g stall speed that
sugests that the wing is generating more lift than the weight of the
glider for some reason; if the stick is near the back-stop at 50 kts there
is more elevator downforce so the wing will have to compenstate for that.
The pull from the rope may be slightly down if I'm in high tow,
especially with a long heavy rope with a slight bow in it, but to raise
the stall speed from say 42 to 50 kts (* 1.2) is equivalent to an increase
in load on the wing from 1g to about 1.4g. At a take-off weight of say 475
kg this is equivalent to an additional 190kg or another 418 lb. Where
could this come from??? The downthrust from the elevator fighting the tow
rope?

[The Real Doctor]

On 15 Mar, 00:45, Big Wings > wrote:
> At 11:24 14 March 2009, The Real Doctor wrote:
>
> > Summary: in a steady climb or glide, the lift needed from a
> > glider's wing is as equal to the weight as makes no
> > difference ...
>
> Hmmm. *In a glider with C of G in the correct place the tail plane will be
> producing a down thrust, hence the wing will need to generate lift greater
> than the weight of the glider when in more or less level flight.

If the glider has been well designed,. the tail will be
producingupboard no down- (or up-) thrust at best L/D. The tail load
is still small as the speed varies from best L/D: look at how small
tails generally are and how light the fixing compared to those of the
wings. Very ball park figures: the tail is typically 1/15 of the span
of the wing and 1/2 of the chord. That's 1/30 of the area, so all
other things being equal you wouldn't evry expect the tail to produce
more than about 1/30 of the lift the wing can. So even in the worst
case - a high speed dive or zoom) the tail force probably doesn't
change the wing loading by more than +/- 3%.

> The more
> back stick, the more down thrust and the harder the wing will need to work
> to maintain level flight.

Remember, by the way, that in normal flight the tail produces *down*
force in a dive (stick forward) and *up* force in a zoom. Yes, it's
cambered the wrong way - don't dive or zoom, kids, it ain't efficient.

> What I have observed on slow tows, and has been reported by others in this
> thread, is that the ailerons are ineffective, the glider tends to wallow,
> the stick is a long way back and the nose high - even though the speed is
> above the normal 1G stalling speed. *These seem to be symptoms of an
> accelerated stall or incipient spin - but the rope pulling ahead seems to
> stop the glider rotating into a spin with the associated wing drop.

It would be interesting to know if the glider really *is* near the
stall or if it just exhibits some of the characteristics of being near
the stall. Hmm, there's an interesting experiment there ... I'm not
sure that I'd care to do it.

The behaviour of the Pirat on aerotow is rather different, by the way.
At low speeds it handles very nicely, but if the tuggy is a bit
enthusiastic the ailerons get horribly heavy and ineffective. Quite
the opposite of wallowing, really. At the same speed off tow they are
light and responsive. I'm guessing that the tug downwash affects the
centre of the wing more, effectively increasing the washout.
Meanwhile, I just cajole tuggies into flying with the CHT just below
the red ...

Ian

[Bob Cook[_2_]]

At 13:43 14 March 2009, The Real Doctor wrote:

>Bad example, since tow planes pull - give or take a wee bit -
>horizontally, regardless of climb angle.


Well, our tow plane certainly does not pull horizontally!

I figure it pulls at about a 10:1 angle or about 5.5 degrees.

So if you didn't like my last example here is some more for you:

If we took a 900# glider and pulled it up at 5.5 degrees climb angle, the
lift would be 895#......

Just for ****s and giggles, if we had a (lot) more more powerful towplane
we could get the following:

Climb angle 10 degrees......lift 886#
Climb angle 20 degrees......lift 845#
Climb angle 30 degrees......lift 779#
Climb angle 40 degrees......lift 689#
Climb angle 50 degrees......lift 578#
Climb angle 60 degrees......lift 450#
Climb angle 70 degrees......lift 307#
Climb angle 80 degrees......lift 156#
Climb angle 90 degrees......lift 0#

As climb angle increases, lift decreases!

Same for dive angle!

Granted, at "normal" climb and descent angles the reduction in lift is
tiny, but it is a reduction non the less. Just trying to dispell the
rumor that lift somehow must increase in a climb.

Cookie

[Bob Cook[_2_]]

At 17:24 14 March 2009, sisu1a wrote:
>
>But typically glider's noses, on tow, are unnaturally high (and thus
>AoA is higher...) for a given airspeed,

Not true!

The angle of attack does not have to be high just because the nose is
high. The direction of flight is forward and upward. The angle of
attack is the angle of the wing VS the oncoming airflow, NOT the angle of
the nose relative to the ground.

Remember the towplane is adding power (thrust) to the equation.

With lots of power, a plane and climb rapidly with a relatively low angle
of attack. (so can a glider on tow)

A glider being towed at a relatively high speed will have a relatively low
angle of attack. A glider being towed at a relatively slow speed will
have a relatively high angle of attack. This is independant of the angle
of the glider's nose to the ground and independant of the glider's climb
angle (Direction of flight).


Cookie

[Mike the Strike]

I took a good look at the geometry of my tow today. The tow line
appeared to be pulling down on the nose at about an angle of twenty
degrees. At 65 knots, the tow line formed a catenary to the towplane
with significant sag in the line.

It should be possible to model the angles of the line and angles of
attack of the towplane and glider, but I suspect some of the
simplistic arguments have not explained the phenomenon because they
haven't taken full account of the complex geometry of the tow.

At around 50 to 55 knots, I am unable to maintain high tow and sink
into low tow with no elevator authority and reduced aileron control.
My free-flight stall speed is below 40 knots. Gurus please explain.

Mike

[Mike Schumann]

You are confusing a lack of elevator authority with stalling. The two are
completely different phenomenon.

Mike Schumann

"Mike the Strike" > wrote in message
...
>I took a good look at the geometry of my tow today. The tow line
> appeared to be pulling down on the nose at about an angle of twenty
> degrees. At 65 knots, the tow line formed a catenary to the towplane
> with significant sag in the line.
>
> It should be possible to model the angles of the line and angles of
> attack of the towplane and glider, but I suspect some of the
> simplistic arguments have not explained the phenomenon because they
> haven't taken full account of the complex geometry of the tow.
>
> At around 50 to 55 knots, I am unable to maintain high tow and sink
> into low tow with no elevator authority and reduced aileron control.
> My free-flight stall speed is below 40 knots. Gurus please explain.
>
> Mike
>

[Mike the Strike]

On Mar 14, 9:36*pm, "Mike Schumann" <mike-nos...@traditions-
nospam.com> wrote:
> You are confusing a lack of elevator authority with stalling. *The two are
> completely different phenomenon.
>
> Mike Schumann
>
> "Mike the Strike" > wrote in ...
>
> >I took a good look at the geometry of my tow today. *The tow line
> > appeared to be pulling down on the nose at about an angle of twenty
> > degrees. At 65 knots, the tow line formed a catenary to the towplane
> > with significant sag in the line.
>
> > It should be possible to model the angles of the line and angles of
> > attack of the towplane and glider, but I suspect some of the
> > simplistic arguments have not explained the phenomenon because they
> > haven't taken full account of the complex geometry of the tow.
>
> > At around 50 to 55 knots, I am unable to maintain high tow and sink
> > into low tow with no elevator authority and reduced aileron control.
> > My free-flight stall speed is below 40 knots. *Gurus please explain.
>
> > Mike

Maybe so, but why is poor aileron control in a slow tow similar to
that experienced in an incipient stall? What are the symptoms of
running out of elevator authority and why does this happen on tow but
not in free flight?

No-one has satisfactorily explained this.


Mike

[Bob Cook[_2_]]

Mike,

If the tow line formed a 20 degree angle to the glider, (from tow plane
upward to glider) the glider would have to be 72 feet ABOVE the tow
plane.

Well, really allowing for the angle of climb, it would be 72 feet above
the "direction of flight" of the tow plane. If the tow plane could
climb at a 20 degree angle you would be level with the tow plane, but this
is not possible, If the tow plane climbed at a more reasonable angle of 5
degrees you would be 54 feet above the tow plane.

This, in my opinion, would be dangerous!

I like to tow with the tow plane's wheels sitting on the horizon. This
means that the glider is about 5 feet BELOW the towplane, and the angle of
the tow rope is about 1.5 degrees UPWARD from glider to tow plane.

I despise the term "high tow" as it implies positioning the glider
"high" and above the tow plane. I use the term, ""low tow" for
flying below the wake and the term "normal tow" for flying level with,
or slightly below, the tow plane, but above the wake.

In my opinion flying above the tow plane is forbidden!!

Cookie






At 04:07 15 March 2009, Mike the Strike wrote:
>I took a good look at the geometry of my tow today. The tow line
>appeared to be pulling down on the nose at about an angle of twenty
>degrees. At 65 knots, the tow line formed a catenary to the towplane
>with significant sag in the line.
>
>It should be possible to model the angles of the line and angles of
>attack of the towplane and glider, but I suspect some of the
>simplistic arguments have not explained the phenomenon because they
>haven't taken full account of the complex geometry of the tow.
>
>At around 50 to 55 knots, I am unable to maintain high tow and sink
>into low tow with no elevator authority and reduced aileron control.
>My free-flight stall speed is below 40 knots. Gurus please explain.
>
>Mike
>
>

[Bob Cook[_2_]]

Stall is a factor angle of attack, not airspeed.

You could say that your MINIMUM possible stall speed is under 40 knots,
but you could easily stall at higher speeds IF you get the angle of attack
high enough. Just pull back on the stick harder!

Cookie


At 04:07 15 March 2009, Mike the Strike wrote:
>My free-flight stall speed is below 40 knots. Gurus please explain.

[Bob Cook[_2_]]

Has anybody actually stalled during an aero tow?


Cookie

At 04:36 15 March 2009, Mike Schumann wrote:
>You are confusing a lack of elevator authority with stalling. The two
are
>completely different phenomenon.
>
>Mike Schumann
>
>"Mike the Strike" wrote in message
...
>>I took a good look at the geometry of my tow today. The tow line
>> appeared to be pulling down on the nose at about an angle of twenty
>> degrees. At 65 knots, the tow line formed a catenary to the towplane
>> with significant sag in the line.
>>
>> It should be possible to model the angles of the line and angles of
>> attack of the towplane and glider, but I suspect some of the
>> simplistic arguments have not explained the phenomenon because they
>> haven't taken full account of the complex geometry of the tow.
>>
>> At around 50 to 55 knots, I am unable to maintain high tow and sink
>> into low tow with no elevator authority and reduced aileron control.
>> My free-flight stall speed is below 40 knots. Gurus please explain.
>>
>> Mike
>>
>
>
>

[Bob Cook[_2_]]

For purposes of analysis, the angle at which the tow rope meets the glider
is the angle to consider that THRUST is acting on the glider. A rope can
only be in tension. It cannot impart any rotational "moments" to the
glider.

If the tow rope has a big sag or belly in it, consider the angle at which
the rope meets the glider, not the angel of the glider relative to the tow
plane.

The sag in the rope is caused by the weight of the rope itself.

Cookie


At 04:07 15 March 2009, Mike the Strike wrote:
>I took a good look at the geometry of my tow today. The tow line
>appeared to be pulling down on the nose at about an angle of twenty
>degrees. At 65 knots, the tow line formed a catenary to the towplane
>with significant sag in the line.
>
>It should be possible to model the angles of the line and angles of
>attack of the towplane and glider, but I suspect some of the
>simplistic arguments have not explained the phenomenon because they
>haven't taken full account of the complex geometry of the tow.
>
>At around 50 to 55 knots, I am unable to maintain high tow and sink
>into low tow with no elevator authority and reduced aileron control.
>My free-flight stall speed is below 40 knots. Gurus please explain.
>
>Mike
>
>

[Ian[_2_]]

On Thu, 12 Mar 2009 22:09:24 +0200, Bruce wrote:

> Bruce wrote:

> In case anyone thought that the MG/UL has too little grunt. Follow the
> link below.
>
> The story behind it is - experienced pilot in getting current again on
> Janus forgot the thing has a drogue chute. On a check flight the Janus
> is towed up from an airfield (Orient) at 5100" MSL at around 20
> centigrade.
> As you can see the Samba accelerates slowly - then the drogue chute
> deploys. (one bump too many )
> Tuggie happens to be enormously experienced CFI - uses his head and
> abuses the Rotax a little. Combination manages to make a circuit and the
> Janus gets dropped over the threshold. Samba engine did not over heat or
> suffer any apparent damage, although it was kept in the time limited
> maximum power range for the entire circuit. For information , low down
> there are very few options straight out on 36 that would not include
> reducing the Janus to kit form if he had been dropped before getting
> back to the runway.
>
> http://www.youtube.com/watch?v=4_w3ngf3-R8&feature=related

I actually witnessed that incident from a vantage point midway down the
runway. I did not notice at the time anyone with a video camera but you
can't be to careful these days!

I was not expecting to see two sets of wings climbing out from the dip
and the end of the runway. I realy thought we would be picking up broken
fibre glass that morning. One thing you missed out in the commentry, the
Janus' radio was also discovered to be not working. So the pilot did not
hear radio calls advising him of the situation. The tug pilot did hear
the radio excitement but became aware of the situation when he saw the
shadow of the drag chute. The glider pilot first became aware of the drag
chute when he climbed out of the glider after the landing.

The Samba, with its low mass, low drag airframe coupled with a VP prop
certainly has enough excess thrust to provide the aircraft at the end of
the rope a decent climb rate. The limitations are:

- Low power to mass of glider + tug combination, leads to slow
acceleration.

- Low mass of tug relative to glider leaves little control authority for
handling out of possition gliders on tow.

- VP prop is a high maintenance item.

- The Samba airframe is designed to an AUM limit of 450kg (including two
pilots, engine and fuel). It is very light, has lots of carbon and is
strong in all the right places, but it has limited resilience for
handling club abuse which can test things in ways that the designer did
not anticipate.

Our club decided not to buy one. We did get feedback from other operators
including those at Gariep. Perhaps if you have an owner/tuggie operation
and you tow mainly experienced pilots in single seaters, it is a good
option.

For a club operation with a whole group of tuggies sharing the duties of
towing 2 seater trainers with a whole bunch of different instructors and
every level of student, I think the Samba may have a hard time.

Does anybody know what the certification situation is for towing with
ultralight catorgory aircraft (450kg AUW) is in Europe these days? How
many tow planes are certified for tugging and what mass of glider are
they permitted to tow?

Ian

[Martin Gregorie[_4_]]

On Sat, 14 Mar 2009 18:08:23 -0700, The Real Doctor wrote:

> The behaviour of the Pirat on aerotow is rather different, by the way.
> At low speeds it handles very nicely, but if the tuggy is a bit
> enthusiastic the ailerons get horribly heavy and ineffective. Quite the
> opposite of wallowing, really. At the same speed off tow they are light
> and responsive. I'm guessing that the tug downwash affects the centre of
> the wing more, effectively increasing the washout. Meanwhile, I just
> cajole tuggies into flying with the CHT just below the red ...
>
How does the downwash angle vary with position along the wing span? Since
the lift distribution varies across the span its unlikely that the
downwash angle is constant. As an additional factor, even a single seat
glider's span is larger than the tugs span, so the glider's tips (and
ailerons) will be in a somewhat different airflow to the inboard sections
of its wing.

If the downwash angle behind the tug reduces as you move outboard along
its wing then that's effectively washin at the glider's tips, i.e. an
increased AOA, so during slow tow the glider may be nearly tip stalling,
which would account for the symptoms people have mentioned: poor aileron
response and apparent loss of lift. However, it doesn't explain the
behavior of Ian's Pirat, so I must have missed something obvious.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

[The Real Doctor]

On 15 Mar, 04:07, Mike the Strike > wrote:

> At around 50 to 55 knots, I am unable to maintain high tow and sink
> into low tow with no elevator authority and reduced aileron control.
> My free-flight stall speed is below 40 knots. *Gurus please explain.

Type and hook position, please ...

Ian

[The Real Doctor]

On 15 Mar, 05:30, Bob Cook > wrote:

> If the tow line formed a 20 degree *angle to the glider, (from tow plane
> upward to glider) the glider would have to be 72 feet *ABOVE the tow
> plane.

Only if the tow rope were straight. He has already said that it was in
a clearly visible catenary curve.

Ian

[Derek Copeland[_2_]]

Stick well back and won't further raise the nose, controls ineffective and
a feeling of 'wallowing'. All classic symptoms of being close to the
stall! If you tow on a belly hook (all my own glider has) the stick has be
further forward to prevent the glider trying to 'winch launch' and
directional control during the ground run is a bit more difficult, but the
other symptoms stay the same.

Derek Copeland

At 00:45 15 March 2009, Big Wings wrote:
>
>Hmmm. In a glider with C of G in the correct place the tail plane will
be
>producing a down thrust, hence the wing will need to generate lift
greater
>than the weight of the glider when in more or less level flight. The
more
>back stick, the more down thrust and the harder the wing will need to
work
>to maintain level flight.
>
>Now my Discus turbo stalls in free flight in the low 40s (kts). With
the
>engine out and the thrust line well above the fuselage, thus pushing the
>nose down, the best climb speed in the POH is given as 49 to 54 kts
>depending on weight. If I let if fall to about 45 kts it is still
>perfectly controllable - just less efficient. However aerotowing on the
>nose hook at that speed would not be a happy experience. I have never
>done a slow tow on a belly hook so don't know if the symptoms are the
>same.
>
>What I have observed on slow tows, and has been reported by others in
this
>thread, is that the ailerons are ineffective, the glider tends to
wallow,
>the stick is a long way back and the nose high - even though the speed
is
>above the normal 1G stalling speed. These seem to be symptoms of an
>accelerated stall or incipient spin - but the rope pulling ahead seems
to
>stop the glider rotating into a spin with the associated wing drop.
>
>I wish I knew the answer, but if I have the symtoms of an accelerated
>stall in more or less straight level flight above the 1g stall speed
that
>sugests that the wing is generating more lift than the weight of the
>glider for some reason; if the stick is near the back-stop at 50 kts
there
>is more elevator downforce so the wing will have to compenstate for that.

>The pull from the rope may be slightly down if I'm in high tow,
>especially with a long heavy rope with a slight bow in it, but to raise
>the stall speed from say 42 to 50 kts (* 1.2) is equivalent to an
increase
>in load on the wing from 1g to about 1.4g. At a take-off weight of say
>475
>kg this is equivalent to an additional 190kg or another 418 lb. Where
>could this come from??? The downthrust from the elevator fighting the
tow
>rope?
>
>
>

[The Real Doctor]

On 15 Mar, 05:45, Bob Cook > wrote:
> For purposes of analysis, the angle at which the tow rope meets the glider
> is the angle to consider that THRUST is acting on the glider. *A rope can
> only be in tension. *It cannot impart any rotational "moments" to the
> glider.

Unless the hook is on the centreline and the tow rope is pulling
stright ahead, it certainly can apply a moment. Simplest case: low
down belly hook, pull straight ahead, pitch up.

Ian

[The Real Doctor]

On 15 Mar, 01:15, Bob Cook > wrote:
> At 13:43 14 March 2009, The Real Doctor wrote:
>
> >Bad example, since tow planes pull - give or take a wee bit -
> >horizontally, regardless of climb angle.
>
> Well, our tow plane certainly does not pull horizontally!
>
> I figure it pulls at about a 10:1 angle or about 5.5 degrees.

The only thing that determines the angle of the pull is the angle of
the tow rope at the glider end. This will be slightly affected by the
caternary sag in the cable, but will by and large depend on the
relative positions of glider and tug.

In high tow, the glider is more or less level with the tug, so the tow
force is more or less horizontal. The tug does NOT pull the glider up:
the glider pulls itself up and the tug supplies the energy. It's just
the same as with a climbing powered aircraft: the prop does not pull
it up to any significant extent - it allows the wings to pull it up.

Ian

[Bob Cook[_2_]]

Good point!

Re-reading my own posts I came to the same conclusion this morning.

So it is possible to be in "good" tow position, and have the rope impart
a forward and downward pull (relative) to the glider!

20 degrees still seems like a lot to me. I guess it depends on the weight
of the rope, and the drag on the glider.

Cookie




At 10:58 15 March 2009, The Real Doctor wrote:
>On 15 Mar, 05:30, Bob Cook wrote:
>
>> If the tow line formed a 20 degree =A0angle to the glider, (from tow
>plan=
>e
>> upward to glider) the glider would have to be 72 feet =A0ABOVE the tow
>> plane.
>
>Only if the tow rope were straight. He has already said that it was in
>a clearly visible catenary curve.
>
>Ian
>

[Bob Cook[_2_]]

Let me clarify:

The rope itself is in pure tension.

Yes,this tension force applied to the glider, combined with other forces
acting on the glider can form a moment. (on the glider)

If the rope were magically "rigid" like an I beam, and welded to the
glider, the it could impart moments to the glider. But a rope is
flexible.

When forces applied to the glider act in a direction other than through
the CG, moments are formed in the glider.

Like your example, tow rope attached low, so the "thrust" acts on a line
below the CG. Drag acts in the opposite direction but on a line above
the CG. (like a high winged glider) A nose up moment is formed.

But there is still not moment transmitted directly form the rope to the
glider since the rope is flexible and the tow ring would not allow it
anyway.

Cookie


At 11:00 15 March 2009, The Real Doctor wrote:
>On 15 Mar, 05:45, Bob Cook wrote:
>> For purposes of analysis, the angle at which the tow rope meets the
>glide=
>r
>> is the angle to consider that THRUST is acting on the glider. =A0A
rope
>c=
>an
>> only be in tension. =A0It cannot impart any rotational "moments" to
the
>> glider.
>
>Unless the hook is on the centreline and the tow rope is pulling
>stright ahead, it certainly can apply a moment. Simplest case: low
>down belly hook, pull straight ahead, pitch up.
>
>Ian
>

[Bob Cook[_2_]]

Ok, we do have to consider the angle with which the rope meets the glider,
not the angle of the glider relative to the tow plane.

So if the glider /towplane combination is climbing at say 5 degees, it is
reasonable to see how the rope may be pullling exactly horizontally, or
even a bit downward. (due to sag)

Cookie





The tow plane does At 11:06 15 March 2009, The Real Doctor wrote:
>On 15 Mar, 01:15, Bob Cook wrote:
>> At 13:43 14 March 2009, The Real Doctor wrote:
>>
>> >Bad example, since tow planes pull - give or take a wee bit -
>> >horizontally, regardless of climb angle.
>>
>> Well, our tow plane certainly does not pull horizontally!
>>
>> I figure it pulls at about a 10:1 angle or about 5.5 degrees.
>
>The only thing that determines the angle of the pull is the angle of
>the tow rope at the glider end. This will be slightly affected by the
>caternary sag in the cable, but will by and large depend on the
>relative positions of glider and tug.
>
>In high tow, the glider is more or less level with the tug, so the tow
>force is more or less horizontal. The tug does NOT pull the glider up:
>the glider pulls itself up and the tug supplies the energy. It's just
>the same as with a climbing powered aircraft: the prop does not pull
>it up to any significant extent - it allows the wings to pull it up.
>
>Ian
>

[Mike the Strike]

To clarify some more - we tow behind a Pawnee using a 120-foot rope
and my Discus 2 has a nose hook. On tow, the Discus has a notable
nose-up attitude, so the 20 degrees I report is the apparent angle
from my seat. I am positioned immediately behind the towplane just
above the wake. I did have one tow last year in which I was unable to
maintain low tow and had to release.

The pull of the towrope can exert a moment if the force vector is not
aligned with the center of mass of the glider.

The catenary depends on the weight of the rope, but is very notable.

Mike

[Bruce]

Paul

There is a large scale vortex dimer operating behind any aircraft, and
particularly behind high wing loading, heavy short winged things like
Pawnees.

The wake we fly above in high tow is the turbulent propeller wake, but
we would have to be impossibly high and/or far back to avoid the
downward moving centre section of the dimer.

I saw a picture using smoke trails that demonstrates the scale and power
of this some years back -
http://www.nasa.gov/audience/forstudents/k-4/dictionary/Vortex.html

There is a more impressive video at
http://www.youtube.com/watch?v=uy0hgG2pkUs&NR=1

So - given that you are flying in a field of air that has a significant
downward component, maybe you do have a higher angle of attack on the
wings.


Bottom line is that even in the smooth air above the propwash you are
still in air affected by the tug.

Bruce


sisu1a wrote:
>> Agreed. My money is on the towplane wake.
>
>
> I put my monies on the elevator authority/AoA ratio. We fly above
> the wing wake (USA...) in most cases, in relatively clean air, but
> sometimes in the clean air below it. Box the wake, it will tell you
> where it is and where it isn't...
>
> But typically glider's noses, on tow, are unnaturally high (and thus
> AoA is higher...) for a given airspeed, in addition to being more
> forcefully held there, both effects of course due to the rope's
> pull. The elevator is the same size whether on tow or free flight
> though, so the authority it can exert against the countering forces is
> proportionately lower than in free flight...
>
> The fix is the same regardless of why though- more speed... please!
> (wings rocking in vain...)
>
> -Paul

[[email protected]]

On Mar 15, 8:30*am, Bob Cook > wrote:
> Let me clarify:
>
> The rope itself is in pure tension.
>
> Yes,this tension force applied to the glider, combined with other forces
> acting on the glider can form a moment. (on the glider)
>
> If the rope were magically "rigid" like an I beam, and welded to the
> glider, the it could impart moments to the glider. *But a rope is
> flexible.
>
> When forces applied to the glider act in a direction other than through
> the CG, moments are formed in the glider.
>
> Like your example, tow rope attached low, so the "thrust" acts on a line
> below the CG. * Drag acts in the opposite direction but on *a line above
> the CG. * (like a high winged glider) A nose up moment is formed.
>
> But there is still not moment transmitted directly form the rope to the
> glider since the rope is flexible and the tow ring would not allow it
> anyway.
>
> Cookie
>
> At 11:00 15 March 2009, The Real Doctor wrote:
>
>
>
> >On 15 Mar, 05:45, Bob Cook *wrote:
> >> For purposes of analysis, the angle at which the tow rope meets the
> >glide=
> >r
> >> is the angle to consider that THRUST is acting on the glider. =A0A
> rope
> >c=
> >an
> >> only be in tension. =A0It cannot impart any rotational "moments" to
> the
> >> glider.
>
> >Unless the hook is on the centreline and the tow rope is pulling
> >stright ahead, it certainly can apply a moment. Simplest case: low
> >down belly hook, pull straight ahead, pitch up.
>
> >Ian- Hide quoted text -
>
> - Show quoted text -

Maybe the moment caused by the towhook location relative to the center
of drag helps explain the elevator authority issue. At slower speeds,
elevator is less effective allowing this moment to overtake it. The
aileron issue may be similar to what we experience on take-off. High-
alpha (sitting up on gear with tail on ground) combined with
relatively low-speed airflow over the wings and the effect feels very
much like initial take-off roll. As someone mentioned, the pull of the
rope keeps the glider in this state, yet prevents the stall break.

Just a complete guess on my part. Interesting discussion. I had a slow
tow last year and experienced the same phenomenon.

[ZL]

Mike the Strike wrote:
> To clarify some more - we tow behind a Pawnee using a 120-foot rope
> and my Discus 2 has a nose hook. On tow, the Discus has a notable
> nose-up attitude, so the 20 degrees I report is the apparent angle
> from my seat. I am positioned immediately behind the towplane just
> above the wake. I did have one tow last year in which I was unable to
> maintain low tow and had to release.
>
> The pull of the towrope can exert a moment if the force vector is not
> aligned with the center of mass of the glider.
>
> The catenary depends on the weight of the rope, but is very notable.
>
> Mike
>
I would bet you are in the top of the wake, not just above it. Matches
the symptoms. I have done the same thing since I like to fly very close
to the top of the wake. If you fly to put the towplane the same place on
the canopy (not uncommon in mountainous areas with no well defined
horizon), a slower tow speed will have you riding low and in the wake.
Guess how I know. Flaps really help here.

By all the reports I've read on the D2, seeing the towplane from normal
high tow position below about 65 kts is a problem. So going to a little
higher tow position may not be a sane option.

Your shortish tow rope may exacerbate this since you will need to be
higher to be above the wake, the wake will be stronger and the angles
will all be exaggerated compared to a longer rope you may well be used to.

The best option may indeed be more speed on tow. Or get some stinking
flaps :)

-Dave

[sisu1a]

> There is a large scale vortex dimer operating behind any aircraft, and
> particularly behind high wing loading, heavy short winged things like
> Pawnees.

Hmm, I'm of the understanding that we use Pawnees because they are so
lightly wingloaded (relative to other tugs) and have such good power/
weight ratios when not full of bug juice and spray gear. I thought
this is also what allows them to happily fly too slow for our tastes
as well. While a Pawnee is perfectly content tugging at 55mph, I'm
not.


> The wake we fly above in high tow is the turbulent propeller wake, but
> we would have to be impossibly high and/or far back to avoid the
> downward moving centre section of the dimer.

I disagree, I think we are flying well clear of this phenomenon, at
least in high tow. To me, the video you link illustrates just how far
below and behind the a/c this is taking place. The C-5 flies over, and
well past, and they even skip some time in the footage before the
vorticies reach down where the smokers are, which really shows the
downward trailing shape of this effect. It does not compute that you
would be subject to this effect if you were level with it and not
overly close behind (Isn't there a ratio of minimum rope length to
wingspan that is used as a rule of thumb?)

> I saw a picture using smoke trails that demonstrates the scale and power
> of this some years back -http://www.nasa.gov/audience/forstudents/k-4/dictionary/Vortex.html

Neat pic, (very neat actually...) but tells little in the way of what
is happening to a glider on normal tow. A side shot, and one well
clear of the ground would show us what we are actually looking for for
this discussion, but this plane is in ground effect and the shot is
from behind. All bets are off when the disturbed air can't escape
below the flightpath where it wants to go... (and we are thus stuck
flying in this disturbance, which I don't recall as being very
disturbing either) Pretty pic, but somewhat useless as evidence for
this debate, or at least for what I am describing, which is not the
mechanics of towing while in ground effect but rather why gliders feel
like they are on the verge of a stall while on tow despite being well
above normal stall speeds.

> There is a more impressive video athttp://www.youtube.com/watch?v=uy0hgG2pkUs&NR=1
>
> So - given that you are flying in a field of air that has a significant
> downward component, maybe you do have a higher angle of attack on the
> wings.

I think you are misinterpreting the photo/video and drawing incorrect
conclusions form them. Intuition (dangerous in aerodynamics, I
know..) tells me that if your vortex dimer was indeed striking the
glider as you suggest, the downward moving air would actually serve to
effectively decrease AoA since it would be striking the top of the
wing and not the bottom. I still think we fly pretty clean air, above
or below the really affected air, and are only suffering the
butterffly effect of this phenomenon when not actually inside or right
on the edge of the wake.

> Bottom line is that even in the smooth air above the propwash you are
> still in air affected by the tug.

Affected? Sure. But my money is STILL on the pitching up of the nose
due to the rope's pull as being the primary cause of the sensation of
being on the verge of a stall while on tow, since the pull of the rope
is causing the glider to be drug through the air at an unnaturally
high AoA for any given airspeed, while at the same time drastically
reducing elevator effectiveness from reduced airflow and the fact the
nose is tethered. This increased AoA also greatly affects aileron
performance as well, since they too are operating at higher AoA's for
any given speed.

As mentioned elsewhere here, flaps most certainly help this effect
too, by pitching your nose back down some and thus reducing your AoA
for whatever given speed the tug is pulling at.

Unflapped,
-Paul

PS. the propwash to wingwash ratio should be pretty easy to figure
out. The main wing has to support the entire a/c (couple thousand
pounds) while the little spinning wing only needs to provide thrust.
(couple hundred pounds?) Which do YOU think is dominating the scene?
Unless there are tugs out there approaching 1:1 (thrust:weight) the
main wing is the main show, and hence the main contributor of
disturbed air. Ian, please do your power off tug test and please
post to youtube! ;) Be sure to box the wake too though, because it
IS most certainly there, prop or not.

[The Real Doctor]

On 15 Mar, 13:15, Bob Cook > wrote:

> So it is possible to be in "good" tow position, and have the rope impart
> a forward and downward pull (relative) to the glider! *

I'd expect high tow to give a downwards pull and low tow to give an
upwards one.

> 20 degrees still seems like a lot to me. *I guess it depends on the weight
> of the rope, and the drag on the glider.

Seems a lot to me as well. What are these people towing with - chain?

Ian

[The Real Doctor]

On 15 Mar, 17:18, sisu1a > wrote:

> Hmm, I'm of the understanding that we use Pawnees because they are so
> lightly wingloaded (relative to other tugs) and have such good power/
> weight ratios when not full of bug juice and spray gear. I thought
> this is also what allows them to happily fly too slow for our tastes
> as well. While a Pawnee is perfectly content tugging at 55mph, I'm
> not.

The vortex strength is inversely proportional to the airspeed. For a
free vortex, the lift per metre is the vorticity times the free stream
velocity. For an aircraft, the vortex strength is therefore
approximately weight / (airspeed x span).

> PS. the propwash to wingwash ratio should be pretty easy to figure
> out. The main wing has to support the entire a/c (couple thousand
> pounds) while the little spinning wing only needs to provide thrust.
> (couple hundred pounds?) *Which do YOU think is dominating the scene?

Each blade of a Pawnee propeller is about 1m long. Each Pawnee wing is
about 5m long. With the engine at 3000rpm, the tip velocity will be
about 300m/s, which is about 150kt. 200bhp (150kW) at 60kt (30m/s) is
5kN. The maximum takeoff weight of a Pawnee is about 12.5kN.

So ... 0.4 times the force, 0.2 times the span, 2.5 times the
airspeed ... the propeller vorticity will be around 80% of the wing
vorticity. Ball park.

Then you have to remember that the effects of the vortex shedding are
felt, mostly, in a cylinder of about twice the diameter of the span,
and that the air velocity is inversely proportional to the distance
from the vortex. The propeller's vortex street is going to be about 4m
across, the wing's about 20m across....

Finally, there is a danger of confusing two things here. The vortex
wake of a lifting surface is *not* the same as the turbulent wake.
It's bigger and lasts longer.

Ian

[Nyal Williams[_2_]]

Help; what is "dimer" ?


At 14:09 15 March 2009, Bruce wrote:
>Paul
>
>There is a large scale vortex dimer operating behind any aircraft, and
>particularly behind high wing loading, heavy short winged things like
>Pawnees.
>
>The wake we fly above in high tow is the turbulent propeller wake, but
>we would have to be impossibly high and/or far back to avoid the
>downward moving centre section of the dimer.
>
>I saw a picture using smoke trails that demonstrates the scale and power

>of this some years back -
>http://www.nasa.gov/audience/forstudents/k-4/dictionary/Vortex.html
>
>There is a more impressive video at
>http://www.youtube.com/watch?v=uy0hgG2pkUs&NR=1
>
>So - given that you are flying in a field of air that has a significant
>downward component, maybe you do have a higher angle of attack on the
>wings.
>
>
>Bottom line is that even in the smooth air above the propwash you are
>still in air affected by the tug.
>
>Bruce
>
>
>sisu1a wrote:
>>> Agreed. My money is on the towplane wake.
>>
>>
>> I put my monies on the elevator authority/AoA ratio. We fly above
>> the wing wake (USA...) in most cases, in relatively clean air, but
>> sometimes in the clean air below it. Box the wake, it will tell you
>> where it is and where it isn't...
>>
>> But typically glider's noses, on tow, are unnaturally high (and thus
>> AoA is higher...) for a given airspeed, in addition to being more
>> forcefully held there, both effects of course due to the rope's
>> pull. The elevator is the same size whether on tow or free flight
>> though, so the authority it can exert against the countering forces is
>> proportionately lower than in free flight...
>>
>> The fix is the same regardless of why though- more speed... please!
>> (wings rocking in vain...)
>>
>> -Paul
>

[Nyal Williams[_2_]]

Help; what is "dimer" ?


At 14:09 15 March 2009, Bruce wrote:
>Paul
>
>There is a large scale vortex dimer operating behind any aircraft, and
>particularly behind high wing loading, heavy short winged things like
>Pawnees.
>
>The wake we fly above in high tow is the turbulent propeller wake, but
>we would have to be impossibly high and/or far back to avoid the
>downward moving centre section of the dimer.
>
>I saw a picture using smoke trails that demonstrates the scale and power

>of this some years back -
>http://www.nasa.gov/audience/forstudents/k-4/dictionary/Vortex.html
>
>There is a more impressive video at
>http://www.youtube.com/watch?v=uy0hgG2pkUs&NR=1
>
>So - given that you are flying in a field of air that has a significant
>downward component, maybe you do have a higher angle of attack on the
>wings.
>
>
>Bottom line is that even in the smooth air above the propwash you are
>still in air affected by the tug.
>
>Bruce
>
>
>sisu1a wrote:
>>> Agreed. My money is on the towplane wake.
>>
>>
>> I put my monies on the elevator authority/AoA ratio. We fly above
>> the wing wake (USA...) in most cases, in relatively clean air, but
>> sometimes in the clean air below it. Box the wake, it will tell you
>> where it is and where it isn't...
>>
>> But typically glider's noses, on tow, are unnaturally high (and thus
>> AoA is higher...) for a given airspeed, in addition to being more
>> forcefully held there, both effects of course due to the rope's
>> pull. The elevator is the same size whether on tow or free flight
>> though, so the authority it can exert against the countering forces is
>> proportionately lower than in free flight...
>>
>> The fix is the same regardless of why though- more speed... please!
>> (wings rocking in vain...)
>>
>> -Paul
>

[Nyal Williams[_2_]]

Help; what is "dimer" ?


At 14:09 15 March 2009, Bruce wrote:
>Paul
>
>There is a large scale vortex dimer operating behind any aircraft, and
>particularly behind high wing loading, heavy short winged things like
>Pawnees.
>
>The wake we fly above in high tow is the turbulent propeller wake, but
>we would have to be impossibly high and/or far back to avoid the
>downward moving centre section of the dimer.
>
>I saw a picture using smoke trails that demonstrates the scale and power

>of this some years back -
>http://www.nasa.gov/audience/forstudents/k-4/dictionary/Vortex.html
>
>There is a more impressive video at
>http://www.youtube.com/watch?v=uy0hgG2pkUs&NR=1
>
>So - given that you are flying in a field of air that has a significant
>downward component, maybe you do have a higher angle of attack on the
>wings.
>
>
>Bottom line is that even in the smooth air above the propwash you are
>still in air affected by the tug.
>
>Bruce
>
>
>sisu1a wrote:
>>> Agreed. My money is on the towplane wake.
>>
>>
>> I put my monies on the elevator authority/AoA ratio. We fly above
>> the wing wake (USA...) in most cases, in relatively clean air, but
>> sometimes in the clean air below it. Box the wake, it will tell you
>> where it is and where it isn't...
>>
>> But typically glider's noses, on tow, are unnaturally high (and thus
>> AoA is higher...) for a given airspeed, in addition to being more
>> forcefully held there, both effects of course due to the rope's
>> pull. The elevator is the same size whether on tow or free flight
>> though, so the authority it can exert against the countering forces is
>> proportionately lower than in free flight...
>>
>> The fix is the same regardless of why though- more speed... please!
>> (wings rocking in vain...)
>>
>> -Paul
>

[Big Wings]

At 21:00 15 March 2009, Nyal Williams wrote:
>Help; what is "dimer" ?

A pair of vortices?

[Mike the Strike]

I dyslexed the tow rope length. It should have been 200 feet, not 120
feet, although they can vary between 190 and 230 (we use a longer rope
when launching off the dirt).

I have towed enough to know where the wake is, thank you very much!

[Alan Garside]

Can I add another question, why does the adverse yaw at 60 on tow appear to
be more than 60 in free flight, many students learing to aerotow have
difficulty with the rudder.


At 00:27 16 March 2009, Mike the Strike wrote:
>I dyslexed the tow rope length. It should have been 200 feet, not 120
>feet, although they can vary between 190 and 230 (we use a longer rope
>when launching off the dirt).
>
>I have towed enough to know where the wake is, thank you very much!
>

[Bob Cook[_2_]]

At 19:58 15 March 2009, The Real Doctor wrote:

>I'd expect high tow to give a downwards pull and low tow to give an
>upwards one.

So, that implies that there is a position, on tow, where the rope would
neither impart an upward pull nor impart a downward pull.

This would certainly make the vector analysis simpler. Unfortunately, I
bet this position would be just about in the top of the wake somewhere.

I did go over to our tow plane yesterday, while it was sitting on the
ground, and I pulled on the loose end of the rope to see how hard a pull
would give how much sag. It took suprisingly little force to lift the
center of the rope off the ground. Figure my end of the rope was 4 feet
off the ground and the tow plane end was only 1 foot off the ground, I got
the center of the rope easily about 2 feet off the ground. Point being:
the angle with which the rope intersected with me was very small.

I also weighed a tow rope. Under 5 lbs, with two Schweizer rings, and an
adaptor with a tost ring. So I figure, in a "level" tow the weight of
the rope imparts only about 2 lbs down force to the glider. So you need
generate about two more lbs of lift to counter act this. Signifagant? i
don't think so.

A also paid attention to the rope on a couple of tows, and also noted that
although there is a noticable sag in the rope, it was really not all that
much.

I also noted, in the case of a 2-33 and a Blanik L-13 (the crap that I
usually have to fly) that BOTH of these gliders fly NOTICIBLY nose down on
tow! (which menas the rope is imparting a nose up force vector, even in
normal, tow position. )

Anyone who has flow a 2-33 knows that it takes CONSIDERABLE forward stick
force when on tow.

I have experienced the "too slow tow" phenomonon in the '33. (never
got a too-slow in the Blanik)

Again, my point being that I am still not convinced that anyone has
completly explained the phenomonon, that we all agree exists!

Cookie







forward and downward pull (relative) to the glider! =A0
>
>I'd expect high tow to give a downwards pull and low tow to give an
>upwards one.

>
>Seems a lot to me as well. What are these people towing with - chain?
>
>Ian
>
>

[Bob Cook[_2_]]

I have not noticed any difference in adverse yaw on tow vs free flgiht. I
will pay attention to that and see.

On tow however, students often seem to try to "steer" with the stick
(only).

This of course leads to adverse yaw, which causes to student to further
try to steer with the stick adding further adverse yaw, etc. It can get
out of hand quickly.

In fact, on tow, "steering" should be done with (almost) only rudder! I
find that application of rudder will yeild bank angle as well. (dihedral
effect, advancing wing etc.)

I tell students to use the sitck in order to match the glider's wings to
the tow planes wings (bank angle) and use the rudder to "point" the
nose where desired. (They wrongly try to point the nose using the stick
and adverse yaw gets them every time)

I have noticed that while turning on tow, the stick is often to the
outside of the turn, preventing "overbanking" tendancy!.

I also tell students, use 80% rudder and 20% stick to demonstrate the need
for rudder on tow, and minimal stick forces.

Here is another one. In normal tow position, I notice that the stick has
to be slightly LEFT of center. The glider has a tendancy to want to roll
right. I beleive this is because to tug's wake has a rotation to it.
(even above the wake) Students tend to mechanically "center" the stick,
and the glider will roll right. Experienced pilots naturally put the
stick whereven necessary to achieve wings level. I often ask them if
they were holding left stick. Invariably they say , "I dunno"

Cookie









At 10:15 16 March 2009, Alan Garside wrote:
>Can I add another question, why does the adverse yaw at 60 on tow appear
to
>be more than 60 in free flight, many students learing to aerotow have
>difficulty with the rudder.
>

[Derek Copeland[_2_]]

K13s and Bocians (both types I fly regularly) also aerotow in a noticeably
nose down attitude. The Bocian is such a slow glider that it unlikely that
any normal tug could fly slowly enough to get close to its stalling speed,
without stalling itself, but the K13 definitely starts 'wallowing' below
about 52knots, whereas its normal free flight stalling speed is about
36knots.

Derek Copeland

At 12:15 16 March 2009, Bob Cook wrote:
>
>I also noted, in the case of a 2-33 and a Blanik L-13 (the crap that I
>usually have to fly) that BOTH of these gliders fly NOTICIBLY nose down
on
>tow! (which menas the rope is imparting a nose up force vector, even in
>normal, tow position. )
>
>Anyone who has flow a 2-33 knows that it takes CONSIDERABLE forward
stick
>force when on tow.
>
>I have experienced the "too slow tow" phenomonon in the '33. (never
>got a too-slow in the Blanik)
>
>Again, my point being that I am still not convinced that anyone has
>completly explained the phenomonon, that we all agree exists!
>
>Cookie
>
>
>
>
>
>
>
>forward and downward pull (relative) to the glider! =A0
>>
>>I'd expect high tow to give a downwards pull and low tow to give an
>>upwards one.
>
>>
>>Seems a lot to me as well. What are these people towing with - chain?
>>
>>Ian
>>
>>
>

[The Real Doctor]

On 16 Mar, 10:15, Alan Garside > wrote:
> Can I add another question, why does the adverse yaw at 60 on tow appear to
> be more than 60 in free flight, many students learing to aerotow have
> difficulty with the rudder.

Do you think it's an aerodynamic problem? I'd always thought, based on
my own experience, that it came from having a much clearer indication
of yaw (the tow rop) than normal and overcorrecting some PIOs into the
system.

But I hadn't really considered the aerodynamics. Hmmm.

Ian

[[email protected][_2_]]

On Mar 14, 8:43*am, The Real Doctor > wrote:
> On 14 Mar, 13:15, Bob Cook > wrote:
>
> > To beter understand how the lift gets less as the climb angle gets
> > greater, let's look at teh "extreme". Consider a glider attached by a
> > nose hook to a huge construction crane. *The crane operator *applies POWER
> > to the lifting cable and the glider is slowly lifted, vertically into the
> > air.
>
> Bad example, since tow planes pull - give or take a wee bit -
> horizontally, regardless of climb angle.
>
> Ian

Never been towed behind an Ag-cat, have you?

Nothing horizontal about that evolution!

Kirk
66

[Bruce]

Sorry Nyal - excessive vocab use. Dimer == related pair of.

In this case two vortices - one off each wingtip that interact to create
a roughly symmetrical "geared disk" shape behind the wing. With the
downward part of the vortex from each wingtip merging with the downward
flow from the other.

If you drive behind a (modern / streamlined not SUV) car in the rain or
snow you can see the dimer it creates. Formula one and Nascar rear wings
also create impressive examples...



Nyal Williams wrote:
> Help; what is "dimer" ?
>
>
> At 14:09 15 March 2009, Bruce wrote:
>> Paul
>>
>> There is a large scale vortex dimer operating behind any aircraft, and
>> particularly behind high wing loading, heavy short winged things like
>> Pawnees.
>>
>> The wake we fly above in high tow is the turbulent propeller wake, but
>> we would have to be impossibly high and/or far back to avoid the
>> downward moving centre section of the dimer.
>>
>> I saw a picture using smoke trails that demonstrates the scale and power
>
>> of this some years back -
>> http://www.nasa.gov/audience/forstudents/k-4/dictionary/Vortex.html
>>
>> There is a more impressive video at
>> http://www.youtube.com/watch?v=uy0hgG2pkUs&NR=1
>>
>> So - given that you are flying in a field of air that has a significant
>> downward component, maybe you do have a higher angle of attack on the
>> wings.
>>
>>
>> Bottom line is that even in the smooth air above the propwash you are
>> still in air affected by the tug.
>>
>> Bruce
>>
>>
>> sisu1a wrote:
>>>> Agreed. My money is on the towplane wake.
>>>
>>> I put my monies on the elevator authority/AoA ratio. We fly above
>>> the wing wake (USA...) in most cases, in relatively clean air, but
>>> sometimes in the clean air below it. Box the wake, it will tell you
>>> where it is and where it isn't...
>>>
>>> But typically glider's noses, on tow, are unnaturally high (and thus
>>> AoA is higher...) for a given airspeed, in addition to being more
>>> forcefully held there, both effects of course due to the rope's
>>> pull. The elevator is the same size whether on tow or free flight
>>> though, so the authority it can exert against the countering forces is
>>> proportionately lower than in free flight...
>>>
>>> The fix is the same regardless of why though- more speed... please!
>>> (wings rocking in vain...)
>>>
>>> -Paul

[Nyal Williams[_2_]]

I know this is not alt.usage.english, but what is the etymology of this
word? Is it slang? Jargon? Engineer language? Is it Di-mer or Dim-er or
dimer... something or other? I'm just a poor musicologist trying to figure
stuff out.

At 14:27 16 March 2009, Bruce wrote:
>Sorry Nyal - excessive vocab use. Dimer == related pair of.
>
>In this case two vortices - one off each wingtip that interact to create

>a roughly symmetrical "geared disk" shape behind the wing. With the
>downward part of the vortex from each wingtip merging with the downward
>flow from the other.
>
>If you drive behind a (modern / streamlined not SUV) car in the rain or
>snow you can see the dimer it creates. Formula one and Nascar rear wings

>also create impressive examples...
>
>
>
>Nyal Williams wrote:
>> Help; what is "dimer" ?
>>
>>
>> At 14:09 15 March 2009, Bruce wrote:
>>> Paul
>>>
>>> There is a large scale vortex dimer operating behind any aircraft, and

>>> particularly behind high wing loading, heavy short winged things like

>>> Pawnees.
>>>
>>> The wake we fly above in high tow is the turbulent propeller wake, but

>>> we would have to be impossibly high and/or far back to avoid the
>>> downward moving centre section of the dimer.
>>>
>>> I saw a picture using smoke trails that demonstrates the scale and
>power
>>
>>> of this some years back -
>>> http://www.nasa.gov/audience/forstudents/k-4/dictionary/Vortex.html
>>>
>>> There is a more impressive video at
>>> http://www.youtube.com/watch?v=uy0hgG2pkUs&NR=1
>>>
>>> So - given that you are flying in a field of air that has a
significant
>
>>> downward component, maybe you do have a higher angle of attack on the

>>> wings.
>>>
>>>
>>> Bottom line is that even in the smooth air above the propwash you are

>>> still in air affected by the tug.
>>>
>>> Bruce
>>>
>>>
>>> sisu1a wrote:
>>>>> Agreed. My money is on the towplane wake.
>>>>
>>>> I put my monies on the elevator authority/AoA ratio. We fly above
>>>> the wing wake (USA...) in most cases, in relatively clean air, but
>>>> sometimes in the clean air below it. Box the wake, it will tell you
>>>> where it is and where it isn't...
>>>>
>>>> But typically glider's noses, on tow, are unnaturally high (and
thus
>>>> AoA is higher...) for a given airspeed, in addition to being more
>>>> forcefully held there, both effects of course due to the rope's
>>>> pull. The elevator is the same size whether on tow or free flight
>>>> though, so the authority it can exert against the countering forces
is
>>>> proportionately lower than in free flight...
>>>>
>>>> The fix is the same regardless of why though- more speed... please!
>>>> (wings rocking in vain...)
>>>>
>>>> -Paul
>

[Bruce]

monomer
Dimer
Trimer
....


Nyal Williams wrote:
> I know this is not alt.usage.english, but what is the etymology of this
> word? Is it slang? Jargon? Engineer language? Is it Di-mer or Dim-er or
> dimer... something or other? I'm just a poor musicologist trying to figure
> stuff out.
>
> At 14:27 16 March 2009, Bruce wrote:
>> Sorry Nyal - excessive vocab use. Dimer == related pair of.
>>
>> In this case two vortices - one off each wingtip that interact to create
>
>> a roughly symmetrical "geared disk" shape behind the wing. With the
>> downward part of the vortex from each wingtip merging with the downward
>> flow from the other.
>>
>> If you drive behind a (modern / streamlined not SUV) car in the rain or
>> snow you can see the dimer it creates. Formula one and Nascar rear wings
>
>> also create impressive examples...
>>
>>
>>
>> Nyal Williams wrote:
>>> Help; what is "dimer" ?
>>>
>>>
>>> At 14:09 15 March 2009, Bruce wrote:
>>>> Paul
>>>>
>>>> There is a large scale vortex dimer operating behind any aircraft, and
>
>>>> particularly behind high wing loading, heavy short winged things like
>
>>>> Pawnees.
>>>>
>>>> The wake we fly above in high tow is the turbulent propeller wake, but
>
>>>> we would have to be impossibly high and/or far back to avoid the
>>>> downward moving centre section of the dimer.
>>>>
>>>> I saw a picture using smoke trails that demonstrates the scale and
>> power
>>>> of this some years back -
>>>> http://www.nasa.gov/audience/forstudents/k-4/dictionary/Vortex.html
>>>>
>>>> There is a more impressive video at
>>>> http://www.youtube.com/watch?v=uy0hgG2pkUs&NR=1
>>>>
>>>> So - given that you are flying in a field of air that has a
> significant
>>>> downward component, maybe you do have a higher angle of attack on the
>
>>>> wings.
>>>>
>>>>
>>>> Bottom line is that even in the smooth air above the propwash you are
>
>>>> still in air affected by the tug.
>>>>
>>>> Bruce
>>>>
>>>>
>>>> sisu1a wrote:
>>>>>> Agreed. My money is on the towplane wake.
>>>>> I put my monies on the elevator authority/AoA ratio. We fly above
>>>>> the wing wake (USA...) in most cases, in relatively clean air, but
>>>>> sometimes in the clean air below it. Box the wake, it will tell you
>>>>> where it is and where it isn't...
>>>>>
>>>>> But typically glider's noses, on tow, are unnaturally high (and
> thus
>>>>> AoA is higher...) for a given airspeed, in addition to being more
>>>>> forcefully held there, both effects of course due to the rope's
>>>>> pull. The elevator is the same size whether on tow or free flight
>>>>> though, so the authority it can exert against the countering forces
> is
>>>>> proportionately lower than in free flight...
>>>>>
>>>>> The fix is the same regardless of why though- more speed... please!
>>>>> (wings rocking in vain...)
>>>>>
>>>>> -Paul

[The Real Doctor]

On 16 Mar, 13:28, " > wrote:
> On Mar 14, 8:43*am, The Real Doctor > wrote:

> > Bad example, since tow planes pull - give or take a wee bit -
> > horizontally, regardless of climb angle.

> Never been towed behind an Ag-cat, have you?

265 horse Pawnee count?

> Nothing horizontal about that evolution!

Tug wheels on the horizon, glider just above the prop wash, just like
everything else.

Ian

[The Real Doctor]

On 16 Mar, 13:00, Derek Copeland > wrote:

> ... but the K13 definitely starts 'wallowing' below
> about 52knots, whereas its normal free flight stalling speed is about
> 36knots.

OK, here's my latest theory. Gliders have bigger wingspans than tugs.
Therefore the outer bit of each glider wing is in the upwards moving
bit of the tug's tip vortices, and the centre bit is in the downwards
going bit. Effective result: much higher angle of attack at the tips,
particularly since the nose has to come up to maintain AoA at the
centre. Hence wash-in, tips near stall, downgoing aileron actually
stalling, reduced control effectiveness, wallowing.

Questions: does it happen as much out to one side hen boxing the wake?
Does it happen when the tug - a motorglider - has the same span as the
tug?

Ian

[danlj]

On Mar 8, 11:03*am, Brad > wrote:
> I know this has come up before in RAS. But thought I would bring up
> the subject again.
>
> For a club looking at long term projections, which at some point will
> include either sticking a "new" engine on a Pawnee or getting rid of
> it, does it make sense to start evaluating getting a 2 place
> motorglider to serve as a tug and also as a touring/training tool?
>
> Can a MG tug pull a loaded 2 place Blanik on a standard day at SL,
> from a 1800' grass strip? Or perhaps such a tug could be used to tow
> the members single place ships, and the heavier/ 2-place ships stow
> behind the clubs remaining pawnee?
>
> We are blessed with 3 towplanes in our club, there are upcoming
> factors that will/are causing us to look at several different
> scenarios and am wondering if tossing a MG into the mix might be one
> such solution.
>
> Brad

Ten - twelve years ago we had a little 4-man club that towed a fully-
loaded Blanik L-13 off a *paved* runway with a Piper Pacer. I think it
had a 135hp engine. I think the ground roll on asphalt for the Pacer
was about 2000'. We had grass available, and really didn't want to
find out how long the ground roll would be, partly because the
obstacle-clearance distance was very poor due to the anemic climb
rate. On a hot day, it might be 1 kt, and really needed thermals to
get up. There was no climb in sink. Our field has nearby landable farm
terrain, and believe me, I always made sure I knew exactly which field
I'd be landing in if the Pacer had an engine problem, and had a hand
on the yellow handle all the way to 500+ ft agl.

It's the "fully loaded Blanik" phrase that give me pause more than the
"motoglider as tug" phrase.

As other responders have implied, the proof of the pudding is very
much in the eating. Testing is always a good idea.

[sisu1a]

> OK, here's my latest theory. Gliders have bigger wingspans than
tugs.
> Therefore the outer bit of each glider wing is in the upwards moving
> bit of the tug's tip vortices, and the centre bit is in the downwards
> going bit. Effective result: much higher angle of attack at the tips,
> particularly since the nose has to come up to maintain AoA at the
> centre. Hence wash-in, tips near stall, downgoing aileron actually
> stalling, reduced control effectiveness, wallowing.
>
> Questions: does it happen as much out to one side hen boxing the wake?
> Does it happen when the tug - a motorglider - has the same span as the
> tug?
>
> Ian

This thread is interesting and I thought it deserved a new title, even
though I have nothing to really add to it at this point (except that
rudder effectiveness is also reduced with a tethered nose...)

-Paul

PS. the rope's tugging of the nose is quite severe at the outside
positions while boxing the wake, and greatly affects control authority
(especially in draggy fat ships with ineffective controls like a
2-33...), so the experiment you propose might not yield as much
insight as we'd like.

[Nyal Williams[_2_]]

Ah, chemistry; I never did that either. Shoulda tried Wikipedia first; was
sure it was some sort of shortened up slang. Thanks.


At 15:13 16 March 2009, Bruce wrote:
>monomer
>Dimer
>Trimer
>....
>
>
>Nyal Williams wrote:
>> I know this is not alt.usage.english, but what is the etymology of
this
>> word? Is it slang? Jargon? Engineer language? Is it Di-mer or
Dim-er
>or
>> dimer... something or other? I'm just a poor musicologist trying to
>figure
>> stuff out.
>>
>> At 14:27 16 March 2009, Bruce wrote:
>>> Sorry Nyal - excessive vocab use. Dimer == related pair of.
>>>
>>> In this case two vortices - one off each wingtip that interact to
>create
>>
>>> a roughly symmetrical "geared disk" shape behind the wing. With the

>>> downward part of the vortex from each wingtip merging with the
downward
>
>>> flow from the other.
>>>
>>> If you drive behind a (modern / streamlined not SUV) car in the rain
or
>
>>> snow you can see the dimer it creates. Formula one and Nascar rear
>wings
>>
>>> also create impressive examples...
>>>
>>>
>>>
>>> Nyal Williams wrote:
>>>> Help; what is "dimer" ?
>>>>
>>>>
>>>> At 14:09 15 March 2009, Bruce wrote:
>>>>> Paul
>>>>>
>>>>> There is a large scale vortex dimer operating behind any aircraft,
>and
>>
>>>>> particularly behind high wing loading, heavy short winged things
like
>>
>>>>> Pawnees.
>>>>>
>>>>> The wake we fly above in high tow is the turbulent propeller wake,
>but
>>
>>>>> we would have to be impossibly high and/or far back to avoid the
>>>>> downward moving centre section of the dimer.
>>>>>
>>>>> I saw a picture using smoke trails that demonstrates the scale and
>>> power
>>>>> of this some years back -
>>>>> http://www.nasa.gov/audience/forstudents/k-4/dictionary/Vortex.html
>>>>>
>>>>> There is a more impressive video at
>>>>> http://www.youtube.com/watch?v=uy0hgG2pkUs&NR=1
>>>>>
>>>>> So - given that you are flying in a field of air that has a
>> significant
>>>>> downward component, maybe you do have a higher angle of attack on
the
>>
>>>>> wings.
>>>>>
>>>>>
>>>>> Bottom line is that even in the smooth air above the propwash you
are
>>
>>>>> still in air affected by the tug.
>>>>>
>>>>> Bruce
>>>>>
>>>>>
>>>>> sisu1a wrote:
>>>>>>> Agreed. My money is on the towplane wake.
>>>>>> I put my monies on the elevator authority/AoA ratio. We fly
above
>>>>>> the wing wake (USA...) in most cases, in relatively clean air, but
>>>>>> sometimes in the clean air below it. Box the wake, it will tell
you
>>>>>> where it is and where it isn't...
>>>>>>
>>>>>> But typically glider's noses, on tow, are unnaturally high (and
>> thus
>>>>>> AoA is higher...) for a given airspeed, in addition to being more
>>>>>> forcefully held there, both effects of course due to the rope's
>>>>>> pull. The elevator is the same size whether on tow or free
flight
>>>>>> though, so the authority it can exert against the countering
forces
>> is
>>>>>> proportionately lower than in free flight...
>>>>>>
>>>>>> The fix is the same regardless of why though- more speed...
please!
>>>>>> (wings rocking in vain...)
>>>>>>
>>>>>> -Paul
>

[Bob Cook[_2_]]

This theory certainly explains the poor aileron control or wallowing at
slow towing speeds, even though it seems the wing, (on average) is no
where near the stall angle of attack.

I have seen guys try to fly model planes with wash in instead of wash out
in the wings. Doesn't work too well!


Cookie





At 15:33 16 March 2009, The Real Doctor wrote:
>On 16 Mar, 13:00, Derek Copeland wrote:
>
>> ... but the K13 definitely starts 'wallowing' below
>> about 52knots, whereas its normal free flight stalling speed is about
>> 36knots.
>
>OK, here's my latest theory. Gliders have bigger wingspans than tugs.
>Therefore the outer bit of each glider wing is in the upwards moving
>bit of the tug's tip vortices, and the centre bit is in the downwards
>going bit. Effective result: much higher angle of attack at the tips,
>particularly since the nose has to come up to maintain AoA at the
>centre. Hence wash-in, tips near stall, downgoing aileron actually
>stalling, reduced control effectiveness, wallowing.
>
>Questions: does it happen as much out to one side hen boxing the wake?
>Does it happen when the tug - a motorglider - has the same span as the
>tug?
>
>Ian
>

[Bob Cook[_2_]]

Agree with Ian,

I thought we agreed that the force of the tow rope acts at whatever angle
the rope meets the glider. This means it is a factor the RELATIVE
position of the glider to the towplane, NOT the direction of flight.

That being said, I once was at an airshow where Oscar Bosch was towed by
the famous plane called "Sampson". Sampson held all kinds of "time to
climb" records.

Old Oscar never really caught up with Sampson, and was in the lowest low
tow I think I have ever seen. There was plenty of up thrust on that tow
rope!


Cookie








At 15:29 16 March 2009, The Real Doctor wrote:
>On 16 Mar, 13:28, " wrote:
>> On Mar 14, 8:43=A0am, The Real Doctor wrote:
>
>> > Bad example, since tow planes pull - give or take a wee bit -
>> > horizontally, regardless of climb angle.
>
>> Never been towed behind an Ag-cat, have you?
>
>265 horse Pawnee count?
>
>> Nothing horizontal about that evolution!
>
>Tug wheels on the horizon, glider just above the prop wash, just like
>everything else.
>
>Ian
>

[bildan]

On Mar 16, 9:33*am, The Real Doctor > wrote:
> On 16 Mar, 13:00, Derek Copeland > wrote:
>
> > ... but the K13 definitely starts 'wallowing' below
> > about 52knots, whereas its normal free flight stalling speed is about
> > 36knots.
>
> OK, here's my latest theory. Gliders have bigger wingspans than tugs.
> Therefore the outer bit of each glider wing is in the upwards moving
> bit of the tug's tip vortices, and the centre bit is in the downwards
> going bit. Effective result: much higher angle of attack at the tips,
> particularly since the nose has to come up to maintain AoA at the
> centre. Hence wash-in, tips near stall, downgoing aileron actually
> stalling, reduced control effectiveness, wallowing.
>
> Questions: does it happen as much out to one side hen boxing the wake?
> Does it happen when the tug - a motorglider - has the same span as the
> tug?
>
> Ian

I fly a 20 meter glider that sometimes is loaded with 60 gallons of
water and I have to say that I've not noticed this effect when towing
behind either a Pawnee or a Cub. Aileron control on tow is no worse
than in free flight. The slower Cub is easier to follow if anything.
Over the years, I've been towed by everything from a 85HP 7AC champ to
a 400HP Pawnee Brave. It's all pretty much the same - just follow the
tug.

What I have noticed is that a glider in high tow will tend to drift
laterally toward the center which is generally a good thing. I
haven't noticed any of the pitch related stuff people are talking
about.

On one occasion a pilot was complaining that his glider needed more
than 80 knots on tow to feel "right". I flew the glider and radioed
the tow pilot to reduce towing speed until he started complaining
about control authority but I saw no problems with this glider. The
only thing I could point to was the pilots tendency to over control
his glider and to use positive flap "to see the tow plane better". I
suspect the excess flap was limiting aileron authority. He would have
been better off with some cushions.

[The Real Doctor]

On 16 Mar, 16:41, danlj > wrote:

> Ten - twelve years ago we had a little 4-man club that towed a fully-
> loaded Blanik L-13 off a *paved* runway with a Piper Pacer.

I was once approaching my then club - Borders GC - to land when I saw
a Bocian lining up for takeoff behind the club's Pawnee 150. "Oh, this
should be interesting" I thought and I found a bit of lift to park in
and watch the fun.

The fun turned out to be the combination making three full circuits of
the airfield, never getting above a couple of hundred feet, then the
tug waving off and the two landing together (no on-tow landings in the
UK, unlike France).

Nice wee tug for a single seater, and whisper quiet with a four blade
prop and silencer, but it couldn't pull the skin off a two-seater rice
pudding. It later had the engine upped to 160hp (as far as the CAA
would go without an STC), a 2-blade prop and lost the silencer.

Ian

[The Real Doctor]

On 16 Mar, 17:08, sisu1a > wrote:

> This thread is interesting and I thought it deserved a new title, even
> though I have nothing to really add to it at this point (except that
> rudder effectiveness is also reduced with a tethered nose...)

Thanks! I'm finding this interesting too. I might have to tuft a wing
and go towing later this year.

> PS. the rope's tugging of the nose is quite severe at the outside
> positions while boxing the wake, and greatly affects control authority
> (especially in draggy fat ships with ineffective controls like a
> 2-33...), so the experiment you propose might not yield as much
> insight as we'd like.

Good point. It's a lot easier with a belly hook and I think AN-2s can
tow off the interplane struts, which would give a bit of offset.
Unfortunately I don't know anyone with an AN-2 ...

Ian

[Mike the Strike]

I ran a quick calculation of the expected catenary sag in a towrope.
With a 200-foot nylon/poly towrope and 75 pounds of tension, the sag
amounts to less than a foot, ignoring aerodynamic forces. Under
steady towing conditions, it can probably be ignored.

Mike

[Bob Cook[_2_]]

Mike,

I agree that the sag of the rope is small and can probably be ignored.

My calculations do not show drag values as high as 75 lbs however, so the
sag may be (a little) more that your calculations.

I figure a modern glider will have something in the neighborhood of 30 lbs
drag, while an old "boat" of a glider will be in the 50 lb range. (Am I
missing something here?)

I calculated the sag effect in the rope a different way. I figure a
typical 200' poly rope weighs about 4 lbs. The tow plane supports one
end, while the glider supports the other. I figured in a 2 lb down force
on the glider. Since, during tow, lift is essentially the same as weight,
you have to get an extra 2 lbs of lift to compensate for the weight of the
rope. If you figure the weight of the glider in the 700 to 900 lb range,
it is a tiny percentage of extra lift needed due to the rope. (like 2/10
of one percent.)

Same is true if the rope pulls (slightly) upward or downward, the change
in lift is tiny.

Now consider free flight in a 45 degree bank. Lift is increased by 40%.
The glider still will fly pretty well at relatively slow speeds in a 45
degree bank. Now look at 60 degree bank where lift is doubled......

Just a couple more thoughts about the forces on that tow rope:

When the tug is flying without a glider, the rope does not hang straight
down. The "drag" on the rope causes the rope to fly at quite a flat
angle. We could say that some of the weight of the rope is being
supported by the air drag on the rope. But then when a glider is on tow,
and the rope is essentially level, with a sag, I guess the front half of
hte rope is being lifted by air drag, while the back half of the rope is
being pushed down by air drag. So I think that just adding the weight of
the rope as a downward component will work in calcualtions.

Long way to get to the point however. I don't see how the tow rope can
be the culprit in this "too slow tow" situation.

Cookie









At 23:30 16 March 2009, Mike the Strike wrote:
>I ran a quick calculation of the expected catenary sag in a towrope.
>With a 200-foot nylon/poly towrope and 75 pounds of tension, the sag
>amounts to less than a foot, ignoring aerodynamic forces. Under
>steady towing conditions, it can probably be ignored.
>
>Mike
>
>

[Mike the Strike]

I calculated the rope tension as 25 pounds of aerodynamic drag (40:1
for a 1,000 pound glider) and 50 pounds to provide the force necessary
for the glider to climb. That's in the right ball park. The tension
obviously increases with the rate of climb since you have to impart
energy to the glider for it to ascend.

Mike

[Derek Copeland[_2_]]

I understand that Bill's glider is a Nimbus 2, but don't know which
version.

I used to have a share in an early Nimbus 2 with an all flying tailplane
and just a belly hook. It was a bit twitchy in pitch, but otherwise very
easy to aerotow. I think the reasons for this are that this type has a
very large wing area (with a corresponding low stalling speed), and the
long wings give plenty of lateral damping. When retrieving from fields,
with negative flap selected, the wings would often level themselves in the
propwash when the tuggie opened up the throttle, even before the glider
started to roll. The little Standard Cirrus I own now is actually much
more of a handful on aerotow, particularly if you have to start without a
wingtip runner. It normally stalls at just below 40knots in free flight,
but doesn't feel comfortable below 60knots on tow.

I have noticed that in general, big span gliders seem to be easier to
aerotow than little gliders, despite the reduced aileron effectiveness.
Maybe this is because the outer part of the wings (where the ailerons are
located) are outside most of the slipstream of the usually much smaller
span tugs. This might also explain why tows behind motor gliders with
bigger wingspans seem to be more difficult, if the tug's slipstream is
the main problem.

Derek Copeland

At 20:45 16 March 2009, bildan wrote:
>>
>I fly a 20 meter glider that sometimes is loaded with 60 gallons of
>water and I have to say that I've not noticed this effect when towing
>behind either a Pawnee or a Cub. Aileron control on tow is no worse
>than in free flight. The slower Cub is easier to follow if anything.
>Over the years, I've been towed by everything from a 85HP 7AC champ to
>a 400HP Pawnee Brave. It's all pretty much the same - just follow the
>tug.
>
>What I have noticed is that a glider in high tow will tend to drift
>laterally toward the center which is generally a good thing. I
>haven't noticed any of the pitch related stuff people are talking
>about.
>
>On one occasion a pilot was complaining that his glider needed more
>than 80 knots on tow to feel "right". I flew the glider and radioed
>the tow pilot to reduce towing speed until he started complaining
>about control authority but I saw no problems with this glider. The
>only thing I could point to was the pilots tendency to over control
>his glider and to use positive flap "to see the tow plane better". I
>suspect the excess flap was limiting aileron authority. He would have
>been better off with some cushions.
>

[Peter Higgs]

Hi Mike, what are you towing.. A Horsa Glider?
75 pounds x (L/D) of 40 = 3000 Lbs
I would think the tow rope 'Thrust' would be nearer 20-25 lbs.
Even so, your sag only increases to 3 ft.

Pilot Pete

At 23:30 16 March 2009, Mike the Strike wrote:
>I ran a quick calculation of the expected catenary sag in a towrope.
>With a 200-foot nylon/poly towrope and 75 pounds of tension, the sag
>amounts to less than a foot, ignoring aerodynamic forces. Under
>steady towing conditions, it can probably be ignored.
>
>Mike
>
>

[The Real Doctor]

On 17 Mar, 10:00, Peter Higgs > wrote:
> Hi Mike, what are you towing.. *A Horsa Glider?
> 75 pounds x (L/D) of 40 * = * 3000 Lbs
> I would think the tow rope 'Thrust' would be nearer 20-25 lbs.

We covered this a few posts back. During the two the glider's flight
path is - obviously - inclined upwards, so the lift vector is inclined
backwards, giving a horizontal component of lift for the tug to
overcome. If the glider is doing 60kt, climbing at 4kt and weighs
1000lb then this backwards lift is close to 1000 * sin (arctan (4/60))
which is 67lb.

Ian

[Bob Cook[_2_]]

Ian,

Of course I was missing something! (I think better in the morning than
evening.)

When you figure in the DIRECTION OF FLIGHT (upward and forward) the
Thrust vector (tension on the rope) has to be greater than drag alone.

Our tow plane can tow a 1000# glider at about 1:10, so the pull on the
rope would be even a little more than you calculated, (1:15).

I do the vector calcualtions with the following "standards":

Gravity acts downward, lifts acts perpendicular to the direction of
flight, drag acts parallel to the direction of flight. Although it is
nice to think of thrust as acting parallel to the direction of flight, we
have shown that this is not necesarily true and thrust can act slightly
upward or downward compared to the direction of flight.

I think that we can conclude the following: On tow, lift is almost
unchanged, even at different climb angles. Even if the thrust from the
rope is not parallel to the direction of flight, lift is hardly changed.

The tension on the rope, however varies with the direction of flight, is
greater than drag alone, and is signifigant.

Cookie





At 10:15 17 March 2009, The Real Doctor wrote:
>On 17 Mar, 10:00, Peter Higgs wrote:
>> Hi Mike, what are you towing.. =A0A Horsa Glider?
>> 75 pounds x (L/D) of 40 =A0 =3D =A0 3000 Lbs
>> I would think the tow rope 'Thrust' would be nearer 20-25 lbs.
>
>We covered this a few posts back. During the two the glider's flight
>path is - obviously - inclined upwards, so the lift vector is inclined
>backwards, giving a horizontal component of lift for the tug to
>overcome. If the glider is doing 60kt, climbing at 4kt and weighs
>1000lb then this backwards lift is close to 1000 * sin (arctan (4/60))
>which is 67lb.
>
>Ian
>

[Bob Cook[_2_]]

Ian,

I do think of this (Climbing flight) as the "lift vector being tilted
rearward."

But we should add (vector addition) the force of gravity (downward) the to
force of lift (upward and rearward).

We find that these two forces nearly cancel each other except for a
realtively small resultant force, parallel to the line of flight.

This resultant force could be added to drag, therefore thrust would have
to be the sum of this force plus drag, but in the opposite direction.


Cookie






At 10:15 17 March 2009, The Real Doctor wrote:

>We covered this a few posts back. During the two the glider's flight
>path is - obviously - inclined upwards, so the lift vector is inclined
>backwards, giving a horizontal component of lift for the tug to
>overcome. If the glider is doing 60kt, climbing at 4kt and weighs
>1000lb then this backwards lift is close to 1000 * sin (arctan (4/60))
>which is 67lb.
>
>Ian
>

[Bob Cook[_2_]]

OK, all this talk about the aerodynamics of towing brings me to something
about the aerodynamics of gliding!

The following question came up at a recent club meeting. I believe the
question was originally on the Bronze Badge Exam.

Q) What force provides the forward motion necessary to move a glider
through the air?

a) lift
b) centripetal force
c) gravity


I contend that it is a flawed question, and that none of the choices for
answers are (completly) correct!

Any thoughts?


Cookie

[Andreas Maurer]

On Sun, 15 Mar 2009 10:36:59 +0200, Ian > wrote:

>Does anybody know what the certification situation is for towing with
>ultralight catorgory aircraft (450kg AUW) is in Europe these days? How
>many tow planes are certified for tugging and what mass of glider are
>they permitted to tow?

Most ultralights in Germany are certified to tow gliders, most of
them are limited to 650 kg AUW of the glider, some of them to 750 kg.

Bye
Andreas

[The Real Doctor]

On 17 Mar, 12:30, Bob Cook > wrote:
> OK, all this talk about the aerodynamics of towing brings me to something
> about the aerodynamics of gliding!
>
> The following question came up at a recent club meeting. *I believe the
> question was originally on the Bronze Badge Exam.
>
> Q) What force provides the forward motion necessary to move a glider
> through the air?
>
> a) lift
> b) centripetal force
> c) gravity
>
> I contend that it is a flawed question, and that none of the choices for
> answers are (completly) correct!

It's a flawed question because, as Newton pointed out, NO force is
required for steady motion.

Mind you, it's amazing how many people think the power source for
soaring flight is gravity.

Ian

[The Real Doctor]

On 17 Mar, 12:30, Bob Cook > wrote:

> But we should add (vector addition) the force of gravity (downward) the to
> force of lift (upward and rearward).
>
> We find that these two forces nearly cancel each other except for a
> realtively small resultant force, parallel to the line of flight.
>
> This resultant force could be added to drag, therefore thrust would have
> to be the sum of this force plus drag, but in the opposite direction.

Agreed.

Ian

[Darryl Ramm]

On Mar 17, 8:53*am, The Real Doctor > wrote:
> On 17 Mar, 12:30, Bob Cook > wrote:
>
> > OK, all this talk about the aerodynamics of towing brings me to something
> > about the aerodynamics of gliding!
>
> > The following question came up at a recent club meeting. *I believe the
> > question was originally on the Bronze Badge Exam.
>
> > Q) What force provides the forward motion necessary to move a glider
> > through the air?
>
> > a) lift
> > b) centripetal force
> > c) gravity
>
> > I contend that it is a flawed question, and that none of the choices for
> > answers are (completly) correct!
>
> It's a flawed question because, as Newton pointed out, NO force is
> required for steady motion.
>
> Mind you, it's amazing how many people think the power source for
> soaring flight is gravity.
>
> Ian

No force? Newton's first law tells you that for constant velocity the
forward component of lift must match the drag (or the glider would
accelerate or decelerate, and if so then the second law tells you how
much).

There is drag force so a forward force is required to counter that or
the glider would decelerate, and something has to overcome that force.
And that is the forward component of lift. Answer (a) is clearly the
best answer and this question as is I think is a suitable test of a
basic understanding of what is going on.

Gravitational potential energy gained by soaring (climbing in lift) or
by winching/towing/motorglider engine is the power source for gliding
flight. Thankfully we have several ways to charge that power source.

Darryl

[Bob Cook[_2_]]

Ian,

You are of course correct that once an object is in motion, there is no
force required to keep it in motion.

This is why the SUM of all forces acting on a glider in unaccelerated
flight is zero.



Accelerated flight: forces are not in balance, hence change, or
acceleration (speed and / or direction).

Now it did take some (unbalanced) force(s) to get the glider moving in the
first place.

But why does the glider take on the flight path that it does?

If we could say "drop" a glider from a helium balloon. The glider would
at first have no motion at all.

Eventually the glider would achieve a nearly horizontal flight path,
(40:1) with the correct control inputs. Initially there would be
unbalanced forces, resulting in increasing velocity, and changing
direction.

Once the forces became "in balance" (their sum = zero) the glider would
achieve steady flight.


My point is that Gravity alone will not have the above effect.

I don't like your use of the word power below however. Force, energy and
power are not the same. People often confuse them.

Cookie




At 15:53 17 March 2009, The Real Doctor wrote:
>On 17 Mar, 12:30, Bob Cook wrote:
>> OK, all this talk about the aerodynamics of towing brings me to
>something
>> about the aerodynamics of gliding!
>>
>> The following question came up at a recent club meeting. =A0I believe
>the
>> question was originally on the Bronze Badge Exam.
>>
>> Q) What force provides the forward motion necessary to move a glider
>> through the air?
>>
>> a) lift
>> b) centripetal force
>> c) gravity
>>
>> I contend that it is a flawed question, and that none of the choices
for
>> answers are (completly) correct!
>
>It's a flawed question because, as Newton pointed out, NO force is
>required for steady motion.
>
>Mind you, it's amazing how many people think the power source for
>soaring flight is gravity.
>
>Ian
>

[The Real Doctor]

On 17 Mar, 16:16, Darryl Ramm > wrote:

> Gravitational potential energy gained by soaring (climbing in lift) or
> by winching/towing/motorglider engine is the power source for gliding
> flight.

So what's the power source when climbing in a thermal, when
gravitational potential enegry is decreasing?

If you take a 500m launch, you have 4,905kJ/kgof potential energy to
play with - no more and no less. As we all know, that's about enough
energy to keep a glider flying for ten minutes or so. For flights of
any longer, another power source is needed, and though gravitational
potential may be used as a store from time to time, it is only the
store, not the source.

Ian

[The Real Doctor]

On 17 Mar, 17:30, Bob Cook > wrote:

> I don't like your use of the word power below however. *Force, energy and
> power are not the same. *People often confuse them.

> >Mind you, it's amazing how many people think the power source for
> >soaring flight is gravity.

I don't. I used "power" because I meant "power"!

Ian

[Bob Whelan[_3_]]

Darryl Ramm wrote:
> On Mar 17, 8:53 am, The Real Doctor > wrote:
>> On 17 Mar, 12:30, Bob Cook > wrote:
>>
>>> OK, all this talk about the aerodynamics of towing brings me to something
>>> about the aerodynamics of gliding!
>>> The following question came up at a recent club meeting. I believe the
>>> question was originally on the Bronze Badge Exam.
>>> Q) What force provides the forward motion necessary to move a glider
>>> through the air?
>>> a) lift
>>> b) centripetal force
>>> c) gravity
>>> I contend that it is a flawed question, and that none of the choices for
>>> answers are (completly) correct!
>> It's a flawed question because, as Newton pointed out, NO force is
>> required for steady motion.
>>
>> Mind you, it's amazing how many people think the power source for
>> soaring flight is gravity.
>>
>> Ian
>
> No force? Newton's first law tells you that for constant velocity the
> forward component of lift must match the drag (or the glider would
> accelerate or decelerate, and if so then the second law tells you how
> much).
>
> There is drag force so a forward force is required to counter that or
> the glider would decelerate, and something has to overcome that force.
> And that is the forward component of lift. Answer (a) is clearly the
> best answer and this question as is I think is a suitable test of a
> basic understanding of what is going on.
>
> Gravitational potential energy gained by soaring (climbing in lift) or
> by winching/towing/motorglider engine is the power source for gliding
> flight. Thankfully we have several ways to charge that power source.
>
> Darryl
Yebbut...imagine a glider magically inserted into earth's atmosphere
after global warming has removed all grabbity. What gets it moving?

Bob - going to gather my chicken's eggs, now - W.

[Mike the Strike]

All of the energy to maintain a glider in steady flight through still
air is derived from gravity - there is no other source of energy.

Potential energy of the glider is converted to kinetic energy in the
form of forward motion in order to overcome drag. In a 1000 lb
sailplane with a 40:1 glide, the wings will develop about 25 lbs of
forward thrust to overcome drag and about 1,000.3 lbs of total lift.
The glide angle is about 1.5 degrees.

I don't understand why people have a problem understanding this - it's
a very similar problem to a ball rolling down an inclined plane.

Just basic physics!

Mike

[Darryl Ramm]

On Mar 17, 11:13*am, The Real Doctor >
wrote:
> On 17 Mar, 16:16, Darryl Ramm > wrote:
>
> > Gravitational potential energy gained by soaring (climbing in lift) or
> > by winching/towing/motorglider engine is the power source for gliding
> > flight.
>
> So what's the power source when climbing in a thermal, when
> gravitational potential enegry is decreasing?
>
> If you take a 500m launch, you have 4,905kJ/kgof potential energy to
> play with - no more and no less. As we all know, that's about enough
> energy to keep a glider flying for ten minutes or so. For flights of
> any longer, another power source is needed, and though gravitational
> potential may be used as a store from time to time, it is only the
> store, not the source.
>
> Ian

Say what? Gravitational potential energy *increases* not decreases as
the glider climbs.

And that "store" is the transfer mechanism that allows the glider to
glide. There is no other mechanism. With no gravity, even with an
atmosphere (which would be difficult to arrange), your glider could
not glide. Even if you could arrange to launch the glider with some
airspeed drag would eventually slow it down to a dead stop. Any air
currents would move the glider around but you would not be able to
soar/glider based on those. (OK some forms of dynamic soaring might be
possible).

When you are climbing in a thermal the source of power (or increase in
energy, if you prefer but power is a perfectly correct term) is the
force of the raising airmass lifting the glider against gravitational
pull. The energy gained is just the difference in gravitational
potential energy between the different altitudes, the average power
spent doing this is that difference in potential energy divided by the
time to climb. Yes the force to deliver that energy/power came from
somewhere i.e. the glider had a tiny but real effect on the raising
air mass.

Want some ball park numbers? A 400kg (880 pounds) glider climbing in
a strong thermal at 5 meters per second (~ 10 knots). Is gaining
energy at a rate = g * M * v ~ 9.8 m/s2 * 400 kg * 5 m/s = 19 kW or 26
horsepower.

Guy's this has wandered into junior-high school level physics. Time to
either let Bob's chickens' contribute to this thread or let it die.

Darryl

[Surfer!]

In message
>, The
Real Doctor > writes
>On 17 Mar, 12:30, Bob Cook > wrote:
>> OK, all this talk about the aerodynamics of towing brings me to something
>> about the aerodynamics of gliding!
>>
>> The following question came up at a recent club meeting. *I believe the
>> question was originally on the Bronze Badge Exam.
>>
>> Q) What force provides the forward motion necessary to move a glider
>> through the air?
>>
>> a) lift
>> b) centripetal force
>> c) gravity
>>
>> I contend that it is a flawed question, and that none of the choices for
>> answers are (completly) correct!
>
>It's a flawed question because, as Newton pointed out, NO force is
>required for steady motion.
>
>Mind you, it's amazing how many people think the power source for
>soaring flight is gravity.
>
>Ian

If gravity isn't acting why does the glider need to produce lift with
it's wings?

--
Surfer!
Email to: ramwater at uk2 dot net

[KevinFinke]

Indeed this has degraded into junior high physics and if I remember
most junior high students got poor grades in physics.... I think I'll
pull out the chickens...

If Bob's chicken is sliding down a hill.. what force causes it to
continue sliding?

It is gravity. There is no forward component of lift that contributes
to forward motion. By definition lift is perpendicular to the
direction of motion and drag is parallel. How can lift make anything
move forward if it acts perpendicular to the line of motion? In steady
state gliding flight, Net Lift is equal to Weight times the cosine of
the gliding angle. This value will "always" be less than weight.
Please don't try to sound smart and point out the obvious answer that
if the angle is zero, lift equals weight. True, but then it fails to
satisfy the condition: Steady state gliding flight.

As for the thread that this originally morphed into. Why does my
glider handle so poorly when being pulled slowly on tow? The answer
lies in the dynamics of the tow. It's not obvious when the towing
force is steady state. The clue is in how the glider behaves when the
tow force is changing because of gusts and the fact that you are
trying to maintain position behind the towplane during a climb. The
wing will see a greater variation in angle of attack as a function of
changes in rope tension at low airspeed than it does at higher speed.
Combining this effect with low dynamic pressure results in reduced
aileron effectiveness and therefore poor handling.

-Kevin

[Darryl Ramm]

On Mar 17, 2:41*pm, KevinFinke > wrote:
> Indeed this has degraded into junior high physics and if I remember
> most junior high students got poor grades in physics.... I think I'll
> pull out the chickens...
>
> If Bob's chicken is sliding down a hill.. what force causes it to
> continue sliding?
>
> It is gravity. There is no forward component of lift that contributes
> to forward motion. By definition lift is perpendicular to the
> direction of motion and drag is parallel. How can lift make anything
> move forward if it acts perpendicular to the line of motion? In steady
> state gliding flight, Net Lift is equal to Weight times the cosine of
> the gliding angle. This value will "always" be less than weight.
> Please don't try to sound smart and point out the obvious answer that
> if the angle is zero, lift equals weight. True, but then it fails to
> satisfy the condition: Steady state gliding flight.
>
> As for the thread that this originally morphed into. Why does my
> glider handle so poorly when being pulled slowly on tow? The answer
> lies in the dynamics of the tow. It's not obvious when the towing
> force is steady state. The clue is in how the glider behaves when the
> tow force is changing because of gusts and the fact that you are
> trying to maintain position behind the towplane during a climb. The
> wing will see a greater variation in angle of attack as a function of
> changes in rope tension at low airspeed than it does at higher speed.
> Combining this effect with low dynamic pressure results in reduced
> aileron effectiveness and therefore poor handling.
>
> -Kevin

Sorry back to the hen house for you. Lift is *not* perpendicular to
the direction of motion. Lift is perpendicular to the airflow. That
small difference is going to get you in trouble in these arguments.

The chicken sliding down the hill is propelled forward by the
component of the force pushing up on it from the sloped surface in
response to gravity pushing it down onto that surface. Gravity
operates vertically down through the center of mass and without that
sloping surface the chicken would not move forward. There is a
component of this force vector pushing forward and the chicken moves
forward down the slope.

This is exactly analogous to the lift vector pointed forward on the
glider and the forward component of that vector providing the force to
overcome drag in forward flight. Gravity operates on the glider though
it's center of mass, without that lift vector pointed forward the
glider would not (continue to) move forward.

Gravity, without the thrust vector leaning forward, does not explain
how a glider glides through the air, however Gravity is what provides
the power/energy to do this (or store for that energy if you prefer).

The above discussion applies equally to chickens and African or
European swallows.

However I actually think you are onto a good line of reasoning about
the on-tow issue.

Darryl

[Nyal Williams[_2_]]

Jim White had it right, way back several weeks ago. What makes it work is
MAGIC.



At 22:41 17 March 2009, Darryl Ramm wrote:
>On Mar 17, 2:41=A0pm, KevinFinke wrote:
>> Indeed this has degraded into junior high physics and if I remember
>> most junior high students got poor grades in physics.... I think I'll
>> pull out the chickens...
>>
>> If Bob's chicken is sliding down a hill.. what force causes it to
>> continue sliding?
>>
>> It is gravity. There is no forward component of lift that contributes
>> to forward motion. By definition lift is perpendicular to the
>> direction of motion and drag is parallel. How can lift make anything
>> move forward if it acts perpendicular to the line of motion? In steady
>> state gliding flight, Net Lift is equal to Weight times the cosine of
>> the gliding angle. This value will "always" be less than weight.
>> Please don't try to sound smart and point out the obvious answer that
>> if the angle is zero, lift equals weight. True, but then it fails to
>> satisfy the condition: Steady state gliding flight.
>>
>> As for the thread that this originally morphed into. Why does my
>> glider handle so poorly when being pulled slowly on tow? The answer
>> lies in the dynamics of the tow. It's not obvious when the towing
>> force is steady state. The clue is in how the glider behaves when the
>> tow force is changing because of gusts and the fact that you are
>> trying to maintain position behind the towplane during a climb. The
>> wing will see a greater variation in angle of attack as a function of
>> changes in rope tension at low airspeed than it does at higher speed.
>> Combining this effect with low dynamic pressure results in reduced
>> aileron effectiveness and therefore poor handling.
>>
>> -Kevin
>
>Sorry back to the hen house for you. Lift is *not* perpendicular to
>the direction of motion. Lift is perpendicular to the airflow. That
>small difference is going to get you in trouble in these arguments.
>
>The chicken sliding down the hill is propelled forward by the
>component of the force pushing up on it from the sloped surface in
>response to gravity pushing it down onto that surface. Gravity
>operates vertically down through the center of mass and without that
>sloping surface the chicken would not move forward. There is a
>component of this force vector pushing forward and the chicken moves
>forward down the slope.
>
>This is exactly analogous to the lift vector pointed forward on the
>glider and the forward component of that vector providing the force to
>overcome drag in forward flight. Gravity operates on the glider though
>it's center of mass, without that lift vector pointed forward the
>glider would not (continue to) move forward.
>
>Gravity, without the thrust vector leaning forward, does not explain
>how a glider glides through the air, however Gravity is what provides
>the power/energy to do this (or store for that energy if you prefer).
>
>The above discussion applies equally to chickens and African or
>European swallows.
>
>However I actually think you are onto a good line of reasoning about
>the on-tow issue.
>
>Darryl
>
>
>
>

[Bob Cook[_2_]]

At 18:16 17 March 2009, The Real Doctor wrote:

Ian,

YOU did use the term "power" correctly.

What I didn't like about it was that the question I posed referred to
force. (OK, so you gave us some additional information)

I agree that a sailplane, in gliding flight, in still air, has no
"power" at all. (Although, as you said, some wrongly believe that
gliders are "gravity powered")

By some of the responses, I think I am correct in assuming that some
confuse power, energy, and force.

So I again ask, (not to Ian, but to some of the others who answered my
question with "gravity",) how can gravity alone, a force which acts
vertically downward, impart forward motion to a glider or anything for
that matter?


Cookie


>
>I don't. I used "power" because I meant "power"!
>
>Ian
>

[Nyal Williams[_2_]]

I'd like to turn this around since I'm not a physicist or an engineer.
What force causes a ball to roll down an inclined plane?


At 00:00 18 March 2009, Bob Cook wrote:
>At 18:16 17 March 2009, The Real Doctor wrote:
>
>Ian,
>
>YOU did use the term "power" correctly.
>
>What I didn't like about it was that the question I posed referred to
>force. (OK, so you gave us some additional information)
>
>I agree that a sailplane, in gliding flight, in still air, has no
>"power" at all. (Although, as you said, some wrongly believe that
>gliders are "gravity powered")
>
>By some of the responses, I think I am correct in assuming that some
>confuse power, energy, and force.
>
>So I again ask, (not to Ian, but to some of the others who answered my
>question with "gravity",) how can gravity alone, a force which acts
>vertically downward, impart forward motion to a glider or anything for
>that matter?
>
>
>Cookie
>
>
>>
>>I don't. I used "power" because I meant "power"!
>>
>>Ian
>>
>

[Darryl Ramm]

On Mar 17, 5:00*pm, Bob Cook > wrote:
> At 18:16 17 March 2009, The Real Doctor wrote:
>
> Ian,
>
> YOU did use the term "power" correctly.
>
> What I didn't like about it was that the question I posed referred to
> force. *(OK, so you gave us some additional information)
>
> I agree that a sailplane, in gliding flight, in still air, *has no
> "power" at all. *(Although, as you said, some wrongly believe that
> gliders are "gravity powered")
>
> By some of the responses, I think I am correct in assuming that some
> confuse power, energy, and force.
>
> So I again ask, (not to Ian, but to some of the others who answered my
> question with "gravity",) how can gravity alone, a force which acts
> vertically downward, impart forward motion to a glider or anything for
> that matter?
>
> Cookie
>
>
>
> >I don't. I used "power" because I meant "power"!
>
> >Ian
>
>

Sigh. No a glider moving through still air even at a constant rate
expends power. A glider has gravitational potential energy, the rate
it converts that potential energy to overcome drag is an expenditure
of power and you can talk about that as horsepower or kW etc. We plot
these things now as polars, just with different display units.

As I explained, Gravity provides the energy but the forward motion
comes from the forward component of the lift vector. The large part of
the lift vector is resisting the pull of gravity and as a side effect
the horizontal part moves the glider forward, and conveniently the
glider needs to move forward to create lift. Lucky thing that.

Analogous to the chicken sliding down a ramp, gravity there also only
acts through the center of mass yet the chicken does not fall
vertically. I think this is obvious to any chicken sliding down a
ramp. You need a chicken and a ramp and do the experiment then ask the
chicken to explain why.

Darryl

[Jim Logajan]

Darryl Ramm > wrote:
> Analogous to the chicken sliding down a ramp, gravity there also only
> acts through the center of mass yet the chicken does not fall
> vertically. I think this is obvious to any chicken sliding down a
> ramp. You need a chicken and a ramp and do the experiment then ask the
> chicken to explain why.

That analogy is most fowl.

I felt compelled to google the words chicken and gliders and found this:

http://blog.modernmechanix.com/2006/12/19/airborne-chickens-roost-in-glider-nose/

[Mike Schumann]

That's an excellent analogy.

Mike Schumann

"Nyal Williams" > wrote in message
...
> I'd like to turn this around since I'm not a physicist or an engineer.
> What force causes a ball to roll down an inclined plane?
>
>
> At 00:00 18 March 2009, Bob Cook wrote:
>>At 18:16 17 March 2009, The Real Doctor wrote:
>>
>>Ian,
>>
>>YOU did use the term "power" correctly.
>>
>>What I didn't like about it was that the question I posed referred to
>>force. (OK, so you gave us some additional information)
>>
>>I agree that a sailplane, in gliding flight, in still air, has no
>>"power" at all. (Although, as you said, some wrongly believe that
>>gliders are "gravity powered")
>>
>>By some of the responses, I think I am correct in assuming that some
>>confuse power, energy, and force.
>>
>>So I again ask, (not to Ian, but to some of the others who answered my
>>question with "gravity",) how can gravity alone, a force which acts
>>vertically downward, impart forward motion to a glider or anything for
>>that matter?
>>
>>
>>Cookie
>>
>>
>>>
>>>I don't. I used "power" because I meant "power"!
>>>
>>>Ian
>>>
>>

[Nyal Williams[_2_]]

It is past mid-night and all this is making me more goofy than usual;
howzabout renaming the thread? (Some of us are certainly tripped up.)

OK, I apologize.


>At 03:30 18 March 2009, Mike Schumann wrote:
>That's an excellent analogy.
>
>Mike Schumann
>
>"Nyal Williams" wrote in message
...
>> I'd like to turn this around since I'm not a physicist or an
engineer.
>> What force causes a ball to roll down an inclined plane?
>>
>>
>> At 00:00 18 March 2009, Bob Cook wrote:
>>>At 18:16 17 March 2009, The Real Doctor wrote:
>>>
>>>Ian,
>>>
>>>YOU did use the term "power" correctly.
>>>
>>>What I didn't like about it was that the question I posed referred to
>>>force. (OK, so you gave us some additional information)
>>>
>>>I agree that a sailplane, in gliding flight, in still air, has no
>>>"power" at all. (Although, as you said, some wrongly believe that
>>>gliders are "gravity powered")
>>>
>>>By some of the responses, I think I am correct in assuming that some
>>>confuse power, energy, and force.
>>>
>>>So I again ask, (not to Ian, but to some of the others who answered my
>>>question with "gravity",) how can gravity alone, a force which acts
>>>vertically downward, impart forward motion to a glider or anything for
>>>that matter?
>>>
>>>
>>>Cookie
>>>
>>>
>>>>
>>>>I don't. I used "power" because I meant "power"!
>>>>
>>>>Ian
>>>>
>>>
>
>
>

[Bob Cook[_2_]]

The resultant of TWO forces.

The force of gravity downward, plus the force of the ramp pushing upward
and forward, "normal" (at a right angle) to the ramp surface.

This resultant force is parallel to the ramp. When this force is
unbalanced, the ball will begin to roll and pick up speed until drag
becomes equal to that resultant force. Then velocity will remain
constant, and all forces are in balance, and the sum of the forces =
zero.

For glider, substitute lift for the ramp.

Cookie


>
>"Nyal Williams" wrote in message
...
>> I'd like to turn this around since I'm not a physicist or an
engineer.
>> What force causes a ball to roll down an inclined plane?

[Bob Cook[_2_]]

Well, at least I did learn a lot from the previous thread, thanks mostly to
Ian who helped me "tidy up" some of my conceptions.

Yes this is Jr High physics. I think if we took the final exam, Ian would
get an "A", while I would get a "C" and most of the guys would do
worse!


Since everybody liked my "gravity" question so much, here is more from
my pet peeve department.

Q) Two identical gliders on final approach. Glider A has spoilers closed.
Glider B opens spoilers. Glider B will make a steeper approach because
"spoilers reduce lift". True or false? And why.

Q) Two identical gliders on final approach. Glider A has flaps retracted.
Glider B has flaps extended. Glider B will make a steeper approach
because "flaps increase lift". True or false? And why.

Cookie

[Jim Beckman[_2_]]

At 18:22 17 March 2009, Bob Whelan wrote:

>Yebbut...imagine a glider magically inserted into earth's atmosphere
>after global warming has removed all grabbity. What gets it moving?

It won't move. Why would it?

Jim Beckman

[Derek Copeland[_2_]]

In free unaccelerated flight with no thrust, i.e. no aerotow, winch, or
turbo, a glider IS essentially gravity powered. The resultant force of
gravity plus wing lift, angled very slightly forward, opposes drag. Thus a
glider runs down a very slight slope through the air. The less drag there
is, the flatter the glide angle becomes.

Both airbrakes and large positive flap angles increase the drag, so the
glider has to run down a steeper slope to maintain speed.

Airbrakes both reduce lift, by disrupting the airflow over part of the
wing, and increase drag, so the answer to that question is obvious.

Large amounts of positive flap increases lift, but also increases drag to
a much greater extent.

Derek Copeland


At 12:30 18 March 2009, Bob Cook wrote:
>Well, at least I did learn a lot from the previous thread, thanks mostly
to
>Ian who helped me "tidy up" some of my conceptions.
>
>Yes this is Jr High physics. I think if we took the final exam, Ian
would
>get an "A", while I would get a "C" and most of the guys would do
>worse!
>
>
>Since everybody liked my "gravity" question so much, here is more from
>my pet peeve department.
>
>Q) Two identical gliders on final approach. Glider A has spoilers
closed.
> Glider B opens spoilers. Glider B will make a steeper approach because
>"spoilers reduce lift". True or false? And why.
>
>Q) Two identical gliders on final approach. Glider A has flaps
retracted.
> Glider B has flaps extended. Glider B will make a steeper approach
>because "flaps increase lift". True or false? And why.
>
>Cookie
>

[Wayne Paul]

"Derek Copeland" > wrote in message
...
> In free unaccelerated flight with no thrust, i.e. no aerotow, winch, or
> turbo, a glider IS essentially gravity powered. The resultant force of
> gravity plus wing lift, angled very slightly forward, opposes drag. Thus a
> glider runs down a very slight slope through the air. The less drag there
> is, the flatter the glide angle becomes.
>
> Both airbrakes and large positive flap angles increase the drag, so the
> glider has to run down a steeper slope to maintain speed.
>
> Airbrakes both reduce lift, by disrupting the airflow over part of the
> wing, and increase drag, so the answer to that question is obvious.
>
> Large amounts of positive flap increases lift, but also increases drag to
> a much greater extent.
>
> Derek Copeland
>

Just to add a bit to Derek's analysis. I fly a HP-14 which doesn't have
spoilers and totally relies of large span flaps for glide slope control.
The increase in lift is negligible as the flaps are extended beyond 45
degrees to 90 degrees. However, the increase in the drag produced is
massive.

At 50 kts with flaps set at 0 my HP-14 has a glide ratio about 38:1;
however, with the flaps at 90 degrees the glide ratio at 50 kts is some
where between 2:1 to 3:1.

http://www.soaridaho.com/Schreder/Stories/Schreder_on_Flaps.htm

Wayne
HP-14 "6F"
http://www.soaridaho.com/Schreder


> At 12:30 18 March 2009, Bob Cook wrote:
>>Well, at least I did learn a lot from the previous thread, thanks mostly
> to
>>Ian who helped me "tidy up" some of my conceptions.
>>
>>Yes this is Jr High physics. I think if we took the final exam, Ian
> would
>>get an "A", while I would get a "C" and most of the guys would do
>>worse!
>>
>>
>>Since everybody liked my "gravity" question so much, here is more from
>>my pet peeve department.
>>
>>Q) Two identical gliders on final approach. Glider A has spoilers
> closed.
>> Glider B opens spoilers. Glider B will make a steeper approach because
>>"spoilers reduce lift". True or false? And why.
>>
>>Q) Two identical gliders on final approach. Glider A has flaps
> retracted.
>> Glider B has flaps extended. Glider B will make a steeper approach
>>because "flaps increase lift". True or false? And why.
>>
>>Cookie
>>

[Doug Hoffman]

Derek Copeland wrote:
> In free unaccelerated flight with no thrust, i.e. no aerotow, winch, or
> turbo, a glider IS essentially gravity powered. The resultant force of
> gravity plus wing lift, angled very slightly forward, opposes drag. Thus a
> glider runs down a very slight slope through the air. The less drag there
> is, the flatter the glide angle becomes.

Nicely worded answer.

On several occasions I've had (non glider pilot) friends ask me why does
it help when we make our gliders heavier with water ballast. Seems
counter-intuitive.

I'm thinking that a proper explanation is in terms of the gravitational
force in a similar fashion to what you describe. Higher mass = higher
gravitational force (F=MA). Hence the glider is "pulled down the slope"
by a higher force. The glide angle is no better, but we can glide
faster at essentially the same glide angle which is an advantage (normal
caveats about thermal climb ability trade-off). A more complete answer
might also discuss the higher drag at higher speed interplay, but that
could probably be left out as a simplification. Perhaps a further
discussion of the classic experiment where in a vacuum a feather and a
rock will fall to earth at the same rate because the acceleration of
gravity is a constant (I know, but within limits it *is* a constant).
But in the presence of air the "air-drag" on the feather is relatively
high compared to the relatively low gravity "down-pull" due to its low mass.

Comments on this explanation are welcomed/sought. I thought I would
find a well worded description of this in Reichmann but it isn't there
that I can see. TIA

Regards,

-Doug

[Darryl Ramm]

On Mar 18, 8:20*am, Doug Hoffman > wrote:
> Derek Copeland wrote:
> > In free unaccelerated flight with no thrust, i.e. no aerotow, winch, or
> > turbo, a glider IS essentially gravity powered. The resultant force of
> > gravity plus wing lift, angled very slightly forward, opposes drag. Thus a
> > glider runs down a very slight slope through the air. The less drag there
> > is, the flatter the glide angle becomes.
>
> Nicely worded answer.
>
> On several occasions I've had (non glider pilot) friends ask me why does
> it help when we make our gliders heavier with water ballast. *Seems
> counter-intuitive.
>
> I'm thinking that a proper explanation is in terms of the gravitational
> force in a similar fashion to what you describe. *Higher mass = higher
> gravitational force (F=MA). *Hence the glider is "pulled down the slope"
> by a higher force. *The glide angle is no better, but we can glide
> faster at essentially the same glide angle which is an advantage (normal
> caveats about thermal climb ability trade-off). *A more complete answer
> might also discuss the higher drag at higher speed interplay, but that
> could probably be left out as a simplification. *Perhaps a further
> discussion of the classic experiment where in a vacuum a feather and a
> rock will fall to earth at the same rate because the acceleration of
> gravity is a constant (I know, but within limits it *is* a constant).
> But in the presence of air the "air-drag" on the feather is relatively
> high compared to the relatively low gravity "down-pull" due to its low mass.
>
> Comments on this explanation are welcomed/sought. *I thought I would
> find a well worded description of this in Reichmann but it isn't there
> that I can see. *TIA
>
> Regards,
>
> -Doug

Remembering that the power for the glider is coming from the
gravitational potential energy, so it is correct that a higher mass
glider has more energy and this is where the increased L/D does come
from. However you can't do an analysis quite like that to explain the
results. A good discussion of this for L/D for powered aircraft is is
in "Mechanics of Flight" by Phillips, if you read the "Power Failure
and Gliding Flight" chapter you will get a pretty good picture of what
happens, even if not really discussing glider wind loading.

Google books has extracts on line at

http://books.google.com/books?id=6-_iGbJHM-8C&pg=PP1&dq=mechanics+of+flight

and available from Amazon.

http://www.amazon.com/Mechanics-Flight-Warren-F-Phillips/dp/0471334588

It's expensive but very good.

Darryl

[[email protected]]

On Mar 18, 6:45*am, Derek Copeland > wrote:
> In free unaccelerated flight with no thrust, i.e. no aerotow, winch, or
> turbo, a glider IS essentially gravity powered. The resultant force of
> gravity plus wing lift, angled very slightly forward, opposes drag. Thus a
> glider runs down a very slight slope through the air. The less drag there
> is, the flatter the glide angle becomes.
>
> Both airbrakes and large positive flap angles increase the drag, so the
> glider has to run down a steeper slope to maintain speed.
>
> Airbrakes both reduce lift, by disrupting the airflow over part of the
> wing, and increase drag, so the answer to that question is obvious. *
>
> Large amounts of positive flap increases lift, but also increases drag to
> a much greater extent.
>
> Derek Copeland
>
> At 12:30 18 March 2009, Bob Cook wrote:
>
> >Well, at least I did learn a lot from the previous thread, thanks mostly
> to
> >Ian who helped me "tidy up" some of my conceptions.
>
> >Yes this is Jr High physics. *I think if we took the final exam, Ian
> would
> >get an "A", while I would get a "C" and most of the guys would do
> >worse!
>
> >Since everybody liked my "gravity" question so much, here is more from
> >my pet peeve department.
>
> >Q) Two identical gliders on final approach. *Glider A has spoilers
> closed.
> > Glider B opens spoilers. *Glider B will make a steeper approach because
> >"spoilers reduce lift". *True or false? *And why.
>
> >Q) Two identical gliders on final approach. *Glider A has flaps
> retracted.
> > Glider B has flaps extended. *Glider B will make a steeper approach
> >because "flaps increase lift". * True or false? And why.
>
> >Cookie

Actually the amount of lift in both cases is unchanged (once the
approach is stabilized) Total lift equals the weight of the glider -
otherwise the glider would experience a vertical acceleration. There
are some vector effects from the angle of the glideslope, but I'm
pretty sure they are secondary in most cases. What changes is the lift
coefficient. In the case of spoilers the parts of the wing that are
not affected by the spoilers operate at a higher lift coefficient to
hold the glider up - this produces more induced drag, on top of the
drag of the spoilers themselves.

With flaps extended the lift coefficient only goes up as the speed
goes down. Again the main effect on glideslope is the drag of the
flaps, not a change in lifting force.

For the technically inclined, the lift formula is 1/2pV^2SCl, where p
(rho) is the density of air, V is velocity, S is wing area and Cl is
lift coefficient. Flaps let you achieve a higher Cl at a lower
airspeed by changing the characteristics of the airfoil, spoilers
force you to fly at a higher Cl because the effective S goes down. If
you are already at max Cl for the wing and deploy spoilers you will
accelerate downward until you generate enough additional airspeed to
stop it.

9B

[The Real Doctor]

On 17 Mar, 19:04, Darryl Ramm > wrote:

> Say what? Gravitational potential energy *increases* not decreases as
> the glider climbs.

Sorry, typo.

> And that "store" is the transfer mechanism that allows the glider to
> glide.

No it's not. The gravity force is necessary, but it's perfectly
possible to soar for extended periods and distances without adding to
or drawing from the potential energy store. It happens any time we fly
level - along a wavebar, running a ridge, following a cloud street.

> There is no other mechanism. With no gravity, even with an
> atmosphere (which would be difficult to arrange), your glider could
> not glide.

You miss, as so many people do, the point. Because the force of
gravity is necessary for a glider to work, people assume that gravity
somehow "powers" the flight. Which it does not.

> Guy's this has wandered into junior-high school level physics.

No, it has wandered into junior high school level misconceptions about
physics!

Here's another one for you. Does a glider turn (normally) by (a)
rolling (b) pitching (c) yawing or (d) other?


Ian

[The Real Doctor]

On 18 Mar, 00:45, Nyal Williams > wrote:
> I'd like to turn this around since I'm not a physicist or an engineer.
> What force causes a ball to roll down an inclined plane?

Well, not gravity, or at least not just gravity, because gravity moves
things downwards, not along. So the answer is really that two forces
are involved:

1. Gravity, less the vertical component of the reaction force, moves
it down and

2. The horizontal component of the reaction force moves it along

Ian

[The Real Doctor]

On 18 Mar, 12:30, Bob Cook > wrote:

> Q) Two identical gliders on final approach. *Glider A has spoilers closed.
> *Glider B opens spoilers. *Glider B will make a steeper approach because
> "spoilers reduce lift". *True or false? *And why.

Sort of true. All other things being equal, spoilers reduce lift, but
the system adjusts itself, or is adjusted, so the the amount of lift
increases again (if you're accelerating towards the ground you're
Doing It Wrong!) but with increased drag.

Q) Two identical gliders on final approach. *Glider A has flaps
retracted.
> *Glider B has flaps extended. *Glider B will make a steeper approach
> because "flaps increase lift". * True or false? And why.

Same sort of thing, really. Vertical equilibrium (ie constant descent
rate) is reached with a higher drag, so more energy lost, so the
descent rate is higher.

But I se the paradox you're getting at - one device decreases lift,
one increases it and yet they both have the same effect. The key is in
drag: air brakes force you and flaps permit you to fly in a higher
drag regime, and that's what makes the approach steeper. Lift never
makes a difference - because it acts, by definition, at right angles
to the flight path it never does or needs work.

Ian
Ian
> Cookie

[The Real Doctor]

On 18 Mar, 13:45, Derek Copeland > wrote:
> In free unaccelerated flight with no thrust, i.e. no aerotow, winch, or
> turbo, a glider IS essentially gravity powered.

Not true. A glider can fly perfectly happily while increasing its
portential energy - exactly the opposite of being gravity powered.

Ian

[The Real Doctor]

On 18 Mar, 01:38, Darryl Ramm > wrote:

> As I explained, Gravity provides the energy...

Then you will need to explain how gravity provides the energy when the
glider is climbing.

Ian

[The Real Doctor]

On 17 Mar, 18:33, Mike the Strike > wrote:
> All of the energy to maintain a glider in steady flight through still
> air is derived from gravity - there is no other source of energy.

And how often do we fly in still air? For that matter, how often do we
build gliders on the top of hills, fly them to the bottom and then
abandon them. A very little energy may be stored as gravitational
potential, but gravity certainly isn't the source of the energy!

Ian

[The Real Doctor]

On 17 Mar, 21:41, KevinFinke > wrote:

> If Bob's chicken is sliding down a hill.. what force causes it to
> continue sliding?

Can't be gravity, or not just gravity, because chickens, like apples,
fall straight down when gravity gets hold of them.

How can gravity push something forwards?

> It is gravity. There is no forward component of lift that contributes
> to forward motion.

What do you think overcomes the horizontal component of drag, then?

> In steady
> state gliding flight, Net Lift is equal to Weight times the cosine of
> the gliding angle.

No it's not.You forgot something!

Ian

[The Real Doctor]

On 17 Mar, 23:45, Nyal Williams > wrote:
> Jim White had it right, way back several weeks ago. *What makes it work is
> MAGIC. *

I'm convinced that helicopters don'tactually fly: they just induce
hallucinations in people so they think flying is occuring. Some
stroboscopic effect, probably.

Ian

[Jim Logajan]

The Real Doctor > wrote:
> On 18 Mar, 01:38, Darryl Ramm > wrote:
>
>> As I explained, Gravity provides the energy...
>
> Then you will need to explain how gravity provides the energy when the
> glider is climbing.

Just curious, but are you being pedantic?

[Darryl Ramm]

On Mar 18, 3:34*pm, Jim Logajan > wrote:
> The Real Doctor > wrote:
>
> > On 18 Mar, 01:38, Darryl Ramm > wrote:
>
> >> As I explained, Gravity provides the energy...
>
> > Then you will need to explain how gravity provides the energy when the
> > glider is climbing.
>
> Just curious, but are you being pedantic?

Just an extreme case of rasterbation. If he keeps it up he will go
blind.

Darryl.

[KevinFinke]

Ian, "The Real Doctor" Out of curiosity, what exactly do you have a
doctorate in?

Aside from that.... In order to seek clarity in all of these
discussions I suspect that we have a mis-understanding because we are
trying to discuss these using two different reference frames. If
that's the case, then that would explain a lot.

I hope that we are all in agreement about the three forces acting on a
glider. For simplicity they are lift(L), drag(D) and weight(W=mg). As
has been corrected by Darryl, I agree that it is correct that lift, by
definition, is perpendicular to the airflow. However, for a glider in
steady state gliding flight, airflow and direction of motion are
parallel. Any body have any problems so far? I'm hoping this will get
me out of the hen house...

If we align the axis system such that weight is vertical and the
descent angle is theta. The equilibrium equations are:

Vert. Axis 0 = L*cos(theta) + D*sin(theta) - W
Horz. Axis 0 = L*sin(theta) - D*cos(theta)

I'm guessing this is the source of the Lift providing the horizontal
motion argument. Clearly there is no gravity term in that component.
But the motion isn't strictly horizontal or vertical with these
equations. It is both, and therefore I would advocate a simplified set
where the direction of motion is the basis for the axis system.
Therefore....

If the axis system is aligned along the lift vector the equations
simplify to: (For the sliding block this tends to be the convention
that most books I own present) Replace L with N for Normal.

Lift Axis 0 = L - W*cos(theta)
Drag Axis 0 = D - W*sin(theta)

Any objections so far? I sure hope not. I can't imagine how....

The nice thing about convention 2 is that the lift and drag vectors
are isolated variables in the equation, and the weight is already
known so it's easy to solve the other values.

L = W*cos(theta) and D=W*sin(theta)

I can even rearrange the equations in set 1 and get the same
relationships. So, what in the world am I missing when I say Lift =
Weight * cos (glide angle)? Ian, you are the real doctor. I'll confess
my ignorance. I don't want to guess, cause I just don't know what
answer you're looking for, but what did I forget?

The other advantage of using convention 2 is in describing the motion
of the system. The object is constrained to the plane, and therefore
you can get rid of the "vertical" axis in this example and look at the
equation with one dimension. Because the lift force or normal force
constrains the object to the plane, you'll have no accelerations or
displacements in this direction, for a steady state example. In this
case that would be it's glide path. The only equation left is D = W*sin
(theta) So again I argue, Lift, because it is perpendicular to the
direction of motion, can not provide the motive force! The motive
force is governed by a balance between gravity, drag, and the glide
angle. Don't get me wrong, I'm not saying lift isn't important. It is
very important to making the glider stay on a glide path. Maybe this
is just a chicken before the egg argument. I can see the circularity
of the discussion. Why do those chickens keep coming up.... :)

This would be a whole lot easier to explain with pictures. So I'll
cite a reference...If anybody has a copy of the BGA Manual: "Gliding:
Theory of Flight", please reference the discussion of forces on flight
in Chapter 4. The book goes through a very good explanation of how
gravity provides the motive force for gliding. It's an excellent book
and I highly recommend it. If only it had a discussion of forces on
tow....

-Kevin

PS I think we need some good flying weather so that we all get out of
the house and away from the computer....

[Derek Copeland[_2_]]

A glider in steady free flight is always descending through the air in
which it is flying. If you can fly in air that is going up as fast or
faster than the sink rate of the glider, then you can gain potential
height energy. Isn't that what soaring flight is all about?

Derek Copeland
(Qualified gliding instructor with Gold C and 3 Diamonds BTW)

At 22:07 18 March 2009, The Real Doctor wrote:
>On 17 Mar, 19:04, Darryl Ramm wrote:
>
>> Say what? Gravitational potential energy *increases* not decreases as
>> the glider climbs.
>
>Sorry, typo.
>
>> And that "store" is the transfer mechanism that allows the glider to
>> glide.
>
>No it's not. The gravity force is necessary, but it's perfectly
>possible to soar for extended periods and distances without adding to
>or drawing from the potential energy store. It happens any time we fly
>level - along a wavebar, running a ridge, following a cloud street.
>
>> There is no other mechanism. With no gravity, even with an
>> atmosphere (which would be difficult to arrange), your glider could
>> not glide.
>
>You miss, as so many people do, the point. Because the force of
>gravity is necessary for a glider to work, people assume that gravity
>somehow "powers" the flight. Which it does not.
>
>> Guy's this has wandered into junior-high school level physics.
>
>No, it has wandered into junior high school level misconceptions about
>physics!
>
>Here's another one for you. Does a glider turn (normally) by (a)
>rolling (b) pitching (c) yawing or (d) other?
>
>
>Ian
>

[Jim White[_2_]]

Having read all 170 posts in this thread I am still happily ignorant of the
theory of flight!

Jim

[The Real Doctor]

On 18 Mar, 22:34, Jim Logajan > wrote:
> The Real Doctor > wrote:
>
> > On 18 Mar, 01:38, Darryl Ramm > wrote:
>
> >> As I explained, Gravity provides the energy...
>
> > Then you will need to explain how gravity provides the energy when the
> > glider is climbing.
>
> Just curious, but are you being pedantic?

No, I'm being serious in a usenet-is-not-reallife sort of way. I
introduced this, you see, as a common misconception in the gliding
world. Lots of people think that gliders are "powered by gravity", and
they aren't!

Ian

[The Real Doctor]

On 18 Mar, 22:59, Darryl Ramm > wrote:
> On Mar 18, 3:34*pm, Jim Logajan > wrote:
>
> > The Real Doctor > wrote:
>
> > > On 18 Mar, 01:38, Darryl Ramm > wrote:
>
> > >> As I explained, Gravity provides the energy...
>
> > > Then you will need to explain how gravity provides the energy when the
> > > glider is climbing.
>
> > Just curious, but are you being pedantic?
>
> Just an extreme case of rasterbation. If he keeps it up he will go
> blind.

Go on then. Explain how "gravity provides the energy" when a glider is
climbing ...

Ian

[The Real Doctor]

On 19 Mar, 00:03, KevinFinke > wrote:
> Ian, "The Real Doctor" *Out of curiosity, what exactly do you have a
> doctorate in?

Numerical solution of non-linear diffusion problems.Which is actually
a lot more interesting than it sounds.

> I can even rearrange the equations in set 1 and get the same
> relationships. So, what in the world am I missing when I say Lift =
> Weight * cos (glide angle)?

*Smacks head*

Sorry, it was late. I misread, and thought you'd done the old W = L cos
(theta) mistake (W, of course, = L cos(theta) + D sin(theta).

My apologies.

Ian

[The Real Doctor]

On 19 Mar, 07:45, Jim White > wrote:
> Having read all 170 posts in this thread I am still happily ignorant of the
> theory of flight!

Aircraft stay up because they're afraid of the ground.

That said, you might find it interesting to get hold of the TV
programme and video which the Open University made on gliding
mathematics some years ago. The course in question, "MST207 Applied
Mathematics" is defunct, but a fair number of copies of the AV
material lurk around gliding clubs.

Ian

[Doug Hoffman]

Darryl Ramm wrote:
> On Mar 18, 8:20 am, Doug Hoffman > wrote:
>> Derek Copeland wrote:
>>> In free unaccelerated flight with no thrust, i.e. no aerotow, winch, or
>>> turbo, a glider IS essentially gravity powered. The resultant force of
>>> gravity plus wing lift, angled very slightly forward, opposes drag. Thus a
>>> glider runs down a very slight slope through the air. The less drag there
>>> is, the flatter the glide angle becomes.
>> Nicely worded answer.
>>
>> On several occasions I've had (non glider pilot) friends ask me why does
>> it help when we make our gliders heavier with water ballast. Seems
>> counter-intuitive.
>>
>> I'm thinking that a proper explanation is in terms of the gravitational
>> force in a similar fashion to what you describe. Higher mass = higher
>> gravitational force (F=MA). Hence the glider is "pulled down the slope"
>> by a higher force. The glide angle is no better, but we can glide
>> faster at essentially the same glide angle which is an advantage (normal
>> caveats about thermal climb ability trade-off). A more complete answer
>> might also discuss the higher drag at higher speed interplay, but that
>> could probably be left out as a simplification. Perhaps a further
>> discussion of the classic experiment where in a vacuum a feather and a
>> rock will fall to earth at the same rate because the acceleration of
>> gravity is a constant (I know, but within limits it *is* a constant).
>> But in the presence of air the "air-drag" on the feather is relatively
>> high compared to the relatively low gravity "down-pull" due to its low mass.
>>
>> Comments on this explanation are welcomed/sought. I thought I would
>> find a well worded description of this in Reichmann but it isn't there
>> that I can see. TIA
>>
>> Regards,
>>
>> -Doug
>
> Remembering that the power for the glider is coming from the
> gravitational potential energy, so it is correct that a higher mass
> glider has more energy and this is where the increased L/D does come
> from.

I thought the best L/D stayed about the same. It just occurs at higher
speed with ballast.

> However you can't do an analysis quite like that to explain the
> results. A good discussion of this for L/D for powered aircraft is is
> in "Mechanics of Flight" by Phillips, if you read the "Power Failure
> and Gliding Flight" chapter you will get a pretty good picture of what
> happens, even if not really discussing glider wind loading.

OK. Thanks for that. Is a proper explanation that complicated? I was
hoping to avoid a $140 answer. :-)

Regards,

-Doug



>
> Google books has extracts on line at
>
> http://books.google.com/books?id=6-_iGbJHM-8C&pg=PP1&dq=mechanics+of+flight
>
> and available from Amazon.
>
> http://www.amazon.com/Mechanics-Flight-Warren-F-Phillips/dp/0471334588
>
> It's expensive but very good.
>
> Darryl
>
>

[Bob Cook[_2_]]

You can change the reference or axis system all you want, But then you
also change the direction of "down".


Gravity acts downward and no other diredtion.


Lift plus grivity act together and form a resultant force parallel to the
direction of flight. Drag acts in the opposite direction at equal
magnitude.

Gravity (alone) is not the force that provides forward motion to a
glider.

A simple three vector diagram will show this.


Cook

At 00:03 19 March 2009, KevinFinke wrote:
>Ian, "The Real Doctor" Out of curiosity, what exactly do you have a
>doctorate in?
>
>Aside from that.... In order to seek clarity in all of these
>discussions I suspect that we have a mis-understanding because we are
>trying to discuss these using two different reference frames. If
>that's the case, then that would explain a lot.
>
>I hope that we are all in agreement about the three forces acting on a
>glider. For simplicity they are lift(L), drag(D) and weight(W=mg). As
>has been corrected by Darryl, I agree that it is correct that lift, by
>definition, is perpendicular to the airflow. However, for a glider in
>steady state gliding flight, airflow and direction of motion are
>parallel. Any body have any problems so far? I'm hoping this will get
>me out of the hen house...
>
>If we align the axis system such that weight is vertical and the
>descent angle is theta. The equilibrium equations are:
>
>Vert. Axis 0 = L*cos(theta) + D*sin(theta) - W
>Horz. Axis 0 = L*sin(theta) - D*cos(theta)
>
>I'm guessing this is the source of the Lift providing the horizontal
>motion argument. Clearly there is no gravity term in that component.
>But the motion isn't strictly horizontal or vertical with these
>equations. It is both, and therefore I would advocate a simplified set
>where the direction of motion is the basis for the axis system.
>Therefore....
>
>If the axis system is aligned along the lift vector the equations
>simplify to: (For the sliding block this tends to be the convention
>that most books I own present) Replace L with N for Normal.
>
>Lift Axis 0 = L - W*cos(theta)
>Drag Axis 0 = D - W*sin(theta)
>
>Any objections so far? I sure hope not. I can't imagine how....
>
>The nice thing about convention 2 is that the lift and drag vectors
>are isolated variables in the equation, and the weight is already
>known so it's easy to solve the other values.
>
>L = W*cos(theta) and D=W*sin(theta)
>
>I can even rearrange the equations in set 1 and get the same
>relationships. So, what in the world am I missing when I say Lift =
>Weight * cos (glide angle)? Ian, you are the real doctor. I'll confess
>my ignorance. I don't want to guess, cause I just don't know what
>answer you're looking for, but what did I forget?
>
>The other advantage of using convention 2 is in describing the motion
>of the system. The object is constrained to the plane, and therefore
>you can get rid of the "vertical" axis in this example and look at the
>equation with one dimension. Because the lift force or normal force
>constrains the object to the plane, you'll have no accelerations or
>displacements in this direction, for a steady state example. In this
>case that would be it's glide path. The only equation left is D = W*sin
>(theta) So again I argue, Lift, because it is perpendicular to the
>direction of motion, can not provide the motive force! The motive
>force is governed by a balance between gravity, drag, and the glide
>angle. Don't get me wrong, I'm not saying lift isn't important. It is
>very important to making the glider stay on a glide path. Maybe this
>is just a chicken before the egg argument. I can see the circularity
>of the discussion. Why do those chickens keep coming up.... :)
>
>This would be a whole lot easier to explain with pictures. So I'll
>cite a reference...If anybody has a copy of the BGA Manual: "Gliding:
>Theory of Flight", please reference the discussion of forces on flight
>in Chapter 4. The book goes through a very good explanation of how
>gravity provides the motive force for gliding. It's an excellent book
>and I highly recommend it. If only it had a discussion of forces on
>tow....
>
>-Kevin
>
>PS I think we need some good flying weather so that we all get out of
>the house and away from the computer....
>
>
>
>

[Bob Cook[_2_]]

Not many took a stab at the "spoilers and flaps" questons!

Ian pretty much got it right, and at least saw the paradox in my
questions, How can extra lift (flaps)increase glide slope while, reduced
lift (spoiler) also increases glide slope?

THere are two misconceptions in the above question.

The answer to both original questions is FALSE.

Spoilers do not "reduce lift". Spoilers increase drag. As drag
increases, glide slope steepens.

Spoilers redistribute lift, but not reduce lift.

Flaps do not "increase lift". Flaps increase drag. As drag increases,
gilde slope steepens.

Flaps change the coeffecient of lift, but not lift.

Another question:

Q) Two gliders, one is 40:1 racer and glider two is 20:1 trainer. Both
weigh 800#

Glider one has twice the lift of glider two. True or flase and why.


Cookie



At 22:17 18 March 2009, The Real Doctor wrote:
>On 18 Mar, 13:45, Derek Copeland wrote:
>> In free unaccelerated flight with no thrust, i.e. no aerotow, winch,
or
>> turbo, a glider IS essentially gravity powered.
>
>Not true. A glider can fly perfectly happily while increasing its
>portential energy - exactly the opposite of being gravity powered.
>
>Ian
>

[Bob Cook[_2_]]

Ok I'll bite.....

Turning flight is accelerated flight, so the forces acting on the glider
are "unbalanced".

Since I am a firm believer that you can't change gravity (although some
of you out there try to in your analysis), gravity remains constant. If
we keep airpseed constant then drag should not change either. So we have
to change lift in order to get acceleration.

So I say we have to change the direction of lift, inward toward the center
of the circle. (centripetal force). This should be accomplished by
rolling.

Since we have added a "Horizontal component of lift", total lift must be
increased. This normally requires additional nose up force on stick or
trim. (angle of attack) You might call this pitch, but I don't think
the actual pitch attitude changes, just the stick forces and angle of
attack.

Yawing can be conidered to be necessary, or at least correction for
adverse yaw.

But to rephrase the question,

Q) What force causes a glider to turn.
A) Lift

Cookie

>>Here's another one for you. Does a glider turn (normally) by (a)rolling
(b) pitching (c) yawing or (d) other?


Ian

[Bob Cook[_2_]]

Ian,

Misconceptions sure die hard. Many "Cannot handle the truth".

Here' s another one.

Q) A glider is in circling flight. The glider circles because there is a
horizontal component of lift. This horizontal component of lift is
balanced by an equal and opposite force, centrifugal force.

True or False and why?

Cookie

[Doug Hoffman]

Bob Cook wrote:
> You can change the reference or axis system all you want, But then you
> also change the direction of "down".
>
>
> Gravity acts downward and no other diredtion.
>
>
> Lift plus grivity act together and form a resultant force parallel to the
> direction of flight. Drag acts in the opposite direction at equal
> magnitude.
>
> Gravity (alone) is not the force that provides forward motion to a
> glider.

Maybe we are wordsmithing/semanticizing a bit. Take away gravity and
air movement and place a glider somewhere up in the still air (I know,
suspend disbelief for a moment) and let go of the glider. No initial
motion is given to the glider, it is just "suspended in air". How much
lift and/or forward motion do we then get? None. Add gravity to the
same scenario and the glider will then move forward (after an initial
drop). So perhaps gravity *is* the sole force required for forward
motion. Of course we need air as well. But still air is a gaseous
mass, not a force or even a source of force. The reaction force caused
by the combination of gravity (sole source of force) and the presence of
air (air is not a force) leads to the forward motion of the glider.

Regards,

-Doug

[Bob Cook[_2_]]

I just gotta laugh. When Internet "arguements" get heated or go nowhere,
people resort to a comparison of "qualifications".

As if the guy with the longer resume is somehow always correct.

BTW, my qualificatons: Airport Bum.

Cookie


>
>At 00:03 19 March 2009, KevinFinke wrote:
>>Ian, "The Real Doctor" Out of curiosity, what exactly do you have a
>>doctorate in?

[Derek Copeland[_2_]]

I think you would find that a glider flying though air would stop moving
pretty quickly if you could turn off gravity! There would still be lift
and drag until it stopped, but nothing to drive it forward.

Derek Copeland


At 12:00 19 March 2009, Bob Cook wrote:

>Gravity acts downward and no other direction.
>
>
>Lift plus grivity act together and form a resultant force parallel to
the
>direction of flight. Drag acts in the opposite direction at equal
>magnitude.
>
>Gravity (alone) is not the force that provides forward motion to a
>glider.
>
>A simple three vector diagram will show this.
>
>
>Cook
>

[Bob Cook[_2_]]

Dude!



You can pretend there is no gravity. You can pretend there is no air.

But there IS gravity, and there IS air.

Gravity is a downward force. Things move downward due to gravity,
including gliders.

But the question was not "what gives a glider motion?", it was "what
gives a glider FORWARD motion?"

Sorry, can't do it without LIFT. (don't forget drag either)

Three forces act on a glider. Not one, not two, THREE.

Answer to question again: The resultant force of lift added to gravity,
balanced by drag.

Cookie


At 12:53 19 March 2009, Doug Hoffman wrote:
>Bob Cook wrote:
>> You can change the reference or axis system all you want, But then
you
>> also change the direction of "down".
>>
>>
>> Gravity acts downward and no other diredtion.
>>
>>
>> Lift plus grivity act together and form a resultant force parallel to
>the
>> direction of flight. Drag acts in the opposite direction at equal
>> magnitude.
>>
>> Gravity (alone) is not the force that provides forward motion to a
>> glider.
>
>Maybe we are wordsmithing/semanticizing a bit. Take away gravity and
>air movement and place a glider somewhere up in the still air (I know,
>suspend disbelief for a moment) and let go of the glider. No initial
>motion is given to the glider, it is just "suspended in air". How much

>lift and/or forward motion do we then get? None. Add gravity to the
>same scenario and the glider will then move forward (after an initial
>drop). So perhaps gravity *is* the sole force required for forward
>motion. Of course we need air as well. But still air is a gaseous
>mass, not a force or even a source of force. The reaction force caused
>by the combination of gravity (sole source of force) and the presence of

>air (air is not a force) leads to the forward motion of the glider.
>
>Regards,
>
>-Doug
>

[Big Wings]

At 12:30 19 March 2009, Bob Cook wrote:

Clearly not a geologist then. I wonder why they do gravity surveys if it
is a constant?

>Ok I'll bite.....
>
>Since I am a firm believer that you can't change gravity (although some
>of you out there try to in your analysis), gravity remains constant.

[Darryl Ramm]

On Mar 19, 1:18*am, The Real Doctor > wrote:
> On 18 Mar, 22:59, Darryl Ramm > wrote:
>
> > On Mar 18, 3:34*pm, Jim Logajan > wrote:
>
> > > The Real Doctor > wrote:
>
> > > > On 18 Mar, 01:38, Darryl Ramm > wrote:
>
> > > >> As I explained, Gravity provides the energy...
>
> > > > Then you will need to explain how gravity provides the energy when the
> > > > glider is climbing.
>
> > > Just curious, but are you being pedantic?
>
> > Just an extreme case of rasterbation. If he keeps it up he will go
> > blind.
>
> Go on then. Explain how "gravity provides the energy" when a glider is
> climbing ...
>
> Ian


A glider "climbs" when you pull back on the stick and converts kinetic
energy to gravitational potential energy, but that does not get you
far since you can't create within that closed system. The glider also
"climbs" - (maybe you should think of "lifted" if "climb" confuses
you) by a rising air mass and that does increase the glider's
gravitational potential energy. You can then utilize that energy to go
places. There is no other coupling between raising air and the glider
somehow magically using that to get energy go places. Are you confused
by the case of flying in zero sink? That's no different the raising
air just happens to match the sink rate, gravity is still required/is
the coupling mechanism. And a glider while being lifted in a thermal
or wave etc. is still expending gravitational potential energy to
maintain forward flight, it's just being lifted faster than it
descends.

Replace drag with the effort of running, and potential energy with
kinetic (but it lets me invoke chickens again).... If a chicken runs
backwards in a stationary train it is expending a certain amount of
energy (glider sinking in still air). As the train picks up speed and
exceeds the chicken's speed the net speed of the chicken moves forward
(glider is now being lifted in lift), and the chicken gains an
increase in net energy however the chicken is still expending the same
energy to walk to the back of the train (the glider is still using
gravitational potential energy to fly). Don't like that, think of a
ball rolling down an infinitely long inclined ramp and the ramp being
raised faster than the ball falls. What way does the ball move? to an
observer on the ground? Is the ball giving up gravitational potential
energy to slide down the ramp relative to an observer on the ramp?
(yes). Is the ball gaining net energy? (yes).


So how many ways do people need to keep answering the same pedantic
question you keep asking? Do the ultimate thought experiment, turn off
gravity and the glider will just float along with moving air currents
but will be unable to glide anywhere.


Darryl

[Big Wings]

Further to earlier post. Gravity is measured in Gals (in honour of
Galileo), 1 Gal = an acceleration of 1 cm/sec2. Average gravity at the
surface is 980.654321 Gals (or so). At the poles it is more at about 983
Gals and on equatorial mountain tops only 977 Gals - a difference of about
0.6%. There are significant local variations. Now if we start to take
height into account...

By the way, what happened to the discussion about slow tows?


At 14:15 19 March 2009, Big Wings wrote:
>At 12:30 19 March 2009, Bob Cook wrote:
>
>Clearly not a geologist then. I wonder why they do gravity surveys if
it
>is a constant?
>
>>Ok I'll bite.....
>>
>>Since I am a firm believer that you can't change gravity (although
some
>>of you out there try to in your analysis), gravity remains constant.
>

[Jim Logajan]

The Real Doctor > wrote:
> On 18 Mar, 22:34, Jim Logajan > wrote:
>> The Real Doctor > wrote:
>>
>> > On 18 Mar, 01:38, Darryl Ramm > wrote:
>>
>> >> As I explained, Gravity provides the energy...
>>
>> > Then you will need to explain how gravity provides the energy when the
>> > glider is climbing.
>>
>> Just curious, but are you being pedantic?
>
> No, I'm being serious in a usenet-is-not-reallife sort of way. I
> introduced this, you see, as a common misconception in the gliding
> world. Lots of people think that gliders are "powered by gravity", and
> they aren't!

So what does power gliders? Seems like you are the only one who knows the
real answer.

[Jim Logajan]

The Real Doctor > wrote:
> On 19 Mar, 00:03, KevinFinke > wrote:
>> Ian, "The Real Doctor" *Out of curiosity, what exactly do you have a
>> doctorate in?
>
> Numerical solution of non-linear diffusion problems.Which is actually
> a lot more interesting than it sounds.

I didn't know doctorates were awarded quite that way. I always thought they
were awarded in broad fields like math or physics or computer science.

Did you develop some new numerical techniques for such problems? Was your
work done in a math or computer science department?

[Jim Logajan]

Bob Cook > wrote:
> Gravity is a downward force. Things move downward due to gravity,
> including gliders.

Aren't there some things that move upward due to gravity? Even upward and
sideways too?

> But the question was not "what gives a glider motion?", it was "what
> gives a glider FORWARD motion?"
>
> Sorry, can't do it without LIFT. (don't forget drag either)
>
> Three forces act on a glider. Not one, not two, THREE.

But isn't there only one potential energy field operative? Not two, or
three, just one? Didn't Aharonov and Bohm show that potentials are more
"fundamental" than forces, so that we should really be considering the
system using potentials?

Is somebody keeping score on the pedantry? I hope I got at least a couple
points!

[Bob Whelan[_3_]]

Jim Beckman wrote:
> At 18:22 17 March 2009, Bob Whelan wrote:
>
>> Yebbut...imagine a glider magically inserted into earth's atmosphere
>> after global warming has removed all grabbity. What gets it moving?
>
> It won't move. Why would it?
>
> Jim Beckman
>
Indeed. But I find it easier to magically insert the glider than to
magically turn off gravity. That's why my chicken has a soft landing
place for her eggs.

And being 100% non-facetious in this paragraph, it appears some folks'
misconceptions may have been clarified to various degrees as a result of
this thread...put me in the camp that believes clear thought is a good
thing. Once a person grasps the essential role of gravity in powering
sailplanes, the lifty bits become distinctly easier to grasp, despite
the pesky intrusion of (ahem) plane geometry into resolving resultant
forces.

Regards,
Bob W.

[Jim Logajan]

Bob Cook > wrote:
> Since I am a firm believer that you can't change gravity (although
> some of you out there try to in your analysis)

They're just using thought experiments:

http://en.wikipedia.org/wiki/Thought_experiment

[[email protected]]

On Mar 19, 9:39*am, Darryl Ramm > wrote:
> On Mar 19, 1:18*am, The Real Doctor > wrote:
>
>
>
> > On 18 Mar, 22:59, Darryl Ramm > wrote:
>
> > > On Mar 18, 3:34*pm, Jim Logajan > wrote:
>
> > > > The Real Doctor > wrote:
>
> > > > > On 18 Mar, 01:38, Darryl Ramm > wrote:
>
> > > > >> As I explained, Gravity provides the energy...
>
> > > > > Then you will need to explain how gravity provides the energy when the
> > > > > glider is climbing.
>
> > > > Just curious, but are you being pedantic?
>
> > > Just an extreme case of rasterbation. If he keeps it up he will go
> > > blind.
>
> > Go on then. Explain how "gravity provides the energy" when a glider is
> > climbing ...
>
> > Ian
>
> A glider "climbs" when you pull back on the stick and converts kinetic
> energy to gravitational potential energy, but that does not get you
> far since you can't create within that closed system. The glider also
> "climbs" - (maybe you should think of "lifted" if "climb" confuses
> you) by a rising air mass and that does increase the glider's
> gravitational potential energy. You can then utilize that energy to go
> places. There is no other coupling between raising air and the glider
> somehow magically using that to get energy go places. Are you confused
> by the case of flying in zero sink? That's no different the raising
> air just happens to match the sink rate, gravity is still required/is
> the coupling mechanism. And a glider while being lifted in a thermal
> or wave etc. is still expending gravitational potential energy to
> maintain forward flight, it's just being lifted faster than it
> descends.
>
> Replace drag with the effort of running, and potential energy with
> kinetic (but it lets me invoke chickens again).... If a chicken runs
> backwards in a stationary train it is expending a certain amount of
> energy (glider sinking in still air). As the train picks up speed and
> exceeds the chicken's speed the net speed of the chicken moves forward
> (glider is now being lifted in lift), and the chicken gains an
> increase in net energy however the chicken is still expending the same
> energy to walk to the back of the train (the glider is still using
> gravitational potential energy to fly). Don't like that, think of a
> ball rolling down an infinitely long inclined ramp and the ramp being
> raised faster than the ball falls. What way does the ball move? to an
> observer on the ground? *Is the ball giving up gravitational potential
> energy to slide down the ramp relative to an observer on the ramp?
> (yes). Is the ball gaining net energy? (yes).
>
> So how many ways do people need to keep answering the same pedantic
> question you keep asking? Do the ultimate thought experiment, turn off
> gravity and the glider will just float along with moving air currents
> but will be unable to glide anywhere.
>
> Darryl

I use walking down the up escalator as my analogy - works with people
who spend a lot of time at the shopping mall. I have demonstrated it
to onlookers on rare occasions, but this usually upsets the mall cops.

Of course if you think about the analogy the energy for the whole
operation comes from the electric motors that drive the steps and
carry everyone's sorry butts to the next floor up. In soaring the
power is provided by the sun heating the air that rises (or forms
pressure systems that creates wind that flows over mountains). Gravity
and aerodynamics are just the way we turn that energy into a combined
forward and downward motion.

9B

9B

[Jim Logajan]

Bob Cook > wrote:
> Not many took a stab at the "spoilers and flaps" questons!

Probably because it was a no-win situation.

> Spoilers do not "reduce lift". Spoilers increase drag. As drag
> increases, glide slope steepens.
>
> Spoilers redistribute lift, but not reduce lift.

You're claim conflicts with that in the FAA "Pilot's Handbook of
Aeronautical Knowledge":

"Found on many gliders and some aircraft, high drag devices called
spoilers are deployed from the wings to spoil the smooth airflow,
reducing lift and increasing drag."

http://www.faa.gov/library/manuals/aviation/pilot_handbook/media/PHAK%20-%20Chapter%2003.pdf

> Flaps do not "increase lift". Flaps increase drag. As drag
> increases, gilde slope steepens.
>
> Flaps change the coeffecient of lift, but not lift.

Again, from the PHAK, same chapter:

"Flaps are the most common high-lift devices used on aircraft. These
surfaces, which are attached to the trailing edge of the wing, increase
both lift and induced drag for any given AOA."

(But I see you're being pedantic. Now you make a distinction between
"lift" and "coefficient of lift".)

> Another question:
>
> Q) Two gliders, one is 40:1 racer and glider two is 20:1 trainer.
> Both weigh 800#
>
> Glider one has twice the lift of glider two. True or flase and why.

Another no-win situation, since you don't indicate what they are doing.
Are they sitting stationary on the ground? Then both have zero lift. Are
they turning and if so, are the turn radii and descent rates different?

[The Real Doctor]

On 19 Mar, 09:19, Doug Hoffman > wrote:

> I thought the best L/D stayed about the same. *It just occurs at higher
> speed with ballast.

Yup. I think this is a good one to think about in terms of angle of
attack.

The only two things - normally - you can change to change lift are
lift coefficient (via AoA) and speed. If you generate the extra lift
needed to keep a heavier glider up by flying faster and keeping CL the
same then CD will also be the same. D will therefore increase by
exactly the same proportion as L and L/D stays the same.

Ian

[The Real Doctor]

On 19 Mar, 12:15, Bob Cook > wrote:

> Spoilers redistribute lift, but not reduce lift.

They reduce the lift over part of the wing. Whether the overall lift
increases, decreases or stays the same depends on what the pilot does.

> Flaps change the coeffecient of lift, but not lift.

If you keep everything else the same then they do change lift. Every
tried dumping your landing flap just before the flare ...?

> Q) *Two gliders, one is 40:1 racer and glider two is 20:1 trainer. Both
> weigh 800#
>
> Glider one has twice the lift of glider two. *True or flase and why.

Depends what they are doing.

Ian

[The Real Doctor]

On 19 Mar, 12:45, Bob Cook > wrote:

> Q) A glider is in circling flight. *The glider circles because there is a
> horizontal component of lift. *This horizontal component of lift is
> balanced by an equal and opposite force, centrifugal force.
>
> True or False and why?

I hope you're not getting hung up on the old centripetal/centrifugal
debate. Centripetal force is just as real in a stationary axix system
as centrifugal is in a turning one!

By and large (ignoring a few second-order effects) what you wrote is
fine.

Ian

[The Real Doctor]

On 19 Mar, 16:39, Darryl Ramm > wrote:

> A glider "climbs" when you pull back on the stick and converts kinetic
> energy to gravitational potential energy,

Technically that's a "zoom", not a climb.

> The glider also
> "climbs" - (maybe you should think of "lifted" if "climb" confuses
> you) by a rising air mass and that does increase the glider's
> gravitational potential energy.

But people keep telling me that it's conversion of PE to drag which
keeps the glider flying. How can that be happening when the PE is
increasing?

> So how many ways do people need to keep answering the same pedantic
> question you keep asking?

Well, some accurate ways would make a good start!

It amazes and depresses me how prevalent the nonsensical believe that
"gravity powers gliders" extends in the gliding world.

> Do the ultimate thought experiment, turn off
> gravity and the glider will just float along with moving air currents
> but will be unable to glide anywhere.

Cars won't be able to work either, since no gravity => no weight => no
friction at the tyres. Would you say that gravity powers cars?

Ian

PS Thanks for your concern about my confusion. I don't really go in
for arguments from authority, but would it allay your concern to know
that I have taught and examined fluid dynamics at two major UK
universities for over twenty years?

[The Real Doctor]

On 19 Mar, 17:57, wrote:

> Of course if you think about the analogy the energy for the whole
> operation comes from the electric motors that drive the steps and
> carry everyone's sorry butts to the next floor up. In soaring the
> power is provided by the sun heating the air that rises (or forms
> pressure systems that creates wind that flows over mountains). Gravity
> and aerodynamics are just the way we turn that energy into a combined
> forward and downward motion.

Give the man a coconut!

Ian

[The Real Doctor]

On 19 Mar, 17:20, Jim Logajan > wrote:
> The Real Doctor > wrote:
>
> > On 19 Mar, 00:03, KevinFinke > wrote:
> >> Ian, "The Real Doctor" *Out of curiosity, what exactly do you have a
> >> doctorate in?
>
> > Numerical solution of non-linear diffusion problems.Which is actually
> > a lot more interesting than it sounds.
>
> I didn't know doctorates were awarded quite that way. I always thought they
> were awarded in broad fields like math or physics or computer science.

Well, technically it's just a doctorate over here. It's the thesis
title and contents which matter.

> Did you develop some new numerical techniques for such problems?

Yup.

> Was your
> work done in a math or computer science department?

Engineering!

Ian

[The Real Doctor]

On 19 Mar, 17:04, Jim Logajan > wrote:

> So what does power gliders? Seems like you are the only one who knows the
> real answer.

The sun.

Also, initially, internal combustion engines, stretched rubber or, if
you buy an Electrostart and choose your contract carefully, nuclear
fission!

Ian

[Martin Gregorie[_4_]]

On Thu, 19 Mar 2009 17:00:04 +0000, Big Wings wrote:

> Further to earlier post. Gravity is measured in Gals (in honour of
> Galileo), 1 Gal = an acceleration of 1 cm/sec2. Average gravity at the
> surface is 980.654321 Gals (or so). At the poles it is more at about
> 983 Gals and on equatorial mountain tops only 977 Gals - a difference of
> about 0.6%. There are significant local variations. Now if we start to
> take height into account...
>
I notice that's not an SI unit. Is the Gal some sort of engineering or
specialist geological unit? Its a pretty big unit for gravimetry, since
the maximal variation at the earths surface is only 6 Gals.

The SI unit is uM/s^2 (micrometers per sec squared), or 0.1 milliGals,
which looks more in tune with what satellites and aerial surveys are
measuring these days.

> By the way, what happened to the discussion about slow tows?
>
Got too slow and stalled in.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

[Jim Logajan]

The Real Doctor > wrote:
> On 19 Mar, 17:04, Jim Logajan > wrote:
>
>> So what does power gliders? Seems like you are the only one who knows
>> the real answer.
>
> The sun.

So you were being pedantic after all.

[Martin Gregorie[_4_]]

On Thu, 19 Mar 2009 13:09:30 -0500, Jim Logajan wrote:

> You're claim conflicts with that in the FAA "Pilot's Handbook of
> Aeronautical Knowledge":
>
> "Found on many gliders and some aircraft, high drag devices called
> spoilers are deployed from the wings to spoil the smooth airflow,
> reducing lift and increasing drag."
>
Printing it in some official or semi-official publication doesn't make it
right.

There is a question in the UK Bronze badge written paper about the
proportion of lift provided by the top and bottom surfaces of a wing
that's just as wrong. The so-called "correct" answer is 70/30, but as a
wing is a device for imparting momentum to an air mass its a meaningless
question.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

[Jim Logajan]

The Real Doctor > wrote:
> It amazes and depresses me how prevalent the nonsensical believe that
> "gravity powers gliders" extends in the gliding world.

We all owe you a debt of gratitude for showing us how ignorant we are.

> Cars won't be able to work either, since no gravity => no weight => no
> friction at the tyres. Would you say that gravity powers cars?

I think pedantic trolling powers cars:
http://blog.modernmechanix.com/2008/02/12/prop-driven-car-makes-85-mph/

> I don't really go in for arguments from authority, but

I removed the contradiction in order to protect small children.

[Mike Schumann]

Weight is a force????? Did I miss something in HS Physics? Weight = Mass.

Mike Schumann

"KevinFinke" > wrote in message
...
> Ian, "The Real Doctor" Out of curiosity, what exactly do you have a
> doctorate in?
>
> Aside from that.... In order to seek clarity in all of these
> discussions I suspect that we have a mis-understanding because we are
> trying to discuss these using two different reference frames. If
> that's the case, then that would explain a lot.
>
> I hope that we are all in agreement about the three forces acting on a
> glider. For simplicity they are lift(L), drag(D) and weight(W=mg). As
> has been corrected by Darryl, I agree that it is correct that lift, by
> definition, is perpendicular to the airflow. However, for a glider in
> steady state gliding flight, airflow and direction of motion are
> parallel. Any body have any problems so far? I'm hoping this will get
> me out of the hen house...
>
> If we align the axis system such that weight is vertical and the
> descent angle is theta. The equilibrium equations are:
>
> Vert. Axis 0 = L*cos(theta) + D*sin(theta) - W
> Horz. Axis 0 = L*sin(theta) - D*cos(theta)
>
> I'm guessing this is the source of the Lift providing the horizontal
> motion argument. Clearly there is no gravity term in that component.
> But the motion isn't strictly horizontal or vertical with these
> equations. It is both, and therefore I would advocate a simplified set
> where the direction of motion is the basis for the axis system.
> Therefore....
>
> If the axis system is aligned along the lift vector the equations
> simplify to: (For the sliding block this tends to be the convention
> that most books I own present) Replace L with N for Normal.
>
> Lift Axis 0 = L - W*cos(theta)
> Drag Axis 0 = D - W*sin(theta)
>
> Any objections so far? I sure hope not. I can't imagine how....
>
> The nice thing about convention 2 is that the lift and drag vectors
> are isolated variables in the equation, and the weight is already
> known so it's easy to solve the other values.
>
> L = W*cos(theta) and D=W*sin(theta)
>
> I can even rearrange the equations in set 1 and get the same
> relationships. So, what in the world am I missing when I say Lift =
> Weight * cos (glide angle)? Ian, you are the real doctor. I'll confess
> my ignorance. I don't want to guess, cause I just don't know what
> answer you're looking for, but what did I forget?
>
> The other advantage of using convention 2 is in describing the motion
> of the system. The object is constrained to the plane, and therefore
> you can get rid of the "vertical" axis in this example and look at the
> equation with one dimension. Because the lift force or normal force
> constrains the object to the plane, you'll have no accelerations or
> displacements in this direction, for a steady state example. In this
> case that would be it's glide path. The only equation left is D = W*sin
> (theta) So again I argue, Lift, because it is perpendicular to the
> direction of motion, can not provide the motive force! The motive
> force is governed by a balance between gravity, drag, and the glide
> angle. Don't get me wrong, I'm not saying lift isn't important. It is
> very important to making the glider stay on a glide path. Maybe this
> is just a chicken before the egg argument. I can see the circularity
> of the discussion. Why do those chickens keep coming up.... :)
>
> This would be a whole lot easier to explain with pictures. So I'll
> cite a reference...If anybody has a copy of the BGA Manual: "Gliding:
> Theory of Flight", please reference the discussion of forces on flight
> in Chapter 4. The book goes through a very good explanation of how
> gravity provides the motive force for gliding. It's an excellent book
> and I highly recommend it. If only it had a discussion of forces on
> tow....
>
> -Kevin
>
> PS I think we need some good flying weather so that we all get out of
> the house and away from the computer....
>
>
>

[Bob Cook[_2_]]

Dear Big,

You are on to something here. Get one of those geo surveys, and fly your
glider only over areas of higher gravity. This extra gravity will give
your glider extra power, and you will be the best glider pilot!

Cookie

At 14:15 19 March 2009, Big Wings wrote:
>At 12:30 19 March 2009, Bob Cook wrote:
>
>Clearly not a geologist then. I wonder why they do gravity surveys if
it
>is a constant?
>
>>Ok I'll bite.....
>>
>>Since I am a firm believer that you can't change gravity (although
some
>>of you out there try to in your analysis), gravity remains constant.
>

[Bob Cook[_2_]]

Ian,

I was trying to point out a misconception that the "horizontal component
of lift" in a turn is somehow balanced by some other force.

If it were, the glider would not turn.

A turn is an acceleraton, requiring unbalanced forces. Lift is greater
than the sum of gravity plus drag.

Cookie

At 18:25 19 March 2009, The Real Doctor wrote:
>On 19 Mar, 12:45, Bob Cook wrote:
>
>> Q) A glider is in circling flight. =A0The glider circles because there
>is=
> a
>> horizontal component of lift. =A0This horizontal component of lift is
>> balanced by an equal and opposite force, centrifugal force.
>>
>> True or False and why?
>
>I hope you're not getting hung up on the old centripetal/centrifugal
>debate. Centripetal force is just as real in a stationary axix system
>as centrifugal is in a turning one!
>
>By and large (ignoring a few second-order effects) what you wrote is
>fine.
>
>Ian
>

[Bob Cook[_2_]]

OK. badly worded question.

Here I was trying to point out that DRAG is the major difference between a
40:1 glider and a 20: glider. Not lift, not gravity.


Drag determines the direction of flight.

More drag= steeper glide (spoilers, flaps)

Less drag = flatter glide

Cookie



At 18:23 19 March 2009, The Real Doctor wrote:
>On 19 Mar, 12:15, Bob Cook wrote:
>
>> Spoilers redistribute lift, but not reduce lift.
>
>They reduce the lift over part of the wing. Whether the overall lift
>increases, decreases or stays the same depends on what the pilot does.
>
>> Flaps change the coeffecient of lift, but not lift.
>
>If you keep everything else the same then they do change lift. Every
>tried dumping your landing flap just before the flare ...?
>
>> Q) =A0Two gliders, one is 40:1 racer and glider two is 20:1 trainer.
>Both
>> weigh 800#
>>
>> Glider one has twice the lift of glider two. =A0True or flase and why.
>
>Depends what they are doing.
>
>Ian
>

[Doug Hoffman]

Mike Schumann wrote:
> Weight is a force????? Did I miss something in HS Physics? Weight = Mass.
>

Yeah. This is often a confusing issue. If one is dealing with just the
metric system then I think the terminology is clear and unambiguous. A
kilogram is clearly a unit of mass, and a newton is clearly a unit of
force. Here in the good old USA we have the pound-force and the
pound-mass. Oh boy. I was taught that the unit of mass in English
units is the slug. But I have yet to see a bathroom scale with a
readout in slugs. :-)

When people talk about "on the moon we would weigh less" they have to be
referring to force (not mass) and indeed our bathroom scales would
reflect force and not mass if used on the moon.

I just wish the US would perform its conversion to metric units for
*everything*. The sooner the better. But that job is being handled by
our highly efficient government. Don't hold your breath. :-)

Regards,

-Doug

p.s. The US auto industry voluntarily converted everything to hard
metric years ago because we have to work with suppliers from around the
world and sell product the same. We couldn't wait for our government to
get that job done. Remember in 1978 when the US government was
orchestrating the conversion? The interstate freeways even had all of
the kph speed limit signs installed but covered. At the last minute
some senator decided he didn't like the idea of converting and
everything was scuttled and here we are.

[Bob Cook[_2_]]

Bob,

If we are gonna play what if.....

What if there was no lift? (like a brick,)

Sure it would move because of gravity, but in what direction? Down! Not
forward!

So tell me again how gravity imparts a forward force to a glider?


Although "no gravity is not possible, "no lift" is easily
demonstrated:

Next time you fly, try this. Push forward on the stick. Keep pushing
until your angle of attack goes to zero (no lift). Tell me which way your
glider flies!

Cookie


>> At 18:22 17 March 2009, Bob Whelan wrote:
>>
>>> Yebbut...imagine a glider magically inserted into earth's atmosphere

>>> after global warming has removed all grabbity. What gets it moving?
>>
>> It won't move. Why would it?
>>
>> Jim Beckman

[Bob Cook[_2_]]

Ian,

If we draw vector force diagrams of two identical gliders, one with flaps
extended, and the other with flaps retracted, we can easily see that
"lift" is essentially the same in both cases.

The direction of flight will be steeper with flaps extended, and drag will
be signifigantly greater.

As for spoilers, vector diagrams will also show lift is not reduced
(except for a tiny, tiny amount) but drag is greatly increased and the
fligth path is steepened.

Remember were are talking about stabilized unaccelerated flight here.

Sudden application or retraction of flaps might have MOMENTARY changes in
lift, but not once stabilized unacellerated flight is resumed.

Misconceptions commonly found in publications and professed by many
"experts":

Gravity powers a glider and provides forward motion.
Flaps increase lift
Spoilers reduce lift
Centrifugal force turns a glider
Aircraft climb due to increased lift
Aircraft descend due to reduced lift

Any more?

Cookie



At 01:00 20 March 2009, Bob Cook wrote:
>OK. badly worded question.
>
>Here I was trying to point out that DRAG is the major difference between
a
>40:1 glider and a 20: glider. Not lift, not gravity.
>
>
>Drag determines the direction of flight.
>
>More drag= steeper glide (spoilers, flaps)
>
>Less drag = flatter glide
>
>Cookie
>
>
>
>At 18:23 19 March 2009, The Real Doctor wrote:
>>On 19 Mar, 12:15, Bob Cook wrote:
>>
>>> Spoilers redistribute lift, but not reduce lift.
>>
>>They reduce the lift over part of the wing. Whether the overall lift
>>increases, decreases or stays the same depends on what the pilot does.
>>
>>> Flaps change the coeffecient of lift, but not lift.
>>
>>If you keep everything else the same then they do change lift. Every
>>tried dumping your landing flap just before the flare ...?
>>
>>> Q) =A0Two gliders, one is 40:1 racer and glider two is 20:1 trainer.
>>Both
>>> weigh 800#
>>>
>>> Glider one has twice the lift of glider two. =A0True or flase and
why.
>>
>>Depends what they are doing.
>>
>>Ian
>>
>

[Bob Cook[_2_]]

This one is for Ian so you others don't read this, OK?

Ian,

I have invented a "gravity powered machine". It uses weight(s) as fuel.
Once the fuel is exhaused, the "used" fuel is fully recoverable and can
simply be put back in the "tank", and the machine will start running
again!

Here is how it works.

Take a bucket and place 100 steel balls into it.
Take another bucket and place 100 steel balls into it, but also place 1/2
of a steel ball into it. These steel balls are the "fuel"

String a rope over a pulley from the peak of the barn. Tie the buckets to
each end of the rope, about 10' high. Naturally the heavier bucket will
fall to the ground, lifting the light bucket. Remove one steel ball from
the heavy bucket making it now the lighter bucket. Now the other bucket
will fall to the ground, remove one seel ball, etc. The machine will go
up and down until you run out of steel balls.

But don't worry, just pick up the steel balls and place them in the
buckets again, have fun all day with gravity power!

(Same way a glider is powered by gravity!)


Cookie






At 18:38 19 March 2009, The Real Doctor wrote:
>On 19 Mar, 17:04, Jim Logajan wrote:
>
>> So what does power gliders? Seems like you are the only one who knows
>the
>> real answer.
>
>The sun.
>
>Also, initially, internal combustion engines, stretched rubber or, if
>you buy an Electrostart and choose your contract carefully, nuclear
>fission!
>
>Ian
>

[Doug Hoffman]

Bob Cook wrote:

> If we are gonna play what if.....
>
> What if there was no lift? (like a brick,)
>
> Sure it would move because of gravity, but in what direction? Down! Not
> forward!

That is correct and demonstrates a useful "thought exercise".


> So tell me again how gravity imparts a forward force to a glider?

By itself it cannot. The presence of gravity *and* air are required for
gliding (forward) motion.


> Although "no gravity is not possible,

Sure it is. Place your glider in outer space. Likely? No. Possible?
Certainly.


> "no lift" is easily demonstrated:

And lift(as we are using the term) without gravity is not possible.

Regards,

-Doug

[Bob Cook[_2_]]

Doug,

I was about to give you a "gold star" but then found some more
misconceptions.

There is gravity in outer space. Anywhere there is matter, with mass,
there is Gravity. The moon is held in orbit by the earth's gravity.
Earth in solar orbit, etc.

Lift would be possible without gravity, (if it was possible to have no
gravity.) If you placed an airplane in air, but no gravity, and provided
a propelling force (engine), the wing could certainly produce lift without
gravity . Without gravity, lift would be unbalanced, resulting in your
aircraft doing successive loops!

Air (or other "fluid" if we are taking this into outer space), is needed
to provide lift. But the forces involved in gliding flight are still
three.....lift, drag, and gravity.

Although thought provoking exerceses, better to say "what is" than
"what if.".....

Cookie





At 01:39 20 March 2009, Doug Hoffman wrote:
>Bob Cook wrote:
>
>> If we are gonna play what if.....
>>
>> What if there was no lift? (like a brick,)
>>
>> Sure it would move because of gravity, but in what direction? Down!
>Not
>> forward!
>
>That is correct and demonstrates a useful "thought exercise".
>
>
>> So tell me again how gravity imparts a forward force to a glider?
>
>By itself it cannot. The presence of gravity *and* air are required for

>gliding (forward) motion.
>
>
>> Although "no gravity is not possible,
>
>Sure it is. Place your glider in outer space. Likely? No. Possible?
> Certainly.
>
>
>> "no lift" is easily demonstrated:
>
>And lift(as we are using the term) without gravity is not possible.
>
>Regards,
>
>-Doug
>

[Doug Hoffman]

Bob Cook wrote:
> Doug,
>
> I was about to give you a "gold star" but then found some more
> misconceptions.
>
> There is gravity in outer space. Anywhere there is matter, with mass,
> there is Gravity. The moon is held in orbit by the earth's gravity.
> Earth in solar orbit, etc.

You are picking knits and missing the overall points. Clearly I meant
place it in space where gravity is negligible.


> Lift would be possible without gravity, (if it was possible to have no
> gravity.) If you placed an airplane in air, but no gravity, and provided
> a propelling force (engine), the wing could certainly produce lift without
> gravity . Without gravity, lift would be unbalanced, resulting in your
> aircraft doing successive loops!

Another knit. This discussion is about *gliding*.


> Air (or other "fluid" if we are taking this into outer space), is needed
> to provide lift. But the forces involved in gliding flight are still
> three.....lift, drag, and gravity.
>
> Although thought provoking exerceses, better to say "what is" than
> "what if.".....

OK Bob. You want to see things just one way and apparently nothing else
anyone says is going to sway you. No problem here. Have fun. I think
I am done with this.

Regards,

-Doug

[Jim White[_2_]]

Cookie...are you drinking enough? The weather in your part of the world
must be bad as you do not seem to leave much time for flying.

Jim


At 01:30 20 March 2009, Bob Cook wrote:
>This one is for Ian so you others don't read this, OK?
>
>Ian,
>
>I have invented a "gravity powered machine". It uses weight(s) as
fuel.
> Once the fuel is exhaused, the "used" fuel is fully recoverable and
can
>simply be put back in the "tank", and the machine will start running
>again!
>
>Here is how it works.
>
>Take a bucket and place 100 steel balls into it.
>Take another bucket and place 100 steel balls into it, but also place
1/2
>of a steel ball into it. These steel balls are the "fuel"
>
>String a rope over a pulley from the peak of the barn. Tie the buckets
to
>each end of the rope, about 10' high. Naturally the heavier bucket
will
>fall to the ground, lifting the light bucket. Remove one steel ball
from
>the heavy bucket making it now the lighter bucket. Now the other bucket
>will fall to the ground, remove one seel ball, etc. The machine will go
>up and down until you run out of steel balls.
>
>But don't worry, just pick up the steel balls and place them in the
>buckets again, have fun all day with gravity power!
>
>(Same way a glider is powered by gravity!)
>
>
>Cookie
>
>
>
>
>
>
>At 18:38 19 March 2009, The Real Doctor wrote:
>>On 19 Mar, 17:04, Jim Logajan wrote:
>>
>>> So what does power gliders? Seems like you are the only one who knows
>>the
>>> real answer.
>>
>>The sun.
>>
>>Also, initially, internal combustion engines, stretched rubber or, if
>>you buy an Electrostart and choose your contract carefully, nuclear
>>fission!
>>
>>Ian
>>
>

[The Real Doctor]

On 20 Mar, 01:39, Doug Hoffman > wrote:
> Bob Cook wrote:

> > Although "no gravity is not possible,
>
> Sure it is. *Place your glider in outer space. *Likely? *No. *Possible?
> * Certainly.

Or just fly a vomit-comet style ballistic trajectory.

> And lift(as we are using the term) without gravity is not possible.

Aircraft in a 90 degree bank can still produce lift ...

Ian

[The Real Doctor]

On 20 Mar, 01:00, Bob Cook > wrote:
> Ian,
>
> I was trying to point out a misconception that the "horizontal component
> of lift" in a turn is somehow balanced by some other force.
>
> If it were, the glider would not turn.

It all depends on your reference frame. To someone on the ground, an
unbalanced sideways force gives rise to the necessary acceleration.
But to an observer moving with the glider - the pilot, say - there is
no sideways acceleration /of the glider/ (the rest of the world may,
of course, be doing something). To the moving observer, an equal an
opposite centrifugal force provides the necessary balance.

We engineers like modelling with moving reference frames and
centrifugal forces because it turns dynamics problems into statics
problems, which are generally simpler. Physicists, and particularly
school physics teachers, traditionally get terribly upset by the idea
of centrifugal force.

Ian

[The Real Doctor]

On 19 Mar, 19:48, Martin Gregorie >
wrote:

> There is a question in the UK Bronze badge written paper about the
> proportion of lift provided by the top and bottom surfaces of a wing
> that's just as wrong. The so-called "correct" answer is 70/30, but as a
> wing is a device for imparting momentum to an air mass its a meaningless
> question.

Oh no. Not that one.

At the surface of the wing, it exerts a force on the air mass. A long
distance away (typically > 2 chord lengths) it's a momentum change. In
between the effect of the wing is a pressure change /and/ a momentum
change.

Overall, the integrated pressure across the top surface is about 70%
of the total lift force, and the intergrated pressure across the
bottom surface is about 30% of the total lift.

Significance? Irregularities on the top surface will reduce lift by
more than the same irregularities on the bottom surface. Hence top-
surface-only airbrakes: they are more effective there than underneath,
because they destroy more lift, necessitating a bigger and draggier
change of AoA to compensate.

Ian

[The Real Doctor]

On 20 Mar, 01:15, Bob Cook > wrote:

> If we draw vector force diagrams of two identical gliders, one with flaps
> extended, and the other with flaps retracted, we can easily see that
> "lift" is essentially the same in both cases.

What happens if you extend the flaps on a glider while keeping
everything else (AoA, airspeed) constant?

Ian

[The Real Doctor]

On 19 Mar, 19:41, Jim Logajan > wrote:
> The Real Doctor > wrote:
>
> > On 19 Mar, 17:04, Jim Logajan > wrote:
>
> >> So what does power gliders? Seems like you are the only one who knows
> >> the real answer.
>
> > The sun.
>
> So you were being pedantic after all.

No I wasn't. Unless you think that "the sun" and "gravity" are somehow
the same power source.

Don't feel bad about this. The misapprehensions run very deep, and a
huge number of gliding instructors really don;t understand this stuff.
Not that it matters, really, because you don't need to know
aerodynamics to fly a glider!

Ian

[The Real Doctor]

On 20 Mar, 07:30, Jim White > wrote:
> Cookie...are you drinking enough? The weather in your part of the world
> must be bad as you do not seem to leave much time for flying.

Dunno where he is, but I'm in Scotland. Need I say more?

Ian

[Martin Gregorie[_4_]]

On Fri, 20 Mar 2009 02:39:17 -0700, The Real Doctor wrote:

> On 19 Mar, 19:48, Martin Gregorie >
> wrote:
>
>> There is a question in the UK Bronze badge written paper about the
>> proportion of lift provided by the top and bottom surfaces of a wing
>> that's just as wrong. The so-called "correct" answer is 70/30, but as a
>> wing is a device for imparting momentum to an air mass its a
>> meaningless question.
>
> Oh no. Not that one.
>
> At the surface of the wing, it exerts a force on the air mass. A long
> distance away (typically > 2 chord lengths) it's a momentum change. In
> between the effect of the wing is a pressure change /and/ a momentum
> change.
>
> Overall, the integrated pressure across the top surface is about 70% of
> the total lift force, and the intergrated pressure across the bottom
> surface is about 30% of the total lift.
>
> Significance? Irregularities on the top surface will reduce lift by more
> than the same irregularities on the bottom surface. Hence top-
> surface-only airbrakes: they are more effective there than underneath,
> because they destroy more lift, necessitating a bigger and draggier
> change of AoA to compensate.
>
True enough, but the point I was trying (badly) to make is that the lift
is due to the whole wing section and shouldn't be apportioned to the two
surfaces as a number taught to neophytes. Apart from anything else this
breaks down when you consider the pressure distribution across the top
surface. Why not also teach an arbitrary percentage of lift generated by
the LE suction spike at high Cl?


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

[Bob Cook[_2_]]

Doug,

You're the one who took the coversation into aouter space, not me.

Yes, I only look at this one way. In physics, its black or white,there is
pretty much not a lot of "gray area".

But I think you got my point that the question on the Bronze Badge exam is
flawed, and none of the answers are correct.

Cookie



At 04:17 20 March 2009, Doug Hoffman wrote:
>Bob Cook wrote:
>> Doug,
>>
>> I was about to give you a "gold star" but then found some more
>> misconceptions.
>>
>> There is gravity in outer space. Anywhere there is matter, with mass,
>> there is Gravity. The moon is held in orbit by the earth's gravity.
>> Earth in solar orbit, etc.
>
>You are picking knits and missing the overall points. Clearly I meant
>place it in space where gravity is negligible.
>
>
>> Lift would be possible without gravity, (if it was possible to have no
>> gravity.) If you placed an airplane in air, but no gravity, and
>provided
>> a propelling force (engine), the wing could certainly produce lift
>without
>> gravity . Without gravity, lift would be unbalanced, resulting in
your
>> aircraft doing successive loops!
>
>Another knit. This discussion is about *gliding*.
>
>
>> Air (or other "fluid" if we are taking this into outer space), is
needed
>> to provide lift. But the forces involved in gliding flight are still
>> three.....lift, drag, and gravity.
>>
>> Although thought provoking exerceses, better to say "what is" than
>> "what if.".....
>
>OK Bob. You want to see things just one way and apparently nothing else

>anyone says is going to sway you. No problem here. Have fun. I think
>I am done with this.
>
>Regards,
>
>-Doug
>

[Bob Cook[_2_]]

That would not be possible......only "momentary". You can't "keep"
"everything else" constant.

Yes, there would be a "momentary" increase in lift......but......

Glider would experience "acceleration", change in velocity, and or
direction. (Most likely glider would "nose up" (change in direction)
then "slow down" (change in velocity) finally resume steady flight with
greater drag giving steeper glide angle.

Once "stabilized" flight is resumed, vector analysis shows flaps do not
increase lift. Yes, the coefficient of lift has changed but not the
actual lift (other than a really, really tiny reduction due to new flgith
path)

Similar for spoilers

Cookie




At 09:40 20 March 2009, The Real Doctor wrote:
>On 20 Mar, 01:15, Bob Cook wrote:
>
>> If we draw vector force diagrams of two identical gliders, one with
>flaps
>> extended, and the other with flaps retracted, we can easily see that
>> "lift" is essentially the same in both cases.
>
>What happens if you extend the flaps on a glider while keeping
>everything else (AoA, airspeed) constant?
>
>Ian
>

[Surfer!]

In message
>, The
Real Doctor > writes
>On 20 Mar, 07:30, Jim White > wrote:
>> Cookie...are you drinking enough? The weather in your part of the world
>> must be bad as you do not seem to leave much time for flying.
>
>Dunno where he is, but I'm in Scotland. Need I say more?

I sincerely hope you were flying on the 22nd February. Fantastic wave
day, and a Sunday!

--
Surfer!
Email to: ramwater at uk2 dot net

[Bob Cook[_2_]]

In the words of Foghorn Leghorn, "That was a joke son, a joke. How come
you're not laughin'?"

Anyway, we have some of the best soaring weather here in Blairstown. Just
check out the OLC!

As you might have noticed my posts are usually in the early morning or
late evening. The middle of my day is filled up with flying! (Lucky me)
But, beyond RAS and flying I have no life at all!

Cookie



At 09:45 20 March 2009, The Real Doctor wrote:
>On 20 Mar, 07:30, Jim White wrote:
>> Cookie...are you drinking enough? The weather in your part of the
world
>> must be bad as you do not seem to leave much time for flying.
>
>Dunno where he is, but I'm in Scotland. Need I say more?
>
>Ian
>

[The Real Doctor]

On 20 Mar, 12:20, Surfer! > wrote:
> In message
> >, The
> Real Doctor > writes
>
> >On 20 Mar, 07:30, Jim White > wrote:
> >> Cookie...are you drinking enough? The weather in your part of the world
> >> must be bad as you do not seem to leave much time for flying.
>
> >Dunno where he is, but I'm in Scotland. Need I say more?
>
> I sincerely hope you were flying on the 22nd February. *Fantastic wave
> day, and a Sunday!

Alas not. The glider is awaiting some work and I don;t normally fly in
the winter anyway.

Ian

[[email protected]]

You guys have proven 2 fundamental truths.

1) In any engineering discussion. If you don't define terms at the
start, you just chase yourself around in a circle.

2) Pilots don't need to know much about aerodyanmics to fly well.

Oh yeah,

3) Flight instructors explain these things more to make a point than
to be accurate :-)


Todd Smith
3S

[The Real Doctor]

On 20 Mar, 13:29, wrote:
> You guys have proven 2 fundamental truths.
>
> 1) *In any engineering discussion. *If you don't define terms at the
> start, you just chase yourself around in a circle.

What, exactly, do you mean by "circle"?

> 2) *Pilots don't need to know much about aerodyanmics to fly well.

I suspect it only distracts from the task at hand.

> 3) Flight instructors explain these things more to make a point than
> to be accurate :-)

And how. I just wish they'd tell the rest of us what the point is ...

Ian

[Jim Beckman[_2_]]

At 00:45 20 March 2009, Bob Cook wrote:
>
>You are on to something here. Get one of those geo surveys, and fly
your
>glider only over areas of higher gravity. This extra gravity will give
>your glider extra power, and you will be the best glider pilot!

Well, that should work. The important thing is that you have to find and
use thermals only in *low* gravity areas, and then do your inter-thermal
cruising in the *high* gravity areas. Next best thing to dynamic
soaring.

Jim Beckman

[Jim Beckman[_2_]]

At 19:48 19 March 2009, Martin Gregorie wrote:
>On Thu, 19 Mar 2009 13:09:30 -0500, Jim Logajan wrote:
>
>> You're claim conflicts with that in the FAA "Pilot's Handbook of
>> Aeronautical Knowledge":
>>
>> "Found on many gliders and some aircraft, high drag devices called
>> spoilers are deployed from the wings to spoil the smooth airflow,
>> reducing lift and increasing drag."
>>
>Printing it in some official or semi-official publication doesn't make
it
>right.

But it *does* make it the right answer to the written test when that test
is given by that same authority. For the test, concentrate not on what is
necessarily true, but on what the FAA wants for an answer. After the
written is out of the way, just learn to fly the aircraft.

Jim Beckman

[Doug Hoffman]

The Real Doctor wrote:
> On 20 Mar, 01:39, Doug Hoffman > wrote:

>> And lift(as we are using the term) without gravity is not possible.
>
> Aircraft in a 90 degree bank can still produce lift ...

Without gravity how will you get the glider into a bank (or even to
move)? Answer = not possible.

Regards,

-Doug

[Wayne Paul]

"The Real Doctor" > wrote in message
...
> On 20 Mar, 01:15, Bob Cook > wrote:
>
>> If we draw vector force diagrams of two identical gliders, one with flaps
>> extended, and the other with flaps retracted, we can easily see that
>> "lift" is essentially the same in both cases.
>
> What happens if you extend the flaps on a glider while keeping
> everything else (AoA, airspeed) constant?
>
> Ian

I have really been trying to avoid getting involved in this thread. It seem
too much like "I'm smart and you're not". However, I just want to point out
that to maintain the same AoA while lowering the flaps will require the
pilot to lower the nose of the aircraft. This is due to the change to the
wing's effective angle of incidence.

I really question if it is posible to lower to flaps while keeping the AoA
and airspeed constant. I know that maintaining a constant airspeed of 50
kts while lowering the flaps to 90 degrees on my HP-14 results in a 45+
degree nose down attitude. I am positive the wing's AoA changes while doing
so.

Respectfully,

Wayne
HP-14 "6F"
http://www.soaridaho.com/Schreder

[Nyal Williams[_2_]]

A physicist acquantance who wrote a book on Newton and aerodynamics said
that the mathematics was taken over from electrical theory because the
equations balanced, but that they don't explain what is cause and what is
effect. That's the feeling I get when you start talking about something
called theta as if it were proof.

Seems to me we have a new theology going here -- something akin to
pre-milleniumism versus post-milleniumism.


At 13:29 20 March 2009, wrote:
>You guys have proven 2 fundamental truths.
>
>1) In any engineering discussion. If you don't define terms at the
>start, you just chase yourself around in a circle.
>
>2) Pilots don't need to know much about aerodyanmics to fly well.
>
>Oh yeah,
>
>3) Flight instructors explain these things more to make a point than
>to be accurate :-)
>
>
>Todd Smith
>3S
>

[The Real Doctor]

On 20 Mar, 14:38, Doug Hoffman > wrote:
> The Real Doctor wrote:
> > On 20 Mar, 01:39, Doug Hoffman > wrote:
> >> And lift(as we are using the term) without gravity is not possible.
>
> > Aircraft in a 90 degree bank can still produce lift ...
>
> Without gravity how will you get the glider into a bank (or even to
> move)? *Answer = not possible.

An engine. You'll notice that I wrote "aircraft" and not "gliders"...

Ian

[Bob Kuykendall]

On Mar 19, 6:08 pm, Doug Hoffman > wrote:

> I just wish the US would perform its conversion to metric units for
> *everything*. The sooner the better. But that job is being handled by
> our highly efficient government. Don't hold your breath. :-)

Kind of a hijack, but what I wish is that aircraft hardware
manufacturers would get their crap together and produce a coherent
metric equivalent of the AN system of common aircraft hardware.

The magic of the AN hardware system is not that they offer any
particularly high strength (they don't; for the most part AN bolts are
equivalent to Grade 5 hardware store bolts) or any particularly high
precision (again, they're about the same as the bolts at Ace or True
Value). The magic is that AN bolts have just enough thread for a nut
and somewhere between 0" and about 0.125" of washers, and that they
come in length increments of 0.125". Those two elements let you create
a nice, tidy bolted joint of virtually any practical grip length, and
not have the threaded portion of the bolt loaded in shear, and not
have a bunch of threads hanging out of the nut. Furthermore, common AN
hardware is very attractively priced, for the most part you can buy
them from any of several aircraft parts outlets at the same or even
lower prices as Grade 5 bolts at a mom&pop hardware store.

By contrast, metric aircraft hardware has no coherent system of
markings, thread lengths, and grip lengths. It averages twice or
thrice the cost of AN hardware when you can find it, and is available
from only a few outlets. When you need a replacement bolt for your
European aircraft, you have virtually no choice but to order it
directly from the manufacturer at huge markups and with huge shipping
charges.

I like the metric system, and I like metric hardware. I appreciate
that even American cars are, by and large, assembled with metric nuts
and bolts these days. But given the choice between about $500 worth of
AN hardware per aircraft and twice or thrice that in metric nuts and
bolts that offer no greater utility, hmmm, I think I'll go with the
less expensive option.

End rant.

Thanks, Bob K.
http://www.hpaircraft.com/hp-24

[Doug Hoffman]

Bob Kuykendall wrote:
> On Mar 19, 6:08 pm, Doug Hoffman > wrote:
>
>> I just wish the US would perform its conversion to metric units for
>> *everything*. The sooner the better. But that job is being handled by
>> our highly efficient government. Don't hold your breath. :-)
>
> Kind of a hijack, but what I wish is that aircraft hardware
> manufacturers would get their crap together and produce a coherent
> metric equivalent of the AN system of common aircraft hardware.
>
> The magic of the AN hardware system is not that they offer any
> particularly high strength (they don't; for the most part AN bolts are
> equivalent to Grade 5 hardware store bolts) or any particularly high
> precision (again, they're about the same as the bolts at Ace or True
> Value). The magic is that AN bolts have just enough thread for a nut
> and somewhere between 0" and about 0.125" of washers, and that they
> come in length increments of 0.125". Those two elements let you create
> a nice, tidy bolted joint of virtually any practical grip length, and
> not have the threaded portion of the bolt loaded in shear, and not
> have a bunch of threads hanging out of the nut. Furthermore, common AN
> hardware is very attractively priced, for the most part you can buy
> them from any of several aircraft parts outlets at the same or even
> lower prices as Grade 5 bolts at a mom&pop hardware store.
>
> By contrast, metric aircraft hardware has no coherent system of
> markings, thread lengths, and grip lengths. It averages twice or
> thrice the cost of AN hardware when you can find it, and is available
> from only a few outlets. When you need a replacement bolt for your
> European aircraft, you have virtually no choice but to order it
> directly from the manufacturer at huge markups and with huge shipping
> charges.
>
> I like the metric system, and I like metric hardware. I appreciate
> that even American cars are, by and large, assembled with metric nuts
> and bolts these days. But given the choice between about $500 worth of
> AN hardware per aircraft and twice or thrice that in metric nuts and
> bolts that offer no greater utility, hmmm, I think I'll go with the
> less expensive option.
>
> End rant.

Hi Bob,

Yes. There may be some niche areas like aircraft hardware that would at
least require legacy support for a period of years. Makes me wonder
what Boeing/Cessna and others are doing now and plan to do in the future
in that respect.

Regards,

-Doug

Btw, American designed cars and trucks do more than just assemble with
metric fasteners. Nominal dimensions are typically, e.g., 100 mm for a
bracket width instead of 4.0". We call that "hard metric" design. Some
user interface items like wheel lug nuts may still be SAE.

[Uncle Fuzzy]

On Mar 20, 4:58*pm, Doug Hoffman > wrote:
> Bob Kuykendall wrote:
> > On Mar 19, 6:08 pm, Doug Hoffman > wrote:
>
> >> I just wish the US would perform its conversion to metric units for
> >> *everything*. *The sooner the better. *But that job is being handled by
> >> our highly efficient government. *Don't hold your breath. *:-)
>
> > Kind of a hijack, but what I wish is that aircraft hardware
> > manufacturers would get their crap together and produce a coherent
> > metric equivalent of the AN system of common aircraft hardware.
>
> > The magic of the AN hardware system is not that they offer any
> > particularly high strength (they don't; for the most part AN bolts are
> > equivalent to Grade 5 hardware store bolts) or any particularly high
> > precision (again, they're about the same as the bolts at Ace or True
> > Value). The magic is that AN bolts have just enough thread for a nut
> > and somewhere between 0" and about 0.125" of washers, and that they
> > come in length increments of 0.125". Those two elements let you create
> > a nice, tidy bolted joint of virtually any practical grip length, and
> > not have the threaded portion of the bolt loaded in shear, and not
> > have a bunch of threads hanging out of the nut. Furthermore, common AN
> > hardware is very attractively priced, for the most part you can buy
> > them from any of several aircraft parts outlets at the same or even
> > lower prices as Grade 5 bolts at a mom&pop hardware store.
>
> > By contrast, metric aircraft hardware has no coherent system of
> > markings, thread lengths, and grip lengths. It averages twice or
> > thrice the cost of AN hardware when you can find it, and is available
> > from only a few outlets. When you need a replacement bolt for your
> > European aircraft, you have virtually no choice but to order it
> > directly from the manufacturer at huge markups and with huge shipping
> > charges.
>
> > I like the metric system, and I like metric hardware. I appreciate
> > that even American cars are, by and large, assembled with metric nuts
> > and bolts these days. But given the choice between about $500 worth of
> > AN hardware per aircraft and twice or thrice that in metric nuts and
> > bolts that offer no greater utility, hmmm, I think I'll go with the
> > less expensive option.
>
> > End rant.
>
> Hi Bob,
>
> Yes. *There may be some niche areas like aircraft hardware that would at
> least require legacy support for a period of years. *Makes me wonder
> what Boeing/Cessna and others are doing now and plan to do in the future
> in that respect.
>
> Regards,
>
> -Doug
>
> Btw, American designed cars and trucks do more than just assemble with
> metric fasteners. *Nominal dimensions are typically, e.g., 100 mm for a
> bracket width instead of 4.0". *We call that "hard metric" design. *Some
> user interface items like wheel lug nuts may still be SAE.- Hide quoted text -
>
> - Show quoted text -

While we're 'Ranting'.... Bob, that would be nice, wouldn't it? It
took me 2 tries to get the right bolt from Grob when I went throught
my control system a few years ago.
DON'T get me started on US auto makers. I have two Dodge Trucks. I
HATE the fact that evey time I get under one to work on it, I need to
take BOTH metric and SAE tools. Pick a STANDARD! My favorite example
was an exhaust stud on a 1998 Olds Intrigue. Engine side - SAE
exhaust flange side - metric.
Rant off.
I feel better now.

[Myles]

On Mar 20, 1:32 am, The Real Doctor > wrote:

> Physicists, and particularly
> school physics teachers, traditionally get terribly upset by the idea
> of centrifugal force.
>
> Ian

Physicsists make equally snarky comments about engineers, Ian. My
engineering dynamics professor at UC, Berkeley was adamant in
opposing the perpetuation of the centrifugal force myth. It's phony
physics and can lead to seriously erroneous conclusions.

Myles

[The Real Doctor]

On 21 Mar, 05:21, Myles > wrote:
> On Mar 20, 1:32 am, The Real Doctor > wrote:
>
> > Physicists, and particularly
> > school physics teachers, traditionally get terribly upset by the idea
> > of centrifugal force.

> Physicsists make equally snarky comments about engineers, Ian. *

Indeed. Bu hey, the mathematicians despise us all.

> My
> engineering dynamics professor at UC, Berkeley was *adamant in
> opposing the perpetuation of the centrifugal force myth. *It's phony
> physics and can lead to seriously erroneous conclusions.

It's a perfectly useful tool if applied correctly. That normally means
within a moving axis system, and getting there is not always simple.
You always have to decide whether it's going to be easier overall to
use the difficult model with the simple setup (stationary axes) or the
simple model with the difficult setup (moving axes).

It's the same in fluids - normally we model a glider by holding it
still and letting the air move past, but that's not always the best
way, or the easiest way. Mind you, I'm a typical lazy engineer, so for
me best = easiest in about 99% of cases.

Ian

[The Real Doctor]

On 21 Mar, 03:17, Uncle Fuzzy > wrote:

> While we're 'Ranting'.... Bob, that would be nice, wouldn't it? *It
> took me 2 tries to get the right bolt from Grob when I went throught
> my control system a few years ago.

You think that;s bad? Try getting specialist fasteners for old Polish
gliders. Or, on the ground, getting fasteners in the odd 5mm-but-not-
M5 thread that Citroen used. I've seen refurbished original Citroen
nuts and bolts at €4 per item - not per set!

Ian

[Jim Beckman[_2_]]

At 11:44 21 March 2009, The Real Doctor wrote:
>
>You think that;s bad? Try getting specialist fasteners for old Polish
>gliders. Or, on the ground, getting fasteners in the odd 5mm-but-not-
>M5 thread that Citroen used. I've seen refurbished original Citroen
>nuts and bolts at =804 per item - not per set!

I have to keep my Whitworth wrenches on hand when working on my
Austin-Healey 100. Some of the basic stuff that came out of the parts
bins, like suspension and transmission, uses Whitworth hardware, while the
rest works with SAE stuff. It's still possible to buy Whitworth threaded
fasteners, but you have to know where to look. Are there any British
gliders left that are old enough to have used Whitworth hardware?

Jim Beckman

[TonyV[_2_]]

Myles wrote:
> On Mar 20, 1:32 am, The Real Doctor > wrote:
>
>... My engineering dynamics professor at UC, Berkeley was adamant in
> opposing the perpetuation of the centrifugal force myth. It's phony
> physics and can lead to seriously erroneous conclusions.

Yeah, yeah, centrifugal force is a reaction to a centripetal force, I
know the difference,.... I don't care. The layman understands the former
term and not the latter. To the typical student, I'll use the term
"centrifugal". If he's a physicist, I'll say "centripetal". :-)

Tony V.