Helium bubbles used to show bird aerodynamics

[John DeRosa OHM Ω http://aviation.derosaweb.net]

https://youtu.be/2sh8_3-R90I

At 0:58 in the video the bird's wingtip vortices are described as "This helps provide lift". This statement caught my eye.

However, Wikipedia (https://en.wikipedia.org/wiki/Wingtip_device) mentions that "Wingtip devices increase the lift generated at the wingtip (by smoothing the airflow across the upper wing near the tip) and reduce the lift-induced drag caused by wingtip vortices, improving lift-to-drag ratio.

A contradiction?

- John non-aero-e DeRosa

[Bret Hess]

These are journalists describing science. I think more accurate would be "these vortices are a consequence of the process of generating lift with that wing". They create drag, which we don't want, but they come with the lift which we want.

An analogy would be to say "the rubber you leave on the road from your tires creates all the thrust for your car". Tearing up the tires and heating the road from the slipping rubber rob some of the engine's energy, but you can't go forward without them.

[Martin Gregorie[_6_]]

On Mon, 24 Feb 2020 23:21:59 -0800, Bret Hess wrote:

> These are journalists describing science. I think more accurate would be
> "these vortices are a consequence of the process of generating lift with
> that wing". They create drag, which we don't want, but they come with
> the lift which we want.
>
>
Yes, that's fair. A wing can't generate lift without generating tip
vortices. The energy used in spinning them up adds to the aircraft's
sinking speed, but clever wing design, which may include winglets and/or
wingtip shaping as well as a carefully designed wing planform can reduce
the energy that goes into spinning up tip vortices and hence will reduce
the sinking speed of the aircraft.

For a very readable account, try "Affandi Darlington on winglets", which
first appeared on PPrune but may have vanished unless The Wayback Machine
has a copy. I have a local copy I can send if you can't find it anyplace
else.

Peter Masak's "Winglet Design for Sailplanes" and Wil Schuemann's "A new
wing planform with improved low-speed performance" are both worth
reading. A web search will find the Masak paper and Wil Schuemann's paper
is in the Soaring Symposium archive.

When I developed my 'Delta-G' series of F1A class competition free flight
gliders, I used a combination of Wil Schuemann's planform ideas, first
seen by glider pilots on the S-H Discus, combined with Hoerner wingtips,
also common on gliders from the first glass airframes until winglets took
over, and had decent competition results with this series of models.


--
Martin | martin at
Gregorie | gregorie dot org

[Richard Livingston]

On Monday, February 24, 2020 at 10:33:46 PM UTC-6, John DeRosa OHM Ω http://aviation.derosaweb.net wrote:
> https://youtu.be/2sh8_3-R90I
>
> At 0:58 in the video the bird's wingtip vortices are described as "This helps provide lift". This statement caught my eye.
>
> However, Wikipedia (https://en.wikipedia.org/wiki/Wingtip_device) mentions that "Wingtip devices increase the lift generated at the wingtip (by smoothing the airflow across the upper wing near the tip) and reduce the lift-induced drag caused by wingtip vortices, improving lift-to-drag ratio.
>
> A contradiction?
>
> - John non-aero-e DeRosa

There are always wing tip vortices when a wing is generating lift, but they can be tight and concentrated, like a horizontal tornado, or broad and diffuse. The tight and concentrated carry more energy that the diffuse vortices, even though the vorticity (and the generated lift) is the same.

Rich L

[2G]

On Wednesday, February 26, 2020 at 6:26:53 AM UTC-8, Richard Livingston wrote:
> On Monday, February 24, 2020 at 10:33:46 PM UTC-6, John DeRosa OHM Ω http://aviation.derosaweb.net wrote:
> > https://youtu.be/2sh8_3-R90I
> >
> > At 0:58 in the video the bird's wingtip vortices are described as "This helps provide lift". This statement caught my eye.
> >
> > However, Wikipedia (https://en.wikipedia.org/wiki/Wingtip_device) mentions that "Wingtip devices increase the lift generated at the wingtip (by smoothing the airflow across the upper wing near the tip) and reduce the lift-induced drag caused by wingtip vortices, improving lift-to-drag ratio.
> >
> > A contradiction?
> >
> > - John non-aero-e DeRosa
>
> There are always wing tip vortices when a wing is generating lift, but they can be tight and concentrated, like a horizontal tornado, or broad and diffuse. The tight and concentrated carry more energy that the diffuse vortices, even though the vorticity (and the generated lift) is the same.
>
> Rich L

It is unclear here whether it was the journalist reaching the wrong conclusion or the scientists feeding her a bad conclusion. Obviously, vortices don't create lift for the bird (although flocks use the vortices of the leading bird for additional lift).

Tom

[Chris Behm]

Also not correct is the statement that a glider with a lifting tail would be unstable.
Most earlier freeflight model gliders did have lifting tails, and no in flight controls.
R,
Chris

[Martin Gregorie[_6_]]

On Wed, 18 Mar 2020 22:44:42 -0700, Chris Behm wrote:

> Also not correct is the statement that a glider with a lifting tail
> would be unstable. Most earlier freeflight model gliders did have
> lifting tails, and no in flight controls.
>
What do you mean by 'early'? :-)

A more correct statement would be 'all current competition free flight
models have lifting tails'.

I used to design my own F1A and F1J/1/2A models as well as building them,
and all had lifting tails.

My F1A towline gliders had their CG at 55% of mean wing chord. The
stabiliser operated at a positive lift coefficient of 0.05, which for the
sections I used (B8403, 7% Clark Y and Woebbeking), put the stabiliser
smack in the middle of its minimum drag bucket. Win-Win!

I used a 10 degree swept back LE on the wing's outer panels, straight TE
and raked Hoerner tips. This combination does two things. The sharp angle
where the tip, raked at 30 degrees with the TE longer than LE, meets the
TE tends to localise the tip vortex. The spanwise flow encouraged by the
swept outer LE and the upper tip surface rolling down to meet the lower
surface at a sharp edge tends. In theory these push the tip vortex
further outboard, so increasing effective aspect ratio, but who knows for
sure? However, the design was easy to fly and trim and won its share of
contests.

My F1J design (small stab, long moment, VIT and autorudder) flew best
with the CG at 65% of mean chord, so it used a similar trim setup to my
F1A gliders, while the 1/2A was a modified traditional model (George
French '1/2A Train'), so it had a shorter moment arm and large (35% of
wing) stab. It was also fitted with VIT and autorudder and liked having
its CG at 80% of mean wing chord.

All three designs were stable in wind and turbulent conditions, easy to
trim and fly, and had good contest records.


--
Martin | martin at
Gregorie | gregorie dot org

[[email protected]]

Are these lifting tails creating upward lift during low speed flight, close to stall speed?

Or... are they only providing upward forces at high speed during the climb, transitioning to a downward force during slow speed flight after powerloss?

[Martin Gregorie[_6_]]

On Thu, 19 Mar 2020 06:23:40 -0700, jjdk737 wrote:

> Are these lifting tails creating upward lift during low speed flight,
> close to stall speed?
>
> Or... are they only providing upward forces at high speed during the
> climb, transitioning to a downward force during slow speed flight after
> powerloss?

Depends on the model: my F1A gliders used a fixed stabiliser trim for all
phases of the flight: launch, circle towing to find lift, a good, hard
zoom launch and the glide. Rudder setting on tow depends on line tension
(straight with load on the line, circling to check thermals with slack
line, and slight turn into glide circle with the hook open ready for
release.

My power toys had timer controlled vertical trim and rudder as well as
motor stop. Climb is a very steep right hand spiral with some down trim
relative to glide and a bit of left rudder to keep the nose up. At motor
stop the F1J's timer applied a lot more down to bunt over to glide
attitude and then retrimmed up for glide in a right hand circle. The 1/2A
was similar, but without the bunt transition from climb to glide.

So yes, all three types glided with the tailplane providing lift. All
free flight competition models are better thought of as tandem wing
aircraft with both wings providing lift. That was more obvious in the old
days, when very large tailplanes, up to 35-50% of the wing area, with
short moment arms, 3-3.5 times wing chord, were used. Now tailplanes are
around 20% of the wing area and the moment arms are about 5 times the
wing chord. All free flight models are trimmed to fly at minimum sink
trim and to, hopefully, stay in the thermal you launch them into.

Free flight competitions are often flown when gliders belonging to
sensible pilots stay in their trailers. In fact, some of the best
competitions have been flown in overcast, calm conditions with very
little light lift available. However, there's a 9 m/s limit on wind speed
(32 kph, 23 kts) in Internationals and rain seldom stops play unless its
heavy enough to prevent timekeepers from seeing models. On somewhere like
Sculthorpe where runway 05 is 8800ft (9800ft to the boundary fence) and
models are launched from the SW end taxiway, its fairly normal to pick
them up in the next one or two fields out when flying to a 3 minute
maximum: the scoring flight time is 180 seconds and the dethermaliser
timer releases a second or two later. This gives full stabiliser up at
about 45-60 degrees, which stalls the model and holds it stalled,
converting it into a rigid parachute with a 4-5 m/s descent rate.

my F1As, which are now old technology, used carbon D-boxes and spars
and 7mm diameter hardened steel wing joiners. The models were a little
heavy at around 430g (class minimum is 410g), but the tow hook unlatched
at 16kg tension and I would have been pulling around 25-30 kg at release:
they'd gain around 10m in a half-spiral zoom climb when I let go of the
bottom of the line to release the model. With 100 lb Spectra towline
(essentially no stretch) the unlatch tension needed to be at least 16kg
to prevent accidental unlatch when towing on rough ground and/or in gusty
conditions.


Anyway, thats probably far more than you ever wanted to know!


--
Martin | martin at
Gregorie | gregorie dot org

[Eric Greenwell[_4_]]

Martin Gregorie wrote on 2/25/2020 2:10 AM:
> When I developed my 'Delta-G' series of F1A class competition free flight
> gliders, I used a combination of Wil Schuemann's planform ideas, first
> seen by glider pilots on the S-H Discus, combined with Hoerner wingtips,
> also common on gliders from the first glass airframes until winglets took
> over, and had decent competition results with this series of models.

I flew hand launch gliders in early60's. My best glider had a planform identical
to the original Discus. It was called the "Sweepette". The cover on this article
shows the 1960 version:

https://indoornewsandviews.files.wordpress.com/2012/10/inav-113.pdf

I wonder if Wil Schumann was inspired by the Sweepette, or some earlier version of
that planform?
--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1

[Eric Greenwell[_4_]]

Martin Gregorie wrote on 3/19/2020 8:11 AM:
> On Thu, 19 Mar 2020 06:23:40 -0700, jjdk737 wrote:
>
>> Are these lifting tails creating upward lift during low speed flight,
>> close to stall speed?
>>
>> Or... are they only providing upward forces at high speed during the
>> climb, transitioning to a downward force during slow speed flight after
>> powerloss?
>
>
> So yes, all three types glided with the tailplane providing lift. All
> free flight competition models are better thought of as tandem wing
> aircraft with both wings providing lift. That was more obvious in the old
> days, when very large tailplanes, up to 35-50% of the wing area, with
> short moment arms, 3-3.5 times wing chord, were used. Now tailplanes are
> around 20% of the wing area and the moment arms are about 5 times the
> wing chord. All free flight models are trimmed to fly at minimum sink
> trim and to, hopefully, stay in the thermal you launch them into.

I flew hand-launch, towed, and power FF in the early '60s. After a detour to race
sports cars, I ended up sitting in gliders instead building them.

How do you determine the tail is lifting in gliding flight? And wouldn't be more
efficient to have the larger wing provide all the lift, and just use the tailplane
to provide stability?

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1

[Martin Gregorie[_6_]]

On Thu, 19 Mar 2020 08:28:33 -0700, Eric Greenwell wrote:

> I flew hand launch gliders in early60's. My best glider had a planform
> identical to the original Discus. It was called the "Sweepette". The
> cover on this article shows the 1960 version:
>
> https://indoornewsandviews.files.wordpress.com/2012/10/inav-113.pdf
>
I know it well: Lee Hines has been a good friend ever since 1983, when I
was on the way back from the Australian WC and stopped over on my way
home to fly in some Californian comps. I did the MaxMen's comp at Taft
and the Sierra Cup at Sacramento, travelling with Lee.

I got 2nd in in the Sierra Cup that year - an epic event with 10 rounds
flown Saturday and Sunday morning. Flyoffs started at 1:30 PM on a hot,
thermally afternoon. We flew 4,5,6 and 7 minute flyoffs before those with
clockwork timers dropped out en mass. By the 10 minute round it was
cooling off and only Walt Ghio and I were left. We both blobbed it, but I
dropped more than he did.

Halcyon Days!

> I wonder if Wil Schumann was inspired by the Sweepette, or some earlier
> version of that planform?
>
Lee seems to have developed the Sweepette in the late 60s and got the
Model Of The Year award for it in 1970.

I have a copy of Wil's article on modifying an ASW-12 so he could fly it
in Standard Class, and that modification included his new planform, but
annoyingly, the article doesn't show a date. The ASW-12 was introduced in
1965, so its at least possible that Wil was first with that planform.
AFAIK Lee has never followed developments in fullsize gliding in any
detail. He was always more interested in Formula 1 GP racing, and spent a
season as a Goodyear gofer (and got into the partys) when Jimmy Clark,
Graham Hill, etc were to top drivers.

Do you know when Wil was flying that modified ASW-12?


--
Martin | martin at
Gregorie | gregorie dot org

[[email protected]]

"How do you determine the tail is lifting in gliding flight?"

Agree. Do we have wind tunnel testing to prove the theory that your fixed stab is always providing upwards forces on the aft end of the fuselage?

I used do some free flight modeling.
Mostly Dick Mathis designs.

[Martin Gregorie[_6_]]

On Thu, 19 Mar 2020 08:43:45 -0700, Eric Greenwell wrote:

> How do you determine the tail is lifting in gliding flight? And wouldn't
> be more efficient to have the larger wing provide all the lift, and just
> use the tailplane to provide stability?

Good question, but I think flight stability gives the answer for FF
models. They're all trimmed for minimum sink, it being a duration event
with distance covered being a matter of wind and thermal strength and
what time you put on the d/t timer, so they all glide slowly at min.sink
to maximise flight time.

We also know from wind tunnel tests, etc. that the Centre of Pressure (CP)
of almost all airfoils is around 33% chord at slow speed.

So, for FF models, if your CG is behind 33%, then the tail *must* be
producing lift for stable flight.

Similarly, we know that while aircraft trimmed that way can be extremely
stable, they aren't necessarily controllable, but that they are if the CG
is in front of the CP, which requires downforce from the tail for stable
flight, so all manned aircraft are set up like that. This is particularly
obvious if you look at any of the earlier Boeing airliners: the tailplane
has quite a noticeable negative incidence *and* has an inverted cambered
airfoil, to the amount of downforce it produces will be considerable.

Back to gliders, yes, the less downforce you need from the tailplane, the
more efficient the glide becomes, but the more squirrelly it becomes as
you move the CG back.

The other way of getting efficiency is through leverage. If you lengthen
the tail boom you need progressively less downforce at its rear end to
balance the nose-down tendency. This alone means the tail needs to
produce less downforce, and so reduces the drag the goes with producing
it. It also means you can make the tailplane smaller, so reducing its
surface drag.

Putting all the surface in the wing seems to produce stability issues,
which I won't pretend to understand. All you can say is that tailless
gliders have all had issues, mostly connected with high speed stability.
I remember Rudy Opitz reporting that his father found that the Horten
SIV.b developed a nasty high speed pitch oscillation well below Vne and
that this affected his on-task speed. The Akaflieg Karlsruhe's AK-10 also
had this problem. It has affected powered tailless aircraft too - that's
what killed Geoffrey De Havilland in the DH.108 and nearly got Eric
"Winkle" Brown as well.


--
Martin | martin at
Gregorie | gregorie dot org

[Martin Gregorie[_6_]]

On Thu, 19 Mar 2020 09:33:40 -0700, jjdk737 wrote:

> "How do you determine the tail is lifting in gliding flight?"
>
> Agree. Do we have wind tunnel testing to prove the theory that your
> fixed stab is always providing upwards forces on the aft end of the
> fuselage?
>
I don't have access to a wind tunnel, but Mark Drela does and may will
have done that, However, I have photos that have a bearing on it.

When I drew up my glider series I made calculations of where the wing
wake should be at the tail location and how thick the wake was at that
point. I wanted to be sure that the tailplane was outside the wing wake
because my flying mates and I had already discovered that an F1A isn't
trimmable if the tailplane is inside the wing wake at its normal flight
attitude.

Anyway, I did the calcs, drew the lines on the CAD screen and adjusted
the fuselage height and wing AOA to put the tailplane comfortably below
the wing wake - I like zero dihedral inner wing panels with all the
dihedral on the shorter tips, which occupy about 1/3 of the semispan, so
wing/tail wake interference would be a problem if it ocurred.

A long time later I checked this by making a rake to let me see the
airflow behind the wing and onto the tailplane. Here's a description and
pictures:

https://www.gregorie.org/freeflight/wing_wake_visualization

Notice in both pictures that the streamers are deflected up in front of
the tail - I think that's a fairly clear indication that lift is being
generated.

Here are some wind tunnel pics showing similar effects, but the flow
deflection is a lot less because flow velocities will much higher than a
FF models gliding speed. An F1A glider's gliding speed is quite low: in
flat calm its easy to run alongside and just behind one and then reach in
and grab the tail boom just behind the wing is it sinks past your waist.

In case it isn't obvious, these models weigh just over 410 grams. They
are generally 2.2 to 2.5 metres in span and fly are about 10 mph (16 kph,
4.5 m/s). Sinking speed is around 0.3 m/s, giving a glide ratio of about
1:16.


Anyway, here are the wind tunnel links:

https://www.aa.washington.edu/AERL/KWT/techguide/flowviz

https://www.youtube.com/watch?v=IIHFkp1aAOI




> I used do some free flight modeling.
> Mostly Dick Mathis designs.



--
Martin | martin at
Gregorie | gregorie dot org

[jfitch]

On Thursday, March 19, 2020 at 9:33:44 AM UTC-7, wrote:
> "How do you determine the tail is lifting in gliding flight?"
>
> Agree. Do we have wind tunnel testing to prove the theory that your fixed stab is always providing upwards forces on the aft end of the fuselage?
>
> I used do some free flight modeling.
> Mostly Dick Mathis designs.

There is a misconception that a requirement for stability is that the tail must be providing a downward load. In fact all that is required is that the lift slope of the tail be higher than the wing with increasing angle of attack. That guarantees a righting moment with any angle of attack disturbance.

All this talk about free flight models reminds me of family connections. My brother won the world competition in F1B rubber powered free flight a few years back (first US win since the mid '50s). These aircraft have surprisingly sophisticated powertrains using secret and carefully hoarded rubber motors driving folding, feathering, torque sensing prop hubs. The latest have wings that are folded in half for the high speed climb, and unfold for the glide. They typically have lifting tails.

[Tango Whisky]

Absolutely. On a modern glider, this slope goes through zero at the angle of attack of best L/D (zero lift = minimum drag). Below that speed, the tailplane produces lift, above it produces downward force.

[Eric Greenwell[_4_]]

Tango Whisky wrote on 3/21/2020 1:37 AM:
> Absolutely. On a modern glider, this slope goes through zero at the angle of attack of best L/D (zero lift = minimum drag). Below that speed, the tailplane produces lift, above it produces downward force.
>
Curiously, that is not (we're told) how the model gliders are trimmed: the tail is
lifting at minimum sink. That seems inefficient to have a small wing producing
lift instead the big wing, with it's lower drag from a larger aspect ratio.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1

[Martin Gregorie[_6_]]

On Sat, 21 Mar 2020 01:37:35 -0700, Tango Whisky wrote:

> Absolutely. On a modern glider, this slope goes through zero at the
> angle of attack of best L/D (zero lift = minimum drag). Below that
> speed, the tailplane produces lift, above it produces downward force.

At a slight tangent:

If you're in a COVID-19 lockdown and need something to do with your
hands, you can do a lot worse that teach yourself some basic flight
stability rules while having fun making stuff.

All you need is:

- a copy of "Circular Airflow" by Frank Zaic ABE Books
https://www.abebooks.com
have a few copies if you don't have one (and you won't unless you were
a keen FF model flyer). Other second hand bookstores should also have
copies

- a sheet or two of 1/16" balsa

- modelling clay or electrical solder to use as nose weight

- some glue (white PVA, cellulose cement or a decent brand of
cyanoacrylate such as Zap)

- a suitable knife. I like the snap-off blade sort you can find in DIY
stores.

- maybe some glass-headed pins to hold stuff in place while glue dries

The section of the book called "Spiral Stability Demonstration" (starts
on page 49 in my copy) shows how to make very simple hand launch gliders
and use them to see the effect of side area, vertical and horizontal tail
size, etc. The models are small, light and slow enough to be flown in any
reasonable sized room or in your garden on a calm day. Its easy enough to
extend this to an investigation of CG position relative to wing chord and
the associated up or download on the tailplane.


Couple of other links:

- if you liked experimenting with the Zaic test models, you might like
Easy Mini, a small, light catapult launched balsa glider. Its interesting
because, when correctly set up, that same fixed trim works at high speed
in a spiral climb and equally well after it slows down into a slow
floating glide. You can lose these upwards of you fire them into a
thermal. They are suitable to use with scout groups, after-school hobby
groups, and, of course, young gliding club associates.

https://www.gregorie.org/freeflight/easymini/

- a debunking job I did to prove that an ancient Egyptian wooden bird was
probably a kids toy or a decoration and certainly not anything that
actually flew. Proving that was a lot of fun:

https://www.gregorie.org/freeflight/saqqara_bird



--
Martin | martin at
Gregorie | gregorie dot org

[Martin Gregorie[_6_]]

On Sat, 21 Mar 2020 06:40:45 -0700, Eric Greenwell wrote:

> Curiously, that is not (we're told) how the model gliders are trimmed:
> the tail is lifting at minimum sink. That seems inefficient to have a
> small wing producing lift instead the big wing, with it's lower drag
> from a larger aspect ratio.
>
Its not as inefficient as you might think: several people, as well as
myself, have found that a well designed model with a long tail moment has
a fairly low coefficient of lift on the tail. At a normal glide trim the
parasitic drag of the tail is greater then its drag due to lift. The
tailplane is working at a Cl of around 0.05, which puts it pretty much in
the centre of its minimum drag bucket, while the wing will be operating
at a Cl of 1.1 - 1.2.

We don't care what the glide slope of a gliding model is like since its
not going anyplace, just circling in the thermal it was launched into.
All we care about trimming it to glide at min. sink speed. Contests are
won and lost on total airtime recorded during the event.


--
Martin | martin at
Gregorie | gregorie dot org

[[email protected]]

So Martin, as a totally “model” ignorant observer lol, question, the key is all about getting minsink rate the absolute lowest possible irregardless of l/d?
Dan

[Eric Greenwell[_4_]]

Martin Gregorie wrote on 3/21/2020 7:04 AM:
> On Sat, 21 Mar 2020 06:40:45 -0700, Eric Greenwell wrote:
>
>> Curiously, that is not (we're told) how the model gliders are trimmed:
>> the tail is lifting at minimum sink. That seems inefficient to have a
>> small wing producing lift instead the big wing, with it's lower drag
>> from a larger aspect ratio.
>>
> Its not as inefficient as you might think: several people, as well as
> myself, have found that a well designed model with a long tail moment has
> a fairly low coefficient of lift on the tail. At a normal glide trim the
> parasitic drag of the tail is greater then its drag due to lift. The
> tailplane is working at a Cl of around 0.05, which puts it pretty much in
> the centre of its minimum drag bucket, while the wing will be operating
> at a Cl of 1.1 - 1.2.
>
> We don't care what the glide slope of a gliding model is like since its
> not going anyplace, just circling in the thermal it was launched into.
> All we care about trimming it to glide at min. sink speed. Contests are
> won and lost on total airtime recorded during the event.

It's called a "lifting tail" even though it is producing very little lift, and is
producing that lift with a high drag penalty from the parasitic drag? Confusing...

- What is the advantage for trimming it with a small positive lift instead of zero
lift?
- How about using a smaller horizontal trimmed for a higher L/D? That would lower
the parasitic drag and the overall drag of the horizontal, while still producing
the lift needed for stability.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1

[Eric Greenwell[_4_]]

Martin Gregorie wrote on 3/21/2020 6:42 AM:
> All you need is:
>
> - a copy of "Circular Airflow" by Frank Zaic ABE Books
> https://www.abebooks.com
> have a few copies if you don't have one (and you won't unless you were
> a keen FF model flyer). Other second hand bookstores should also have
> copies

That was my bible over 60 years ago. Twenty years ago, I finally gave it to a
college friend that's still building models, even now. What is the "bible" for
modelers now?

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1

[Martin Gregorie[_6_]]

On Sat, 21 Mar 2020 07:31:14 -0700, Eric Greenwell wrote:

> Martin Gregorie wrote on 3/21/2020 7:04 AM:
>> On Sat, 21 Mar 2020 06:40:45 -0700, Eric Greenwell wrote:
>>
> It's called a "lifting tail" even though it is producing very little
> lift, and is producing that lift with a high drag penalty from the
> parasitic drag? Confusing...
>
Recap: the tail has to lifting if the model's CG is behind the wing's
centre of pressure (CP). We also know from wind tunnel studies etc, that
the CP of a wing operating near min sink is close to 33% of the chord
behind the LE. All the fixed trim FF competition models I've built or
flown have their CG at around 55% (towline gliders and rubber powered) or
in the 80-90% range ('chuck' or catapult gliders, power models). VIT
models (those where a timer reduces the AOA of the tail when it stops the
motor) will have the CG 10-20% further forward than those with fixed trim.

In all these cases the CG is behind the CP when the model is gliding, so
the tail most be lifting or the model would simply pitch up and stall.

> - What is the advantage for trimming it with a small positive lift
> instead of zero lift?
>
Good question. Its due to launch requirements. If fixed trim model glides
with downforce on its tail, then simply speeding it up will make it pitch
up and stall. Thats an unavoidable consequence of speeding up a fixed
trim model.

If the model has a lifting tail it will pitch up relatively slowly
because thats what the combination of wing and tail sections combined
with decalage (the difference in geometric AOA between wing and tail) is
designed and trimmed to do. This, combined with a small amount of wing
twist (all FF models use wash-in on the wing on the inside of the glide
circle) and rudder setting can be arranged to convert excess speed into a
spiral climb rather than a straight pitch-up or loop and, as the model
slows down the turn opens out and it settles into a circling glide at min.
sink speed.

The glider turn is fairly open. Around 40 seconds per circle is about
right. Too tight a turn raises the sinking speed while too open a circle
may let it wander out of the thermal.

Launch behaviour is a major design input because, with the exception of
large, open class rubber powered models which seen to climb and glide at
very much the same speed (I've never flown these, so don't know for sure,
but climb and glide speeds look very similar), all FF models are launched
a lot faster than they glide to get them nice and high in the thermal
you've just picked. Power models climbing speed is at least 2-3 times
faster than they glide, hand launched gliders are thrown as hard as
possible and F1A towline gliders are towed as fast as you can run while
the model flies a catch-up arc at the top of a 50m towline, so must be
travelling into the wind about twice as fast as the person launching it.
As a result it will pulling 15-40 kg line tension at launch: you really
hammer them off to gain as much height as possible. The maximum permitted
towline length is 50m under 5kg tension and, depending on the model and
how hard you can launch, it will gain another 10m to 60m before settling
into its glide.


--
Martin | martin at
Gregorie | gregorie dot org

[Martin Gregorie[_6_]]

On Sat, 21 Mar 2020 07:14:21 -0700, uneekcowgirl wrote:

> So Martin, as a totally “model” ignorant observer lol, question, the key
> is all about getting minsink rate the absolute lowest possible
> irregardless of l/d?
>
Spot on!

Flying a competition consists of making a previously announced number of
flights during the day. Each flight is timed from when its launched until
it lands. The winner is the person with the highest total flight time.

Simple!

Modifiers:
To avoid losing models, there is a predetermined maximum scoring time for
each flight. All models carry a 'dethermaliser', universally called a
'd/t'. This is typically a timer that, then it trips, moves the entire
tailplane to a 45 degree 'up elevator' angle. This stalls the model and
keeps it stalled so it descends vertically relative to the air: its
normal gliding sink speed will be around 0.3 m/s but after the d/t has
popped its descent rate will be in the 4-5 m/s range, which is usually
enough to drop it out of even a strong thermal.

The 'max' flight time gives the possibility of, on a good day, having
several people who have all maxed out, i.e. flew for the maximum time in
all flights. This is sorted out with a flyoff involving just the flyers
with full scores, held in the evening when both wind and lift should be
weaker. In a small competition this is a single unlimited duration
flight, timed to the ground.

Internationals and World Championship events are a little different.
Theese fly 7 rounds during the day, each being a 50-60 minute period
during which every competitor makes one flight. Its usual to have several
people maxing out, so flyoff rounds are flown in the evening the
determine the winner. These are flown in much shorter rounds, typically
20 minutes each, and with the max time going up, starting from 5 minutes
and increasing by 2 minutes for each successive flyoff. If there are
still several flyers with full scores when it gets too dark to fly, a
final 10 minute round is flown at dawn the next day.



--
Martin | martin at
Gregorie | gregorie dot org

[Martin Gregorie[_6_]]

On Sat, 21 Mar 2020 07:38:34 -0700, Eric Greenwell wrote:

> Martin Gregorie wrote on 3/21/2020 6:42 AM:
>> All you need is:
>>
>> - a copy of "Circular Airflow" by Frank Zaic ABE Books
>> https://www.abebooks.com have a few copies if you don't have one
>> (and you won't unless you were a keen FF model flyer). Other second
>> hand bookstores should also have copies
>
> That was my bible over 60 years ago. Twenty years ago, I finally gave it
> to a college friend that's still building models, even now. What is the
> "bible" for modelers now?

Thats still worth reading if you want to design your own models, but you
do need to remember that materials and ideas have moved on since that was
written: nobody had glass or carbon back than and everything was still
covered with modelling tissue and lightweight silk and nylon.

Jim Baguley, a Brit, wrote an excellent set of articles in Aeromodeller
magazine back in the '60s and one of the Swiss flyers wrote a good series
on FreeFlight News in, I think, the late 70s. Both are still worth
finding and reading.

NFFS (American) has a seminal series of annual Symposia, starting in 1968
and still going strong - I have a complete set. Its packed with good
ideas and the Models Of The Year are worth studying.

Articles in Free Flight News, SCAT News (Californian - formerly on paper,
now online as SCAT Electronic News) and Vol Libre (French, stopped a
decade ago) are all good sources, but the best source of design ideas are
still in the bars and various International and World-class competitions.



--
Martin | martin at
Gregorie | gregorie dot org

[[email protected]]

Thanks Martin, from what I am reading, this type of experimenting can actually be a lifetime pursuit playing with all of the multiple variables that combine for success, or that don’t lol.
Dan

[Martin Gregorie[_6_]]

On Sat, 21 Mar 2020 18:15:55 -0700, uneekcowgirl wrote:

> Thanks Martin, from what I am reading, this type of experimenting can
> actually be a lifetime pursuit playing with all of the multiple
> variables that combine for success, or that don’t lol.
>
Yes, this often happens. Many of us started flying models, usually
remarkably badly, at age 10 or so, enjoyed the challenge and stuck with
it. I dropped out in 2000 when the soaring bug bit, but most of the
people I was flying and competing with in the 70s are still flying unless
they've gotten ill or dropped off the perch. I'm thinking I might have a
crack at building and flying FF scale, which don't go nearly as far, when
its time to hang up my parachute.

Competitive FF is a good way of keeping fit. Flying in typical UK
conditions means you can easily cover 10+ miles a day, and that's just
retrieving the model after a flight, so all the galloping round with a
towline model on the line, looking for a thermal, is an extra bit that
the rubber and power guys don't do.


--
Martin | martin at
Gregorie | gregorie dot org