Nice skyline article

[BB]

Readers of this group might like a very nice article on turn radius and
handicapping by Judah Milgram

http://skylinesoaring.org/NEWSLETTER/2005/February/

Though mostly about handicapping, it also shows very nicely why you
should not thermal at the "minimium sink speed" corresponding to the
level flight polar, but instead should usually thermal right on the
edge of a stall. (Going slower means you can achieve the same turn
radius with a lower bank angle.)

John Cochrane

BB

[Andreas Maurer]

On 24 Feb 2005 09:27:21 -0800, "BB" >
wrote:

>Though mostly about handicapping, it also shows very nicely why you
>should not thermal at the "minimium sink speed" corresponding to the
>level flight polar, but instead should usually thermal right on the
>edge of a stall. (Going slower means you can achieve the same turn
>radius with a lower bank angle.)

Try to thermal once at the edge of a stall in a glider like, say,
ASW-20, ASW-24, and you'll find out immediately that your conclusion
is not universally applicable. :)





Bye
Andreas

[Bruce]

Andreas Maurer wrote:
> On 24 Feb 2005 09:27:21 -0800, "BB" >
> wrote:
>
>
>>Though mostly about handicapping, it also shows very nicely why you
>>should not thermal at the "minimium sink speed" corresponding to the
>>level flight polar, but instead should usually thermal right on the
>>edge of a stall. (Going slower means you can achieve the same turn
>>radius with a lower bank angle.)
>
>
> Try to thermal once at the edge of a stall in a glider like, say,
> ASW-20, ASW-24, and you'll find out immediately that your conclusion
> is not universally applicable. :)
>
>
>
>
>
> Bye
> Andreas
Ditto Std Cirrus - only time I dare go really slow is when there are big weak
thermals with no gusts. Otherwise it is only a case of when you are going to get
that auto rotation feeling. And the recovery from a wing drop when slow is time
and altitude consuming. As the article says, pays to attend to how the aircraft
flies. I am much more comfortable a little faster and a little higher bank angle
in the Cirrus.

[Terry]

As the article says, pays to attend to how the aircraft
> flies. I am much more comfortable a little faster and a little higher
bank angle
> in the Cirrus.

================================================== ==================================

The article dances around the question, at what speed do we thermal?

Students hate the "it depends" that this type of question drives, but
for my students, I say at the minimum sinking speed for your bank
angle. Since turning polars are not published I approximate this by
multiplying the minimum sinking speed by the load factor. Maybe not
completely and aerodynamically correct, but a workable rule while
flying around.

Terry Claussen
Estrella

[BB]

> The article dances around the question, at what speed do we thermal?
>
> Students hate the "it depends" that this type of question drives, but
> for my students, I say at the minimum sinking speed for your bank
> angle. Since turning polars are not published I approximate this by
> multiplying the minimum sinking speed by the load factor. Maybe not
> completely and aerodynamically correct, but a workable rule while
> flying around.

That's exactly what the article shows is wrong. I found it interesting
because I always found myself flying slower than "minimum sink adjusted
for bank angle". I noticed the same among most contest pilots, but I
always felt like I might be doing something wrong since I had been
taught the same logic.

By flying somewhat slower, on the "backside" of the polar (though not
of course to the point of losing control, buffeting, dropping wings
etc) you climb better. You want the minimum sink for a given TURN
RADIUS not a minimum sink for a given BANK ANGLE. By flying slower, you
get the same radius turn with a lower bank angle.

For example, in most standard/15 m gliders straightline "minimum sink"
is about 45 kts. Yet most pilots thermal at 45-47 kts even in 30-45
degree bank. These speeds are well below "minimum sink" for the given
bank angles.

John Cochrane
BB

John Cochrane BB

[[email protected]]

Andreas Maurer wrote:
> On 24 Feb 2005 09:27:21 -0800, "BB" >
> wrote:
>
> >Though mostly about handicapping, it also shows very nicely why you
> >should not thermal at the "minimium sink speed" corresponding to the
> >level flight polar, but instead should usually thermal right on the
> >edge of a stall. (Going slower means you can achieve the same turn
> >radius with a lower bank angle.)
>
> Try to thermal once at the edge of a stall in a glider like, say,
> ASW-20, ASW-24, and you'll find out immediately that your conclusion
> is not universally applicable. :)
> >
>
> Bye
> Andreas

Comment: I've gotta dissagree with you on this. Both mentioned gliders,
when properly tuned, respond very well to this kind of technique. '20
in particular if flaps and ailerons are very well sealed and good
winglets are used. '24 needs improved winglets and ,in my opinion, the
"B mod" on the leading edge. Do these and it climbs very well, mostly
due to improved ability to pull harder thus giving smaller circle.
13 years in '20's, 13 years in '24. Lotza work done on both.
UH

[Bill Daniels]

"BB" > wrote in message
oups.com...
>
> > The article dances around the question, at what speed do we thermal?
> >
> > Students hate the "it depends" that this type of question drives, but
> > for my students, I say at the minimum sinking speed for your bank
> > angle. Since turning polars are not published I approximate this by
> > multiplying the minimum sinking speed by the load factor. Maybe not
> > completely and aerodynamically correct, but a workable rule while
> > flying around.
>
> That's exactly what the article shows is wrong. I found it interesting
> because I always found myself flying slower than "minimum sink adjusted
> for bank angle". I noticed the same among most contest pilots, but I
> always felt like I might be doing something wrong since I had been
> taught the same logic.
>
> By flying somewhat slower, on the "backside" of the polar (though not
> of course to the point of losing control, buffeting, dropping wings
> etc) you climb better. You want the minimum sink for a given TURN
> RADIUS not a minimum sink for a given BANK ANGLE. By flying slower, you
> get the same radius turn with a lower bank angle.
>
> For example, in most standard/15 m gliders straightline "minimum sink"
> is about 45 kts. Yet most pilots thermal at 45-47 kts even in 30-45
> degree bank. These speeds are well below "minimum sink" for the given
> bank angles.
>
> John Cochrane
> BB
>
> John Cochrane BB
>

John, I'm sure there are situations where that applies but the reduction in
turn radius is not great for a small reduction in airspeed. For example,
reducing the airspeed from 50 to 45 knots in a 45 degree bank decreases the
radius by only a little more than 40 feet.

See the Turn Radius Calculator:
http://www.soarcsa.org/thinking_pages/soaring/turn_radius/turn_rad_knots.htm


Bill Daniels

[Kevin Christner]

BB wrote:
> Readers of this group might like a very nice article on turn radius
and
> handicapping by Judah Milgram
>
> http://skylinesoaring.org/NEWSLETTER/2005/February/
>
> Though mostly about handicapping, it also shows very nicely why you
> should not thermal at the "minimium sink speed" corresponding to the
> level flight polar, but instead should usually thermal right on the
> edge of a stall. (Going slower means you can achieve the same turn
> radius with a lower bank angle.)
>
> John Cochrane
>
> BB

My LS1c climbed like a rock under 46kts. Think it has somthing to do
with a flow separation on the all flying tail. On the other hand, my
Ka8 went up like a rocket at 34kts indicated (min sink around 38). I
think you need to determine these things on a case-by-case basis.

[Udo Rumpf]

> By flying somewhat slower, on the "backside" of the polar (though not
> of course to the point of losing control, buffeting, dropping wings
> etc) you climb better. You want the minimum sink for a given TURN
> RADIUS not a minimum sink for a given BANK ANGLE. By flying slower, you
> get the same radius turn with a lower bank angle.
>
> For example, in most standard/15 m gliders straightline "minimum sink"
> is about 45 kts. Yet most pilots thermal at 45-47 kts even in 30-45
> degree bank. These speeds are well below "minimum sink" for the given
> bank angles.
>
> John Cochrane
> BB
>
> John Cochrane BB

John
I use a different approach.
In case of my sHP18 I determent the minimum sink
polar for a know wing loading x 1.4 g load at a 45deg. bank angle. With that
I establish the speed for my bank. 43kt 8lb/sqft will give me a speed at
45 deg. bank of about ~ 51kt. This allows the glider to stay within the
sweet
spot and gives good control with the least amount of control drag.
Reducing the speed further would not gain anything.

My ASW24 previously owned by Hank has a minimum sink of 45kt at 6.4 lb/sqft
as per R. Johnson but the minimum speed in this case can be pushed back to
about
40kt. due to the relative flat bottom of the polar as well as better
winglets and modified
leading edge. this would give a speed of 43kt
at 7.4 lb/sqft. The 45 deg bank speed would about 51kt.
As it turns out, I fly it between 50-52kt.
Regards
Udo

[Andreas Maurer]

On 25 Feb 2005 05:59:35 -0800, wrote:

>Comment: I've gotta dissagree with you on this. Both mentioned gliders,
>when properly tuned, respond very well to this kind of technique. '20
>in particular if flaps and ailerons are very well sealed and good
>winglets are used. '24 needs improved winglets and ,in my opinion, the
>"B mod" on the leading edge. Do these and it climbs very well, mostly
>due to improved ability to pull harder thus giving smaller circle.
>13 years in '20's, 13 years in '24. Lotza work done on both.

Interesting. What winglets do you use?
I got only about 700 hrs in the 20 and maybe 200 hrs in the 24, but
both didn't climb very well below 47-50 kts. The latter is equipped
with the factory winglets, btw.

The 20 was regarded as one of the best performing 20's ever (before
the ero of winglets). It climbed really well, but I made sure that I
stayed away from stall speed.


Bye
Andreas

[[email protected]]

Andreas Maurer wrote:
> On 25 Feb 2005 05:59:35 -0800, wrote:
>
> >Comment: I've gotta dissagree with you on this. Both mentioned
gliders,
> >when properly tuned, respond very well to this kind of technique.
'20
> >in particular if flaps and ailerons are very well sealed and good
> >winglets are used. '24 needs improved winglets and ,in my opinion,
the
> >"B mod" on the leading edge. Do these and it climbs very well,
mostly
> >due to improved ability to pull harder thus giving smaller circle.
> >13 years in '20's, 13 years in '24. Lotza work done on both.
>
> Interesting. What winglets do you use?
> I got only about 700 hrs in the 20 and maybe 200 hrs in the 24, but
> both didn't climb very well below 47-50 kts. The latter is equipped
> with the factory winglets, btw.
>
> The 20 was regarded as one of the best performing 20's ever (before
> the ero of winglets). It climbed really well, but I made sure that I
> stayed away from stall speed.
>
>
> Bye
> Andreas

Winglets I referred to are those I began developing in 1993. Udo uses
them on my old ship as do quite number of others. They significantly
improve the flow at the tip and permit much more agressive turning when
needed. Much better than the .4M factory ones. Thus my comment
differing with your opinion.
The '20 also responded very well when I added winglets. More than I
expected. Probably due to wide tip chord.
I'm off the track of this thread. John is very much correct in his
observations- as usual.
UH

[Bill Daniels]

"T o d d P a t t i s t" > wrote in message
...
> "Bill Daniels" > wrote:
>
> >John, I'm sure there are situations where that applies but the reduction
in
> >turn radius is not great for a small reduction in airspeed. For example,
> >reducing the airspeed from 50 to 45 knots in a 45 degree bank decreases
the
> >radius by only a little more than 40 feet.
>
> You are missing the point. You don't reduce airspeed to get
> a tighter turn radius. You reduce airspeed to get a
> shallower bank in the *same* turn radius. The shallower
> bank, even when flown at an airspeed less than the min sink
> speed for that bank, produces a lower sink rate than the
> same turn radius flown faster at a higher bank angle.
> Flying at min sink for the bank angle is not the optimum.
>
> >See the Turn Radius Calculator:
>
>http://www.soarcsa.org/thinking_pages/soaring/turn_radius/turn_rad_knots.ht
m
>
> See Jud's article.

I read the article and I don't agree with it. I think it's much better to
fly the minimum sink airspeed for the bank angle. Lets do the math.

A 45 degree bank at 43 knots results in a turn radius of 164.1 feet. For
me, that's a fairly standard thermalling turn that takes 14 seconds. That's
minimum sink in my Nimbus 2C if flown dry and it turns inside almost
everyone including the 12 meter ships.

At 40 knots and 40 degrees bank the turn radius is 169.3 feet. That
actually INCREASES the turn radius and it means I have to wobble along just
above stall at a high sink rate. That's a very bad deal from both a soaring
and safety standpoint.

Maybe if the starting point is someone that flies a 45 degree bank at 65
knots with a 375.1 foot radius, it looks different. Reducing the airspeed
to 45 while reducing the bank to 30 results in a radius of 311.3 feet or a
radius reduction of 63.8 feet. That's significant but just reducing the
speed to 45 knots turn is better still.

In fact, 45 - 45 is a good turn for most gliders. At a 45 degree bank,
there is little increase in G load so the sink doesn't increase much at all.
I'm beginning to think there is a big payoff for an angle of attack
indicator so we can just fly AOA and not have to compute these things.

Bill Daniels

[[email protected]]

> I read the article and I don't agree with it. I think it's much
better to
> fly the minimum sink airspeed for the bank angle. Lets do the math.

> Bill Daniels

With a little reflection it is *obvious* that it cannot be optimal to
fly a given radius circle at a certain bank angle and airspeed if there
is a different combination of airspeed and bank angle giving a lower
sink rate for that radius of circle. If there is a different
combination yielding the *same radius* and *lower sink rate* then all
the higher sink rate combinations for that radius are not optimal.

If we are optimized then we are at the minimum sink rate for the circle
radius we are flying - period.

Judah's beautiful graphs and Reichman's 70's vintage "Cross Country"
both make it clear that the optimum will be found somewhat on the back
side of minimum sink speed for the optimal bank angle. Severe mushing
descent speed and sub-minimum controllable airspeed as potential
solutions are exagerated straw men.

But I did not understand from Reichman that the optimal speed actually
decreases initially with increasing bank angle/decreasing radius. So
this feature of the data in my ASW-20C pilot's manual was always a bit
of a mystery to me.

Thanks to Judah for graphing optimal speed versus radius directly for
several types, making the situation clear.

How to find the optimal radius though! Wouldn't it be a wonderfully
convenient coincidence if at the optimal radius the overbanking
tendency of the glider was exactly balanced by the lift gradient trying
to unroll the glider? Would this work out for some particular span? I
could probably notice when the stick was in the center, and it would
sure be nice to know I was flying right. Someone please do the math
and let me know.

Jonathan

[Bill Daniels]

> wrote in message
oups.com...
> > I read the article and I don't agree with it. I think it's much
> better to
> > fly the minimum sink airspeed for the bank angle. Lets do the math.
>
> > Bill Daniels
>
> With a little reflection it is *obvious* that it cannot be optimal to
> fly a given radius circle at a certain bank angle and airspeed if there
> is a different combination of airspeed and bank angle giving a lower
> sink rate for that radius of circle. If there is a different
> combination yielding the *same radius* and *lower sink rate* then all
> the higher sink rate combinations for that radius are not optimal.
>
> If we are optimized then we are at the minimum sink rate for the circle
> radius we are flying - period.
>
> Judah's beautiful graphs and Reichman's 70's vintage "Cross Country"
> both make it clear that the optimum will be found somewhat on the back
> side of minimum sink speed for the optimal bank angle. Severe mushing
> descent speed and sub-minimum controllable airspeed as potential
> solutions are exagerated straw men.
>
> But I did not understand from Reichman that the optimal speed actually
> decreases initially with increasing bank angle/decreasing radius. So
> this feature of the data in my ASW-20C pilot's manual was always a bit
> of a mystery to me.
>
> Thanks to Judah for graphing optimal speed versus radius directly for
> several types, making the situation clear.
>
> How to find the optimal radius though! Wouldn't it be a wonderfully
> convenient coincidence if at the optimal radius the overbanking
> tendency of the glider was exactly balanced by the lift gradient trying
> to unroll the glider? Would this work out for some particular span? I
> could probably notice when the stick was in the center, and it would
> sure be nice to know I was flying right. Someone please do the math
> and let me know.
>
> Jonathan
>

The original issue was that gains could be achieved by reducing bank angle
and flying slower. More specifically, that the same radius could be
achieved at a lower sink at a lower bank and speed.

My point is that any gains are very, very small and likely to place the
pilot at risk of a stall/spin in rough air. (If I know a pilot is
attempting this, I won't be flying under him in a gaggle.)

All I'm saying is that small reductions in airspeed below min sink have
little effect on turn radii. Bank angle has a far larger effect. I've done
the math and plotted the results to scale to prove it to myself. A small
turn radius is good but it's best achieved with bank not reductions in
airspeed below min sink.

Accurate centering has a much greater effect on average climb rate than a
tiny reduction in turn radius achieved by a small reduction in airspeed.
Minimum sink is a solid airspeed that provides good control authority for
centering the thermal and is a much better bet for most pilots.

Almost any pilot will benefit more by practicing a "standard" 45 degree bank
at minimum sink airspeed for that bank angle than by reaching for a few feet
reduction in turn radius by flying slower.

Bill Daniels

[Bob Korves]

"T o d d P a t t i s t" > wrote in message
...
(snip)
> IMHO, the best thermalling performance for your glider can
> be found by flying against another glider, not by cranking
> numbers. Nonetheless, Jud's article sheds some new light on
> why I seem to find the best performance when flying slower
> than min sink for my bank angle.
>

I agree with Todd. The best way to learn how to make your glider climb well
is by flying in gaggles with other gliders, especially during contests. You
will find out what works pretty quickly, and the best pilots are usually
willing to talk to you (after the contest!) to help you improve. Reading
charts and doing the math helps a lot with understanding "how things work",
and this was a very good article and well worth the read. To learn how to
really make a glider climb, however, you need to go fly one, preferably far
from home, where it really matters.

BTW, real world thermals are not usually like the model!
-Bob Korves