ASW 20 SPIN CHARACTERISTICS

On Sat, 17 Jul 2004 10:52:50 -0700, Eric Greenwell
wrote:

Jack wrote:
Bruce Hoult wrote:

In article ,
Martin Gregorie wrote:


...the load being carried by the wing is

at least as important as the AoA.


[snippage]

...if you don't insist on trying to support the load...then you can

be in perfect control and not stalled at as low an airspeed as you like.

Bruce, it would appear that you and Martin are in agreement.

Appearances can be deceiving...

If you look at the Coefficient of lift diagrams for airfoils, you see
that it is dependent only on AOA, not load. In other words, a wing will
stall at the same AOA at .5 G, 1 G, 2 G, etc. I think this is what Bruce
is saying. Martin is wrong to say the load is as important as AOA, and
that is why some ras posters think we should have AOA indicators in our
gliders.

Sure, Cl is dependent entirely on AoA, but is not a linear
relationship throughout the range:

- It is linear at small angles.
- When the AoA is high enough for the upper surface flow
to start to separate the Cl tends to a constant value with
increasing AoA.
- If the AoA continues to increase even further you reach
a point at which the Cl starts to decline, reaching zero
at an AoA of 90 degrees.

However, my understanding is that a stall occurs when the lift
generated by the wing drops below the load the wing is required to
support.

For a given wing the generated lift is proportional to the Cl and to
the square of the speed, so at a fixed AoA you can reduce the speed
until the lift is no longer sufficient for flight, at which point the
wing stalls. If the aircraft weight is reduced then so is the stalling
speed: it doesn't matter whether this reduction is due to dumping
ballast or to pushing over to generate reduced G forces. If you put
water in a glider you raise its stalling speed but you don't
necessarily change the AoA at which it stalls.

Hence my comment that the load on the wing is as important as AoA for
*stalling* behaviour. I was not talking about the aerodynamic
characteristics of the wing section - of course!

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

On Sat, 17 Jul 2004 12:18:58 +1200, Bruce Hoult
wrote:

No it's not. There isn't a "maximum angle of attack". There is only an
"angle of attack for maximum lift". As you approach that angle of
attack the rate of lift increase gets smaller and smaller, then you get
the same amount of lift at slightly increasing angles of attack, and
then with still more angle of attack you get less lift. The more you
take the angle of attack past the point of maximum lift the less lift
you get.

So, yes, it does matter whether you are 2 degrees or 6 degrees past the
angle of attack for maximum lift.

Of course you are correct as ever, Bruce.

I used the term "maximum angle of attack" to define the point where
the airflow separates close to the leading edge, creating a massive
and sudden loss of lift (this situation being usually defined as "the
stall"). Is there a better technical term in English for this?

In scientific terms maximum AoA would be Pi (or 180 degrees) of
course, e.g. in a tailslide.



Bye
Andreas

Martin Gregorie wrote:
If you look at the Coefficient of lift diagrams for airfoils, you see
that it is dependent only on AOA, not load. In other words, a wing will
stall at the same AOA at .5 G, 1 G, 2 G, etc. I think this is what Bruce
is saying. Martin is wrong to say the load is as important as AOA, and
that is why some ras posters think we should have AOA indicators in our
gliders.


Sure, Cl is dependent entirely on AoA, but is not a linear
relationship throughout the range:

- It is linear at small angles.
- When the AoA is high enough for the upper surface flow
to start to separate the Cl tends to a constant value with
increasing AoA.
- If the AoA continues to increase even further you reach
a point at which the Cl starts to decline, reaching zero
at an AoA of 90 degrees.

However, my understanding is that a stall occurs when the lift
generated by the wing drops below the load the wing is required to
support.

This is the usual result of a stall, and is what occurs in the typical
training situation, but it isn't the definition of a stall. Generally, a
stall begins when the airflow starts to separate from the wing at
increasing AOA. It is this separation that keeps the lift from
increasing and sets the maximum lift coefficient.

A wing can be stalled and still produce plenty of lift; for example, in
a high speed pull up done with too much elevator can stall the wing, but
the stalled wing will still have more lift than the weight of the
aircraft because of the high speed.

In a high speed climb after rapid pull up, pushing the stick enough to
give zero G will reduce the lift to zero, but the wing is not stalled
(the airflow is well attached - no separation) even though it can not
support the glider.


For a given wing the generated lift is proportional to the Cl and to
the square of the speed, so at a fixed AoA you can reduce the speed
until the lift is no longer sufficient for flight, at which point the
wing stalls. If the aircraft weight is reduced then so is the stalling
speed: it doesn't matter whether this reduction is due to dumping
ballast or to pushing over to generate reduced G forces. If you put
water in a glider you raise its stalling speed but you don't
necessarily change the AoA at which it stalls.

Hence my comment that the load on the wing is as important as AoA for
*stalling* behaviour.

Perhaps I don't understand this correctly: by load, do you mean
different G loads, or just different aircraft weights? By *stalling
behavior*, do you mean how rapidly the aircraft responds as it stalls,
the amount of buffeting, how the nose is, or...?

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

In article ,
Martin Gregorie wrote:

Sure, Cl is dependent entirely on AoA, but is not a linear
relationship throughout the range:

- It is linear at small angles.
- When the AoA is high enough for the upper surface flow
to start to separate the Cl tends to a constant value with
increasing AoA.
- If the AoA continues to increase even further you reach
a point at which the Cl starts to decline, reaching zero
at an AoA of 90 degrees.

I'm with you on all that.


However, my understanding is that a stall occurs when the lift
generated by the wing drops below the load the wing is required to
support.

No, a stall is when increasing AoA decreases lift. There might well
still be more lift that the weight of the aircraft, especially at high
speed. The only reasons to avoid such stalled flight a

- high drag and thus inefficient
- the aircraft is unstable in roll, making it difficult or
impossible to control.

Presumably you've seen aircraft such as the F/A-18 demonstrate a slow
pass at very high and stalled angle of attack? They are getting some of
their support from the downward component of the engine thrust, of
course, but with an AoA of, say, around 30 degrees it would need a
thrust:weight ratio of around 2 in order for thrust to be enough to
support the entire aircraft weight. It would also require *huge* drag
in order to avoid accelerating at such a thrust level. The F/A-18 has
nowhere near that amount of thrust, so the majority of the support is
clearly still coming from the stalled wings. In that situation the
aircraft is unstable, and would probably be improssible to fly like that
without the computer reacting very quickly to unwanted rolls.

So the F/A-18 can be happily flown in steady-state stalled straight and
level flight primarily because of the computer control and also because
the extra drag is less than the engine thrust available.


For a given wing the generated lift is proportional to the Cl and to
the square of the speed, so at a fixed AoA you can reduce the speed
until the lift is no longer sufficient for flight, at which point the
wing stalls.

Well, ... no :-)

If you maintain a fixed AoA, and the speed is such that the lift is less
than the weight of the aircraft then the aircraft will start to follow a
downwards parabolic path (not as sharply downwards as in a zero-G
pushover, but similar).

What happens next depends on what else (if anything) you are holding
constant.

Suppose, for the sake of concreteness, that you are initially flying
straight and level at 60 knots and you then fix the AoA such that the
wings are producing only half the lift required to support the glider.

Normally a glider will accelerate, increasing the lift (and drag, but
not by much). The extra lift will cause the path to become less sharply
curved downward and things will come to equilibrium (or oscillate
around) the point where the combined lift and drag are equal and
opposite to gravity. For a typical glider polar curve this will happen
at an airspeed of around 1/sqrt(0.5) times 60 knots, or 85 knots, plus
or minus a little due to drag.

So all you've acheived is to change the trimmed speed from 60 to 85
knots.

Or, look at it the other way around. Maybe you were flying straight and
level at 85 knots, and then you somehow instantly decrease the airspeed
to 60 knots (maybe a gust up the tail). The lift is no longer
sufficient to maintain level flight. But the glider doesn't stall. It
just drops the nose and accelerates until it has returned to the trimmed
speed of 85 knots.

In no way are the wings ever stalled.


If you stipulate constant speed as well as constant AoA (presumably via
some large and adjustable drag, magical or otherwise) then the flight
path will become steeper until the combined lift and drag vectors are
again exactly equal to and opposite the gravity vector. This will
result in a much steeper flight path, but still stable.

Let's suppose again that you are at 60 knots and reduce the AoA to
produce only half the lift required for flight and then continue to
maintain exactly 60 knots somehow. Alternatively, suppose you're flying
trimmed for level flight at 85 knots and then apply airbrakes to reduce
and maintain 60 knots, while keeping the same trim (AoA).

What happens?

The AoA/speed are insufficient for flight at 60 knots and so the nose
drops. If you draw up the force vectors then you will find that the
glider will stablize in a 60 degree descent at your desired constant 60
knots. Lift (from the wings) is still 0.5 of the weight just as it was
initially (but it's in a funny direction, tilted 60 degrees forward from
vertical). Drag (from the airbrakes) is 0.866 of the weight, tilted 30
degrees from vertical. The horizontal components of lift and drag are
equal and opposite and cancel out. The vertical force to oppose gravity
comes 25% from the wings and 75% from the airbrakes.

In no way are the wings ever stalled.


No matter what you do, if you start with the wings not stalled then
there is nothing you can do that will stall them while all the time
keeping the AoA constant.

If you see the nose drop and don't like it and pull back on the stick to
try to prevent it then that is an entirely different matter -- you're
increasing the AoA which certainly *can* stall the wings.

-- Bruce

Martin Gregorie wrote in message . ..
On 15 Jul 2004 18:17:04 GMT, Derrick Steed
wrote:

FWIW, on Tuesday evening I decided to investigate turning stall/spin
behaviour in my '20 at a sensible altitude. It was calm and with
little air movement under a high overcast. With the aircraft clean and
flaps at zero (setting 3) I flew some moderately steep turns - about
45 degrees of bank and at speeds ranging down to about 43 kts. This
was completely uneventful - no buffeting, burble or hints of
departure. In short, it flew like a pussycat. I'll try this again by
myself in a turbulent thermal next time because all that series of
turns told me was that in nearly still air my '20 can fly uneventful
turns at stupidly slow airspeeds. By comparison I typically fly at
48-50 kts for that steep a turn in zero flap during normal thermalling
turns.

Dear Martin, In your next flight-test-experiment with the ASW-20, try
a climbing turn stall at good altitude. Bank into a right turn at
60-65 knots and as the right wing goes down pull back fairly hard and
steadily on the stick to make the glider go up and slow down. Keep
pulling back and wow! you will stall over the top and quickly too. I
think that this has been the cause of more than one low altitude
glider crash. I tried this in my '20 when Walt Cannon and I were
having a similar discussion about stalling in the ASW-20. Let me
know how it works and what you think. Rudy Allemann

On Sat, 17 Jul 2004 17:13:42 -0700, Eric Greenwell
wrote:

Martin Gregorie wrote:
If you look at the Coefficient of lift diagrams for airfoils, you see
that it is dependent only on AOA, not load. In other words, a wing will
stall at the same AOA at .5 G, 1 G, 2 G, etc. I think this is what Bruce
is saying. Martin is wrong to say the load is as important as AOA, and
that is why some ras posters think we should have AOA indicators in our
gliders.


Sure, Cl is dependent entirely on AoA, but is not a linear
relationship throughout the range:

- It is linear at small angles.
- When the AoA is high enough for the upper surface flow
to start to separate the Cl tends to a constant value with
increasing AoA.
- If the AoA continues to increase even further you reach
a point at which the Cl starts to decline, reaching zero
at an AoA of 90 degrees.

However, my understanding is that a stall occurs when the lift
generated by the wing drops below the load the wing is required to
support.

This is the usual result of a stall, and is what occurs in the typical
training situation, but it isn't the definition of a stall. Generally, a
stall begins when the airflow starts to separate from the wing at
increasing AOA. It is this separation that keeps the lift from
increasing and sets the maximum lift coefficient.

Usually major airflow separation coincides with a stall and the drag
increase ensures that a stall will happen because of the associated
loss of airspeed. However, flow separation is not the same as a stall.
Many aircraft have quite a high degree of flow separation during low
speed flight. In the model world we assume separation always occurs at
about 60% chord at min.sink and this would appear to be close to the
mark for sailplanes judging by Will Schumann's experiments.

A wing can be stalled and still produce plenty of lift; for example, in
a high speed pull up done with too much elevator can stall the wing, but
the stalled wing will still have more lift than the weight of the
aircraft because of the high speed.

I would normally call that a high drag flight regime rather than a
stall.

In a high speed climb after rapid pull up, pushing the stick enough to
give zero G will reduce the lift to zero, but the wing is not stalled
(the airflow is well attached - no separation) even though it can not
support the glider.

Sure - and I don't think a wing can be stalled in a zero-G situation,
e.g. a model flown in ISS. It probably can't be stalled in vertical
flight either. In both cases the generated lift is necessarily zero
and so is the opposing load on the wing.


For a given wing the generated lift is proportional to the Cl and to
the square of the speed, so at a fixed AoA you can reduce the speed
until the lift is no longer sufficient for flight, at which point the
wing stalls. If the aircraft weight is reduced then so is the stalling
speed: it doesn't matter whether this reduction is due to dumping
ballast or to pushing over to generate reduced G forces. If you put
water in a glider you raise its stalling speed but you don't
necessarily change the AoA at which it stalls.

Hence my comment that the load on the wing is as important as AoA for
*stalling* behaviour.

Perhaps I don't understand this correctly: by load, do you mean
different G loads, or just different aircraft weights? By *stalling
behavior*, do you mean how rapidly the aircraft responds as it stalls,
the amount of buffeting, how the nose is, or...?

By 'load' I mean the instantaneous load applied parallel to the wing's
lift vector. It will be the vector sum of the weight and acceleration
at that instant.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

On Sun, 18 Jul 2004 15:14:37 +1200, Bruce Hoult
wrote:

In article ,
Martin Gregorie wrote:

Sure, Cl is dependent entirely on AoA, but is not a linear
relationship throughout the range:

- It is linear at small angles.
- When the AoA is high enough for the upper surface flow
to start to separate the Cl tends to a constant value with
increasing AoA.
- If the AoA continues to increase even further you reach
a point at which the Cl starts to decline, reaching zero
at an AoA of 90 degrees.

I'm with you on all that.


However, my understanding is that a stall occurs when the lift
generated by the wing drops below the load the wing is required to
support.

No, a stall is when increasing AoA decreases lift. There might well
still be more lift that the weight of the aircraft, especially at high
speed. The only reasons to avoid such stalled flight a

I wouldn't describe that as stalled. Separated flow, yes, and hence
high drag, but not stalled.

- high drag and thus inefficient

of course

- the aircraft is unstable in roll, making it difficult or
impossible to control.

That's not due to the stall, but rather to upper surface flow
separation reducing aileron effectiveness.

Presumably you've seen aircraft such as the F/A-18 demonstrate a slow
pass at very high and stalled angle of attack? They are getting some of
their support from the downward component of the engine thrust, of
course, but with an AoA of, say, around 30 degrees it would need a
thrust:weight ratio of around 2 in order for thrust to be enough to
support the entire aircraft weight. It would also require *huge* drag
in order to avoid accelerating at such a thrust level. The F/A-18 has
nowhere near that amount of thrust, so the majority of the support is
clearly still coming from the stalled wings. In that situation the
aircraft is unstable, and would probably be improssible to fly like that
without the computer reacting very quickly to unwanted rolls.

IIRC an F-18 has unlimited vertical capability when lightly loaded.
Such vertical flight requires no wing lift and a thrust:weight ratio
of 1:1, so it follows that any other flight regime from normal flight
to high-alpha, fully flow-separated attitudes where the wing is
contributing some lift will require less thrust, not more.

So the F/A-18 can be happily flown in steady-state stalled straight and
level flight primarily because of the computer control and also because
the extra drag is less than the engine thrust available.

I think the control regime will be decidedly odd - ailerons should be
pretty ineffective, computer of no computer, and that the aircraft may
well be being flown on a combination of rudder and elevator.

The drag in supersonic flight is so high that almost any Mach 2
aircraft has the thrust to do this. The real issue is control and here
the all-flying tail that the Bell X-1 program discovered also happened
to have the control authority needed to hold the high angle.


For a given wing the generated lift is proportional to the Cl and to
the square of the speed, so at a fixed AoA you can reduce the speed
until the lift is no longer sufficient for flight, at which point the
wing stalls.

Well, ... no :-)

If you maintain a fixed AoA, and the speed is such that the lift is less
than the weight of the aircraft then the aircraft will start to follow a
downwards parabolic path (not as sharply downwards as in a zero-G
pushover, but similar).

I think you'll find that's pretty much what does happen if you watch a
stall from the outside, follow the path taken by the CG and ignore the
attitude changes. I think our perceptions from the inside are very
much affected by the pitch-down, but that's designed in by picking
suitable incidence difference and airfoil characteristics for the
tail. When the lift no longer balances the load on the wing the
aircraft will accelerate downward: A = F/M always applies and the
aircraft will accelerate until the lift once again matches the load on
the wing. If you continue to hold the stick back in a fully stalled
glider it will again achieve a steady state, but a very inefficient
one with a largely separated airflow on the wing. Centreing the stick
lowers the AoA and hence the Cl, so the aircraft again accelerates
downward until it has the airspeed necessary for normal flight.

Hmm, it looks like a terminology thing to me. I'd still maintain that
the stall break comes when the wing can't support the load imposed on
it. I suspect there's no disagreement here. However, I'd also accept
that most pilots would call the following highly separated, though
stable steady state rapid descent a "stalled wing" despite the fact
that the wing is supporting the aircraft. Regardless of what its
called, you'd best not try to land in that condition!

BTW I've deliberately ignored accelerated stalls and high-speed stalls
in this discussion because they are so far from steady state that I
think the dynamics of the situation obscures what's really happening.


What happens next depends on what else (if anything) you are holding
constant.

Suppose, for the sake of concreteness, that you are initially flying
straight and level at 60 knots and you then fix the AoA such that the
wings are producing only half the lift required to support the glider.

Normally a glider will accelerate, increasing the lift (and drag, but
not by much). The extra lift will cause the path to become less sharply
curved downward and things will come to equilibrium (or oscillate
around) the point where the combined lift and drag are equal and
opposite to gravity. For a typical glider polar curve this will happen
at an airspeed of around 1/sqrt(0.5) times 60 knots, or 85 knots, plus
or minus a little due to drag.

So all you've acheived is to change the trimmed speed from 60 to 85
knots.

Or, look at it the other way around. Maybe you were flying straight and
level at 85 knots, and then you somehow instantly decrease the airspeed
to 60 knots (maybe a gust up the tail). The lift is no longer
sufficient to maintain level flight. But the glider doesn't stall. It
just drops the nose and accelerates until it has returned to the trimmed
speed of 85 knots.

In no way are the wings ever stalled.


If you stipulate constant speed as well as constant AoA (presumably via
some large and adjustable drag, magical or otherwise) then the flight
path will become steeper until the combined lift and drag vectors are
again exactly equal to and opposite the gravity vector. This will
result in a much steeper flight path, but still stable.

Let's suppose again that you are at 60 knots and reduce the AoA to
produce only half the lift required for flight and then continue to
maintain exactly 60 knots somehow. Alternatively, suppose you're flying
trimmed for level flight at 85 knots and then apply airbrakes to reduce
and maintain 60 knots, while keeping the same trim (AoA).

What happens?

The AoA/speed are insufficient for flight at 60 knots and so the nose
drops. If you draw up the force vectors then you will find that the
glider will stablize in a 60 degree descent at your desired constant 60
knots. Lift (from the wings) is still 0.5 of the weight just as it was
initially (but it's in a funny direction, tilted 60 degrees forward from
vertical). Drag (from the airbrakes) is 0.866 of the weight, tilted 30
degrees from vertical. The horizontal components of lift and drag are
equal and opposite and cancel out. The vertical force to oppose gravity
comes 25% from the wings and 75% from the airbrakes.

In no way are the wings ever stalled.


No matter what you do, if you start with the wings not stalled then
there is nothing you can do that will stall them while all the time
keeping the AoA constant.

If you see the nose drop and don't like it and pull back on the stick to
try to prevent it then that is an entirely different matter -- you're
increasing the AoA which certainly *can* stall the wings.

-- Bruce

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

On 17 Jul 2004 23:50:41 -0700, (Rudy Allemann)
wrote:

Martin Gregorie wrote in message . ..
On 15 Jul 2004 18:17:04 GMT, Derrick Steed
wrote:

FWIW, on Tuesday evening I decided to investigate turning stall/spin
behaviour in my '20 at a sensible altitude. It was calm and with
little air movement under a high overcast. With the aircraft clean and
flaps at zero (setting 3) I flew some moderately steep turns - about
45 degrees of bank and at speeds ranging down to about 43 kts. This
was completely uneventful - no buffeting, burble or hints of
departure. In short, it flew like a pussycat. I'll try this again by
myself in a turbulent thermal next time because all that series of
turns told me was that in nearly still air my '20 can fly uneventful
turns at stupidly slow airspeeds. By comparison I typically fly at
48-50 kts for that steep a turn in zero flap during normal thermalling
turns.

Dear Martin, In your next flight-test-experiment with the ASW-20, try
a climbing turn stall at good altitude. Bank into a right turn at
60-65 knots and as the right wing goes down pull back fairly hard and
steadily on the stick to make the glider go up and slow down. Keep
pulling back and wow! you will stall over the top and quickly too. I
think that this has been the cause of more than one low altitude
glider crash. I tried this in my '20 when Walt Cannon and I were
having a similar discussion about stalling in the ASW-20. Let me
know how it works and what you think. Rudy Allemann

Any particular reason you picked a right turn? Mine tends to drop the
left wing in a stall.

Your suggestion sounds like a good exercise to try: normally I don't
go below 50 kts at the top of a fast pull-up. Maybe Monday or
Tuesday....

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

Martin Gregorie wrote:

This is the usual result of a stall, and is what occurs in the typical
training situation, but it isn't the definition of a stall. Generally, a
stall begins when the airflow starts to separate from the wing at
increasing AOA. It is this separation that keeps the lift from
increasing and sets the maximum lift coefficient.


Usually major airflow separation coincides with a stall and the drag
increase ensures that a stall will happen because of the associated
loss of airspeed. However, flow separation is not the same as a stall.

Perhaps we are not discussing the same thing. It sounds like you are
talking about "a stall", meaning the aircraft's behavior from the pilots
viewpoint (buffeting, loss of lift, poor control, etc), and I am talking
about the aerodynamic situation during "a stall" (high AOA leading to
flow separation and constant or diminishing lift coefficient).

Many aircraft have quite a high degree of flow separation during low
speed flight. In the model world we assume separation always occurs at
about 60% chord at min.sink and this would appear to be close to the
mark for sailplanes judging by Will Schumann's experiments.

I think our modern airfoils have very little separation at minimum sink,
and certainly far aft of the 60% point. Instead of "separation", perhaps
you mean the transition from laminar flow to turbulent flow? That does
occur somewhere around the 60% point (maybe 70% or so) on modern airfoils.


A wing can be stalled and still produce plenty of lift; for example, in
a high speed pull up done with too much elevator can stall the wing, but
the stalled wing will still have more lift than the weight of the
aircraft because of the high speed.


I would normally call that a high drag flight regime rather than a
stall.

I agree it is not "a stall", but I think is sometimes referred to as
"stalled flight", and the wing is considered "stalled". For some
aircraft, like fighters with their powerful engines, it is a useful
situation. For gliders, I think any time the AOA is high enough to stall
the wing, the glider will suffer "a stall", regardless of the load on it!


--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

On Sun, 18 Jul 2004 08:27:23 -0700, Eric Greenwell
wrote:

Perhaps we are not discussing the same thing. It sounds like you are
talking about "a stall", meaning the aircraft's behavior from the pilots
viewpoint (buffeting, loss of lift, poor control, etc), and I am talking
about the aerodynamic situation during "a stall" (high AOA leading to
flow separation and constant or diminishing lift coefficient).

I think that's partly true. I meant 'A stall' as in what happens as
the wing becomes no longer able to support the aircraft, not what
happens if you keep the stick back and the situation stabilises with a
high but constant descent rate.

I think our main disagreement is whether the aircraft really reaches
the constant Cl, increasing Cd region, let alone the diminishing Cl
region. It may do that, but the AoA would need to be very large indeed
- over 20 degrees at a guess.

I've not played with calibrated AoA indicators. If you have, what AoA
was reached at the stall? I'm curious.

Many aircraft have quite a high degree of flow separation during low
speed flight. In the model world we assume separation always occurs at
about 60% chord at min.sink and this would appear to be close to the
mark for sailplanes judging by Will Schumann's experiments.

I should read back more carefully before hitting SEND. I meant 80%.
Sorry 'bout that.

I think our modern airfoils have very little separation at minimum sink,
and certainly far aft of the 60% point. Instead of "separation", perhaps
you mean the transition from laminar flow to turbulent flow? That does
occur somewhere around the 60% point (maybe 70% or so) on modern airfoils.

Depends on the surface texture and Re number: the turbulent transition
is just behind the hi-point with a paper covered surface and Re =
50,000. I'd guess the separation point was about at the aileron hinge
line on a Discus 1 - otherwise why put the turbulator there? Its job
is to increase the boundary layer energy by forcing a transition from
laminar to turbulent and hence causing separation to be delayed.
Without measuring the wing, that must be in the 80% ballpark.


--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :