Ailerons Stalling?
 

Heya,
I'm doing a presentation on how control systems have developed since the Wright Brothers and I have come up with a question regarding high angles of attack and rolling.
Firstly, I am unsure exactly how an aileron works, is it:
a) Increases/decreases overall curvature of wing hence increasing/decreasing lift of the whole wing
b) Increasing/decreasing the angle of attack of the aileron (which is in effect a small wing itself) causes force to be applied in terms of pressure and so rolls the aircraft
c) a mixture of both
(Sorry if those definitions aren't clear, if anyone wants I will try to clarify them)

Assuming it is b) or c) when an aircraft is in a steep climb close to an angle of attack which will induce a stall, rolling the aircraft right or left (may need to be quite a high roll rate so large deflection) will cause one of the ailerons to increase its angle of attack. As the aircraft is close to a stall angle any increase in AOA of a control surface would lead to the control surface stalling?
In this situation would a roll actually occur based on the fact the wing, which is supposed to roll upwards, has the high angle of attack aileron which may be stalled?
The same question applies to elevator deflection, from a high angle of attack attempting to pitch the nose down would result in an increased angle of attack of the elevators and so the tail would essentially stall so the plane would instead pitch upwards?

Any opinions or information on this would be appreciated as it has got me thinking and I just can't seem to come up with a definite yes or no answer
Regards
Alistair Strong

In a high AOA situation it is perfectly possible to induce an asymmetric stall on the wing you actually want to go up. It's not just the aileron that will stall, but when the angle of attack is very close to stall onset it will sometimes cause the stall state on that wing to kick in a little earlier. Most interesting effects can be noted on twin-engined turboprop aircraft that experience an engine failure and require some aileron input to fly straight and level. Slowing down towards (or even past) blue line speed and applying more aileron will easily lead to a situation opposite of what you are trying to accomplish.
Re elevators: since the horizontal stabilizer on all aircraft has a smaller or even inverted camber when compared to the main wings, this surface cannot be stalled before stalling the wings first. When the main wings have reached a stalled condition, the natural behavior of the aircraft will be to pitch nose down. Unless of course you are flying a poorly designed swept-wing aircraft where the wing tip stalls before the root, which causes the lift center to move forward and cause a pitch-up motion.

Aileron Stall
 

I've had aileron stall once or twice when aerobatting Chipmunk aircraft.

IIRC it gave a slight rocking and sideways snatching of the control column.

The aileron itself stalls and the centre of presure moves rapidly back and forth and unbalances the aileron(s).

Not a normal occurrence - cause - over enthussiastic use of ailerons!

Hang on ... aileron stall?
The position of the aileron can affect the stalling characteristics of the wing, but I have not heard of aileron stall.

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Increases/decreases overall curvature of wing hence increasing/decreasing lift of the whole wing

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Most accurate.

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Increasing/decreasing the angle of attack of the aileron (which is in effect a small wing itself) causes force to be applied in terms of pressure and so rolls the aircraft

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Since the overall flow of the air around the wing is generally parallel to the wing, it’d be tough to argue the aileron has its own AOA.

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As the aircraft is close to a stall angle any increase in AOA of a control surface would lead to the control surface stalling?

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The deflection of the aileron doesn’t increase its own AOA or that of the wing (due to the way that AOA is defined), but the critical AOA of a flapped wing is reduced, so a wing with a downward deflected aileron may indeed stall before the unflapped one. This is often described as the mechanism behind a cross-controlled stall.

However, during the roll, the AOA on the rising wing is reduced due to the roll rate and the AOA on the descending wing is increased due to the roll rate.

So the rising wing, due to the deflected aileron, has a lower critical AOA than the unflapped wing, but due to its motion through the air, has the AOA lowered from non-rolling flight. Which effect wins? I’ve never heard of a stall being induced this way, so I suspect the rolling effect is dominant. During a cross-controlled stall, there is no rolling motion so the aileron can induce a stall. (Speculation only.)

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The same question applies to elevator deflection, from a high angle of attack attempting to pitch the nose down would result in an increased angle of attack of the elevators and so the tail would essentially stall so the plane would instead pitch upwards?

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No. At a high AOA, the AOA of the horizontal stabilizer is actually low, although the lift coefficient is high. And since the horizontal stabilizer normally provides a download, a stall would produce a pitch down.

This is a good model, but leads to a surprising conclusion.

Looking at Fig 100 in Abbott and von Doenhoff, looks like a 0.2c plain flap indeed reduces the stall AoA by about a degree per 12 degrees of flap. But the interesting points arise if you compare a flap deflection of 0 at an AoA of 14 degrees with a higher flap deflection (say 40 degrees) at the same AoA. The wing without flap deflection is not stalled. The wing with flap deflected is stalled -- indeed the stall (Clmax) AoA has reduced -- but it still produces more lift than the unflapped wing! It's very difficult to find a counterexample where the lift goes down when flap is deflected, except at very high flap deflections (e.g. 60 degrees) or beyond the stall of the unflapped aerofoil.

So your ailerons still work, even close to the stall, albeit less effectively.

Blista, your presentation (I wonder who the audience will be..?) needs to focus, at your level, much more on certification requirements than the complex aerodynamics involved.


This is the key to your question, put, as it is, in the broadest terms, and is the cause behind bookworm's finding above.

:hmm:


Alternatively, start at the NASA website to learn how to stop worrying about lift generation and love your lack of academic success!!! ;)

http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html

Good insight!

Nathan: Thankyou very much for that in-depth explination. Just one point to clarify if you could:
"However, during the roll, the AOA on the rising wing is reduced due to the roll rate"
Why is this true? My guess is:
I presume when you use AOA here you don't mean pitch of the wing but presumably direction of the airflow? When the wing rolls upwards the effective direction of airflow is now coming from more above the wing so the result is an apparent lower AOA?
Do you have a "dictionary definition" of AOA so I can be sure I'm not mixing up terms.



Kit d'Rection KG:
My presentation is most definately not at this level. Unfortunately it was quite late when I was writing my presentation and my mind started to wander a bit which results in ridiculous questions like this :ugh:

The presentation I'm doing is for 10minutes on how control has developed in the past 100 years from the Wright Flyer using bodyweight to twist the wings, through to FBW and actuators. As you can see it is much more on how mechanical linkages have changed above anything else.


Bookworm:
"The wing with flap deflected is stalled -- indeed the stall (Clmax) AoA has reduced -- but it still produces more lift than the unflapped wing"

How can a stalled wing produce more lift than an unstalled one or have I mis-read what you were saying?
By extending flaps is it just increasing the effective AOA (see above) of the wing hence why more lift is generated, but there is a lower stall angle, or is there something else going on here?

Thankyou to all that have replied
Regards
Alistair Strong

I don't know what pitch of the wing means. The textbook definition of AOA is the angle between the chordline of the airfoil and the relative wind. The chordline is defined to be the line between the leading edge and the trailing edge of the unaugmented airfoil. That's why flaps won't affect the AOA. (The only time a different definition is used is in stability and control calculations; they use the angle between the zero lift line and the relative wind.)

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When the wing rolls upwards the effective direction of airflow is now coming from more above the wing so the result is an apparent lower AOA?

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Exactly.

His point is that the definition of a stall is when the Clmax has been exceeded. It doesn't mean that all lift goes away. A flapped wing has a higher Clmax than an unflapped wing, so just because the flapped wing's clmax has been exceeded doesn't in any way imply that the quantity of lift is less than an unflapped wing.