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Here is a $C_L$ / $AoA$ curve that I took from Wikipedia.

Cl vs AoA - two points indicated, one before critical angle, one after

The better textbooks say that a stall is that condition in which a further increase in angle of attack will result in a reduction of lift. The point at which that transition happens is known as the critical angle of attack. Theoretically, sustained flight is possible at angles beyond the critical angle of attack - take a look at the chart. If the airplane can sustain level flight at point $A$, it can sustain level flight at point $B$.

Is there a practical way that I can demonstrate sustained flight on the backside of the lift curve without an angle of attack indicator?

Or to ask the question another way,

Is there a practical way to tell when an airplane has exceeded the critical angle of attack without an AoA meter?

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just nitpicking, but drag is much,much larger in B than in A. If you have enough engine power for sustained flight at A, it doesn't imply you can sustain straight & level flight at B. Plus B is statically unstable as DeltaLima has answered already – Radu094 2 days ago

5 Answers 5

up vote 8 down vote accepted

Flight on the backside of the stallcurve is unstable; any (infinite small) increase in AoA will result in less lift. Less lift will cause the flightpath angle to decrease and the AoA to go up even more. To increase the lift, the nose has to be pitched down, but not so much that the stall is recovered otherwise you wouldn't call it sustained flight at the back of the AoA curve.

At the same time you are also on the backside of the powercurve. This means that any reduction in airspeed would cause the drag to increase therefore decelerating the aircraft furthermore.

This double unstable situation would require a lot of fast responses from the pilot, probably causing too high workload. I doubt if it is practically possible to fly on the backside of the liftcurve without special automation.

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Just pull all the way back and let the plane do the rest. Depending on the airfoil, this may or may not work. If stall is characterized by flow separation near the nose (think NACA 5-digit airfoils), the aircraft will pitch down. If you use a modern laminar flow airfoil, chances are good that you can trim this without problem. The missing thrust will be compensated with sink speed. Air France demonstrated that you can keep an A-340 comfortably in this condition for 3 minutes, and the automatization was switched off. – Peter Kämpf May 2 '14 at 13:15
@PeterKämpf I agree with your analysis. A steady spin is another a demonstration of remaining in this B state. In that case the high wing will be a bit to the left of point B, the low wing a bit to the right, but both will be passed the top. But I don't think this categorises as sustained flight though, as AF clearly demonstrated. (And the nitpicker in me says it was an A330, it was not comfortable, and the automation was no switched off but it was in Alternate Law) – DeltaLima May 2 '14 at 13:57
@DeltaLima +1 for the last 2 sentences of your comment. Also, I believe it was more like 6 minutes. Definitely not sustained, though. – reirab Jan 16 at 20:07
@DeltaLima you don't have any glider time? – rbp yesterday
@rbp Just saw your answer, I guess that's the perfect excuse to get some more glider time next summer... :-) – DeltaLima yesterday

Increase the aircraft's pitch in level flight. If you climb, you are on the good side of the curve. If you sink, you are on the bad side. This will only be the initial response of the aircraft, as the airspeed will start to change and further affect the aircraft's behavior.

Without an AOA meter, you would have to look at your artificial horizon and vertical speed.

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If you sink you are not necessarily on the backside of the liftcurve. Your airspeed will be lower than it was in level flight and your AoA will be higher, but not past the critical AoA – DeltaLima Apr 30 '14 at 6:41
Are you saying the effect of the pitch on airspeed is greater than the effect on lift? – fooot Apr 30 '14 at 15:06
If you increase the pitch, you'll initially increase the lift and the aircraft will start to climb and loose airspeed. Less airspeed is less lift so the climb will shallow out over time and might develop into a sink depending on pitch, power and dynamics of the aircraft. – DeltaLima Apr 30 '14 at 15:35
Thanks for your comments, I have clarified my statements. – fooot Apr 30 '14 at 15:40

Is there a practical way that I can demonstrate sustained flight on the backside of the lift curve without an angle of attack indicator?

Set up the aircraft for stalls, do all the stall safety checks. Fly slowly, so that you're near the peak of that curve. Pitch up sharply and confidently while adding power. When you've done the right amount of both, you'll find that the aircraft remains more or less in level flight. Make adjustments to zero the vertical speed, remembering that if you're climbing, you want more up pitch to stop it in this configuration, and vice versa.

P.S. I'm no instructor and my flying hours are limited, but I was taught to do this during my initial training. I suppose how easy this is depends strongly on the aircraft.

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Gliders make sustained flight on the backside of the lift curve all the time while thermalling at 40-50 knots and 50 degrees of bank. You spend a lot of time adjusting the elevator to keep the plane from stalling, a lot of time adjusting the ailerons to keep the bank angle, and a lot of time trying to stay coordinated (or in a slight slip) with your feet. The bigger the wingspan, the more work it is.

The big difference in a glider is that you're usually going up, not down when this happens :)

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The drag at point B in your chart is much higher than at point A, so you might not be able to trim level flight there. If you are willing to sacrifice altitude to compensate for missing thrust, the lift at point B will be sufficient to trim a quasi-steady flight condition.

Flying beyond stall requires sufficient pitch authority, so you might not be able to trim this condition with forward c.g. That said, your pitch response will be much weaker than in normal flight, and you can pull suddenly without much happening. Depending on your c.g. location, you will need to have your yoke/stick/whatever pulled way back to trim this condition, and if some more travel in pull direction remains, it will not help to further increase your angle of attack. Some aircraft will not let you stay in this condition and either drop the nose by themselves (which is good) or drop a wing (which is really bad), so try to experiment with enough altitude for recovery.

My answer is Yes, if the aircraft lets you. Just watch the sink speed.

Hoerner lift coefficient over 180°

Lift coefficient over 180° for several NACA airfoils, taken from Sighard Hoerner's Fluid Dynamic Lift.

@DeltaLima is absolutely correct: While the lift curve slope is negative (the region around point B in your plot), the aircraft will be unstable and will drop out of this alpha range quickly. Note, however, that in the post stall region another section with positive lift curve slope follows, and here you can fly (well, let's better say mush) stably if the pitch control power of your aircraft allows. You will detect that you are in this region when your sink speed is much higher than at around stall, but the aircraft is in a stable state.

Air France demonstrated that you can keep an A330 in this condition for three minutes without any trouble. The trouble only starts when your altitude has run out.

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Miss clicked on the down vote yesterday, noticed today, can't change anymore. sorry. – Antzi 22 hours ago
@Antzi: Thank you for letting me know! – Peter Kämpf 21 hours ago

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