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In the process of increasing angle of attack of a wing, lift is increasing to a maxima at a certain AOA, then decreasing as this AOA is exceeded. This seems to be the current agreed upon definition of the stall point.

Yet the sharp pitch down and transition of the center of lift to 50% chord does not occur until the top lift pocket completely collapses. Is this the true definition of "stall"?

There is also pre-stall buffeting as high AOA turbulence begins to affect the wing trailing edge.

So, are there really 3 parts to this that a pilot must understand, rather than just one?

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  • $\begingroup$ Your last line asks a different question than your title; which is it? Are you looking for a technical definition of stall or for what does the pilot need to know about it? $\endgroup$ – AEhere supports Monica Jul 3 at 12:37
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    $\begingroup$ Actually both. A recent ASE question regarding the difference between a "mush" stall and a sharp pitch down piqued my interest. I have experienced the mushy nose drop in a 172 and have seen very sharp pitch down stalls in staticly unstable rear CG set models. Thanks for your answer below. $\endgroup$ – Robert DiGiovanni Jul 3 at 15:49
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The scientific definition of a stall is as you state in the first line, however in the cockpit that definition isn't actually that useful. Practically there's 2 parts of stalls a pilot needs to be aware of because they are things a pilot experiences, and impact the flying characteristics of the airplane:

  1. Stall buffet: this isn't a stall, but it happens when you're very close to one. Recognizing a buffet may lead to a pilot decreasing AoA before entering a fully developed stall
  2. Flow separation: when the flow separates from the top of the wings it may do so on one wing before the other, causing a wing drop, aileron input may work opposite of intent, and you of course have a high rate of descent. This can feel mild or violent depending on the airplane's characteristics and the speed at which the stall happens

Whether the definition of a stall is when the airflow separates or a change of the center of lift is academic, what a pilots needs to know is how to recognize the condition and recover.

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    $\begingroup$ It's basically like oversteering and understeering when driving. It's not really about the academics, but how it feels and how to recover. $\endgroup$ – Nelson Jul 4 at 1:44
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    $\begingroup$ It's a very good analogy to explain the difference between knowing something and experiencing something @Nelson. Skidding is another apt driving analogy. $\endgroup$ – GdD Jul 4 at 7:47
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EASA's unofficial definition:

Stall means a loss of lift caused by exceeding the aeroplane’s critical angle of attack.

Note: A stalled condition can exist at any attitude and airspeed, and may be recognised by continuous stall warning activation accompanied by at least one of the following:

(a) buffeting, which could be heavy at times;

(b) lack of pitch authority and/or roll control; and

(c) inability to arrest the descent rate.

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A pilot does not need to understand the technical definitions of each stage of a stall. A pilot needs to be able to recognize the indications of the onset of a stall and how to recover from it. This is part of primary training, and it should be part of familiarization training when getting checked out in a new type of aircraft.

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I think the distinction between the aerodynamic definition of a stall and a pilot's experience is that while most diagrams show a cross section, the whole wing doesn't always stall at the same time.

Commonly the wing root is designed to stall first, while the tips are still flying. The result is buffeting from the stalled section and 'mushing' due to the reduction in lift but control is maintained.

If the wing is not designed like this, one tip will stall slightly ahead of the other, resulting in asymmetric lift and a sudden roll, probably flicking the aircraft into a spin.

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