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I wonder, is there a clear definition of stall at all...?
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Captain Doggles gives a good definition:
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Stall is defined as the point where the airfoil's critical angle of attack is exceeded.
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The question is not when does the stall occur but rather how much warning the aircraft gives.
That is the conundrum faced by designers. Today stability and control is a well defined science. Aircraft designers have many more tools to take advantage and flying characteristics are a consideration almost from conception. For example, today designers build aircraft with no stall warning at all, these airplanes can be flown at maximum performance right up to CLmax without aerodynamic penalties of a buffet. To warn the pilot he is nearing a stall, an artificial device called a "stick shaker" is used.
To understand that stall warning, one must understand what buffeting is aerodynamically. It is a rapid secession of flow separation and reattachment.
When that boundary layer is not attached to a portion of the wing that portion is stalled, an airplane in turn is no longer turning at maximum rate. If you read the Spitfire Mk I pilots notes it expressly warns the pilot to ensure he experiences no buffeting in a turn. First of all when the airplane is buffeting, the turn rate is reduced even though the airfoil is not at CLmax. Secondly, the Spitfire has an extremely nasty stall that will spin and the aircraft is susceptible to airframe destruction in an aggravated spin.