A wing does not always stall at the same AoA

[punkalouver]

I seem to remember reading frequently that a wing always stall at the same AoA. A quick search confirms this is what various sources say.

But Boeing Aero #12 says differently.

Aero 12 - Angle of Attack (http://boeing.com/commercial/aeromagazine/aero_12/attack_story.html)

"On most transport category airplanes, the lift that the wing produces is also a function of Mach number, particularly as the airplane approaches transonic speeds typical of cruise flight. Of course, lift at a given AOA will increase with speed, but even at the same airspeed, as Mach number is increased (the speed of sound changes with temperature), lift will increase.

However, higher Mach reduces the maximum lift the wing can attain and the AOA at which stall occurs. This means that as gross weight, altitude, or load factor is increased, the resultant increase in Mach number will cause a stall at a higher speed and lower AOA. This is true even at takeoff and landing speeds with the flaps down."

I'm some of our regular posters knew this all along and perhaps will say that I should have as well. But I thought I would share it with the rest of you anyways.

[Wizofoz]

Yes, true.

The effect is negligible at low altitudes as stall occurs at a low Mach number, but at high altitude, where stall is a higher Mach, the AofA at which it occurs changes (reduces) quite considerably.

[Microburst2002]

It is not a new discovery, only we usually remember incompressible aerodynamics and forget compressibility effects.

Separation occurs earlier in the chord at high mach numbers. Both CL and CD curves vary significantly with mach number when at high mach numbers.

[Miserlou]

And we also usually consider free flow air rather than another case such as in ground effect.