My 2 cents worth: Mcrit for a given airframe will not change with respect to changes in altitude/airspeed/density. What does change however, is the True Airspeed at which that Mcrit number is achieved. As altitude increases all forms of airspeed for a given mach number will decrease (ie IAS, Calibrated, Equivalent, and True). What does not change however is the Mach speed of the airflow which causes the first evidence of local sonic flow. And that can be on ANY part of the aircraft, not just the wing.
It is true that increasing AOA will increase the velocity of airflow over the wing but this does not change the Mcrit number for that airplane, it just reaches that critical number sooner than if you simply accelerated in level flight.
The shape of the airfoil affects the Critical Mach Number.
Last edited by Spoonman; 10-04-2013 at 05:59 PM.
Reason: Left out info