Refer to figure.
Critical Mach Number: (MCRIT) The critical Mach number is the Mach number of the aircraft when the speed of the airflow over some part of the aircraft first reaches the speed of sound.
At low subsonic speeds, the lift coefficient CL is assumed to be constant at a given angle of attack. With increasing Mach number, however, it will vary as shown in figure 1.
At high altitude, a large high-speed jet transport aircraft will be cruising at a speed marginally above its critical Mach number, and it will have a small shock wave on the wing.
If such an aircraft overspeeds, the shock wave will rapidly grow larger, causing the static pressure to increase sharply in the immediate vicinity of the shock wave. The locally increased adverse pressure gradient will cause the boundary layer to separate immediately behind the shock wave. This is called a ‘shock stall’ because it results from a separated boundary layer just as the low speed stall does.
Shock stall occurs when the lift coefficient, as a function of Mach number, reaches its maximum value (for a given angle of attack).
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