Dear Pilots,
first of all: there is no specific Mcrit for ANY airplane!
Definition:
The Free Stream Mach number at which the local velocitey first reaches Mach 1.0 (SONIC) is called the Critical Machnumber (Mcrit). Mcrit is the highest speed at which no parts of the aircraft are SUPERSONIC.
Increased thickness/chord and increased angle of attack cause greater accellerations over the top surface of teh wing; so the critical Mach number will DECREASE with increasing thikness/chord ratio or angle of attack. Which means, that the aircraft will reach Mcrit at a lower Mach number.
At speeds just above the critical Mach number there will be a small region of supersonic airflow on the upper surface. This area is terminated by a shock wave. As the aircraft speed is increased the region of supersonic flow on the upper surface extends and the shockwave (marking the end of the supersonic region) moves rearwards.
When the aircraft speed reaches Mach 1.0 tha airflow is supersonic over the whole of both upeer and lower surfaces. the shockwave (also to be found on the lower wing surface) has moved all the way rearwards to the trailing edge of the wing.
And remember!: LSS (local speed of sound) varies with changes in absolute Temperature! So if temperature rises LSS will increase. And as temperature decreases LSS will decrease as well.
Now let's get back to that question.
Mcrit is not always the same TAS/IAS for a given aircraft. One always has to take into consideration the angle of attack and therefor consider aircraft mass, CG position and wander.
