When does the critical Mach number occur?

The critical Mach number is defined as the lowest Mach number at which airflow over any part of the wing reaches the speed of sound (Mach 1.0). This phenomenon is crucial in aerodynamics because it indicates the onset of compressibility effects, which can lead to significant changes in the aerodynamic characteristics of the aircraft.

When the airflow over the wing reaches Mach 1.0, it generates shockwaves that can alter pressure distribution and increase drag, which pilots must consider during flight operations. Therefore, identifying the critical Mach number is pivotal for understanding how close an aircraft is to its aerodynamic limits and ensuring safe and efficient flight.

The other scenarios presented do not accurately depict the conditions under which the critical Mach number occurs. For instance, airflow starting to stall or reaching a Mach number of 0.75 does not necessarily correlate with the critical Mach number, nor does the ineffectiveness of the elevator directly relate to reaching the critical Mach number of the wing.