Critical Mach number Critical Mach number In aerodynamics, the critical Mach number (Mcr or M* ) of an aircraft is the lowest Mach number at which the airflow over some point of the aircraft reaches the speed of sound, but does not exceed it. At the lower critical Mach number, airflow around the entire aircraft is subsonic. At the upper critical Mach number, airflow around the entire aircraft is supersonic. For an aircraft in flight, the speed of the airflow around the aircraft differs considerably in places from the airspeed of the aircraft; this is due to the airflow having to speed up
Critical Mach number not designed to fly at or above the critical Mach number, the shock waves that form in the airflow over the wing and tailplane are sufficient to stall the wing, render the control surfaces ineffective, or lead to loss of control of the aircraft (such as Mach tuck, when shock waves in the airflow over the elevator send the aircraft into an uncontrollable dive). These problematic phenomena appearing at or above the critical Mach number became known as compressibility. Compressibility led to a number of accidents involving high-speed military and experimental aircraft in the 1930s and 1940s. Although unknown at