Critical Mach number

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The Critical Mach number (Mcr) is the maximum Mach number (airspeed in relation to the speed of sound - Mach 1.0) which a subsonic aircraft can attain whilst still remaining controllable by the pilot.

At the Critical Mach number, local airflow over the airframe reaches the speed of sound (due to the airflow speeding-up to go around various curvatures in the aircraft structure) and creates shock waves sufficient to affect the airflow over the control surfaces, resulting in a loss of control, although the aircraft itself may still be flying subsonically.

Although unknown at the time, the Critical Mach number was the cause of the effect known as the 'sound barrier' which was being observed in the late 1930s/early 1940s.

Subsonic aircraft, e.g., Supermarine Spitfire, BF 109, P-51 Mustang, Gloster Meteor, Me 262, P-80 possess a Critical Mach number and are incapable of safely passing-through the 'sound barrier'.

Transonic aircraft, e.g., Hawker Hunter, F-86 Sabre are designed to have no Critical Mach number and while not possessing sufficient engine power to do so in level flight, can be dived to above the speed of sound and remain controllable.

Likewise, supersonic aircraft, e.g., English Electric Lightning, Lockheed F-104, Mirage III, MiG 21 have no Critical Mach Number, being designed to exceed the speed of sound in level flight and having the engine power to do so.