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Critical Mach Number

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Category: Theory of Flight Theory of Flight
Content source: SKYbrary About SKYbrary
Content control: EUROCONTROL EUROCONTROL


Definition

In aerodynamics, the critical Mach Number (Mcr or Mcrit) of an aircraft is the lowest Mach number at which the airflow over any part of the aircraft reaches the speed of sound.

Discussion

For all aircraft in flight, the airflow around the aircraft is not exactly the same as the airspeed of the aircraft due to the airflow speeding up and slowing down to travel around the aircraft structure.

At the Critical Mach number, local airflow in some areas near the airframe reaches the speed of sound, even though the aircraft itself has an airspeed lower than Mach 1.0. This creates a weak shock wave. At speeds faster than the Critical Mach number, drag coefficient increases suddenly, causing dramatically increased drag in aircraft not designed for transonic or supersonic speeds, and changes to the airflow over the flight control surfaces lead to deterioration in control of the aircraft.

In aircraft not designed to fly at the Critical Mach number, shock waves in the flow over the wing and tailplane are sufficient to stall the wing, make control surfaces ineffective or lead to loss of control.

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