Critical Mach number

Airliners cruise at speeds just below the speed of sound. An important speed which the aircraft flies through just before reaching cruising speed occurs when the air flowing over the wing reaches the speed of sound. It is known as the critical Mach number (Mcr). Further increase in aircraft speed to the cruise value produces a region of supersonic flow which ends in a shock wave. Beyond the cruise speed, at the drag-divergence Mach number, the shock wave, and any attendant separated flow, produces too much drag for economical flight.

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