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Transonic speed is:
  • A
    a speed at which locally an oblique shock wave has developed in the flow along the aeroplane.
  • B
    a speed at which locally around the aeroplane both supersonic and subsonic speeds exist.
  • C
    the speed range between MCRIT and MMO.
  • D
    a speed at which compressibility effects are first noticeable.

Refer to figure.
Figure 1 shows the flow speed ranges with their approximate Mach number values, where: MFS (Free Stream Mach number) is the Mach number of the flow sufficiently remote from an aircraft to be unaffected by it. (In effect, the Mach number of the aircraft through the air). This is the Mach number shown on the aircraft Mach meter.

ML (Local Mach number) is the actual speed of the flow over a surface. When an aircraft flies at a certain MFS the flow over it is accelerated in some places and slowed down in others.

The transonic speed region is located between MCRIT and MDET (Detachment Mach number). In this region, ML is both subsonic and supersonic.

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