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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