Because of the changing response of air at different speeds, it is important to define the various divisions of speed so that we can anticipate the type of reaction to a particular speed.
01. Subsonic
Subsonic flow starts at zero and has its upper limit at Mcrit. Remember that Mcrit is that Mfs at which the highest local Mach number reaches unity. So the subsonic band contains no flow whatsoever that is sonic (equal to the speed of sound) or supersonic (faster than the speed of sound).
02. Transonic
Transonic flow beg ins at Mcrit (that is the Mfs that is Mcrit) and extends up to Mdet. This is the stage at which the bow shockwave attaches to the leading edge and above which all local Mach numbers are supersonic. Therefore the transonic band of Mach free streams is where there is a mixture of both subsonic and supersonic flow around the aerofoil.
03. Supersonic
When all the local Mach numbers surrounding an aerofoil are supersonic (i.e. greater than Mach 1), then the Mfs at that time is considered to be supersonic.
In general terms the subsonic band extends up to about 0.75M and the transonic band from 0.75M to about 1.2M.
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