This compressibility effect changes the amount of resulting force on the aircraft. The characteristics of compressible flow mainly depend on the Mach number. A compressible flow can be classified into four types, namely subsonic, transonic, supersonic and hypersonic based on the Mach number.
Subsonic flow: 0.4 < M < MCR
The air cannot be considered as incompressible anymore, compressibility effects must be taken into consideration. Therefore, aerodynamic characteristics of the flow change quite significantly.
Transonic flow: MCR < M ≤ 1.2
The region is characterised by the appearance of shock waves (wave drag is created). Part of the airflow is below sonic speed; another part is over sonic speed.
Supersonic flow: 1.2 < M ≤ 5
In this region the airflow is over sonic speed everywhere about the aircraft. The aerodynamic characteristics change again.
Hypersonic flow: 5 < M
Due to aerodynamix heating, the air is ionised.
Note: Compressibility is taken into account above M 0.4. You can remember the answer from there.
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