Subsonic flight refers to the air speed range where all the airflow over the aircraft is less than Mach 1.
Bearing in mind that air increases in velocity over curved surfaces, Mach 1 air velocities can occur on some parts of an aircraft before it reaches Mach 1.
As a result, the subsonic flight range is limited to about Mach 0.7.
As soon as air velocity reaches Mach 1 on any curved part of the aircraft a shock wave will form at that point.
There is now a mixture of subsonic and Mach 1 flow over the aircraft, the speed range is referred to as being transonic.
Due to the behavior of air flowing through shock waves this mixture of subsonic and supersonic air flows remains until the aircraft reaches around Mach 1.4.
The transonic speed range is between approximately Mach 0.7 and Mach 1.4.
Above Mach 1.4 the air flow over the aircraft is usually fully supersonic and this is called the supersonic range.
This airspeed range exists up to around Mach 5 beyond which the term hypersonic then applies.
NORMAL SHOCK WAVE
When a shock wave appears at a point of Mach 1 air flow it is described as being normal if it appears at right angles to direction of the air flow causing it.
Air passing through a normal shock wave will be Mach 1 or above on entry but instantly decelerates to below Mach 1 on exit.
As the shock wave is only about one ten thousandth of an inch thick, the change of energy from kinetic to pressure and heat energy is quite dramatic to take place in such a small area.
At a higher Mach number this can lead to severe turbulence and an increase in air temperature behind the wave.
OBLIQUE SHOCK WAVE
If a shock wave makes contact with a sharp edge it will bend backwards forming a wave that is at an angle to the airflow causing it, the airflow will decelerate through the wave, but
remains above Mach 1 on exit – this is referred to as an oblique shockwave.