Mach Number
If you want to compare whether the speed of the aircraft is fast or slow you need a reference. The reference we use in such a case is the speed of sound which is 343 m/s(not in all cases). But how do we exactly compare it?
For this we are gonna introduce a new quantity know as Mach Number. Mach Number is basically the ratio of the speed of the aircraft to the speed of sound. Since it is a ratio it’s a dimension less quantity. If the Mach Number is 1, we call the flight as SONIC flight. If it is less than one, it is known as SUBSONIC flight and if it is greater than 1 it is a SUPERSONIC flight. Well there is one more case too, if the Mach Number is greater than 5(quite a large number) the flight is termed as HYPERSONIC flight which is really fast. Spacecrafts usually use the Hypersonic Speed to get out of the gravitational field of the Earth. But one thing we need to be careful while using the Mach Number is the speed of sound, as it changes with change in temperature by the relation:
This forms the basics of aerodynamics.
