What are different speed types of flow used in identifying Mach number?

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Mach number is the ratio of the velocity of the object with respect to the velocity of sound. Mach number is represented by "M". The formula for M is v/c, where v is the velocity of the object and c is the speed of sound which is approximately 330 m/s. The speed of sound varies due to a number of factors like temperature, altitude etc. thus depending on the surrounding conditions we estimate the value of "c".

  • Mach 1 implies that the body is travelling at a speed equal to the speed of sound. If Mach number < 1 it implies the body travels with speed less than sound. For example, Mach 0.40 implies the body is traveling at 40% of the speed of sound and Mach 2 would mean the object travels with a speed twice that of the speed of sound.

  • With the help of Mach number, various regimes are classified. These regimes have different physical and thermodynamic properties due to their difference in speeds.

  • This classification serves as a basis for many estimations and calculations in aerodynamics to determine the mission requirements or to test new designs.

The various regimes are


  • This regime includes all the lowest Mach nos. M< 0.8 comes under subsonic. Commercial aircraft and propeller driven smaller aircraft fly in this regime.

  • These aircraft have a high aspect ratio and round noses. There is less turbulence and no shock waves are produced at subsonic speeds.


  • In this range, the Mach number varies from 0.8- 1.2. It is called transonic because of the airflow over various parts of an aircraft shifts between subsonic and supersonic.

  • Swept wings are mostly used in transonic flights. Weak shock waves are formed in this regime.


  • The supersonic speed regime is that range of velocities inside which the air current over an airplane is more than Mach 1.

  • In any case, free stream meeting the leading edges is at first decelerated, so that it is marginally more than Mach 1 to guarantee that the flow over the aircraft is supersonic.

  • It is normally acknowledged that the supersonic speed extends begins at a speed more than Mach 1.3.

  • Aircraft intended to fly at supersonic velocities demonstrate huge contrasts in their aerodynamics due to the radical contrasts in the behavior of flow above Mach 1. Sharp edges, Pointy nose, thin aerofoils etc. are such changes.


  • Objects moving at speeds greater than Mach 5 come under the hypersonic regime.

  • Strong and less distant shock waves are produced. Due to high viscous forces, the temperature in this regime is very high and chemical properties change due to excitation or ionization of air molecules.

  • The X-15, at Mach 6.72 is one of the fastest manned aircraft.

High Hypersonic

  • The Mach no. ranges between 10-25. NASA X-43 is one of the fastest aircraft with Mach no. reaching 9.6.

  • At this speeds, thermal protection is very important as the body heats up so much that the atmospheric oxygen starts reacting with the surface of the aircraft.

Reentry speeds

  • Reentry speeds are those speeds with which the objects enter the atmosphere from outer space.

  • These speeds are greater than Mach 25. Ablative heat shields are used which lifts off the hot shock regions from the surface and protecting it from being damaged.

Updated on 26-Jun-2020 09:19:41