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Mach Number

Mach number is the speed of an object moving through the air divided by the speed of sound in those current conditions. The speed of sound is directly related to temperature and density so it is constantly varying. The accepted speed of sound at sea level and standard atmospheric conditions (ISA) is 661.2 knots or 761.4 mph. If an aircraft were moving at the speed Mach 1 at sea level on a standard atmospheric day then it would be flying at 661.2 knots. Mach speed is represented by the equation:

Mach number

where:

  • M = is the Mach number
  • Vs = is the velocity of the source (the object relative to the medium)
  • u = is the velocity of sound in the medium

However, before you can calculate your Mach number you have to know the speed of sound in the medium you are traveling. In this case you would need to know the speed of sound through the air you are currently traveling through. You would find this with the help of the equation:

Mach number

where:

Mach number

There are five different classifications of Mach number with reference to speed. They are:

Subsonic: Less then Mach 1

Sonic: Mach 1

Transonic: Mach 0.8 – Mach 1.2

Supersonic: Mach 1.2 – Mach 5

Hypersonic: Greater then Mach 5

Mach number

Most airliners currently travel around anywhere from Mach 0.6 – Mach 0.9. Military jets of course can travel in some cases at hypersonic speeds. When you do travel greater then Mach one you break the sound barrier and the air around the plane starts to form shock waves. A shock wave is an abrupt pressure difference caused by the compression of air by the aircraft moving forward. The shock wave creates a sonic boom and that is the sound you hear following an aircraft moving at sonic speeds. Depending on the type of aircraft and/or airfoil design certain kinds shock waves can be hazardous and/or beneficial to the aircraft so the speed in which the plane can travel becomes limited.

Explore More Aviation Terminology

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