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• Because of the high drag associated with compressibility effects, aircraft do not cruise near Mach 1. Supersonic conditions occur for Mach numbers greater than one, 1 < M < 3 . Compressibility effects are important for supersonic aircraft, and shock waves are generated by the surface of the object.

• But it can go mach 3. Famously an Egyptian one (admittedly the recce version) legged it across Israeli airspace at a whopping 3.2, ruining the engines according to legend. The MiG-25 was the only mach 3 capable fighter (yes, yes—fighter interceptor if you’re going to be a dick about it) to enter service.

• The Mach number appears as a similarity parameter in many of the equations for compressible flows , shock waves , and expansions . When wind tunnel testing, you must closely match the Mach number between the experiment and flight conditions.

As well as being a razor, mach 3 is a speed. It’s very fast. Flying at mach 3 produces oven-like skin temperatures and requires aircraft with exotic propulsion systems, and structures wrought from unusual metals that refuse to behave as well as aluminium.

Mach number is a function of temperature and true airspeed. Aircraft flight instruments, however, operate using pressure differential to compute Mach number, not temperature. Assuming air to be an ideal gas, the formula to compute Mach number in a subsonic compressible flow is found from Bernoulli's equation for M < 1 (above):

When an aircraft exceeds Mach 1 (i. e. the sound barrier), a large pressure difference is created just in front of the aircraft. This abrupt pressure difference, called a shock wave, spreads backward and outward from the aircraft in a cone shape (a so-called Mach cone).

Because of the high drag associated with compressibility effects, aircraft do not cruise near Mach 1. Supersonic conditions occur for Mach numbers greater than one, 1 < M < 3 . Compressibility effects are important for supersonic aircraft, and shock waves are generated by the surface of the object.