Mach cutoff is a phenomenon of high-altitude

supersonic flight A supersonic aircraft is an aircraft capable of supersonic flight, that is, flying faster than the speed of sound (Mach 1). Supersonic aircraft were developed in the second half of the twentieth century. Supersonic aircraft have been used for r ...

in which the

sonic boom A sonic boom is a sound associated with shock waves created when an object travels through the air faster than the speed of sound. Sonic booms generate enormous amounts of sound energy, sounding similar to an explosion or a thunderclap to ...

generated at speeds not too far above

Mach The Mach number (M or Ma), often only Mach, (; ) is a dimensionless quantity in fluid dynamics representing the ratio of flow velocity past a Boundary (thermodynamic), boundary to the local speed of sound. It is named after the Austrian physi ...

1 never reaches the ground.

Details

Flight is supersonic when the aircraft speed exceeds the speed of sound in the immediately surrounding air. A side-effect is the creation of a sonic shock-wave, heard on the ground in the vicinity as a loud and disturbing bang, known as a sonic boom. For this reason supersonic flight is generally limited to travel over oceans and large seas, and to high altitude. But there is a temperature gradient between the ground (generally a few degrees Celsius) and flight altitude (generally several tens of degrees below zero). This causes the sound waves travelling downwards to diffract towards the horizontal. Accordingly, so long as the flight path is high enough and aircraft speed is not too far above Mach 1, this diffraction causes the sonic boom instead to be heard as subsonic sound; it is called Mach cutoff. Calculating this varies for any flight, as the required diffraction depends on temperature gradient, air pressure gradient and differing wind speeds in the air column.

References

{{aviation-stub Aerodynamics Airspeed Fluid dynamics

Details

See also

References