What does Mach speed mean for describing aircraft speed if the speed of sound changes with temperature?


The aerodynamics change with Mach number.

At low Mach numbers (< ~0.3) , air is considered incompressible; it has constant density along the stream line.

At higher Mach numbers, air is no longer incompressible; the density changes as the air passes over the aircraft. The compressing and expanding of air changes the way the air flows, and consequently it changes the performance of the aircraft.

At Mach numbers approaching 1, parts of the flow around the aircraft may breach the sound barrier. This leads to trans-sonic effects such a shock waves which affects the performance of the aircraft even more.

For subsonic aircraft the maximum speed is typically expressed by two numbers: the maximum calibrated airspeed and the maximum Mach number. The first is related to the dynamic pressure the flow of air creates and is typically the limiting factor at lower altitudes. The Mach number is related to trans-sonic effects and is typically the limiting factor at higher altitudes where are is colder and the speed of sound is lower.

For the pilot it thus important to know both the airspeed and the Mach number.

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    $\begingroup$ It may help to get rid of the notion that "speed of sound" has anything to do with actual sound, and think of it more as "the speed of (shock) waves in air", and I believe it will be easier to see why it is important to know how fast you are in comparison to how fast (shock) waves propagate. The sonic boom is essentially when the speed of the airplane is the same as the speed of waves, which means that the shocks the airplane creates travel at the same speed as the airplane itself, so it will create shocks on top of the shocks and thus adding up the intensity of those shocks. $\endgroup$ – Jörg W Mittag Sep 16 '19 at 21:40
  • $\begingroup$ It is easy to imagine that creating those booms / shocks wreaks all sorts of havoc with the aerodynamics of the plane, and thus knowing how fast you are relative to how fast disturbances of the air move, is crucial. As mentioned in the answer, you can have supersonic airflow over just parts of the airplane even while the airplane is still slightly sub-sonic. $\endgroup$ – Jörg W Mittag Sep 16 '19 at 21:42
  • $\begingroup$ Because the Mach number is a local property of air. For flying the properties of the surrounding air are relevant, not the properties of the standard atmosphere. $\endgroup$ – Dohn Joe Sep 17 '19 at 11:06
  • $\begingroup$ Mach number is also used by ATC to apply separation standards between aircraft. If both aircraft are flying at a constant Mach speed, their separation will remain constant. $\endgroup$ – Mike Sowsun Sep 17 '19 at 19:24

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