The Speed of Sound

 

The speed of sound - referred to as "mach 1.0" is used as a reference for how fast a speedy airplane is moving. But it's a bit of a slippery metric, in terms of the aircraft’s movement through the air (true airspeed).

 

Oddly, the speed of sound in our atmosphere is solely a function of the temperature – and not anything else.

 

It is calculated very simply:

 

mach 1.0 (in knots) = 39 x square_root(temperature in Kelvin)

 

which is ridiculously simple. Kelvin is just Celcius with a bias of +273.

 

Examples:

 

+15C: 39 x sqrt(15+273)   = 662 KTAS or 761 mph

+40C: 39 x sqrt(40+273)   = 690 KTAS or 793 mph

    0C: 39 x sqrt(273)          = 644 KTAS or 741 mph

-20C: 39 x sqrt(-20 + 273) = 620 KTAS or 713 mph

-40C: 39 x sqrt(-40 + 273) = 595 KTAS or 685 mph

-60C: 39 x sqrt(-60 + 273) = 569 KTAS or 654 mph

 

Mach 1.0 can vary by 150 mph depending upon whether it is hot or cold out!

 

--

acboyd@gmail.com

Dec 2014