Ram Air Temperature Rise

As an airplane travels faster, it will start to heat up it’s leading edges and other bits and pieces that are hung out in the breeze.  This may be counter-intuitive to you, if you are familiar with “wind chill”.

Ram air temperature rise is actually due to adiabatic compression (and not friction) at the speeds you are likely to be travelling.  Basically, when you fly along, you are compressing a large mass of air at the leading edges of the aircraft.  This is not unlike pumping up a tire – ever notice the valve gets hot? – and is due to the fact that all of the energy of the air is now concentrated in a small place.

This heating is called “Ram Air Temperature Rise” and is calculated (due to adiabatic compression) as follows:

Ram Air Temp Rise (deg C) = velocity(knots true) squared / 87 squared

Examples:

100 knots: 100^2   / 87^2 =      +1C

150 knots: 150^2   / 87^2 =      +3C

200 knots: 200^2   / 87^2 =      +5C

250 knots: 250^2   / 87^2 =      +8C

300 knots: 300^2   / 87^2 =    +12C

350 knots: 350^2   / 87^2 =    +16C

400 knots: 400^2   / 87^2 =    +21C

500 knots: 500^2   / 87^2 =    +33C

600 knots: 600^2   / 87^2 =    +48C

1000 knots: 1000^2 / 87^2 =  +132C

Basically, if you keep the speed up over 350 knots, you won’t have a problem with airframe icing!

--

acboyd@gmail.com

Dec 2014