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
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!