I was at a dinner party the other night when the topic of air travel came up. We were talking about de-icing in a commercial jet when someone said that they only do this to free up the flaps when the planes are on the ground.
He went on to say that not only are the wings of a passenger jet heated but even in the coldest of weather, when the jet is in flight, wind is passing so quickly over the wing, the friction that the air produces heats the wings to an almost HOT temperature...
I found this hard to believe... The space shuttle on re-entry - yes... but a passenger jet???
Can you shed some light on this for me?
You are right. A commercial passenger jet does not fly at a supersonic speed and any heat generates at its usual cruising speed in the region of 500 to 550 mph is not enough to deice the plane.
In the air, heating is provided by the hot bleed air to the engines cowling and leading edges of the wings.
On the ground, especially during this bad winter season, planes sitting on the ground for a while with accumulated ice on its surface are deiced by special maintenance crew to ensure flaps; wings and body structure are cleared of ice before take off.
If you want to know in details the efficacy of heat generated from skin fiction, (we call it kinetic heating), lets see how much additional heating is attributed to a increase in air speed when trying to prevent the fuel from reaching its freezing point in the Polar Region.
For instance, an increase of 0.01 Mach (around 6 knots or 7 mph) from a plane’s usual cruising speed of around 0.84 Mach will result in a temperature increase of 0.5° to 0.7°C. So it would take approximately 15 minutes to 1 hour for you to observe any noticeable changes in the temperature. The rate of cooling of the fuel is approximately 3°C per hour. A maximum of 12°C per hour is possible under the most extreme cold conditions.
So as you can see, even though skin friction does generate heat, it is insufficient to deice a plane’s wing when caught in icing condition. Yes, you need the hot engine bleed air to do the job!
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