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Old 01-06-2012, 01:49 PM   #11 (permalink)
mort
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Quote:
Originally Posted by winkosmosis View Post
I don't get how this works. Heating causes air to expand, but why would that expansion force be applied to the rear any more than to the front of the radiator? It should resist airflow from the front just as much as it pushes air out the back.
Hi winkosmosis,
An efficient radiator presents a huge drag. So in flight the whole radiator duct system builds a high pressure in front of the intake, and the addition of heat into the lower pressure volume behind the radiator doesn't raise the pressure there enough to back up through the radiator. The path out is through a De Laval nozzle which converts the added heat into thrust. An aircraft engine of this type may need between 7% and 10% of the shaft hp removed as heat from the cooling system. For an engine producing 1500 hp that's like 100 hp through that nozzle. And maybe as high as 60% efficient conversion to thrust. As Arragonis said, the added thrust is about the same magnitude as the added drag. So you get the cooling for free.
-mort

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