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Old 01-06-2012, 01:49 PM   #11 (permalink)
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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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Old 01-06-2012, 03:53 PM   #12 (permalink)
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Quote:
Originally Posted by drmiller100 View Post
I have thought about it - two problems. One, you need flow THROUGH the radiator to cool it. Hard to get flow through the radiator back there.

As for exiting the hot air back to the dead spot, good idea I think.
This might be only true at low speeds (20-30 mph).
At higher speeds (55 mph) enough air will flow from the ducts.
Although most driving might be made at really low speeds during rush hour, so neither front or back radiator placement will provide enough flow by themselves, you need a fan.

Quote:
Originally Posted by jamesqf View Post
This has actually been used in piston-engined aircraft, notably the P-51 Mustang. See Meredith_Effect
Thanks for providing the technical name Meredith effect. Will post more on this soon.

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.
It works the same as turbines and rockets.
Heated air has more pressure thus producing thrust.

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