"Morgans" wrote in message
...

"Charles Talleyrand" wrote in message
oups.com...
On Aug 7, 7:34 am, Bertie the Bunyip wrote:


The drag is induced by the cooling itself.

How can the drag be induced by the cooling itself? If I understand
you, the same cowling with the same air flow shows significantly
different drag depending on whether the engine inside is hot or cold.

I honestly don't understand that.

The air entering the engine compartment, and flowing past the running
engine's hot cooling fins expands at LEAST double.

That is why the exit opening is much larger than the intake.

That is why there have been claims that the P-51 has a positive cooling
drag, that is the heated air exiting actually gives more thrust than the
drag of air entering the radiator passage and going past the radiator.
--
Jim in NC

Double? Naah. It expands, but doesn't double. Boyle's law (PV=NRT) is
based on absolute temperature, which is measured in kelvin. Kelvin is
Celsius plus 273 degrees. Guessing now, if the air temp went from 300k to
350k (or about 80F/27C to about 160F/70C) you'd see volume increase by
(350/300 -1) = 1/6th = about 17%...

KB