"Kyle Boatright" wrote in
:


"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


It's certainly standard practice, at least on airplanes where min
cooling drag is desired, that the outlet be conderably larger than the
inlet...


Bertie