Note: the exhaust area requirement is greater than the
inlet combination of oil and coolant due to the now considerable hotter air
temperature. Rule of thumb: Coolant air inlet opening for 200 HP coolant
cooled engine ~60 sq. in., oil inlet opening ~30 sq. in. equals a total opening
of 90 sq. in. A good place to start with exhaust opening is 1.4 times the
inlets or 126 sq. in. Close the exit area down with cowl flaps to as
little as 80% of the inlet combination at cruise conditions!
I do
understand that an air-cooled installation is a different animal, but by the
end of the day ( or at the cowl exit...for that matter) everything is air
cooled. The interesting thing is that both run a intake/exit ratio of 78%, the
intake (I think you meant ‘exhaust) being bigger - just
about confirming the above statement!
Schmidtbauer mentiones the " rule of
thumb" - exit about 150% of inlet.
I think for our purposes, the 1.5 ratio
is more applicable than 1.28. It can be lower IF you have well shaped entrance
and exit ducting. Typically we focus on entry ducting, and then have
rapid expansion at the exit from the core – sudden expansion pressure
loss; losing all momentum and then, usually a rather sudden acceleration out a
fairly small opening. Definitely less than optimum.
I don't
think there is a great difference on the amout of cooling necessary, as the
efficiency of both engines are fairly close. So some heat goes out the exhaust
and the rest has to be cooled. For sure you need different ducting, but
the amount of heat energy should be about the same and you want to get rid of
it with the least drag, either way.
You’re right. The biggest
difference is the higher rejection temp, and larger delta T of the air on the
air-cooled engines allows for a lower air flow rate.
Al