REFLECTOR: Duct ventillation

[Al Gietzen]

I have reasoned 

that I am building up too much pressure under the cowling.   

 

I'll take the roll of contrarian on the issue.  If you have sufficient air
flow for good cooling, pressure in the cowl is a good thing.  It means the
air has some energy available to accelerate it out the back thereby reducing
drag.  Cooling drag results from the loss of speed (energy) of the air that
is taken in for cooling purposes.  If you could put that air back out at
free stream velocity (the ultimate goal) you have zero cooling drag.

 

At, say, an airspeed of 150 kts the dynamic pressure available is about 14"
H2O.  With the ram scoop I have feeding the in-cowl radiator, the rad is
cooling very effectively, and in spite of the abrupt expansion at the exit
of the rad, I have about 5-6" H20 pressure in the cowl to accelerate the air
out the exit ports.  So in theory, with the rounded shape of the ports, and
the fairing in front of the prop, the air can get back to close to 90 kts at
the exit.  Far from ideal, but in the right direction. (And, yeah; one of
these days I need to try a 'trip strip' at that rounded corner at the end of
the cowl.)

 

Block the wiring ducts at the firewall to prevent the flow of air forward.
This is a good idea anyway, because if ever there is an event of fire in the
cowl, you don't want it going up the ducts.

 

Anyway; that's my story, and I'm sticking to it:-).

 

Al

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