Paul C wrote:
I was thinking of installing two small NACA
ducts on the bottom of my cowling with two hoses coming up to feed air to my
TWM throttlebody. The NACA ducts I purchased from Van's RV (made for
cowling ventilation) already have a male outlet already molded in the
assembly, so you just slide a hose over it and secure with a
clamp
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Paul;
It would be better
to have a single inlet if it can work to minimize flow losses. The inlet
area should be at least 4-5 sq in, and then that area should expand to about
to maybe 4" diameter before the TB entrance for pressure recovery. A
NACA scoop near the back of the plane isn’t going to get you much in the way
of ram effect; but even the best ram scoop is only going to get you about 0.3
– 0.4 psi boost at cruise. Just getting some unencumbered cool air in
there is going to help.
Induction scoops
are a bit tricky because when you start your takeoff roll the inflow is
completely dependant upon being "sucked" in, and you'd like to have minimum
entrance loss. At high speed you want a scoop design that maximizes the
ram pressure, with minimum drag. The two conditions are at
odds.
The best
compromise is a ram scoop with a well rounded entrance lip; about a finger
radius. Attached photo is my shot at a ram scoop.
Al