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