The cowl flaps cut the drag of the intake belly scoop when they are
near closed at high speed. They do this by increasing the pressure in
the whole system and  diverting the unneeded air around the high drag
opening and internal cooling system. Same as a P51. Cowl flaps
also speed up the exit air which adds a bit of thrust that counteracts
the cooling drag by a little bit.

Its got little to do with the drag of the air out let openings per se.

My guess is well designed cowl flaps would be good for around 7 MPH at
200 MPH with a well designed cooling system.

Good explanation.   I finally get it.   Jerry

 
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