Originally Posted by Jim Frisbie
by enlarging the inlet are you also enlarging the exit area?
Most of the RV-s that have cooling problems do so because the exit
area of the cowl is smaller than the inlet, creating back pressure.
The appropriate ratio I believe is 1.17 to one in favor of the exit.
RV-9A, 450+ hrs.
Well........shrinking my exit to create more pressure will be part 2.
I'm hoping the larger lower velocity round inlets in addition to the extra sealing of said inlets and the plenum will keep CHT's in the low 400's on a heat soaked takeoff at 90* and a full power 110 knot climb to altitude. At cruise I should have more pressure differential than needed to cool the engine, I'll need to shrink the exit area to create more pressure in the lower cowling to squirt the air out the exit with less cooling drag.
I measured my original inlet area at roughly 43 sq inches, 43 x 1.17 = 50.31 sq I inches. My original exit subtracting the exhaust pipe area was about 49 sq inches, I nibbled a bit off the end and made it 52 sq inches, both are in the ballpark of your ratio. I don't think cutting a larger opening would yield much of a result as my lower cowling pressures are already quite low.
After reading (several times and still not exactly understanding) NASA CR 3405, AIAA80-1242R I've gleaned these summaries and I'm hoping to gain from:
-Lower Vi/Vo (larger) more forgiving inlets, drag penalty from lower Vi/Vo is very small
-Annular inlets are more forgiving at higher Angles of Attack
-Better diffuser shape aft of the inlets
-Moving the inlets outboard and closer to the prop
-Better sealing with a plenum vs baffle material
-Better sealing of the inlets to the cowl/plenum
-Easier cowling removal and installation
But then again I'm an amateur learning by doing (messing up)!