I think you are missing the pressure inside the cowl Mike. Particularly
if it has cowl flaps that are closed.

Paul Lamar

Paul,

  I assume that an opening into a cowl configured as an inlet should
provide airflow into the cowl. If there is air flowing forward out of an
inlet seems to me that implies there is a significant problem with the cowl
exit that needs to be fixed. It appears from reading Chuck's description of
what he has done in testing that he's reached the same conclusion.

  The P51 pressure distribution you posted implies neutral pressure at the
cowl sides (I've seen a similar diagram for an RV6 which as I recall was
about the same) so I would think any inlet designed to gather dynamic
pressure due to aircraft movement should provide an air intake rather than
an exit. The air pressure in the cowl would have to be very high to
overcome the dynamic pressure and lead to reverse flow wouldnt it? Perhaps
whats being implied here is that a NACA inlet is actually a pretty poor way
to recover dynamic pressure?

  You are correct in that I hadnt considered the additional pressure within
the cowl if there are closed cowl flaps involved. Didnt occur to me as I
intend to use a sealed exit duct on the back side of my rad so
opening/closing cowl flaps should only affect pressure across the rad.

Mike Wills

NACA duct don't work very well in a neutral pressure area. Snorkel scoops
work much better as they get the inlet out in the high velocity area
beyond the boundary layer.

Paul Lamar
 
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