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I have a chart from a technical paper describing the preferred shape for the
NACA duct.  The lip of the NACA duct should not be sharp, but should have a
rounded cross section somewhat like a semi-ellipse with a leading edge radius
that is 9.4% of the duct depth at the edge location.  The total thickness of
the lip is 48% of the duct duct height (seems a lot but true).  

For the example presented, the inlet is 1 inch high by 4 inches wide under
the lip, and the lip is 0.48 thick so that the floor of the duct is 1.48
below the outer fuselage surface.  You seldom see such thick lips at Oshkosh,
but then most of these ducts do not comply with the recommended guidelines.  

I sanded a piece of foam to the duct width, made it 1/8 inch thick, radiused
the leading edge, and then tapered it to zero thickness about 2 inches aft of
the leading edge.  I wrapped this with light weight fiber glass, trimmed and
sanded, and then bonded into the inside of the NACA duct to make the lip
which I then blended in with micro.  It strengthens the vertical wall,
improved pressure recovery, and I believe probably reduces the noise
significantly.  

Fred
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