Ernest Christley wrote

Sid, does the root airfoil of your propeller compare well with what the
factory has?

Don't think there is a "factory standard" (?) ... mine is an MT
(electric) constant-speed, "made for pushers".


Air will flow with any pressure gradient. The factory design looks to
me like it is trying to accelerate the boundary layer along the back
half of the fuselage by having it 'sucked' through by the propeller. Of
course this will only work if the root of your propeller actually sucks.
Some blades are designed to no do anything at the root (Ivo is one, I
believe), and won't give you the pressure gradient you need.

Yes, I have wondered about the possibility that maybe near the hub the
blades might be churning the air so much that there is some slight
back-pressure trying to push the cooling exit air back (forward) into
the cowling exit holes. I had taped some tufts just inside the exit
hole in the left side but none of the photos were able to see this
region.

At this point, I think a test by temporarily fastening some scoops to
the tops of the existing NACA ducts (as Corky posted above) is
relatively easy to do and may disclose a lot.

Sid