Ed,
when in Memphis at the rotor motor flyin a couple of years ago, i took the
attached photos. How did you seal the duct to the cowl to get a smooth
transition?
Bill Schertz
Actually, Bill, on those older ducts, the transition was not all that
smooth.  As you can see from the photo there were some gaps between the duct
and the cowl.  I simple sealed them with some weather stripping foam by
wrapping it around the duct until the thickness sealed the gap.  Clearly
that created some turbulence with the lip of the duct not being tight
against the cowling.

On my new ducts, I took some pain to more carefully match the duct entrance
to the cowling and did somewhat better.  Since I also reduce the duct intake
size, I flared the entrance so the air would have a smoother path rather
than a rather large 1/-1/2" bump.  It was fairly successful except right
next to the prop side of the cowl opening where there is still a small gap
at the inside corners.  But, it should be much better than previous.

I'll take a photo of the new ducts installed on the aircraft in the next day
or so, so you can better see what I am talking about.

Ed