Back in the early 80's I was a co-op student (read,
engineer-in-training) at Fairchild-Swearingen. The group to which I
was assigned was studying the flow through the oil cooler for a PT-6
version of the Metro / Merlin line. They were using NACA-shaped ducts
on the sides of the nacelles to get the air into the plenum to cool
the oil.

It wasn't working, until I dug down into the original NACA references
to discover that the duct lip had a bump - not just a plain radius.

Sort of like this:


slipstream -------------

__________ ____________ surface _____
\ /
_________ | /
\__/ /
/
/
/
/
/
_________________


The lip helps create a low-pressure area inside the duct opening. I
don't think it would work in reverse, except perhaps by blind chance
(sort of like the Davis wing - it happened to be a high-aspect-ratio,
low-drag airfoil. Davis's elaborate equations tunred out to be so
much mathematical hogwash).

My opinion: You want a positive-flow outlet, poke a hole in an
existing low-pressure zone.

"Morgans" wrote in message ...
"R&R Sherwood" wrote in message
...
Reverse NACA duct ... Bad Idea!
Several years ago I read about someone installing a NACA scoop, pointed
end
aft, to remove air from the cabin. I thought I would do the same for my
plane but first decided to test the idea. I built a NACA scoop and ran
high speed water through it in both forward and reverse directions. In
forward the water flowed as expected. In reverse the water exited at
nearly 90 degrees to the slip stream.
I believe a NACA scoop will just add drag, especially to fast
planes....Better to just use a ramped exit.

Russell Sherwood

I also remember someone else coming to that conclusion.

Try the "bump" or an adjustable "cowl type" flap.