If you check Velocity Aircraft's site, you will find that they, too have
gone to downdraft cooling....however, it is with two NACA scoops into a
fiberglass plenum which wraps around the Lycoming cylinders. I have a friend
who has an older Velocity, and converted from the updraft to the newer NACA
downdraft system, and that cured all of his cooling problems.  Just for
information only.  Paul Conner

As I said over and over again NACA ducts are not recommended to feed
radiators. Just because an air cooled engine is successfully cooled by
a NACA duct does not mean it will necessarily work with a radiator.
There is a very big difference in the flow restriction between an
air cooled cylinder and a radiator. A glance at the cylinder fin
spacing will convince anyone with an ounce of common sense that this
is true. Unlike air cooled engines radiators require a high degree
of diffusion. (Conversion of air velocity to air pressure.) Also
unlike air cooled engines the velocity ratio of most radiators
is too high for NACA ducts to work. Respect the difference.

The NACA engineers that invented these things did not just fall off the
aerodynamic turnip truck. They know what they are talking about so please
believe them.

Surprisingly some aircraft manufactures make the same mistakes and
pay the penalty. The Extra 400 at Copper State is a good example
of the mis-application of a NACA ducts to feed heat exchangers.

You can just make out the NACA duct on the right side feeding an
intercooler rad. There is another one on the other side also feeding
an intercooler rad. At least they got the outlets right.

Not all aircraft designers are completely informed when it comes
to NACA ducts. These things have been around for fifty years.
People take them for granted and assume they work for everything.
They don't!

Paul Lamar

I agree Paul.  Like I've said before, the only reason it works on mine
is big
suction from behind.  Eventually I am going to replace the NACA with a
scoop similar to what Spencer's V8 EZ has.
--

Perry Mick
Duckt N7XR
http://www.ductedfan.com

Here are some pictures of Gary Spencer's slick 236 MPH airplane.
The inlet is near perfect but the rad is a bit too close
to the oil pan IMHO. Gary would do well to change it over to a bottom
outlet like Perry, Mike and Jerry are using. As it is now Gary cannot 
apply full throttle unless the OAT is less than 70F without
the engine over heating. The engine would get up to 240
degrees F and at about 200 hours the first set of exhaust valves
went away. Gary now has a strict limit of about 210 degrees F
for the last 120 hours in the hopes that will make the exhaust
valves live. He is also using a thermostat and I have him convinced
(I think) to switch over to cowl flaps :-)

Paul Lamar
 
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