Perry Mick wrote:

  NACAs don't even work that well with aircooled engines.

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

Subject: COZY: NACA
Date: Sat, 12 Oct 2002 13:30:21 -0400
From: "Ron Larock" <rlarock@attbi.com>
Reply-To: "Ron Larock" <rlarock@attbi.com>
To: "Cozy Mailing List" <cozy_builders@canard.com>

Dear Canardians,

It seems everyone I talk to have some kind of problem with the NACA

scoop.

I am researching downdraft cooling similar to George Shell's or arm pit
scoops.

Mike Melville went to arm pit scoops and hasn't had cooling problems

since,

or at least that is what I have read. The downdraft cooling from what I
understand is easier to baffle and cools the way the engine was designed

to

cool.

Arm pit or downdraft gets the cooling air away from the speed brake and
EGT's or CHT's shouldn't be affected by speed brake deployment.

The Berkut uses armpit scoops and cools a larger engine without

problems. I

for one will not be using a NACA scoop. I still have alot of research

before

I decide, but I will use armpit or downdraft cooling.

Ron Larock

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
 
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