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

Isn't it interesting that NACA ducts have been used for EVERYTHING except
the one use that they said they were good for?  Never saw a jet with NACA
inlets.

They do work well for cockpit ventilation inlets though.

Tracy Crook
tcrook@rotaryaviation.com
www.rotaryaviation.com

At the time the NACA papers were written 1947 little was generally
known about turbojets. Turns out turbojets start to become
ram jets in the transonic speed ranges and therein derive a 
bit of additional air compression and consequently efficiency. 
Unfortunately that is where the NACA duct begins to lose efficiency.
Furthermore it was discovered later that turbojets tended
to compressor stall if the intake was partially blocked for some reason
such as high angle of attack or yaw angle. The side mounted NACA duct
as depicted in the original work would have been very sensitive
to yaw angle.

Paul Lamar
 
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