"Corky Scott" wrote in message
...
On Fri, 24 Oct 2003 19:08:30 -0500, Chris W wrote:

Corky Scott wrote:

Chris, what's a reverse NACA duct? My understanding is that they were
designed to work in one direction only, to bring in air with a minimum
of drag.

They don't work well, or at all, when placed on the fuselage or cowl
in a negative pressure area.

I had heard once that on some planes, a reverse or backwards NACA duct
would
be installed in the aft part of the cockpit to improve ventilation by
helping
suck air out of the plane. I also think I remember something about the
design being somewhat different.


--
Chris W

"They that can give up essential liberty
to obtain a little temporary safety
deserve neither liberty nor safety."
-- Benjamin Franklin, 1759 Historical Review of Pennsylvania


I had heard that too, so I contacted NASA about it. Spoke via E-mail
with one of the aerodynamicists. I told them I thought I understood
the principle of the NACA duct and that I'd like to use it to
facilitate removal of cabin air.

He replied that "obviously" I did NOT understand how NACA ducts
worked, or I would not be suggesting that I use one to draw the air
out of the cabin because that's not how they work.

So I meekly asked what might work, instead of a NACA duct and he
replied almost any opening would work, as long as it was placed in a
low pressure area.

You can create such a low pressure area by having a bump just in front
of the opening. The idea of a hollowed out tear drop was mentioned,
this would work. Or, just put a hole in the fuselage connected to the
exit duct aft of the cabin. That's a low pressure area as the
fuselage is narrowing to the tail at that point

Corky Scott

Designing a quiet way to extract air might not be a problem in airplane
cockpits but in sailplanes, exhausting air from the cockpit is often very
noisy. (If it's noisy, I presume it to be draggy.)

The German glider designers are careful to add air intakes but usually
completely ignore air exits - the result is lots of noise but little air
flow through the cockpit.

A really quiet, efficient, low-drag air exit design would be very welcome.
I'm following this thread with interest.

Bill Daniels