Reverse NACA duct

"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.
--
Jim in NC

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

About all that I can remember is that the NACA (or NASA, I don't recall) exit
ducts were essentially rectangular and that the exit ramp maintained a basically
constant width and faired smoothly into the larger surface.

I believe, but am not at all certain, that the exit lip protruded slightly and
was fairly sharp, as compared to the submerged inlet lip which was rounded.

There was considerable discussion of these duct entrances and exits from the
late 1960s until the early 1980s, and numerous examples appeared on
automobiles. Generally, the ones on real factory facing cars were well
designed, and the ones on street machines intended to look fast (such as some
Mustang models) were atrocious.

I couldn't find a solid reference to the exit ports of the submerged ducts, but
considerable work (originally classified, but now unrestricted access) appears
to have been done during the period from late in WWII through the end of the
1940s; and reports are referenced in 1945, 1497, and 1948.

I am not sufficiently motivated at this time to read through even the titles,
but further information is available at: http://naca.larc.nasa.gov/ and
http://naca.larc.nasa.gov/cit.html Reports made after 1958 are in another
directory.

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.

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

"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

Corrie wrote:

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.

You should've kept reading. The original NACA references specifically say *NOT* to use NACA-style
entrances for heat exchangers (oil coolers, radiators).

Russell Kent

Russell Kent wrote:

The original NACA references specifically say *NOT* to use NACA-style
entrances for heat exchangers (oil coolers, radiators).

Do they say why?

Dave 'yellow' Hyde

The original NACA references specifically say *NOT* to use NACA-style
entrances for heat exchangers (oil coolers, radiators).

Do they say why?

Ditto. It seems to me that the air would not care what it's function was at
the time of entrance. Since there have been a few homebuilts that used it for
just this purpose - and seem to have worked - I am/was planning to do the
same.............

Dave Hyde wrote:

Russell Kent wrote:

The original NACA references specifically say *NOT* to use NACA-style
entrances for heat exchangers (oil coolers, radiators).

Do they say why?

It's my understanding that the NACA submerged duct was designed to feed a
jet engine, and as such if the velocity of the air in the duct is not a
significant fraction (like 70%) of the free airstream velocity, then the
duct "looks" like a wart on the fuselage, and the free airstream flows
around it. See NACA-ACR 5i20 at

http://naca.larc.nasa.gov/reports/1945/naca-acr-5i20/

Specifically:
The data obtained indicate that submerged entrances are most suitable for
use with internale-flow systems which diffuse the air only a small amount:
for example, those used with jet motors which have axial-flow compressors.
Where complete diffusion of the air is required, fuselage-nose or
wing-leading-edge inlets may prove to be superior.

And later (pgs. 18-19):
Submerged inlets do not appear to have desirable pressure-recovery
characteristics for use in supplying air to oil coolers, radiators, or
carburetors of conventional reciprocating engines. The required diffusion
of the air and the range of inlet-velocity ratios is too great to give
desirable characteristics at all flight conditions.

If you're determined to use submerged NACA ducts, you might study these
papers to get the best performance:

Russell Kent

Dave Hyde wrote:

Russell Kent wrote:

The original NACA references specifically say *NOT* to use NACA-style
entrances for heat exchangers (oil coolers, radiators).

Do they say why?

Submerged NACA ducts do not allow much air diffusion; they're for feeding
large quantities of air to jet engines. Radiators work best with highly
diffused air (large dynamic pressure recovery). See pgs 18-19 of:

http://naca.larc.nasa.gov/reports/1945/naca-acr-5i20/

If you're determined to have NACA submerged ducts, you might want to study
these NACA reports to get the best duct shape:

http://naca.larc.nasa.gov/reports/1948/naca-rm-a8a20/
http://naca.larc.nasa.gov/reports/1948/naca-rm-a7i30/
http://naca.larc.nasa.gov/reports/1942/naca-report-743/

Russell Kent