Reverse NACA duct

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.

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

Russell Kent wrote:

...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.

Interesting. Intuitively that makes sense, since there's
not a lot of ram-air pressure into the inlet. Still, even
axial-flow compressors *can* generate significant back
pressure and inlet spillage, especially at 'high' speed
and low power settings. I wonder what the stall margin
is like on a NACA-inlet-fed jet.

Dave 'surge' Hyde

Inspection Sunday!

Ehh, I was just a dumb student doing the grunt-work for the "real"
engineers. They hadn't read the report, either.

They also put NACA ducts on both sides of the nacelle, with the result
that air came in the outboard duct, through the plenum, and *out* the
inboard duct without passing through the HE - the venturi effect
between the fuselage and nacelle was that strong.

So, they installed a plate in the plenum to divide it. Then, the air
would go through the outboard half of the HE - and then back up
through the inboard half and out that inboard duct again. (I did the
data reduction on the pressure data from the pressure probed in the
duct, fwiw.)

They finally did away with the inboard duct all together. Might have
made more sense to turn the HE sideways and take advantage of the
pressure drop.


Dave Hyde wrote in message ...
Russell Kent wrote:

...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.

Interesting. Intuitively that makes sense, since there's
not a lot of ram-air pressure into the inlet. Still, even
axial-flow compressors *can* generate significant back
pressure and inlet spillage, especially at 'high' speed
and low power settings. I wonder what the stall margin
is like on a NACA-inlet-fed jet.

Dave 'surge' Hyde

Inspection Sunday!

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