Reverse NACA duct

Sorry for the duplicate posts. Because of an error message, I thought the
first one had disappeared into thin air. :-)

Russell Kent

Russell Kent wrote:

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

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!