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At the risk of invoking PL's name,
anyone else read this months Sport Aviation mag from EAA, and notice an article
on cooling that seems to indicate that NACA's are acceptable and adequate for
aircraft cooling needs? I have no idea regarding the authors credentials, and I
no longer monitor PL's "newsletter".. I was curious more than anything else...
Pauls reaction, others reactions, etc.
Translation.. yes.. I'm stirring
the pot/Trolling... I figure if we are using NACA's on the Velocity, that makes
us somewhat of a NACA supporter..
Dave,
ZZZZZZZZZIIIIIIINNNNNNGGGGGGGG Hit the drag and set the hook!!! I took
the bait.
NACA submerged
inlets will work just fine. Some people have a favorite quote from the
very preliminary first report (NACA-ACR-5I20,
14NOV1945) on these inlets:
"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."
Now if you read nothing else on this subject and
never did any actual building or testing and totally ignored all the
instances where these inlets work just fine you would conclude that they would
never work. You would, however, be totally, completely and utterly
wrong.
Some people fail to cite their sources or
subsequent papers and prefer to copyright the work of others while twisting the
facts to fit their (very wrong) view of the world.
The later and much more thorough investigation on
these inlets was released 13JAN48 (NACA-RM-A7I30)Two of the original
authors wrote this paper as a continuation of the work.
This paper is very good and highly recommended. It
actually gives the design variables and relevant data required to properly
design an efficient, low drag flush inlet.
Quoting directly from this paper on page 18 under
the heading:
Possible Applications For NACA Submerged
Inlets
"Other applications could include some ducting
systems involving cooling and carburetor air. If this type of entrance could be
substituted for the protruding scoop-type of inlet, the aerodynamic neatness of
the aircraft would be greatly enhanced."
There are several gotchas with a flush inlet.
1.) you must not have a thick boundary
layer RELATIVE TO THE SIZE OF THE SCOOP.
2.) you must have the appropriate mass flow ratio
and pressure recovery ratio chosen for your application. This is difficult to do
over a large range with radiators. If you insist on making the cooling system
design with a Mack truck sized radiator using design criteria suitable for fully
laden WWII bombers operating out of North Africa in the middle of summer at Vy,
you will be unable to come up with a flush inlet design that will
work. However, if you design for cruise and our application, you will be able to
make a very nice low drag installation.
3.) You need an appropriate diffuser after
the inlet. This is one problem I have with this paper. They used a very long,
gradual diffuser. Which is probably not a good idea with the flush inlet. The
two vortices make for a very turbulent flow field. This means you will
have a lot of losses from mixing in a long diffuser duct. A short multi
segment 7 deg type or perhaps a shorter K&W streamline diffuser will
probably be a lot better. It's hard to say what the effect of the turbulence on
the streamline diffuser would be. It would take some testing to find out
whether the guide vanes in the shorter 7 deg diffuser would be better or
worse than the K&W streamline type. I do think shorter is better. The
turbulence should help to minimize seperation. Trying to stretch out the process
is just going to make a lot of losses.
I would suggest that the combination of a thick
boundary layer and a less than optimal diffuser make this study CONSERVATIVE....
The relative results are still useful, however.
4.) the lip must be a very thick airfoil shape.
Most people totally screw the pooch here. They also make the ducts way too short
and way too steep. Then they put little bitty inlets in a thick
boundary layer, or a low pressure region.
Guess what? if you do all these things that the
paper says not to do-it won't work. Some people take that to mean they will
never work, ever, anywhere, period.
Data never lies, but liars always use data, and
a few quotes out of context for good measure.
Monty
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