REFLECTOR: Cooling

[Velocity Aircraft Owners and Builders list]

Larry says:

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  I use modified armpit scoops.  They are mounted 3/4" out from the 
fuselage and have an internal venturi for improved pressure recovery.  I 
have included a clip from a Flying Magazine article that explains the 
popularity of misapplication of NACA inlets.

  Larry Coen

  N136LC



  North American's bad experience-and those of others, such as Long-EZ 
builders who tried to use them for engine cooling air-did not put paid to 
the NACA scoop. Today we see them everywhere, and they are widely used in 
precisely the applications-engine induction and cooling air inlets, oil 
radiator inlets, fuel tank vents, cabin ventilators, all sorts of 
mysterious apertures on Firebirds and Lamborghinis-for which the original 
researchers deemed them unsuitable.

  Why? Different designers probably have different motives. Many 
applications require peak performance at only a narrow range of flow rates 
anyway, and so efficiency losses at off-design points can be tolerated. 
Even though they are not really drag-free, submerged inlets certainly 
produce less drag than protruding ones do. They have practical advantages; 
one is that they're easy to fabricate-much easier than a pitot-style scoop 
with a boundary-layer channel. NACA scoops are popular with amateur 
builders, some of whom are probably in the dark about the mechanism of 
their operation, but assume that if they're used on some airplanes, they 
must be good for all.

  Other reasons are more frivolous. NACA scoops look nice. They don't 
appear to clutter up the surface of the airplane the way protruding inlets 
would. They at least seem as if they produce no drag. And that 
double-ogive with its gently sinking floor is an elegant shape that just 
feels aerodynamic in an undefined sort of way. Looks matter. When reason 
fails, follow your heart.


  Read more at 
http://www.flyingmag.com/scoop-naca-scoop#2DDXHD907wXdp5Bi.99

  I use modified armpit scoops.  They are mounted 3/4" out from the 
fuselage and have an internal venturi for improved pressure recovery.  I 
have included a clip from a Flying Magazine article that explains the 
popularity of misapplication of NACA inlets.

  Larry Coen

  N136LC

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Hi Larry - 

All I can do is to tell you of my experience with the roof mounted NACAs. 
I will also (as soon as I can figure out how) post some oil study pictures 
to the VOBA forum. 

For the first 600 or so hours with my STD-RG I had armpits, and I did get 
them to work ... acceptably... if you consider CHTs around 400 - 415 
acceptable.  Careful management in a climb would keep the CHTs below red 
line... barely.  That seemed good enough, even though I knew I should be 
working on better cooling. 

I've written about my cooling misadventures before, but in net what got me 
working on a better solution had to do with oil cooling issues, not CHTs. 
At the time I was working on cooling drag reduction on my airplane (I was 
spending a lot of fun time racing with my hangarmate in his really nice 
Cozy 3 place).  Every drag reduction I did caused my oil temps to get 
hotter... eventually, I couldn't control them.  Along the way I tried a 
whole bunch of different cooling setups. Nothing worked.  My oil temp just 
got worse.  It took close to two years to nail it - it turned out to be 
three bad cylinders.  It's a long story, and looking back at it, it sure 
seems obvious that it had to be an engine problem, but at the time I had 
been working on cooling drag reduction enough to mask the changes in my 
engine.  Anyway, long (very long) story short, I put the roof mounted 
NACAs in.

Now... it is important to understand how a NACA works in order to make a 
NACA work.  In particular, getting the inlet ramp right is really 
critical.  Square edges along the sides are critical.  There are a bunch 
of things like that that I see people mess up in NACA installations that 
preclude them working.  However, if you get them right, they do work, and 
they work really well.

My coldest CHT now is around 220 - 240, and my hottest is right around 
300.  It's arguably too cool... And it's because of the NACAs. 

I'll try to figure out how to post some pictures today.

Thanks again for the article & for your input!

Dave
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