REFLECTOR: Fw: Cooling
Velocity Aircraft Owners and Builders list
reflector at tvbf.org
Wed Jun 25 09:36:19 CDT 2014
Hi again -
Larry, a couple of thoughts (after reading the article). It's a good
article, but it's really focused on maintaining the energy of the incoming
airstream for a jet engine. What we are focused on here isn't that energy
- we're focused on pressure recovery at minimum drag. The article states
that ." Submerged inlets were unsuitable for "oil coolers, radiators, or
carburetors of ... reciprocating engines," the report continued, because
"the required diffusion of the air and the range of inlet-velocity ratios
is too great to give desirable characteristics at all flight conditions.",
and I have no doubt that that is true.
What it's not saying, however, is that a system including the NACAs,
flowing into a diffuser created by ducting that is smooth, straight, and
expanding, and then flowing into a plenum, has more or less drag than an
exposed scoop. Remember, it's the system we've got to look at, not just
the scoop itself. And that we're focused on both overall cooling capacity
and drag minimization.
Secondly, the author states "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". That's just plain wrong. Go try
to find a Long-ez or a Cozy that has a male scoop. There are a few out
there, but the NACA cooling has been standard since my Long-ez was built
in the early 80's. They work great, and have much better performance
because the drag is inherently less and you're not disturbing the airflow
to the prop to the same degree. The key, again, with those airplanes is
that the NACA dumps into an expansion duct that does pressure recovery.
Terry Schubert has done some really great work on cooling and cooling drag
- take a look at the Central States catalog to find good articles.
At the end of the day, what we are all looking for is sufficient cooling
capacity at the lowest possible drag, over all flight regimes. NACAs and
scoops can both do that (as I've seen with my Velocity - they both can
work). A properly installed NACA, however, creates less drag.
My two cents... Great discussion - Thanks!
Dave
Dave T. Nelson
T/L 553-4327, Voice 507-253-4327, Fax 507-253-3648
Program Director, ISC ECAT NPI & Test Engineering
----- Forwarded by Dave T Nelson/Rochester/IBM on 06/25/2014 09:16 AM
-----
From: Dave T Nelson/Rochester/IBM
To: reflector at tvbf.org,
Date: 06/25/2014 06:46 AM
Subject: Re: Cooling
Larry says:
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
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
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.tvbf.org/mailman/private/reflector/attachments/20140625/172462b0/attachment-0001.html>
More information about the Reflector
mailing list