REFLECTOR: Fw: Cooling

Wed Jun 25 10:56:44 CDT 2014

Dave,

My CHT's run about the same as yours which would indicate that both can be made to work.  The article also mentions that on ram type inlets that they need a proper lip at the entrance to work properly.  The kit supplied scoop has the same lips as found on a chicken.  That is why I modified my scoops which I believe improved pressure recovery and reduced drag.  I can top out at 180 kt TAS so the drag can't be too bad.

Larry

From: Velocity Aircraft Owners and Builders list 
Sent: Wednesday, June 25, 2014 9:36 AM
To: reflector at tvbf.org 
Subject: REFLECTOR: Fw: Cooling

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 

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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 

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