REFLECTOR: VGs on cooling air inlet

Mon Aug 11 14:40:58 CDT 2014

I am trying to get more cooling air to my engine.  I have NACA scoops on top.  I have tried scoops on top (ugly and draggy)  and louvers (on the bottom cowl for extraction) and have taken a lot of data.  I will summarize my findings when I am done.  I am doing well with VGs in front of the NACA get air into them.  Still trying different numbers of them and locations.  Any ideas or success?

Photos of 444DX at https://www.davidullman.com/VelocityPhotos.php

David Ullman
N444DX
President EAA 292
541-760-2338
david at davidullman.com

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Subject: Reflector Digest, Vol 111, Issue 41

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Today's Topics:

   1. Re:  Cooling (Velocity Aircraft Owners and Builders list)

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Message: 1
Date: Wed, 25 Jun 2014 11:37:57 -0500
From: Velocity Aircraft Owners and Builders list <reflector at tvbf.org>
To: reflector at tvbf.org
Subject: Re: REFLECTOR: Cooling
Message-ID: <mailman.701.1403673230.2596.reflector at tvbf.org>
Content-Type: text/plain; charset="utf-8"

Hi again...

Larry says:
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
"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."

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

Very interesting... my STD-RG tops out around 186-188 kts TAS flat out and all in.  Now, those numbers are subject to all kinds of errors and variations between our two planes, but it'd sure be interesting if they were truly apples to apples...! 

Thanks - fun conversation.

Dave

Dave T. Nelson
T/L 553-4327, Voice 507-253-4327, Fax 507-253-3648 Program Director, ISC ECAT NPI & Test Engineering

To:
<reflector at tvbf.org>
Subject:
Re: REFLECTOR: Fw: Cooling
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
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