REFLECTOR: VGs to aid in cooling

Wed Oct 1 22:14:59 CDT 2014

I have received many notes in response to my posting about VGs for cooling.  Thanks all.  Please, if you try something, take data before hand and afterward so we can learn some more.  If you hasve a Dynon or similar you can get a data dump and send it to me.  One note I got is worth sharing.  The note is from Bob Jackson and I hope he does not mind me sharing part of it.  After the note, see my response.

Bob's note in part:

In re-reading your VG write-up another time, I'm still a little confused by this diagram (the one that showed the pressures around the VGs taken from the 1961 reference) .  If I'm interpreting it right, it showing the effects of four VGs, arranged as two sets of counter rotating pairs (and there are assumed to be more pairs on each side that aren't shown).  In your case, the centers of the two pairs are 20" apart (D = 20), and the VG of each pair are 5" apart (d = 5), with each VG being 2" high (at it's tallest end?) and 5" long.  And yours are arranged with each pair centered on one of the NACA scoops, as you show in the photo.

It appears to me from the diagram that the 'sweet spot' (where the boundary layer has been reduced the most) is halfway in between the pairs of VGs (where A1 and A2 show the 0.95 isobar depressed well below the tops of the VGs and the free standing boundary layer height), NOT directly behind either set of VGs (where B1 and B2 show the high velocity air now well above the tops of the VGs.  Halfway in between the VG pairs on our Velocity's lines up on the cabin air scoop, not on the engine air NACAs if I'm seeing it correctly.

In your write-up you say this diagram doesn't show what's really happening very well, and I agree -- at least I'm confused!  I'd appreciate a few more words explaining what I'm missing in my interpretation above when you have the chance.

My response:

I too am a little confused by the diagram.  I thought I understood its implications when I put on the VGs but then when I was writing up the results I realized there was much I didn’t understand.  I agree that it shows the best pressure situation between the sets of VGs rather than behind them.  Do note that this diagram is for a flat plate.   However, what is not shown is the direction of the air which is downward between the pairs of VGs.  I could not find a diagram that showed the vector of the velocity, only the total pressure caused by it. So, is it more important that the pressure is low or that the velocity of the air is rolling downward into the ducts?  Don’t know.  If I had realized what you point out earlier (before I made them permanent), I would have done one more set of experiments with three pairs, one in the middle and one to either side of the NACA ducts to see what that does.  

However, being  a pragmatist, I got the cooling I wanted and it was time to stop.  Hopefully someone else can try the other option.

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

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Today's Topics:

   1. Re:  The use of VG to aid cooling (Jerry Teitsma via Reflector)

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Message: 1
Date: Tue, 30 Sep 2014 17:14:54 +0000 (UTC)
From: Jerry Teitsma via Reflector <reflector at tvbf.org>
To: Ron N VelocityXLFG <velocityxl at fastmail.fm>, Reflector
	<reflector at tvbf.org>
Subject: Re: REFLECTOR: The use of VG to aid cooling
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I used large (1" high) VG ahead of the naca scoops on the roof of my Velo to reduce oil temps and they are definitely lower - by about 20 degrees.  I think the easy way to see the affect of the VG is looking at cylinder head temps.  Mine came down noticeably.  Jerry  

----- Original Message -----

From: "Reflector" <reflector at tvbf.org>
To: "Reflector" <reflector at tvbf.org>
Sent: Tuesday, September 30, 2014 10:56:14 AM
Subject: Re: REFLECTOR: The use of VG to aid cooling 

Great Job ! . But now I ask the question how is it so many builders claim the small vg's help when your info so clearly says it does not. I have small vg's  in front of my nacas and was sure they helped till now. :(   I have now put testing my naca intake back on the testing list. Oh well more fun things to experiment with :-) . 

Great article so loaded with great info. 

Ron 

On 9/30/2014 8:48 AM, Matt Bucko via Reflector wrote: 
> Well done David. 
> Matt
> 
> 
> On 9/29/2014 5:09 PM, David Ullman via Reflector wrote: 
>> My Velocity has always run hot.  I have fixed this with the use of 
>> Vortex Generators aiding the flow of air into the NACA ducts on top 
>> of the fuselage.  For a full report go to:
>> 
>> https://davidullman.com/uploads/THE_USE_OF_VORTEX_GENERATORS_TO_ENHAN
>> CE_PUSHER_AIRCRAFT_COOLING.pdf
>> 
>> 
>> Comments appreciated. 
>> 
>> To see recent photos of N444DX hangared in Independence Oregon, go to: 
>> 
>> https://davidullman.com/VelocityPhotos.php
>> 
>> David Ullman
>> David at davidullman.com
>> 
>> 
>> 
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