REFLECTOR: VG Line-up Question

lawrence via Reflector reflector at tvbf.org
Thu Oct 2 06:40:50 CDT 2014


This is been an interesting discussion. Some of the commenters have have reached conclusions not warranted by the experimental conditions. In particular, those that have said "this shows that VG don't work" should understand that the experimental evidence actually shows the exact opposite. Multiple people have shown that VG's do work. What is also clear is that the placement of the VG's is critical and even small changes can have large affects on the efficacy.

> On Oct 2, 2014, at 12:21 AM, Bob Jackson via Reflector <reflector at tvbf.org> wrote:
> 
> Thanks for the response, David -- and of course I don't mind you posting my offline question and your response!
>  
> It will be a few months before I've got my plane back up and flying again to try any more experiments myself with the inlet VGs and cooling.  It's also been an area of experimentation for me over the past three or four years, but less focused on inlet VG's -- which based on your results is looking like at least some misplaced energy.
>  
> When I do my own experiments, I plan to also test the idea that the top of the windshield is really the proper point to consider as the front of the fuselage airfoil, rather than the aircraft nose -- due to the discontinuity presented by the much steeper slope of the windshield.  The windshield 'break' is about 6' ahead of the NACAs (rather than 11' from the nose).  Working the 6' length through the Reynolds number equation you provided results in an approximate boundary layer depth at the NACAs of ~1.3" (rather than ~2.2").
> <image004.jpg>
> And using the alternative 1.3" boundary layer thickness and the design rules you included in your write-up produces the following new layout dimensions:
> ·         1.2" VG height
> ·         3" VG length
> ·         3.25" counter rotating VG spacing
> ·         13" VG pair spacing
>  
> Given these new numbers, my first try would be with three sets of 1.2" x 3" counter rotating VG pairs, in a line about 20" in front of the NACA inlets, which should place the outer pairs just outside of the centerlines of the NACAs.  I would compare that with two sets (instead of three) with each set exactly centered on the NACA inlets, or with your suggested layout if I didn't get the big CHT lowering we're all looking for!  I'm not sure I have the patience and time to construct and conduct the full (really cool!) manometer experiment that you did!
>  
> Thanks for your contribution to improved engine cooling!
> Bob Jackson
> N2XF  XL/RG/XC
>  
> From: David Ullman [mailto:ullman at robustdecisions.com] 
> Sent: Wednesday, October 01, 2014 11:15 PM
> To: 'Bob Jackson'
> Subject: RE: VG Line-up Question
>  
> 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.
>  
> By the way I posted your question and my respoce on the Reflector.  I hope this is OK with you.
>  
> David Ullman
> N444DX
> President EAA 292
> 541-760-2338
> david at davidullman.com
>  
> From: Bob Jackson [mailto:bobj at computer.org] 
> Sent: Wednesday, October 01, 2014 7:01 AM
> To: 'David Ullman'
> Cc: bobj at jaxtechllc.com
> Subject: VG Line-up Question
>  
> Good Morning David,
>  
> My Oregon State degree is in EE.  I'm mainly a Computer Vision and Image Processing kind of guy, and worked here in Orlando for Lockheed Martin as we developed the original night vision and 'smart weapons' avionics products for the military.  Most of what I did was in developing imaging target trackers and automatic target recognizers.
>  
> In re-reading your VG write-up another time, I'm still a little confused by this diagram.  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.
>  
> <image001.jpg>
> 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.
>  
> Thanks,
> Bob Jackson
>  
> PS  I flew out of Tillamook the last two weeks of July this year, and I've flown out of Aurora, Hillsboro and Troutdale in other years.  I haven't made it to Independence yet, but I would like to be there for your fly-in next year if things work out.  Thanks!  
>  
>  
> -----Original Message-----
> From: David Ullman [mailto:ullman at robustdecisions.com]
> Sent: Tuesday, September 30, 2014 9:30 AM
> To: bobj at JaxTechLLC.com
> Subject: RE: REFLECTOR: The use of VG to aid cooling
>  
> Thanks for the words.  Let me know what you learn if you try what I did.  Like to see if the results are repeatable.  Should be. 
>  
> What is your OSU degree in?  I taught in ME for 20 years.  EAA 292 in Independence (7S5, where I now live) has a flyin in August.  Maybe you could come out for that.  I will know the exact date by the end of the year.
>  
>  
> David Ullman
> N444DX
> President EAA 292
> 541-760-2338
> david at davidullman.com
>  
>  
> -----Original Message-----
> From: Bob Jackson (Jax Tech) [mailto:bobj at JaxTechLLC.com]
> Sent: Monday, September 29, 2014 9:30 PM
> To: 'David Ullman'; 'Velocity Aircraft Owners and Builders list'
> Subject: RE: REFLECTOR: The use of VG to aid cooling
>  
> Wow!  Great job on the research and write-up of results, Dave!  Definitely worth reading and applying to your own plane.  You inspire me to go read one of your books!
>  
> We've been using the standard (~1/4" tall) wing and canard VGs in front of our engine NACAs for several years to good effect in lowering our CHTs, but nothing like Dave reports with his super (~2") VGs.  I can appreciate and believe the difference he reports and am eager to try them on our plane.
>  
> This is a great example of the kind of analysis and communication that makes the Velocity (and Experimental aircraft) community special.  Many don't have the background to carry it out, and most of those that do don't have the time, the patience or the kind-heartedness to document and disseminate it back into the community.
>  
> Great job on all accounts, Dave!
>  
> Thanks,
> Bob Jackson
> N2XF
> XL-RG
>  
> PS  I'm an Oregon State 1970 grad, Dave.  I've been reading your posts over the years and wanting to come by to share stories and insights in the summers when I fly my plane out (from my Orlando home base) to visit Oregon family and 'sight see' the West.  Your plane is beautiful and an obvious labor of love.  I'll have to try harder next summer.  your research is an inspiration, not to mention useful!
>  
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