REFLECTOR: VG Line-up Question

Ron N VelocityXLFG via Reflector reflector at tvbf.org
Thu Oct 2 10:12:23 CDT 2014


"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."

I agree 100 percent ....Sometimes things go unnoticed that are different 
from on test /Installation to the next .

Ron

On 10/2/2014 6:40 AM, lawrence via Reflector wrote:
> 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 <mailto: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 <mailto: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 <mailto: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 <mailto: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 <mailto: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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