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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