REFLECTOR: VG Line-up Question

Wed Oct 1 23:21:10 CDT 2014

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").

Velocity_GearUp_PlanView.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.

VGquestion_top2.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

 <mailto:david at davidullman.com> david at davidullman.com

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

From: Bob Jackson (Jax Tech) [ <mailto:bobj at JaxTechLLC.com>
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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