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03-08-2012, 07:59 AM
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Join Date: Dec 2009
Location: Boulder, CO
Posts: 4,435
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Dan, the firewall bottom radius is stock on RV-8s.
It's P/N F-867A, made of .020 or .025 aluminum. The bend radius is 7/16".
Dave
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03-08-2012, 10:12 AM
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Join Date: Sep 2007
Location: pittsburgh pa
Posts: 533
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Dan
Quote:
Originally Posted by DanH
Yep. I just eliminated the corners, but doing so is easier on the -8. However, I suspect if exit velocity was high enough the corners would not matter.
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Agree, high velocity would eventually overcome stagnation/turbulence in this area, but I don't think it is nearly high enough on a standard -6 exit.
Quote:
Originally Posted by DanH
Think about that; a reduction in drag despite an increase in mass flow....... which brings us back to why does it work.
The increase in mass flow should slow the airplane, so you either got a large increase in exit velocity, or a large decrease in form drag, or some of both. So which is it? There's the point....can we convince you to instrument and measure so we learn not just results, but why?
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I think it is most the first (increased exit velocity) and some of the second (reduced form drag).
I would consider instrumenting. My Fairing comes on and off fairly easily (do have to de-cowl though). Didn't volunteer to be part of the effort as my set-up is pretty non-standard: stock inlets, sealed to modified SJ plenum, ducted OC to this exit fairing. Agree that my root cause theory is pretty subjective with no data, other than fairly carefully documented net results.
Quote:
Originally Posted by DanH
BTW, perhaps we should consider a change in nomenclature. Reverse flow is probably a misleading term. Turbulent flow is probably closer to the truth. Oil drop tracks and similar are two-dimensional, while the reality is three-dimensional.
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Agree, not sure that the flow in this region is totally "reverse", but it certainly is turbulent.
Quote:
Originally Posted by DanH
And it by itself brought no change?
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Some, but the increase got lost in the clutter and shoddy documentation.
This hobby is getting engrossing!
__________________
Gary Reed
RV-6 IO-360
WW 200 RV now an Al Hartzell for improved CG
Last edited by gereed75 : 03-08-2012 at 10:32 AM.
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03-08-2012, 10:33 AM
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Senior Curmudgeon
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Join Date: Sep 2005
Location: Dayton Airpark, NV A34
Posts: 15,420
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I wonder how much an after fairing, something like Vetterman has, would change things on
this setup???

__________________
Mike Starkey
VAF 909
Rv-10, N210LM.
Flying as of 12/4/2010
Phase 1 done, 2/4/2011 
Sold after 240+ wonderful hours of flight.
"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
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03-08-2012, 10:41 AM
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Join Date: Sep 2007
Location: pittsburgh pa
Posts: 533
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Mike, Thought about that and have looked pretty close at all of the Vetterman and Elippse (miss that guy!!) versions. My subjective opinion is that most of the gain realized with that set-up comes from reduction of the same pesky turbulence drag at the corners of the exit that my version also accomplishes.
I was hoping to do the same thing as the Vetterman fairing but with what I thought was an easier fabrication.
I also think that smoothing of that corner flow is where the big gains lay, and from here on out it will be much smaller returns on effort. The bigger after body would be a way to smoothly reduce exit area and begin to "exit throttle" mass flow. Hmmmm...
All IMHO of course.
__________________
Gary Reed
RV-6 IO-360
WW 200 RV now an Al Hartzell for improved CG
Last edited by gereed75 : 03-08-2012 at 10:51 AM.
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03-08-2012, 10:52 AM
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Senior Curmudgeon
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Join Date: Sep 2005
Location: Dayton Airpark, NV A34
Posts: 15,420
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I have been sitting here thinking about what may be going on with your setup, and have come up with a couple thoughts.
Think about how a NACA duct works, the sharp edge of the opening causes the boundary flow to "trip", and fall into the duct opening.
Your fairing extension is lengthening the line where the airflow trips over the edge of the outlet, thus lessening the force at any particular location. I suspect someone who is trained in aerodynamics can elaborate a bit better, but it would seem that the same force applied over a greater area may have something to do with it.
As you said, Paul Lipps surely would have been able to give some insight here.
I am thinking of his discussions on the Coanda effect ------ and how it may apply to your adding an after fairing.
May be time to revisit some of the thoughts here.
http://www.vansairforce.com/communit...t=58643&page=6
__________________
Mike Starkey
VAF 909
Rv-10, N210LM.
Flying as of 12/4/2010
Phase 1 done, 2/4/2011 
Sold after 240+ wonderful hours of flight.
"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
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03-08-2012, 11:44 AM
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Join Date: Mar 2005
Posts: 5,685
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Question for Garry Reed
I have looked at your fairing and the references you provided. I see what you are doing with the external vertical surface, the center vane, the cutouts around the pipes and everything I see makes perfect sense. I am curious about the two scalops in the trailing edge of the horizontal part of your fairing they are too inconsistent with the basic smooth lines to be accidental and they obviously will affect the flow coming across that high pressure area and rising into the void. I suspect that is your intent and it looks just right. Did you try different shapes on this edge? Another observation that I should keep to myself, since we are in the same racing class I believe; I can easily visualize the effect of the center vane and it seems to me that this would be enhanced by extending it back farther than the tapered side dams.
Bob Axsom
Last edited by Bob Axsom : 03-08-2012 at 11:47 AM.
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03-08-2012, 12:31 PM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by gereed75
I think it is most the first (increased exit velocity) and some of the second (reduced form drag)....I would consider instrumenting.
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Please. You have something interesting going on. Increased mass flow (evidence being the lower CHT values) and reduced drag are not normally found together. It is worth sorting out (what does what, the why) because your next trick is a mass flow reduction for cruise and/or racing.
An exit velocity measurement as Ken illustrated will tell much of the tale. Do one with the exit fairing and firewall radius, one without each piece, and one without both.
Quote:
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Some, but the increase got lost in the clutter and shoddy documentation.
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I thought so.
Consider the measured exit velocity values Ken graphed in post #1, over 50% of freestream for the stock exit size and about 66% for a throttled exit. At the magic 200 KTAS you would have 130 knots in the exit.....not a great place for a square corner.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
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03-08-2012, 03:59 PM
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Join Date: Sep 2007
Location: pittsburgh pa
Posts: 533
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Bob
I wanted to keep as much horizontal "floor" in the fairing as possible to acheive as much airflow fairing as possible. So for that reason, I tried to keep the radius close into the exhaust pipe as much as possible while also keeping the curve smooth and asthetic.
When I got out to the end, I had some width left to transition at the end. What to do? Cut it off flush?? round it off? I just picked the scalloped, bat wing curves because it looked cool (I thought) and is maybe aerodynamic (works for bats!).
My original design had an overall longer fairing and an even longer center bluff body. I trimmed it all shorter as I went, again basically to get it into asthetic proportion and be faired to the eye. Longer might be better, especially if you are going for a wider center body to reduce exit area more (or to fair more of the nose strut in the case of an A model). Check this example for long and faired http://aerochia.com/parts_images/ES_3_lrg.jpg
Dan, I think that the explanation of how mass flow could go up and still see a decrease in drag lies in just how draggy the stock 6/7/9 exit is. When could you send me a loaner probe??
__________________
Gary Reed
RV-6 IO-360
WW 200 RV now an Al Hartzell for improved CG
Last edited by gereed75 : 03-08-2012 at 04:11 PM.
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03-08-2012, 04:51 PM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by gereed75
When could you send me a loaner probe??
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I'll see about making another.
In the meantime...
(1) order one of these:
http://www.ebay.com/itm/Digital-Mano...item35aeb24f2a
(2) plus whatever you need to install a tee fitting into your aircraft static line and hook up a small vinyl flex line, and
(3) figure how you want to run a few flex lines up into the cockpit.
We'll walk you through the procedure.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
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03-08-2012, 05:39 PM
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Join Date: Feb 2006
Location: Anywhere, USA
Posts: 1,132
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Bob,
This will help everyone visualize the A model problems.
I am consulting some good advise on the gear mount and think
I have a good solution to the problem.
I do like what Gary has done.
I did lose the holder for the cowl halves. This was a one time flight on
this setup and now have a 6" x 10" plate that goes over the split in
the seam.
Not taking very many readings just trying to get the oil temp down
and velocity up in this area.
__________________
Bruce (BOOMER) Pauley
Kathy (KAT) Pauley
RV 7A--"MISS MARIE"--- N177WD (SOLD FLYING)72742
VAF #582-----------------EAA LIFETIME MEMBER
EX -KC-135A -------------BOOM OPERATOR #3633
VAN'S FLIGHT------------#6930
See you in OSHKOSH
http://www.mykitlog.com/users/index....ley&project=84
=VAF= 2006-2020 DUES PAID
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