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  #31  
Old 03-08-2012, 07:59 AM
David Paule David Paule is offline
 
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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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  #32  
Old 03-08-2012, 10:12 AM
gereed75 gereed75 is offline
 
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Location: pittsburgh pa
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Default Dan

Quote:
Originally Posted by DanH View Post
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.
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 View Post
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?

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 View Post
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.
Agree, not sure that the flow in this region is totally "reverse", but it certainly is turbulent.


Quote:
Originally Posted by DanH View Post
And it by itself brought no change?
Some, but the increase got lost in the clutter and shoddy documentation.

This hobby is getting engrossing!
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RV-6 IO-360
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Last edited by gereed75 : 03-08-2012 at 10:32 AM.
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  #33  
Old 03-08-2012, 10:33 AM
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Mike S Mike S is offline
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I wonder how much an after fairing, something like Vetterman has, would change things on
this setup???

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Rv-10, N210LM.

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  #34  
Old 03-08-2012, 10:41 AM
gereed75 gereed75 is offline
 
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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.
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Last edited by gereed75 : 03-08-2012 at 10:51 AM.
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  #35  
Old 03-08-2012, 10:52 AM
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Mike S Mike S is offline
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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
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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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  #36  
Old 03-08-2012, 11:44 AM
Bob Axsom Bob Axsom is offline
 
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Default 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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  #37  
Old 03-08-2012, 12:31 PM
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DanH DanH is online now
 
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Quote:
Originally Posted by gereed75 View Post
I think it is most the first (increased exit velocity) and some of the second (reduced form drag)....I would consider instrumenting.
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:
Some, but the increase got lost in the clutter and shoddy documentation.
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.
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  #38  
Old 03-08-2012, 03:59 PM
gereed75 gereed75 is offline
 
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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??
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Last edited by gereed75 : 03-08-2012 at 04:11 PM.
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  #39  
Old 03-08-2012, 04:51 PM
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DanH DanH is online now
 
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Quote:
Originally Posted by gereed75 View Post
When could you send me a loaner probe??
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.
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  #40  
Old 03-08-2012, 05:39 PM
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Bruce Bruce is offline
 
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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.
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