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  #41  
Old 03-08-2012, 06:23 PM
Bob Axsom Bob Axsom is offline
 
Join Date: Mar 2005
Posts: 5,685
Default Yep There's the complication

I think Mr. Reed has a configuration that will work on our A's though. He has staggered the location where the high speed wind interfaces with the low speed wind coming through the cowling in a way that I think accelerates the low speed wind (when it is coming by the NLG support structure I don't think it is all that fast) by friction from one direction only. Then it is blended with the wind side wind gradually to the end of the fairing. I think the center vane probably helps keep the flow linear and I think it would do it even better if it extended back maybe a foot aft of the side fences. The scallop points have to be deflecting the wind coming across the bottom and may be setting up two small vortices which intuitively I feel is probably better than dragging the equivalent of a half round plate throught the air at some positive angle of attack. Still thinking but I am leaning toward a metal implementation right now.

Bob Axsom
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  #42  
Old 03-08-2012, 07:06 PM
RV8R999 RV8R999 is offline
 
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Quote:
Originally Posted by Bob Axsom View Post
I think Mr. Reed has a configuration that will work on our A's though. He has staggered the location where the high speed wind interfaces with the low speed wind coming through the cowling in a way that I think accelerates the low speed wind (when it is coming by the NLG support structure I don't think it is all that fast) by friction from one direction only. Then it is blended with the wind side wind gradually to the end of the fairing. I think the center vane probably helps keep the flow linear and I think it would do it even better if it extended back maybe a foot aft of the side fences. The scallop points have to be deflecting the wind coming across the bottom and may be setting up two small vortices which intuitively I feel is probably better than dragging the equivalent of a half round plate throught the air at some positive angle of attack. Still thinking but I am leaning toward a metal implementation right now.

Bob Axsom
The only problem I see in this is that anytime freestream flow is the impetus for accelerating low speed air, drag occurs as the net change in momentum is decreased. The low speed air affects the high speed air as well - no free lunch. For drag reduction, the acceleration of the exit air must be accomplished through work done on the system, heat xfer, or an area change. From a pure cooling point of view freestream could be used in many different ways to help increase mass flow.
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  #43  
Old 03-08-2012, 07:48 PM
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Bruce Bruce is offline
 
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Bob,

Letting out race secrets but.
After removing the device that holds the two cowl halves
together the air flow was straight.

With the holder in place the air would move straight back
on the outside and would go back toward the exit in the middle.

Just looking at the oil trail here for the novices.

I will put a fairing on the back side of the NGL to see if I can
speed the air up here.

My .05
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  #44  
Old 03-08-2012, 08:05 PM
ccrawford ccrawford is offline
 
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Location: Denver, CO
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Quote:
Originally Posted by RV8R999 View Post
The only problem I see in this is that anytime freestream flow is the impetus for accelerating low speed air, drag occurs as the net change in momentum is decreased. The low speed air affects the high speed air as well - no free lunch. For drag reduction, the acceleration of the exit air must be accomplished through work done on the system, heat xfer, or an area change. From a pure cooling point of view freestream could be used in many different ways to help increase mass flow.
This could be where the exhaust pulses could be used to speed up the exit air prior to hitting the outside free streaming air. Would probably require a bend in the pipes in the example shown, and having them cut a bit shorter.
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  #45  
Old 03-08-2012, 08:28 PM
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DanH DanH is online now
 
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Quote:
Originally Posted by RV8R999 View Post
The only problem I see in this is that anytime freestream flow is the impetus for accelerating low speed air, drag occurs as the net change in momentum is decreased. The low speed air affects the high speed air as well - no free lunch. For drag reduction, the acceleration of the exit air must be accomplished through work done on the system, heat xfer, or an area change. From a pure cooling point of view freestream could be used in many different ways to help increase mass flow.
I'm just repeating the above so maybe ya'll will read it again.....
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  #46  
Old 03-08-2012, 09:19 PM
Bob Axsom Bob Axsom is offline
 
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Default I suspect the freestream air is affected by th stock outlet

The airplane is moving through the air and work is being done inside the cowl. How the air is routed inside the cowl varies from airplane to airplane and after running a lot of experiments I personally doubt that any of us knows exactly how each unit of air moves from the inlet to the outlet. I have shared my experiments as you are doing and I'll bet you guys come up with some really good math that will allow the prediction of things as others have done before us. I am convinced that I have the best internal flow I can get for my airplane velocity. At the cooling air outlet in the stock configuration there is internal air that is moving in the direction of the airplane relative to the outside air but it is being released out the back so at that point it is certainly not traveling as fast as the airplane. The local outside air is being accelerated in the direction of the airplane as well because of skin friction but probably not as much as the inside air. The skin friction adds heat to the outside air but not as much as the engine and friction heats up the air traveling through the cowl. The immediate curiosity in my mind is, is more tubulance and drag created when these two bodies of air come together around the entire outlet perimeter at one point than if the combination is in line with the exhaust pipes at two small points and some small area action takes place to stabilize those interfaces and and gradually spreads vertically and horizontally to the remainder of the interface. I don't know the answer to that but Mr. Reed's results are compelling enough for me to think about giving it a shot.

Bob Axsom

Last edited by Bob Axsom : 03-09-2012 at 10:17 AM. Reason: typos as usual
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  #47  
Old 03-09-2012, 08:36 AM
gereed75 gereed75 is offline
 
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Location: pittsburgh pa
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Default Bob

I think that you are looking for way to much voodoo in this design. My intent was simply to

A) Reduce turbulence at the cowl exit

and

B) Eventually use the center body as a means to try to tune exit area as a means to throttle mass flow.

I think it does a great job with A. The increased flow rate (evidenced by increased cooling) along with a reduction in drag (I know those two are typically diametrically opposed but I conjecture that the reduction in drag at the outlet via reduction in flow turbulence flow is greater than the increase in drag from increased mass flow rate, more careful testing will tell that tale).

I hope it will help eventually with B.

Beyond that, there may be a small coanda effect happening because of the shape of the firewall/bottom round down, but I certainly would never claim that it produces a propulsive effect. As Ken says, no free lunch.

Look at it for the turbulence reduction and move on.

Have fun
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  #48  
Old 03-09-2012, 10:13 AM
Bob Axsom Bob Axsom is offline
 
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Default Gary I guess I didn't explain myself clearly

What you describe is exactly what I think it is that your outlet provides in the way of improvement. I think you guys are so tightly wound up around the idea of measurement of pressure differentials and flow of the mass of air through the system by measuring little points in the system you apparently lose sight of the fact that the simple reduction of turbulence at the outlet reduces drag which plays a critical part in the:

speed = thrust - drag

equation.

Did you think that I suggested anything else. It has been suggested by someone with an RV-8 that is MUCH faster than our RV-6A that the the heat added to the air in the cooling process could provide some thrust but I have never claimed that. My focus is and has always been on drag reduction and I think the outlet geometry you have created does that. Sorry, you are the one that let the cat out of the bag. 5 to 6 mph increase in speed in deed! If I can get 0.6 kt and break 185 KTAS for the first time I would be more than happy.

Bob Axsom
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  #49  
Old 03-09-2012, 10:39 AM
RV8R999 RV8R999 is offline
 
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Quote:
Originally Posted by Bob Axsom View Post
What you describe is exactly what I think it is that your outlet provides in the way of improvement. I think you guys are so tightly wound up around the idea of measurement of pressure differentials and flow of the mass of air through the system by measuring little points in the system you apparently lose sight of the fact that the simple reduction of turbulence at the outlet reduces drag which plays a critical part in the:

speed = thrust - drag

equation.

Did you think that I suggested anything else. It has been suggested by someone with an RV-8 that is MUCH faster than our RV-6A that the the heat added to the air in the cooling process could provide some thrust but I have never claimed that. My focus is and has always been on drag reduction and I think the outlet geometry you have created does that. Sorry, you are the one that let the cat out of the bag. 5 to 6 mph increase in speed in deed! If I can get 0.6 kt and break 185 KTAS for the first time I would be more than happy.

Bob Axsom
No springs here... nice cool beer on a Friday afternoon waiting for my new CATTO prop to arrive.

Bob - what do you think creates the turbulence? Why does a sharp corner do that? It is precisely because of a delta in flow velocity. The fast air meets the slow air in an abrupt manner and quickly disorganizes. Measuring the internal parameters helps us understand better how we might optimize a particular solution. Without data we are stuck with a trial-n-error process which may or may never lead to any success - any success gained would have been done so without the basis in knowledge and therefore not easily adaptable to other configurations.

My humble .01
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  #50  
Old 03-09-2012, 11:12 AM
Bob Axsom Bob Axsom is offline
 
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Posts: 5,685
Default Exactly!

It would be pretty hard not to recognize that the turbulence is caused by the combination of different air forces. You can try to get your head around it academically or you can just do it and go with what works. I don't care if I can't construct a knowledge model to base success on. A concept proven by testing is good enough for me.

Bob Axsom
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