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  #11  
Old 09-05-2009, 07:27 AM
Capflyer's Avatar
Capflyer Capflyer is offline
 
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Default Exit Area

A few months ago I was trying to find cooling drag solutions and other than in all cases raising temps, there was no measurable speed differences.

First thing I did was to cover the louvers in the lower cowling. Then developed a set of 3 different sized plates that fit over the exit area to reduce the area. I don't have the numbers with me but the inlet to outlet ration prior to starting was about 1:2.5. Again the plates did nothing but raise the temps.

I was about to try reducing the inlet areas with inserts, it had the Sam James inlet rings to the plenum, but had to put the plane up for sale and now she's gone.
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  #12  
Old 09-05-2009, 09:37 AM
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Default

I believe Alex has it right.

An example from a theoretical RV-8 model @8000 ft and 200 mph:

0.3125 sqft inlet and 0.40 sqft exit. Mass flow is 4.66 lbs, exit velocity (for an optimum exit shape) is 147 mph, and drag is 10.8 lbs.

0.25 inlet and 0.40 exit. Mass flow is 3.89 lbs, exit velocity is 128 mph, and drag is 12.7 lbs.

Observe the relationships. Reducing inlet area while maintaining the same exit area reduces exit velocity and increases drag. It's all about loss of momentum.

OK, stick with the stock 0.3125 inlet and reduce exit area to 0.20. Mass flow becomes 2.87, exit velocity becomes 190 and drag becomes 1.3 lbs.

This is the power of a variable exit, or a small fixed exit with its required flat, fast climb.

Notes:

First, don't treat these numbers as actual. Changes to various model assumptions change the actual numbers, but not general relationships.

Second, the designer's task is to turn optimized theory in hard parts. It all goes to **** with lousy pressure recovery, a messy exit, or a dozen other mistakes. Even the best designs can't match optimum values because of practical considerations.

Third, I'm very much just a student at these things, so don't expect deep enlightment from this quarter.
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  #13  
Old 09-06-2009, 08:19 AM
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Default Vetterman's Approach

Mike,

I'm waiting to see if Larry Vetterman's idea proves valid.

http://www.vansairforce.net/vetterman/subcowl_2.pdf
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  #14  
Old 09-06-2009, 10:09 AM
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Ron Lee Ron Lee is offline
 
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Default

Bret, thanks for the link. Is there any factual data on speed improvements?
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  #15  
Old 09-06-2009, 01:31 PM
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hevansrv7a hevansrv7a is offline
 
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Default Sounds good, however..

Quote:
Originally Posted by AlexPeterson View Post
The lack of gain from reducing inlets is probably because "excess" air not taken into the inlets simply splits to go around the cowl. I.e. imagine streamlines going towards the opening(s). At some point just forward of the cowl, a split occurs, sending the streamline either into the cowl or around it. The size of this perimeter is determined by how much the flow rate is into the inlets.

As others have said, the exit area is the key...

If you haven't already, search for cooling exit modification and you will find several fun threads.
Dick Martin's very fast RV-8 with Sam James cowl did, he said, pick up speed from reducing the openings for cooling. Maybe there is more drag from backpressure at the openings rather than from a well streamlined inner ring. Didn't A.J. (last name?) experiment with this on a -6, too?
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  #16  
Old 09-06-2009, 04:05 PM
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N941WR N941WR is offline
 
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Default

Quote:
Originally Posted by smithhb View Post
Mike,

I'm waiting to see if Larry Vetterman's idea proves valid.

http://www.vansairforce.net/vetterman/subcowl_2.pdf
Bret and Mike,

I ran into an RV-7 at a fly-in this spring where the guy made a Larry Vetterman's fairing for the bottom of his fuselage. He said the speed gain was right at five knots, just as Larry said it would be.

I plan on making one once I have my new cowl fitted. But since I'm changing my engine, prop, and everything, you won't be able to compare my gains. However, I will have the option of removing the fairing and see what kind of speed difference I get.

Stand by, I should be flying in about 12 months or so.
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  #17  
Old 09-06-2009, 04:36 PM
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Default

My experience seems to support Dan's calculations.

I was curious about inlet area/drag/speed implications so I "borrowed" several sizes of inlet rings for my James cowl equiped RV9a. The stock rings for my 0360 engine were 4 5/8" diameter so, in addition to the stock rings, I tested 4", 3 3/4", and 3 1/2" rings. Each aluminum ring was the same O.D. but the I.D. was progressively less due to thicker wall area. To deal with the greater thickness w/o additional flat plate area, the thicker rings had a nice smooth radius machined at the forward lip. This common o.d. allowed easy switch out as the "keyway" on the James fg cowl fit all of the rings. Speeds/cht/ot data was collected at the same d. alt and power settings. Four way gps speeds were captured and then run through Kevin Hortons equations to capture tas. The engine data was logged and recovered from my AFS 2500 engine monitor.

I have a full page of data on this but the bottom line was that as I went to the smaller inlets w/o mods to the outlet, my hottest cylinder cht's went up by around 12 degrees and the tas actually went down slightly (maybe 2-3 kts). I went back to my stock rings.

There is much more to this fluid dynamics stuff than meets the eye!!!

Cheers,

db
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  #18  
Old 09-06-2009, 05:01 PM
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hevansrv7a hevansrv7a is offline
 
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Default Thanks, Dave B.

You just saved me some work and some money! Theory is nice but data usually counts for more.
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We can lift ourselves out of ignorance, we can find ourselves as creatures of excellence and intelligence and skill. We can be free! We can learn to fly!" -J.L. Seagull
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  #19  
Old 09-07-2009, 11:26 AM
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DanH DanH is offline
 
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Flight data is good.

Dave, theory does match your observation, but to illustrate the real story I need to expand beyond the two samples offered in the previous post.



I plotted a series of inlet-outlet ratios from one extreme to the other (the scale at the bottom). The plot assumes an exit area of 0.4 sq ft and plots inlet areas from 0.05 sq ft to 0.7 sq ft. If your RV-9 does in fact have an exit area around 0.4, your change in ring sizes shouldn't make much difference in drag. All it would do is change CHT due to a reduction in mass flow. The drag change you made would be between the vertical black lines, large inlet being a 1-1.71 ratio, small inlet being 1-2.98 ratio.

If your exit is closer to 0.3 sq ft, the black lines would shift right; large inlet 1-1.28, small 1- 2.23. That would result in a CHT rise and a drag rise, which is what you report.

Student observation: note cooling capacity (the blue HP line) and mass flow (the magenta line) are proportional. Given a fixed exit area, increasing or decreasing inlet area always results in decreased or decreased CHT...which is exactly what everyone consistently reports here in the forum. However, note cooling drag (momentum loss) is not proportional.

BTW, the yellow line ("exitV') is poorly labeled. It should read "exit velocity ratio". In this example it never quite reaches 1. If it rose above 1 the drag and exitV lines would cross and the result would be thrust. Dream on.....
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Last edited by DanH : 09-07-2009 at 11:39 AM.
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  #20  
Old 09-07-2009, 04:02 PM
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Default

Dan, what happens to the chart if the inlet size is held fixed, and the exit size varied?
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