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  #21  
Old 05-30-2007, 12:00 PM
Bob Axsom Bob Axsom is offline
 
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Default Thanks Paul Lippse

It is always comforting to get your great technical input.

Bob Axsom
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  #22  
Old 05-30-2007, 01:09 PM
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gmcjetpilot gmcjetpilot is offline
 
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Default Parellel

key is getting the exhaust to flow with the free air stream. Rotate that little down turn 180 degrees and weld it back on, so the exhaust exit exit is parallel to the floor. Noise, vibration and exhaust stain affect? Subject to limited data, subjective comments and rumors.
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Last edited by gmcjetpilot : 05-30-2007 at 01:12 PM.
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  #23  
Old 05-30-2007, 01:10 PM
elippse elippse is offline
 
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When a pipe is cut off square on the end, opened or closed, and a harmonic-rich noise source is introduced into one end, the pipe will have a sharp resonance with those harmonics that resonate with the pipe length. Those harmonics will be strong in amplitude and narrow in frequency. Take a short length of 1/4" or 3/8" aluminum tubing (you do have some of that lying around, don't you?), open at each end, and blow over the end of it. Now slice one end at an angle with a length of three times the diameter and blow over the opposite end. Wrap a little piece of paper around the baloney slice to make the tube the same as before slicing it, and try it again. Go back and forth several times and compare the results of the tones that are present and the relative strength of each. With the one I did, there was a very strong tone that emerged on the blunt-end tube when I blew strongly, and a much weaker tone at a higher frequency when I blew softly. With the baloney slice, the lower frequency tone was somewhat weaker, and the higher frequency tone became stronger. Also, there was an instability where the tones would try to go back and forth. Generally, by slicing the end of the exhaust pipe at an angle, you reduce the peak amplitude of a resonance since no longer does all of the pressure wave reflect from a single point. As an experiment, you might try this on your exhaust with two clamp-on extensions, one cut square and the other baloney-sliced, to see if the pounding on the bottom of the fuselage is reduced. Another thing that happens with the baloney slice is that as the gas and the pressure wave begin to emerge from the pipe where the slice begins, they meet opposition from the outside air which slows them down. The pressure wave slows since it goes from a medium of higher sound velocity (you did know that the speed of sound is about 50% faster in your exhaust?) and so it will turn and go away from the line of the exhaust pipe. This pressure wave is usually what causing the pounding, not the gas flow. Somewhat the same takes place in the gas flow. That means that if the baloney slice is away from the bottom of the plane, the exhaust will magically turn away without having to have a turned pipe angled downward, which has more drag. 'Something to experiment with. This is EXPERIMENTAL aviation, non?
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  #24  
Old 05-30-2007, 01:17 PM
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gmcjetpilot gmcjetpilot is offline
 
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Default Interesting

Quote:
Originally Posted by elippse
When a pipe is cut off square on the end, opened or closed, and a harmonic-rich noise source is introduced into one end, the pipe will have a sharp resonance with those harmonics that resonate with the pipe length..................That means that if the baloney slice is away from the bottom of the plane, the exhaust will magically turn away without having to have a turned pipe angled downward, which has more drag. 'Something to experiment with. This is EXPERIMENTAL aviation, non?
Interesting, no reason to not try, but you will lose any residual thrust you might expect from the pipe with the angled cut. Kent Paser "Speed with Econ" has shown 5 mph increase in speed playing with the pipes and "nozzles" at 7,000 feet. Worth a look. Any way speed or noise, you decide.
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  #25  
Old 05-30-2007, 09:18 PM
elippse elippse is offline
 
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I have Kent's book, and he actually made reducing nozzles to fit on the end of the pipes to obtain higher exhaust velocity to obtain thrust. This increased velocity came at the expense of creating back pressure. He obtained the best results at reducing the exhaust diameter from 1 3/4" to 1 1/4" at 7000', but he said the back pressure could reduce his speed at lower altitudes. I wish he would have published his results as the percentage increase each mod gave. You can multiply the increase from each mod to get the overall increase you can obtain from them, but you can not add the mph increases. If one mod gives 3%, another 6%, and a third 10%, then multiplying 1.03X1.06X1.10 would result in 1.20 for a 20% speed increase.
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  #26  
Old 05-31-2007, 09:28 AM
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gmcjetpilot gmcjetpilot is offline
 
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Default Yep that is right

Quote:
Originally Posted by elippse
I have Kent's book, and he actually made reducing nozzles to fit on the end of the pipes to obtain higher exhaust velocity to obtain thrust.
Yep that is correct did I say something different. My point is pointing pipes down at any angle would be counter productive to gaining or recovering any residual thrust. You would agree?

As far as your algebraic math of his data (X*Y*Z=), not sure where you are going, but regarding your complaint of his little book, I do agree data presentation could be better, but I say go do your own test.

You are missing one key point I think Mr. Paser made. The nozzle size that provides some "exhaust thrust" at say 7,000 feet is a detriment at say sea level, where it will cause loss of HP due to restrictive exhaust. He had the idea for a variable nozzle and tried it. However the shape was not ideal. He envisioned the overlapping peddles of a flower design like many jet fighters have but left it there. The design and actuation of such a nozzle would be extensive he conjectured.

Long ago I did some consulting for Pratt & Whitney. Variable nozzles for some "engines" (can't talk about) where being worked on to increase and vector thrust. The physics of acceleration and direction of gases is the same; for every action there is an equal and opposite reaction (Newton).

http://www.youtube.com/watch?v=rYpm5OA78tg
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Last edited by gmcjetpilot : 05-31-2007 at 10:14 AM.
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  #27  
Old 05-31-2007, 09:11 PM
Bob Axsom Bob Axsom is offline
 
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Default I made a test flight this evening

On 5-11-07 I made flew a speed test with the exhaust pipes pointed down and the speed was 170.4 kts this was at 6,000 ft density altitude, three directions using the http://www.us-airrace.org handicap procedure for the conduct of the flight and the NTPS Excel spread sheet with imbedded formulas to calculate out the wind effect. Tonight I repeated the test with the cut off exhaust pipes and the speed was 170.8 kts. I had no noticeable difference in exhaust noise or pounding - probably because of the insulation on the floor and the overlaying carpet.

Other data: RPM=2710, MAP=24.5, temp at 6,000MSL = 17C, flew at 4,600ft MSL for 6,000 ft density altitude per USAR Proc., oil temp. = 180, oil pres. = 80, CHT/EGT 1=374/1309, 2=377/1365, 3=377/1341, 4=338/1309.

Conclusion: Apparent increase in speed and no ill effects. This was worth doing for my airplane.

Bob Axsom
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  #28  
Old 06-02-2007, 04:45 PM
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Nuisance Nuisance is offline
 
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Default yes, it matters!

It took me a little bit to gather the pictures, but now, on my website, down about the middle of the page, is a note about this.

http://www.lazy8.net/speedmods.htm
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  #29  
Old 06-03-2007, 11:28 AM
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gmcjetpilot gmcjetpilot is offline
 
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Default Told Ya, Bob so how much total speed have you gained

Quote:
Originally Posted by Bob Axsom
On 5-11-07 I made flew a speed test with the exhaust pipes pointed down and the speed was 170.4 kts this was at 6,000 ft density altitude, three directions using the http://www.us-airrace.org handicap procedure for the conduct of the flight

Conclusion: Apparent increase in speed and no ill effects. This was worth doing for my airplane. Bob Axsom
Told ya. Nobody listens to me. ......
Seriously where is this US Race "handicap procedure" you mentioned. Went to site and it did not pop out at me. Thanks.

P.S. you're getting real good at doing flight test. You noticed a 0.40 kt gain, cool. Sounds like you got it all down with pretty repeatable results. It would be good to do it over several days, you may have gained more. Bob, what is your TOTAL gain you got so far, for all your mods, since you started on the quest for speed?
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  #30  
Old 06-03-2007, 01:36 PM
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Jerry Cochran Jerry Cochran is offline
 
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Default

Quote:
Originally Posted by hecilopter
Still trying to figure out differences in 2 RV-7A's. Cooling problems are now under control, now to a speed issue. One plane is about 10 mph slower than the other. I have several other threads about our 2 planes, but for all practical purposes, they are identically cowled, baffled, engined, propped, faired and equipped except for the louvres on the bottom of the cowl and removing about 4" from the bottom of the cowl where the exhaust exits due to cooling issues.

Has anyone noticed or experimented with the exhaust pipes and the angle with which they exit the cowl? I noticed that one difference between our airplanes (both equipped with vetterman exhaust) is that my pipes exit the cowl and are parallel to the center of the plane. The pipes on the slower plane are angled pointing to the center of the plane and at each other.

Could this make a difference with drag from the angled pipes or maybe exhaust thrust that is not aimed inline with the airflow? We are going to experiment and twist the pipes to be parallel like mine and see what happens. Just wanted to check to see if anyone else has done this and what the results were.

Thanks!
Since I am currently in the throes of fitting gear leg and wheel fairings, and reading up on how a misalignment can cause lots of drag, I'm wondering if the "slow" -7a took lots of time and measuring addressing this.

A shot in the dark regarding this instance, but might be worth checking. I know of RV-ers who installed their fairings with the plane on the ground, then "eyeballed" the results. This method in my opinion would negate some of the benefits of the fairings.

This may be heresy, but I think Van's method gets them close, but not "dead nuts" as a machinist friend of mine likes to say... Searching the archives of the RV-list, Scott from Van's claims a perfect alignment of these will get you 25mph. The manual says 12, so go figure...

Just food for thought.
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