stunthanger.com
General control line discussion => Open Forum => Topic started by: Andrew Tinsley on January 26, 2010, 06:02:10 AM
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Hallo all
I thought I understood the basic principle of piped engines, but I seem to have got myself thoroughly confused. I always assumed that the pipe was simply a quarter wavelength, at the desired frequency (desired revs). I put some figures into the equation V = n x w. V being the speed of sound in inches per second, n is the revs per second of the engine and w being the wavelength in inches (sorry, I can't find lambda on the keyboard!).
You know, its all easy stuff, the position of the first baffle is a node (or is it an antinode? I can't remember!). I know the speed of sound is different in a hot gas than in air at NTP, but let's ignore this for first rough calculation. Take Brett's VF 40 setup, revs approx 11000 rpm and length to first baffle approx 18.5 inches. If you substitute the revs into V= n x w, then I get a w (wavelength) of around 5 to 6 inches. V in inches per second and n in revs per second........................ Right, either I can't do simple arithmetic or I don't understand simple pipe theory (Both of which are highly likely!!).
Leaving aside the fact that I can't do simple physical calculations (onset of senile dementia?) or perhaps I totally misunderstand standing wave systems, I am still confused about stunt pipes. Eveyone talks about the length from the glowplug to the first baffle, but what about the rest of the pipe? Randy's pipes, show that they have a weird shape, and a stinger. It is pretty obvious that the rest of the pipe is there for a good reason. The re enterant at the end of the pipe shouts of early two stroke motorcycle design, intended to limit revs to preserve the integrity of engines with poor metallergy. Talk of a first baffle implies that there is a second baffle........................................oh dear, perhaps I need to retire to a dark room and take an aspirin!
Anyone out there have a clear concise explanation of the intricacies of a stunt pipe?............................Please?
Andrew.
Isn't it great when you find that you don't understand things after all!
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On speed jobs, they add one heck of a lot of power, make the engine burn A LOT more fuel, and go a lot faster than a normale' engine.
Based on the high percentage of bad engine runs, I conclude that they work mostly by trial & error. The higher math is backed in after the answer has been found through flight testing.
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Dean-O, where are you when we need you?
Ward
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SAE paper 670477, 1967
SAE paper 680594, 1968
SAE paper 690469, 1969
SAE paper 700123, 1970
SAE paper 720156, 1972
Dean Pappas wrote a piece about the stunt application in the May, 1990 Flying Models. It is on the PAMPA Web site in the Members Only section.
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Thanks Howard,
I will try to find the SAE papers, I am afraid that I have no chance of accessing the Dean Pappas article. Living in the UK I have never thought of joining PAMPA. I have only just joined CLAPA (our UK equivalent)!
Much of my question was a little tongue in cheek, but as we say in the UK, the fag packet calculation result was a bit of an eye opener. Still can't see how 18.5 inches is anywhere near a quarter wave on the 40VF setup. I REALLY would like someone to put me out of my misery and show me where I have made my mistake!
Regards,
Andrew
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I believe speed of sound is temperature related. So you need to know temp. in the pipe I guess. Were you using normal atmospheric temp.?
Also s.o.s. varies according the medium in which it is travelling?
Also s.o.s. varies considerably with altitude, due to ??? Is it pressure?
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Just temperature for air.
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Yes, I realise that the speed of sound will vary with temperature and maybe a slight difference for "exhaust gas" coposition. I just took 680mph as the speed of sound. It isn't correct, but should give a very approx answer.
Andrew.
P.S. Many thanks to those of you who have sent info to me, much appreciated.I now need to read it all thoroughly.
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Andrew,
Here is a plot of measurements of the temp in a baffled pipe stunt motor.
Wave speed is going to be higher than 680 mph.
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I don't remember which issue, but Bob Hunt had a visit with Frank McMillen on one of his Stunt News Videos. They had a little talk about the pipes and the pros of using one. But, like Windy stated in one of his articles, a pipe is good if you take time to learn the paculerarities of it. But, when learning or getting better with the pattern, use the old KISS principle. Use known setups. I still have problems with the muffled set ups.
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Gents,
The speed of sound at ambient conditions is 741 mph.
Just my 2 cents.
Bill Ambrose
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Gents,
I need to make a correction. The speed of sound at ambient (60F) is 762 mph. The speed of sound also increases as temperature increases. By using 680 mph in your calculation you are introducing quite a large error. Recalculate based on the number provided here and see if that gets you in the ball park.
Bill
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The high temperature in the pipe doesn't totally account for the wave speed. The exhaust pulse pressure ratio is far greater than that of a sound wave. I'll look up the exact numbers at home, but an accoustic wave is a pressure ratio of about 0.0000x, an exhaust pulse is a pressure ratio of about x. Many orders of magnitude greater and hence has a higher propagation speed. I think a good number for us is in the order of about 1600 feet per second, or about 1100 mph.
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Frank,
Thanks for that information, I am very surprised at the high propagation speed of the pulse. It looks as though I have completely misunderstood the physical basis of calculating the pipe length. It is obviously the pulse propagation speed that is important. I naively assumed that I was dealing with a classic standing wave type problem. It seems that is wide of the mark.
Thanks to all for your input.
Andrew.
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Andrew,
Here is a plot of measurements of the temp in a baffled pipe stunt motor.
Wave speed is going to be higher than 680 mph.
Thats an interesting chart. The peak point at 3" into the header, is that because there is combustion taking place there in the header. What is temperature in the engine combustion chamber
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Allan,
I have always assumed that is the case, finished combustion in the header. It certiantly is good reason leave all the header intact and trim the pipe. The most damage to a pipe is always charing in the first 1-2 inches.
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Hallo all
I thought I understood the basic principle of piped engines, but I seem to have got myself thoroughly confused. I always assumed that the pipe was simply a quarter wavelength, at the desired frequency (desired revs). I put some figures into the equation V = n x w. V being the speed of sound in inches per second, n is the revs per second of the engine and w being the wavelength in inches (sorry, I can't find lambda on the keyboard!).
You know, its all easy stuff, the position of the first baffle is a node (or is it an antinode? I can't remember!). I know the speed of sound is different in a hot gas than in air at NTP, but let's ignore this for first rough calculation. Take Brett's VF 40 setup, revs approx 11000 rpm and length to first baffle approx 18.5 inches. If you substitute the revs into V= n x w, then I get a w (wavelength) of around 5 to 6 inches. V in inches per second and n in revs per second........................ Right, either I can't do simple arithmetic or I don't understand simple pipe theory (Both of which are highly likely!!).
Leaving aside the fact that I can't do simple physical calculations (onset of senile dementia?) or perhaps I totally misunderstand standing wave systems, I am still confused about stunt pipes. Eveyone talks about the length from the glowplug to the first baffle, but what about the rest of the pipe? Randy's pipes, show that they have a weird shape, and a stinger. It is pretty obvious that the rest of the pipe is there for a good reason. The re enterant at the end of the pipe shouts of early two stroke motorcycle design, intended to limit revs to preserve the integrity of engines with poor metallergy. Talk of a first baffle implies that there is a second baffle........................................oh dear, perhaps I need to retire to a dark room and take an aspirin!
Anyone out there have a clear concise explanation of the intricacies of a stunt pipe?............................Please?
Andrew.
Isn't it great when you find that you don't understand things after all!
Andrew,
You may want to double check your math, especially the unit. 11000RPM/ 60sec = 183.3Hz. The speed of sound is 13397in/s at sea level, and therefore a wavelength is 13397/183.3 = 73 inch. ¼ wave length is 18.27in which is pretty close to the real setup.
Regards,
Sean
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Yes, I realise that the speed of sound will vary with temperature and maybe a slight difference for "exhaust gas" coposition. I just took 680mph as the speed of sound. It isn't correct, but should give a very approx answer.
I think that is quite low. 1400 fps is in the ballpark at stunt exhaust temps.
Brett
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Thank you Brett and Shaun!
If you take Brett's figure of 1400 feet per second as the speed of sound and take note of Shaun's 11000/60 as revs per second. (I am sure that I did do the latter in the original calc, but seems I may have loused it up!). Putting the numbers above, in the equation Velocity = frequency x wavelength and solving for wavelength, gives wavelength as 91.64 inches.
Therefore a quarter wavelength is 22.91 inches! Which is more or less exactly the tuned pipe length for the OS 40 VF at speed of 11,000 rpm. So classical theory works exactly, if you put in the right figures!
Thanks to everyone who has contributed to this topic. I have had so much help off and on the forum, that I am overwhelmed. I can now go and build an OS 40 VF ship, without any bothering over theory! My apologies for lousing up the math!
The only thing left is to decide on, is a ship. Impact seems to be the popular one, but Randy's Shrike has been suggested. What does everyone think? I am not very strong. so perhaps a small ship would be in order?
Thanks,
Andrew.
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An Impact with an OS .40 VF is a great combination. I had good luck with mine by just following instructions and subduing my own tendency to theorize.
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Howard,
Once tainted with the science bug, you can't help asking questions! But you are probably correct, I am going with the flow and taking Randy's package for the 40 VF. No more questions, I want to get up and flying without anymore theory!
Schuang and Sean sorry to have combined your names and manage to mispell it! Thanks for pointing out my awful math. Moral of the tale, just stick to the accepted set ups and don't ask dumb questions!
Thanks everyone,
Andrew.