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HelicopterGasser Model RC HelicoptersOther › Tuned exhausts
06-03-2010 09:03 PM  7 years agoPost 1
labont

rrApprentice

sudbury, ont, canada

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I am currently building a Century Radikal with a G20ei in it.

Interestingly I came across a modification to the stock exhaust. Just wish I could find the website now but it involved removing the exit tube completely and brazing in another, larger diameter tube in its place. It was from an airplane guy but he reported a decent increase in higher rpm power with minimal noise gain. I'll end up ordering the recommended pipe eventually but I will try this mod just for curiosity's sake.

To be honest, being comfortable with two-stroke expansion chamber design and its effects on scavenging and port timing ect I have to question just how 'tuned' a tube with baffles in it is, not to mention the 90 degree angle I often see the exhaust has to take as it enters the muffler. This design inherently can not achieve what a proper expansion chamber accomplishes. These seem to be primarily mufflers with a nod to chamber volume which is a level of tuning but only a small part of the package. Which begs the question... what rpm was the pipe and its volume tuned for?

I am reading that 12,500 - 13,000 rpm puts the G20 at the right place to produce the best power for the Radikal but Zenoah says something like 8,000 to 10,000 or something like that for its G20. Was the V2 pipe's volume tuned for this rpm spread? If so maybe something with less volume would be better suited for 13,000.

I noticed the boat guys run expansion chambers. I'm really new to gassers and am wondering what options there are for helis and these engines?

Thanks.

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06-03-2010 09:46 PM  7 years agoPost 2
Justin Stuart (RIP)

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Plano, Texas

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While I have yet to put a tuned pipe on a heli, I know from my experience with airplanes that you need to tune the pipe to be in resonance at about 200 RPM lower than your hover RPM. That way you will start to go "on pipe" when you start to bog. The physics of a tuned pipe is such that (with temperature and atmospheric pressure ignored) it is solely dependent on the length of the pipe. You can bend a tuned pipe into a pretzel shape, and so long as the resonant length is correct, it will work. Installing a bent header will have no real effect on a tuned pipe's resonant frequency. However, if you have a "tuned pipe" which is composed of nothing more than a cylindrical tube with fender washers at both ends, your resonant frequency will be so narrow as to be totally unusable. That is why most tuned pipes have cones at both ends. While the cone will not reflect a pressure/sound wave as effectively as a flat surface, the resonant frequency of the pipe will occur over a larger range of frequencies. This makes the pipe less "peaky".

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06-03-2010 11:12 PM  7 years agoPost 3
labont

rrApprentice

sudbury, ont, canada

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Well, if we are talking about a properly designed expansion chamber the length of the pipe means very little. If you are talking about one of these torpedo types that everyone runs then length has little to do with it as well and comes down to volume to match a certain rpm. If one is hoping for scavenging effect from one of these then that would come down to the header as the canister is basically just a muffler... but a muffler with a certain volume to aid the header.

But, proper expansion chambers are far more complicated in design and have a lot more going on. You can have a short fat pipe and a long slim pipe that will create the same effect so the length doesn't really mean much. It's the volume, length of mid section, length and taper of the convergent and divergent cones which create with equations that take the engine's cc's, bore and stroke, rpm range, intake exhaust and transfer port timing, blowdown time ect...

as far as 90 degree bends... no they wont kill an exhaust system but any restriction in exhaust flow is not a good thing. The 90 degree bend in my nitro exhaust is a sharp 90 degree bend as well.... super bad for flow... but in a canister muffler like this I suspect it's not a big deal since the whole system of baffles and separators are restrictions anyway.

Also, I dont think a cone changes the frequency of the pressure waves I think it changes their shape.

I'm still curious though... maybe one day I will source or maybe build my own expansion chamber design for this engine to make the most power at 12,000 rpm.

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06-04-2010 02:44 AM  7 years agoPost 4
Justin Stuart (RIP)rrMaster - Plano, Texas - My Posts: All  Forum  Topic

You do realize that a French horn which has the entire length of the horn twisted into a compact shape resonates just as good as a horn which is a straight piece of tubing 8 feet long? Tuned pipes don't have to be straight. They just have to have a resonant frequency which corresponds well with the compression cycle of the engine.

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06-04-2010 03:00 AM  7 years agoPost 5
Dr. Fibinotchi

rrKey Veteran

Sioux Falls SD

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hmm

There is a lot of talk on gassers and tuned pipe since we want more power and feel the need. To be honest Its like lighting a fire cracker on a windy day. You might get it lit, but in the end you spend more time tinkering with it then getting it to go consistantly. I had the hanson running good in certain days, but on an average day its temp increase from increase back pressure put it at only par with the motor with a higher compression less squish motor using a more free flowing exhaust system. This was because the pipe was designed to be used at 13k. I changed the pipe volume to acomidate lower rpm range. This of course didn't work as well as a true design change.

There are many different scenarios that can put out the boost of the tuned pipe and only a few that make it worthwhile. Wonder why few at fun flies fly a tuned pipe gasser? I spent way to much time. Unless you have a ill level of tinker gnome in you and have lots of time thats fine. Otherwise go with the more modern physics engine than with the exhaust that will pump out the same.

I can dig up all of the formulas and notes on cone, volume calculations and you can have some fun if this is your fun. Hanson's pipe had the power, but to narrow of a rpm range at too high of rpm, and created aerodynamic issues if you had the model in different positions going fast. Selective gear sets where needed with properly ported motor and gears for the pipe.

When the sun and moon where aligned perfect it was like someone hit the N.O.S. though.

-C

If you want others to be happy, practice compassion. If you want to be happy, practice compassion.

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06-04-2010 03:37 AM  7 years agoPost 6
labont

rrApprentice

sudbury, ont, canada

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Ya thanks for helping me visualizing such an abstract idea, Justin lol. Just kidding with ya

Doc, I have all the formula as well. I have designed and built expansion chambers for my two-stroke race bikes. I was just hoping there was a proven pipe for these G20 engines I guess. I was also hoping to find out when everyone uses these torpedo type pipes. I suspect it is the simplicity they offer.

You're right about the tinkering... I dont mind it but only so much. I think I'll order the Century recommended pipe and also do the mod to the stock muffler and see just how much of a difference there is.

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