Running a Full Wave Pipe

[Doug Moon]

This is kind of long, sorry.

Back in 2018 Frank Williams loaned me an ES composites tuned pipe.

Up until now I have not had a plane with a long enough pipe exit to accommodate the pipe. Enter my new plane in September 2023. It is a Bear modeled after my 2012 Bear but with a pipe tunnel long enough for a PA 75 pipe with a standard rear exit. That plane was good right off the bench. I took it to the TT in Houston just a month after painting it and it did well.  It was not close to final trim but it was a good start.

Spring of 2024 and it's time to really figure this plane out and get ready for the nats. Added some nose weight, took it back out. Added some tip weight, took it back out. Slide the LOs forward and backward and found a good spot. Plane is flying decent now.

I decided to remove the 65/75 Aero Pipe and installed an Eather pipe. This pipe was an improvement right away. The power delivery is smoother and the braking action is good. I am flying on a 13" Eather 4 blade prop. I have tried other props but keep coming back to that one because it has the best flight characteristics across the whole flight.

Another major change for me, direct connect handle. For a long time I have been the big proponent of the bar handle. I like being able to adjust the neutral endlessly until I get it just right. I like the dead feeling around neutral as well, at least I thought I did. A few years ago I bought a Smith Brothers carbon handle kit. It's pretty basic and sets up very easy. It is super light weight. At first flight it really showed me how off the inside and outside turn rates were. I few adjustments and it was very close. I made up many line clips to get the right combo for the best neutral handle setting. I am very sensitive to that setting. Adjusting clips endlessly is not fun. The bar with the cable is so much better for that one single thing. Finally I am able to fly good maneuvers on a direct connect handle and really get a good feel for why people really like these things. I know, I am very late to the party.

All is working well but it's still not great. Not like I have had in the past. I was offered a NIB PA65. I could not get that thing fast enough. I contemplated not running it. Then I thought of all those Tiger 60s stuffed away in drawers somewhere that were never run due to people just sitting on them and now they will never be used.  To heck with that I broke it in and put it in the plane. I am using a brand new Eather 3 blade green. 12.5"x3.75 pitch. Now we are cooking with crisco. Around flight 25 on this motor it's like the plane just took a deep breath and relaxed. Every maneuver just got a little easier.

The PA65 is simply unmatched as a stunt motor. It's the correct weight. Swings the right prop sizes. The motor is very responsive and cycles flawlessly. I have been actively flying the PA75 for many years. I have learned it and know how to run it well. It's a good engine and has tons of power on demand. But it is no PA65.

With the nats dates changed I learned I would once again not be able to attend the nats due to scheduling conflicts. Now it's time to test some stuff and have some fun.  Even though I had that 65 in the plane and working really well I knew there was more there to be had. 

I was looking for something else in my garage when I came across the ES Composite pipe that Frank Williams had loaned me back in 2018. I wonder if it will fit in this plane? I had looked this thing over about 1000 times hoping to use it one day. The finish on this thing is perfect and I have been told it won't cook at the neck where it gets really hot. I fit with a pressure fitting. Check the length and yes it will fit. With its built in exit the pipe it needs a long exit area.

The ES Composite pipe is a full wave pipe. I has diverging and converging cones with no baffle inside. Frank advised me on where to set it up for the initial flights. Will it be louder? Yes. Is it unbearable? No. Not even close. Frank measured his on a 75 and the DB was upper 80s.  I liken it to a loud header muffler. The metal pipes Frank has used in the past are really loud. This is not like that at all. At startup right away things are very different. It is spinning the same prop about 200 rpms faster. I run it up some more to get a good setting. Its on the break on the ground. The pipe rattle is really loud! It sounds AWESOME! On take off the plane is so much more stable. In level flight the motor is cycling ever so slightly on the up and down wind side. Go into the maneuvers and that pipe holds the motor in check the entire time. You hear a hard break and nothing happens. Just steady solid line tension but no speed. On the round 8 intersection you hear the normal little burst but no speed. The moment it gets a burst and starts to rev that pipe shuts it down. When you pull up you can hear the pipe let the rpms go until the pipe kicks in and stops it. It's quite amazing to hear all of this going on. On hard corner exits it revs and stops immediately so there is no bounce out of the maneuver. If that happens it's me making the mistake. The down legs on the square eight are a thing of beauty. It also does not burn off near as much speed in the square eight.

I let one my flying friends go for a ride and he said he could have eaten a peanut butter sandwich on the down leg of the hourglass.    He said that pipe should never come out of my plane.

I am truly amazed and how much of a difference this type of pipe makes over the baffled pipe. I mean my trim on my plane was instantly better by a long way!  It's not even the same thing at all!

Frank has sent me some of his HP charts and the drop off at top RPM is amazing and that directly supports what I am feeling in the air.

This is really fun again and I can't wait to go flying again as the weekend comes. I have burned 5 gallons this year and I only get to fly on the weekend mornings. No more evening flying for us as the bird population is way to large now and they chase the planes. In the mornings they are not around. Weird.

[M Spencer]

Mind your eyebrows , par tikly the pit man

[Matt Colan]

I went flying with Doug a couple weeks ago in some really lousy, 10-15mph turbulent air. We each flew five flights. Those conditions really showed how awesome that pipe is! There was so little windup, him and the airplane flew like it was about a 5mph breeze. It looked so easy! I want one of these pipes, the characteristics it showed when I watched him fly it in that bad air was all the convincing I needed to try one. Next is getting a flight on that plane so I can feel the awesomeness!

[Dave_Trible]

Doug I am making a few test pipes and might make one without baffles to try.   How or from where are you starting for the pipe length?  The start of the convergence cone?   

Dave

[Doug Moon]

Doug I am making a few test pipes and might make one without baffles to try.   How or from where are you starting for the pipe length?  The start of the convergence cone?   

Dave

I measure half way down the converging cone. But in all purposes the measurement is just a ref point for tuning. If youre making your own you can start wherever you want as you will be the only one with that pipe. On a 65 I am at 20.25 half way down the converging cone.

Post pics as you go will be interesting to see what you come up with. Non-baffled is a different game.

[Dave_Trible]

Thanks Doug.  Last night I laid up a set of shells to try a full wave pipe.  I’ll have several to try airborne when I get home from the Nats.  I’ve made clamshell molds initially.  Once joined I put a layer of veil on the outside.  Not pretty but functional.  I have someone making metal mandrels for me to make some the other way.  Work in progress.

Dave

[Dave_Trible]

….

[Dave_Trible]

A finished pipe

[Dave_Trible]

I wonder if the convergence cone will need to be more pronounced……

Dave

[BillLee]

I wonder if the convergence cone will need to be more pronounced……

Dave

I'd like to hear Frank Williams' response to that question.

[Brett Buck]

I am curious why we are calling it  a full-wave pipe. The wavefront runs to the tail cone and reflects back, that is one full wavelength, which is exactly the same as if you have it bounce off a baffle, except that the holes in the baffle and multiple provide the necessary spreading of the return waves (instead of the conicalish reflecting surface which is intended to do the same thing).  The measuring reference for a "conical" reflecting surface is the centroid of the shape, for a true cone it is 1/3 of the way from the "big" part to the point. 20.25" seems a bit long but definitely is about half a wavelength, and for sure not one full wavelength for 1450 FPS, 136 degree exhaust duration, and, say, 10000 rpm. So I think I don't understand the definition.

    Brett

p.s. the return wave/waves echoes are "smeared out" by the conicalish tail cone, the shallow the angle/longer the cone, the more smeared out. Very sharply focused reflections (not smeared, say, a flat plate) give a lot of boost at a very narrow RPM range, tapering it reduces the peak boost but spreads it out over a wider range.  The multiple return waves from the multiple baffles do the same thing

   

[Doug Moon]

I am curious why we are calling it  a full-wave pipe. The wavefront runs to the tail cone and reflects back, that is one full wavelength, which is exactly the same as if you have it bounce off a baffle, except that the holes in the baffle and multiple provide the necessary spreading of the return waves (instead of the conicalish reflecting surface which is intended to do the same thing).  The measuring reference for a "conical" reflecting surface is the centroid of the shape, for a true cone it is 1/3 of the way from the "big" part to the point. 20.25" seems a bit long but definitely is about half a wavelength, and for sure not one full wavelength for 1450 FPS, 136 degree exhaust duration, and, say, 10000 rpm. So I think I don't understand the definition.

    Brett

p.s. the return wave/waves echoes are "smeared out" by the conicalish tail cone, the shallow the angle/longer the cone, the more smeared out. Very sharply focused reflections (not smeared, say, a flat plate) give a lot of boost at a very narrow RPM range, tapering it reduces the peak boost but spreads it out over a wider range.  The multiple return waves from the multiple baffles do the same thing

   

I just called it full wave as this is a more typical "tuned" setup you would see on a 2s vs a baffled pipe. Call it what you want non baffled vs baffled etc. that's what is being discussed.

I can tell you from my experience flying the pipe back to back is 100% NOT the same thing and Frank Williams charts also show it is not the same thing. The non-baffled pipe has an insanely sharp drop off at the peak rpm. The more you press it the farther it falls. The shut down effect is eye opening to say the least. Also the reaction is so much faster than a baffled pipe.  Charts on a baffled pipe show a severe drop off but it levels out very quickly after the initial drop. The non-bafffled pipe doesn't do that.

Another thing I have experienced is better fuel mileage by a noticeable amount. Frank explained the non-baffled pipe will charge the cylinder more effieciantly and there is alot less wasted fuel. I have also notice there is 0 blow-by out the venturi and almost no oil anywhere on my plane at the end of 7 flights. The stinger tip keeps it off of the back of the plane for sure but with baffled pipe there was always blow-by, at least that's what I call it, up around the front and it would end up on the outboard leading edge. Not much but it was there. You know the stuff that turns half of the head brown after a while.

It will be interesting to see how Dave's pipes perform.

[Doug Moon]

I wonder if the convergence cone will need to be more pronounced……

Dave

I am wondering the same thing. I also see a large flat area in the middle. Are you following a specific design idea?

[Dave_Trible]

Not really.   It is patterned after a Macs pipe I ran across based more on diameter for the RO Jett .76 or PA .75.  If that proves an issue I can do something else.   Learning curve here on my part.  I've run the first one on the test stand.  No surprises there.  There are a couple different baffles I cobbled up.   I think I've got that piece sorted out.  If needed I have someone who can make me the typical 'can' setup but I'm trying something more simple to begin.  However maybe this version you have will make that a non-issue.

Dave

[Brett Buck]

I just called it full wave as this is a more typical "tuned" setup you would see on a 2s vs a baffled pipe. Call it what you want non baffled vs baffled etc. that's what is being discussed.

I can tell you from my experience flying the pipe back to back is 100% NOT the same thing and Frank Williams charts also show it is not the same thing. The non-baffled pipe has an insanely sharp drop off at the peak rpm. The more you press it the farther it falls. The shut down effect is eye opening to say the least. Also the reaction is so much faster than a baffled pipe.  Charts on a baffled pipe show a severe drop off but it levels out very quickly after the initial drop. The non-bafffled pipe doesn't do that.

   Yes, the effect is that, for out purposes, motorcycle-style tail cone angles are *far too sharp* and react too much over too small an RPM range. Baffles create a series of reflections at varying times. Frank has a plot of all the different tuning peaks you get. That spreads the returned energy/returned wave over a much wider RPM range. I note that it works that way because the wavelength is much, much longer than the distance between the baffles. The examples you show have what amounts to an *extremely shallow* tail cone angle which smears it out, returning a fraction of the energy from every point along the cone, and making it operate over a usable range.

     Note that the effect of the baffles smearing out the wave is *the sole effect* in a Brian Eather pipe, otherwise the "tail cone" is a dead flat surface that would be completely unusable due to being too "peaky".

     This is all pretty basic wave reflection physics, even the "Two Stroke Tuner's Guide" gets it right enough.

       Brett

[frank williams]

The term “full-wave” is mine, just to distinguish from a traditional baffled stunt pipe.  I also refer to baffled pipes as “muffled megaphones”.  The expansion part of the operation is there, but the wave gets dumped into a muffler (baffles with holes in them), and I think the reflection part of the job gets scrambled.  I’m really not sure what the pressure reflection of the wave looks like off a series of baffles.  At one time I thought I did, but I’m not certain anymore.

The best way to think of what a pipe is doing is to equate the lengths of the expansion cone and contraction cone in terms of time.  A pressure wave traveling down a tube, when it meets a change in section area, will return a reflected wave.  If the section area is increasing (the forward expansion section of the pipe), the returned wave will be an expansion wave.  If the section area is decreasing (contraction tail cone), a compression wave is returned. 

The important thing to keep in mind is that this reflected wave/s is not just a onesie.  The entire length of the expansion or contraction cones of the pipe are returning reflections as the exhaust pulse traverses it.  As the area changes reflections are generated.
 
So, as the exhaust wave travels down the expansion part of the pipe, it is continuously reflecting expansion waves back toward the engine.  Likewise, as the exhaust wave is traveling down the contraction cone of the pipe, it is returning compression waves back to the engine.  So, the length of the cones and the velocity of the exhaust wave, give you an amount of time or duration of the returned reflection wave
.
How fast is an exhaust wave?  Faster than regular sound waves.  Exhaust pulses are classified as waves of finite amplitude.  A sound wave, normal talking, an acoustic wave, has a pressure ratio (peak pressure to atmospheric) of about 1.000002 (P/Pa).  A rifle shot would be a pressure ratio of about 1.02.  This would be about the loudest of acoustic waves.  A typical exhaust pulse has a pressure ratio of about 1.5.  (open face Fox 59 comes to mind).  This 1.5 came from motorcycle literature.  At any rate, one might say that gas particles in these waves “create their own environment”.  I think Brett has said that something like 1450+ ft/sec is a good number.  The pipe designer will build the pipe to match, or be in-sync with, the timing of the opening and closing of the exhaust port and projected wave speeds.

The thing that the baffled pipe screws up is the return of a clean compression wave.  Without a finite length of contraction cone, the return compression waves are in question.  What gets reflected back when the pulse hits a baffle with a hole in it?  There must be a reflected wave, but of what duration.  Is it an instantaneous blip of pressure?   I don’t know

Baffled pipes do alter the engine power curve, which is what we are really after in the long run, however it has been my experience that the un-baffled pipe with a finite amount of contraction cone does produce a better power curve.  When I say better I mean a very steep slope, almost vertical.

I have a neat diagram of the pipe operation from "The Basic Design of Two Stroke Engines" by Gordon Blair.  I'll try to make a copy of it.  This book is really good.  Two Stroke Tuner is ok, but this much more gas dynamics orientated. 

[frank williams]

The pipe Doug is referring to is the pipe that I had Ed Skorepa make.  At the time the only double cone pipe he had was one he made for tether cars.  I got one of those and had to adapt it to a stunt configuration.   Later he took that mandrel and modded it for me into what I thought would be an acceptable stunt pipe.  Pictured below. 

The carbon work is "aerospace grade".  He had them with a straight stinger and a curved stinger.

His web site is gone.  Recently (a couple of month ago) I was able to contact him via facebook.  At that time he said he still had F2B pipes.

[frank williams]

Yes .... the elephant in the room ...... they are louder but from what I've measured well under the 96 Db limit.   Generally in the mid 80's.

[Brett Buck]

The term “full-wave” is mine, just to distinguish from a traditional baffled stunt pipe.  I also refer to baffled pipes as “muffled megaphones”.  The expansion part of the operation is there, but the wave gets dumped into a muffler (baffles with holes in them), and I think the reflection part of the job gets scrambled.  I’m really not sure what the pressure reflection of the wave looks like off a series of baffles.  At one time I thought I did, but I’m not certain anymore.

   It gives you a series of closely-spaced reflections, and a lot of damping (the sum of the echoes is less than the single echo from a flat plate). That's why, even though you have the same rather steep tail cone on, say, a Billy pipe, that's not the only surface, so get a much milder return wave, and its smeared out over a longer period. I can tell you for sure that if you just cut out the baffle section, glue it back together, and try to use the ~40 degree cone, its far, far, to peaky to use, because I did exactly that. Since the baffles are all so close together compared to the wavelength, you can barely detect the fact that there are multiple tuning peaks - but your test data shows the small but detectable effect.

     You example ALSO smears out the reflection over a wide range by having a curved and very shallow reflecting surface, without the additional damping, so you get a much stronger echo but still spread out, so you get a wider RPM range,  which is presumably what Doug feels and why you liked it.

     Brett

p.s. For everyone else, while I am sure Frank understands this, we are not talking about individual molecules travelling down the pipe, bouncing off things, and bouncing back to the exhaust. It's the acoustic waves doing the bouncing and reflecting, each molecule pushes his next-door neighbor a little, that moves the neigbbor, etc, and that little push travels down the pipe much faster than any individual molecule does - at the speed of sound in the hot exhaust, from Frank's test, in the range of 1450 feet/second. Your pipe is a foot and a half long, for a wave to come out the exhaust, travel to the far end, then bounce back, it's 3 feet, 3/1450 = . 00207 or twoish  milliseconds. A particular exhaust molecule actually takes very roughly 1/4 second, but the wave moves through the exhaust in a tiny fraction of the time.

    The idea, of course, is that at some point the time it takes to go to the far end and bounce back lines up with the time it takes for the exhaust port on the engine to open, sending the "crack" of the exhaust from the engine, to the far end, and then the echo of the "crack" get back to the engine just before exhaust closes. The relative high pressure (the echo from the crack) then compresses the charge so it is at a higher pressure even before the piston comes up and compresses it some more. That only happens ideally at one RPM, when timing works out, and at other RPM it either does nothing, or makes it worse. When something comes "on the pipe", that means the RPM is starting to line up, the pipe effect starts boosting it, so the RPM jumps suddenly to the tuned RPM, where it wants to stay. Stunt uses it in a different way, putting enough load on the engine that it can't reach the tuned RPM, and gentle enough to run in a sort-of linear range where if you load the engine, slow it down, it comes closer to in tune and gets a small boost, speed it up, it goes further out of tune, and reduces the boost. The effect is to act as a regulator.

[Doug Moon]

Good stuff on this thread. I like reading this stuff. I really like BB explanation of the molecules pushing on their neighbors that really makes sense.

Frank, as always, bringing very good info to the table!

I am also finding that I am locked into a pretty narrow RPM range for optimal "pipe" usage. I am fine with that I can tune props to work in the range.

[Brett Buck]

Good stuff on this thread. I like reading this stuff. I really like BB explanation of the molecules pushing on their neighbors that really makes sense.

Frank, as always, bringing very good info to the table!

I am also finding that I am locked into a pretty narrow RPM range for optimal "pipe" usage. I am fine with that I can tune props to work in the range.

  Glad you enjoyed it!  The basic ideas are pretty simple, if you looked at the math for a non-trivial case, it tends to get messy. The difference between the degree of "smearing" of the return wave is what you are feeling, the baffles have the effects of not making a clean reflection, even if it is time-shifted the same as with a tail cone. It's like a echo against a canyon wall VS an echo against a curtain or a sheet of foam rubber - the reflecting surfaces have the effect of removing some of the energy of the echo - "crack" VS "thud".

    I became reasonably knowledgeable about wave propagation through pipes by having very well-instrumented tests we conducted on satellites, and helping check out the math models used to predict what happens. Part of this was tracing and measuring what the pressure waves did and where they went inside a much more complex system oe pipes, valves, rubber hoses, etc. Unfortunately, only *after* we had very serious problems with it in-flight. In fact, I got paged and had to go into work for the emergency on the same day I did the first Skyray/20FP test flights, it was like a miracle - and then I got the ominous pager message "Come to the support center right now".

     Brett

[Brett Buck]

Yes .... the elephant in the room ...... they are louder but from what I've measured well under the 96 Db limit.   Generally in the mid 80's.

     Are you using the trick from the "Tuners handbook" where the stinger goes up into the fat part of the pipe - what Randy calls a "resonator". In any case, after-mufflers work.

     Brett

[Dave_Trible]

     Are you using the trick from the "Tuners handbook" where the stinger goes up into the fat part of the pipe - what Randy calls a "resonator". In any case, after-mufflers work.

     Brett

You really must quit coming up with new stuff to try.......the light at the end of the pipe gets further away.......


Dave

[Brett Buck]

You really must quit coming up with new stuff to try.......the light at the end of the pipe gets further away.......


Dave

   If you haven't seen it, the idea it that you can soften the sound by running the exhaust stinger well up into the fat part of the pipe, so you do not transmit the sharpest "cracks" concentrated at the small end of the tail cone. It reflects just as well or better. Another interesting question is how big to make the stinger (diameter) - smaller diameter gives a better reflection and more exhaust boost, larger diameter flows exhaust better. The best 40VF pipes had *tiny*, and I mean really small, maybe 1/4".

      Brett

[Dave_Trible]

I would try to fit the Dubro silicone stinger simply because it’s easier.  The ones I’ve made so far have a small brass tube stub that I put the stinger on.   If I did what you are talking about I likely use a carbon tube.  I use some carbon tube molded into the hub of the props I make that might be big enough.  That will be on the after- Nats list.  I'm exhausting a .76.   I'd hate restrict it too much.   That larger Dubro job I'd think is about the minimum.  I can tell a little difference whether or not I shorten the stinger which tells me a little goes a long ways.

Dave

[Brett Buck]

I would try to fit the Dubro silicone stinger simply because it’s easier.  The ones I’ve made so far have a small brass tube stub that I put the stinger on.   If I did what you are talking about I likely use a carbon tube.  I use some carbon tube molded into the hub of the props I make that might be big enough.  That will be on the after- Nats list.

Dave

   Depends on what engine, or course, but maybe 3-4" of very large arrowshaft for a 40, with maybe 3/4" sticking out the back and the rest inside. The RO-Jett *does not like exhaust restriction* so I am not sure what you would want, maybe 7/16 aluminum tubing.

      Brett

[Dave_Trible]

The carbon tube I have (the larger one to suit a PA shaft diameter) might still be too small.   I didn't find any light aluminum tubing big enough out of a K&S rack.  I have to see what stock carbon tubes are available.   Then take out a loan to buy it (the stuff is pricey).

Dave

[M Spencer]

" I have to see what stock carbon tubes are available.  "

Kiwi Land , theres a Carbon Mnfg. outfit . Does Fishing Rods . as these are Tapered - the reject bin gets you tapered push rods . dirt cheap .
but therefore , if you dont mind a V shallow taper , if you annoy a fisherman enough so he breaks his rod over his knee , youll have some .
alternately , find a fishing rod manufacturer - or the junk pile at a fishing resort .

Heres some Carbon Fibre diggeredoos .




Oh well . Carbon Telescopic Fishing rod . So the chances are five times as good . and you know what to do with the other four . 

[Dan McEntee]

   If you haven't seen it, the idea it that you can soften the sound by running the exhaust stinger well up into the fat part of the pipe, so you do not transmit the sharpest "cracks" concentrated at the small end of the tail cone. It reflects just as well or better. Another interesting question is how big to make the stinger (diameter) - smaller diameter gives a better reflection and more exhaust boost, larger diameter flows exhaust better. The best 40VF pipes had *tiny*, and I mean really small, maybe 1/4".

      Brett

    In my other life when not messing with model airplanes, I messed around with restoring riding and occasionally racing vintage off road motorcycles, or more commonly known as enduro bikes. I have to see if I have those books that Frank mentioned in my library. In my younger days, I used to make and modify pipes for a friend of mine that owned a small motorcycle shop, and he maintained and tuned a 125 Yamaha motocross bike for another business owner's son a few doors down from him. My friend Jerry learned the motorcycle engine trade under Don Vesco, and between what he knew, and I think I had the Gordon Blair book by that time, and we made this kid one of the first snail shaped expansion chambers in the area. It came out of the engine, and snaked around and the stinger pointed down , all in front of the engine. No pipe to burn your leg on or get in the way while working the bike. Only factory bikes had them at that time. It was fast also! Back in those days, the stinger was kind of key t how an individual bike ran. I remember reading in magazines about starting off with a standard length as it came, and then cutting short section off and running acceleration tests to see how it felt on top end, and when performance dropped off, weld the last section back on. Small bore bikes tended to be peaky with power bands like a light switch so no way to tell how low end and mid range was affected, and on a MX track they spent most of the time with the throttle pinned wide open. Off road bikes needed broader power bands and more tractable power, and most importantly, needed to be quiet. The "Less Sound=More Ground"  mantra was in full swing. Those bikes had typical expansion chambers but with different geometry that the motocross pipes, and to keep them quite, there was a can added from the last cone at it's biggest diameter, and that can went back past the end of the stinger, and a silencer/spark arrestor was attached. Guys would cut all that sheet metal off and replaced it with a after market silencer that would save them 5 to 10 pounds sometimes. I have a 1973 250 Penton that I acquired to rebuild that had a modified pipe like Brett described. it was a more or less stock Penton pipe back to where the last cone came to an end, and the stinger was cut off and welded back on with the length going up and into the area of the last cone. I had never heard of that before, but after sending a few emails I found out that is what some guys did back then to lighten up the bike a bit, and still keep the sound to a mild roar. I never ran it that way, and I wanted to restore the bike with an original pipe/silencer/spar arrestor set up, but when I came home with the bike, my oldest brother managed to get a bark out of it!! The bike had been sitting for a long time. The carb wasn't even attached to the bike and was hanging by the throttle cable and just sitting on top of the main cases, but some one must have sprayed something into the cylinder to see it it would fire. My brother was in the habit of just kicking a bike through once or twice to check compression, and when he did that with this bike, it lit and ran a few seconds, much to my surprise!!  It was loud but not excessively so, and had strange note to it. There was a Penton factory rider and R&D mechanic named Carl Cranke and he was supposed to have had something to do with that mode as a suggestion for budget racers to cut some weight and not have to add an after market silencer. I never got a chance to email him to ask about that before he passed away from brain tumor way too young. A lot of observed trials bikes back then just had straight pipes that ran as conveniently located as the factory could to keep it out of the way and then attached a silencer/spark arrestor. Those bikes were looking for mostly very tractable low end and midrange power. Stunt pipes must be a mixture of all of these? More closer to the enduro bike type exhausts I think? I'm thinking that the stinger length could play a small part in fine tuning?  More than just a way to dump the excess oil and exhaust away from the model. interesting stuff. But I have to ask, were not the first generations of stunt pipes built in a conventional way with no baffles? Have things come full circle? I race go karts for a while with my younger brother helping me there. The pipe of choice for most sprint karts back then were Hartmann Blimps. They had an unusual shape and if I remember correctly some pipes ended in a flat section on the end with a cover on the end, and a hole out the back at the 10 o'clock position for the exhaust to exit, much like the baffle stunt pipes but no stinger. The Blimps had a similar shape but a rounded end . I never tried to cut one apart or look deep into them to see what the insides look like.  They had a cupped flange on the ned where they fit over a short length of flex pipe that we could change to meet the conditions of the track. Shorter length for tighter tracks and longer flex pieces for tracks with longer straights, I think. Those pipes might have been closer to what is used in stunt I think. Now enduro karts, like my brother ran at Daytona and the other longer road courses, they had a variable length or "slippy" pipe that you changed the length with a handle on the steering wheel. You worked the length of the pipe in concert with the RPMs of the engine as you went into and came out of turns and down the long straights. I never got the chance to drive one of those!! I might not have been coordinated enough !! Interesting subject for sure.
 
  Type at you later,
   Dan McEntee

[Dan McEntee]

The carbon tube I have (the larger one to suit a PA shaft diameter) might still be too small.   I didn't find any light aluminum tubing big enough out of a K&S rack.  I have to see what stock carbon tubes are available.   Then take out a loan to buy it (the stuff is pricey).

Dave

  Dave;
    If you know what sizes you need, could you just make a mandrel that diameter and "roll your own" tubes from scraps of carbon fiber? Three inches long or so ought to cover a lot of applications I would think.

 Type at you later,
  Dan McEntee

[spare_parts]

Out of curiosity, which resin system are you using for composite pipes?

[Dave_Trible]

  Dave;
    If you know what sizes you need, could you just make a mandrel that diameter and "roll your own" tubes from scraps of carbon fiber? Three inches long or so ought to cover a lot of applications I would think.

 Type at you later,
  Dan McEntee

Guess I could but rather just buy a stick.   Rigidity is another issue.  Most carbon tube uses unidirectional fiber and is going to be better than anything I can make.

Dave

[Dave_Trible]

Out of curiosity, which resin system are you using for composite pipes?

What I am using right now comes from Fiberglast-their 1000 series.  It will set at room temps which is what I need to fabricate in/on an open mold.   Once air cured they go in the oven I built for a couple cycles of heat curing.   I think it will be fine for pipes (so far).  I initially used the 1000 for props,   but wasn't thrilled with it holding finite pitch settings in the heat.  I went to 2000 for that on props.  It will set for days in a container and never set up at room temps.   With the props I use closed molds so they go into the oven in the fluid state and are baked for cure.  This resin is harder and makes temp stable props.   Just can't use it with open molding so the 1000 seems to be OK.

Dave

[frank williams]

Internal stingers? ...... Yes many times ..... on the ES Composites F2B pipe I have not used it
It does reduce the sound ..... I ran them to the high point of the pipe .... where the lowest pressure is
You need a small hole in it internally, at the end of the pipe, so the oil can be eliminated during flight
There is a problem with "fatigue" of the internal stingers, especially with the drain hole ...... carbon stingers were more durable .. aluminum tube would eventually crack off

The purpose of the stinger, obviously,  is to bleed off the pressure in the pipe.  However, you need gas in the pipe for the system to work.  The idea that you should be able to easily blow through the pipe for a better run is problematic .... Its interesting that the CFD simulation programs included in Blair are enlightening .... playing with stinger diameter and length show some massive gains in power out, too much and engine temp goes up..... but as we've always said, power isn't our problem ..... we use the pipe to reshape the power curve and use it as a control system

Bret says the math gets messy ..... an understatement at best

The main point I would emphasize again is that the physical length of the cone (expansion or contraction) translates into the length of time of  the returned wave.  A flat plate reflects the wave, but for what period of time?

[frank williams]

Of all the pipes I've loved before, who traveled in and out my shop .......

Some of the longer stinger pipes .... they were used on a PA61 and for me I thought they were great ....

The metal (OPS I think) pipe on the right I was flying in 2003 in the 30+ mph winds.  I didn't know what an advantage I had at the time .. others that day couldn't make level laps much less fly a pattern.

[frank williams]

A good analogy for the wave, molecule to molecule, interaction, is a slinky stretched out and one end tapped.  The wave transfers from loop to loop down the toy.

Another interesting question is "what is pipe rattle".  The effect that inside the pipe if an expansion wave runs into a compression wave, the particle velocities add together resulting in velocities greater than either one.  The result?  possible shocks formed.

Once again I have a very favorable opinion of the ES comp. F2B pipe.  If you hold the pipe directly up to a light bulb and look down the pipe, no light penetrates the carbon.  Inside the pipe is completely black.  The front end of the pipe does not cook out at all.  I really don't think that there is any resin left in the pipe at all.

[spare_parts]

Another interesting question is "what is pipe rattle".  The effect that inside the pipe if an expansion wave runs into a compression wave, the particle velocities add together resulting in velocities greater than either one.  The result?  possible shocks formed.

Wouldn't two sound waves meeting in phase just increase the amplitude, or cancel if they meet out of phase? Velocity of pressure waves is determined by the medium. I'm not sure what "pipe rattle" is, but it's probably wave interference and resonance of the pipe itself. Shock waves require supersonic flow.

[Dave_Trible]

My first full wave pipe is laid up.  It will go into the oven after the Nats and I’ll get it tested..

Dave

[Brett Buck]

A good analogy for the wave, molecule to molecule, interaction, is a slinky stretched out and one end tapped.  The wave transfers from loop to loop down the toy.

    I have a slinky at my desk (since 1995…) for *exactly that purpose*.  It also walks down stair, alone or in pairs.

     Brett

[Brett Buck]

Another interesting question is "what is pipe rattle".  The effect that inside the pipe if an expansion wave runs into a compression wave, the particle velocities add together resulting in velocities greater than either one.  The result?  possible shocks formed.

   We were discussing the primary effect - a single standing wave acting in resonance, and implicitly, a point source driving function. Of course, there are many other higher-order modes, plus lateral waves rattling around since the exhaust port dimensions are the same order or wider than the header or the front of the pipe, AND, you get a return wave from the far side of the cylinder.   There is abundant evidence of cases where the wave gets so far out of tune that not only does it not hit the exhaust port at the right time, it blows all the way through the exhaust, intake, case, and Venturi.

     If it has baffles, there are multiple discrete waves at different phases, and reflections between the baffles as well (at vastly higher frequencies).    So, there is A LOT of stuff going on besides the one thing we want to happen.

   For the stinger experiment, I suggest using some appropriate silicone tubing to seal the stinger and tube together, and slide it in or out to make the internal end of the stinger wind up in a favorable place, because you might actually make the noise worse, rather than better.

      Brett

[M Spencer]

Generaly , a rattle is resonance .Why the ring dings - ring ding . Echo etc . ( The japs rubber plugged the fins , others used bridges . On the Cylinder & head . Tho : -

However real steel still resonates , if ' its running properly ' ( like a cymbal , slightlyish )
16 Guage muffler er silencer um ' PIPES ' echo & resonate , " On the Mega " even if baffled . slightly . Perforated tube . A series of holes held together by minimal material , that goes Red Hot to ' catalise ' unburnt fuel;
Creating a Vacum . Like a Two stroe tooned pipe . For Grand Prix if not aerobatics , Where its more a rgulator = Or Engine Control manadgement load sensative thingamybob . Shut Off & induction controler. Perhaps .

The Headers resonate too , more , in the normal exhaust case . AS , if youve run headers , as you go onto a hill & the r.pm. goes to  Tourque Peak period , the Pipes RESONATE / CRACK with shockwave .
Sounds like little end rattle or maybe piston slap . Just to ' rattle your nerves ' ( sounds like a mechanical fault . A harsh metalic  rattl rather than knock ) Presumeably pipe ' rattle ' is dimensional interaction
 with shock wave .

If its ' peak tourque ' , that'd beit .

M'Cycle exhausts a whole differant thing , but the Headers on a Good Triumph Twin ( 40 / 45 inch ) WILL .Almost any Two Stroke the echo from the thinwall tuned pipe walls / shock wave striking , is that .

STINGERS on Grand Prix stuff were to cut the cackle , Direct open gas shock emission . Apparently if theres a direct path open at the point ( time ) of combustion - its loud . Some say contry sleeve valve
dosnt have that open combustion shock emitted !

model Boat Tripe : https://www.prestwich.ndirect.co.uk/tunedpipetheory.htm

Somebody ( RD 350 ) thinks this ,

 The stinger is basically a bleed valve/orifice that controls the volume of the exiting gas.

A smaller ID pipe will hold heat in the pipe giving you more peak power (shorter pipe) with the risk of melting pistons if to small.

A larger ID will give you more low to mid by releasing more temp out of the pipe, giving the effect of a longer pipe.

The stringer can be mounted anywhere from the belly section back, it does not need to exit of the end cone.

As for length of stinger it's not so critical as the ID, I've run longer stingers but upped the ID to suit.

You can have internal stingers like I'm having to do on the RG500 pipes I'm doing now.

But after saying all that it's best to run the correct size and length stinger that the pipe was designed to run.

Off STINGERS .  Yuk . could be baloney / inaccurate . Were also ' Flame Trap ' considerations - off road .

https://dragonfly75.com/motorbike/stingersizing.html
looks like the F2B / P. A. stuff .

Better , But STINGERS likely a overcomplication  & destabliseing of run , maybe . on stunt ? or would keeping temperature in there assist . Id assume , down hill , you want it cold . No Burn .

[Tom_Fluker]

"It also walks down stair, alone or in pairs."

Out of all this brilliant conversation, all I got is that slinky song running through my head.

Imagine what the F2A guys are going through since they have to tune to the very, very narrow performance window that is also impacted by the weather.  They change length, tail cone shape and stinger diameter which not only impacts performance but heat then get to also adjust head clearance and props.  Good thing they have a fixed fuel formula.

[GERALD WIMMER]

The stringer can be mounted anywhere from the belly section back, it does not need to exit of the end cone.

Interesting none seem to do this on stunt pipes but a lot of commercial pipes for RC helicopters have the stinger/exit at 2/3rds length of the pipe.

[Dan McEntee]

Interesting none seem to do this on stunt pipes but a lot of commercial pipes for RC helicopters have the stinger/exit at 2/3rds length of the pipe.

    I have to wonder if these heli exhausts are "pipes" or just mufflers?  i remember in the early days of helicopters that one of the most popular "pipes" was a round ball, more or less. they have been all over the place since. I/C powered helis need their power in the upper midrange of their throttle. I guess it depends on when and how you want your power??

  Type at you later,
   Dan McEntee

[spare_parts]

I have to wonder if these heli exhausts are "pipes" or just mufflers?  i remember in the early days of helicopters that one of the most popular "pipes" was a round ball, more or less. they have been all over the place since. I/C powered helis need their power in the upper midrange of their throttle. I guess it depends on when and how you want your power??

They are pipes. Heli have run governed RPM for decades. Often pipes are "folded" to not be 16" long.



 Those old balls might have been tuned in volume only (Helmholtz).

[Brett Buck]

"It also walks down stair, alone or in pairs."

Out of all this brilliant conversation, all I got is that slinky song running through my head.

  You’re welcome!

     Brett

[Dave_Trible]

Today I finally got to do some test runs on the pipes I made before the Nats.  None melted!  I have one I put two separate baffles in.   It ran normally,  maybe a little louder than some others but surely killed the engine when I needled it into the wrong territory.   The second one I removed a ‘can’ from an old Billy pipe and it worked normally.   The third was the FULL WAVE pipe I made.   I didn’t think it was really any louder but when you point the nose down in a maneuver you can order a pizza waiting for it to get to the bottoms!  The sound didn’t really change to my bad ears but the brakes were applied.   I’m not sure I am where the pipe length needs to be but I think I’m liking this.   Doing my test runs in my old Driftwood.

[Dave_Trible]

Driftwood

[Doug Moisuk]

Randy makes a 75 muffler/pipe/header.

[doug coursey]

They are pipes. Heli have run governed RPM for decades. Often pipes are "folded" to not be 16" long.



 Those old balls might have been tuned in volume only (Helmholtz).

THAT IS CALLED A MUSCLE PIPE BUT MORE OF A MUFFLER,IT IS TUNED FOR HIGHER RPM LIKE 15-17 THOUSAND RPM.. WE ALSO HAD WHAT WAS CALLED A GOVERNOR WHICH CONTROLLED THE HEADSPEED OF THE BLADES,THE RPM OF THE MOTOR CHANGED AS THE LOAD CHANGED BUT THE HEADSPEED STAYED THE SAME. WE ALSO HAD SOME TUNED PIPES BY HATORI WHICH WERE MUFFLED AND USED A HEADER ALSO.

https://www.ercmarket.com/hatori-pipe-for-10cc-engines-with-hardware-HAT630/