How to properly calculate and build a resonant tube ???

[Ruslan Kurenkov]

Hello, friends !
I want to calculate the dimensions myself and build a resonant tube with my own hands for my engine Stalker 76 and Stalker 81. Many athletes from different countries want to exploit their engines Stalker with resonant tubes instead of the usual muffler. I also want to try myself how my airplane with a Stalker with a resonant tube will fly and I want to help them, and after successful tests give these athletes the correct dimensions of the resonant tube that will work on their engines. Buy from the manufacturer or make themselves these athletes will decide for themselves when they will have the exact size.
Adjusting the engine speed in the air should be - 8500 rpm.
The start angle of the exhaust is 135 degrees.
I drew a simple drawing of a pipe with an engine and a resonant pipe. In this picture I have indicated all the important dimensions for production. All sizes have a number. I need at least an approximate size for a reference point (for a starting point).
I need any help and any information that will help me, and most importantly - which is not someone’s intellectual property and can be freely posted here on this forum and according to the forum rules.

Regards
Ruslan Kurenkov

[Perry Rose]

I would buy one and cut it apart to find what you need.

[Brett Buck]

Hello, friends !
I want to calculate the dimensions myself and build a resonant tube with my own hands for my engine Stalker 76 and Stalker 81. Many athletes from different countries want to exploit their engines Stalker with resonant tubes instead of the usual muffler. I also want to try myself how my airplane with a Stalker with a resonant tube will fly and I want to help them, and after successful tests give these athletes the correct dimensions of the resonant tube that will work on their engines. Buy from the manufacturer or make themselves these athletes will decide for themselves when they will have the exact size.
Adjusting the engine speed in the air should be - 8500 rpm.
The start angle of the exhaust is 135 degrees.
I drew a simple drawing of a pipe with an engine and a resonant pipe. In this picture I have indicated all the important dimensions for production. All sizes have a number. I need at least an approximate size for a reference point (for a starting point).
I need any help and any information that will help me, and most importantly - which is not someone’s intellectual property and can be freely posted here on this forum and according to the forum rules.

    So, as previously - what sort of effect are you trying to have? You can set it up to provide power boost at 8500 RPM, you can set it up to regulate better at 8500 rpm (meaning you want it tuned for a lower RPM), or something else.

    Per the other thread (on SSW), you should be able to calculate the resonant RPM. For example, set up as a regulator, if you want to operate at 8500 RPM in-flight, I would guess you want to tune it to about 7600-7800 RPM, thats 128 revs per second. Exhaust opens for 135 degrees of that rev, so 2.93 milliseconds. At 1400 feet/second, that is ~4.1 feet, or 49.2". Divide that in two, since it goes to the reflecting surface and back in that time, gives you 24.6". This depends on the 1400 fps speed of sound. Make the pipe diameter larger, the exhaust cools more, speed of sound slows, so the pipe needs to be longer. If it is metal, the heat radiates better, cools off, plastic or graphite/epoxy and it stays warmer and faster. Bill Wisniewski talks about coating his (metal) pipe with insulation to make his length calculations work out.

    I am a bit concerned about the shape you drew -most tuned pipes have a convergent tail cone at a pretty shallow angle, maybe a 45 degree included angle, or more shallow. The tail cone, as a reflective surface, sets how wide the return wave is. A flat surface at 90 degrees to the long axis returns a very sharp echo with an extremely narrow range of operation - a lot of boost over a narrow range. A shallow cone returns a wave that is spread out over time, less boost with a wide range. Internal baffles return multiple waves that can be closely-spaced, or, causing multiple tuning peaks (each baffle returns an echo, and the tail cone returns one, too). Brian Eather pipes look like this but count on a nest of baffles at the end to create a wider return peak.

    If I was doing it, given the low power level, I would probably make the constant diameter section about 1 3/4", and the outlet maybe 3/8" ID. That would be on the small side for a PA75, but we are talking maybe 50% more shaft HP since it runs with a much less efficient prop.

  But again, I can only see this using a large, high-pitch prop, which gives you very high prop efficiency, and corresponding lower performance. As a power enhancement, at low revs, you can only make the prop larger, raising the efficiency even more, which just reduces the performance further. We use tuned pipes primarily to permit the use of very low efficiency props on engines we need to tame, not to "get more power", so you are going to have to determine what your goal is before anyone can give much more guidance. And in any case, it will require *experimentation*, probably extensive, with people able to evaluate the overall results, to get the right answer.

  It's not just an exhaust pipe, it's a package deal. You need it to accomplish some engineering goal, that has to be defined up front.

    Brett

[Ruslan Kurenkov]

I would buy one and cut it apart to find what you need.

Hello Perry
Thank ! I will definitely listen to your advice!

    So, as previously - what sort of effect are you trying to have? You can set it up to provide power boost at 8500 RPM, you can set it up to regulate better at 8500 rpm (meaning you want it tuned for a lower RPM), or something else.

    Per the other thread (on SSW), you should be able to calculate the resonant RPM. For example, set up as a regulator, if you want to operate at 8500 RPM in-flight, I would guess you want to tune it to about 7600-7800 RPM, thats 128 revs per second. Exhaust opens for 135 degrees of that rev, so 2.93 milliseconds. At 1400 feet/second, that is ~4.1 feet, or 49.2". Divide that in two, since it goes to the reflecting surface and back in that time, gives you 24.6". This depends on the 1400 fps speed of sound. Make the pipe diameter larger, the exhaust cools more, speed of sound slows, so the pipe needs to be longer. If it is metal, the heat radiates better, cools off, plastic or graphite/epoxy and it stays warmer and faster. Bill Wisniewski talks about coating his (metal) pipe with insulation to make his length calculations work out.

    I am a bit concerned about the shape you drew -most tuned pipes have a convergent tail cone at a pretty shallow angle, maybe a 45 degree included angle, or more shallow. The tail cone, as a reflective surface, sets how wide the return wave is. A flat surface at 90 degrees to the long axis returns a very sharp echo with an extremely narrow range of operation - a lot of boost over a narrow range. A shallow cone returns a wave that is spread out over time, less boost with a wide range. Internal baffles return multiple waves that can be closely-spaced, or, causing multiple tuning peaks (each baffle returns an echo, and the tail cone returns one, too). Brian Eather pipes look like this but count on a nest of baffles at the end to create a wider return peak.

    If I was doing it, given the low power level, I would probably make the constant diameter section about 1 3/4", and the outlet maybe 3/8" ID. That would be on the small side for a PA75, but we are talking maybe 50% more shaft HP since it runs with a much less efficient prop.

  But again, I can only see this using a large, high-pitch prop, which gives you very high prop efficiency, and corresponding lower performance. As a power enhancement, at low revs, you can only make the prop larger, raising the efficiency even more, which just reduces the performance further. We use tuned pipes primarily to permit the use of very low efficiency props on engines we need to tame, not to "get more power", so you are going to have to determine what your goal is before anyone can give much more guidance. And in any case, it will require *experimentation*, probably extensive, with people able to evaluate the overall results, to get the right answer.

  It's not just an exhaust pipe, it's a package deal. You need it to accomplish some engineering goal, that has to be defined up front.

    Brett

Hello Brett
Thanks for a lot of useful information.
I want to get from the pipe only adjustment of revolutions in the air - 8500 rpm.
Engine - only Stalker 76 (exhaust phase - 135 degrees.)
The propeller is only 3 blades 13.5 / 6.5 - 6 inches.
I do not want to get an increase in power from the pipe.
I do not want to get something else out of the pipe work.
Engine speed on the ground 7600-7800 rpm
The shape of the pipe I took on the photo from the Internet. Probably this is the shape of the pipe Randy Smith produces. If the pipe at the end should taper like F2A pipe, then I need even more sizes.
The most important question for me is what is “pipe length”, between what should be 24.6 inches?
If it is not difficult for you, draw and publish please the shape of the pipe which you consider optimal for Stalker 76.
Also I am interested in the presence of an internal partition P1
If it is needed then at what distance from the glow plug should it be installed and what diameter should there be a hole in it
What is the internal volume of a pipe?
What should be the diameter of a straight pipe section with a length of 1 3/4 in?
I think that in some detail I defined my engineering goal as an aviation engineer by education.

Regards,Ruslan

[Tim Wescott]

If you're doing searches in English, "tuned pipe" will get you a lot more hits than "resonant tube".  Martin Hepperle's site had a tuned pipe calculator, but it was for going fast, not for regulating speed.  There's a lot of other tuned pipe calculators out there -- even some of the ones for motorcycles and boats will work down to model engine sizes (although I haven't checked to see how much agreement there is from one site to another).  I suspect that if you go with Brett's recommendation on setting the target speed that you'll be in the ballpark (because I trust Brett, not because I know what I'm talking about).

[Igor Burger]

Ruslan, I highly recommend to copy existing solution with all details from some working solution, then you can try to experiment and to see if it helps or not.
(And that is not only this problem, it is also about electric, models, airfoils etc.)

There are too many variables, if you start from "opposite side" (for example you try to use >6" pitch props instead 3.5" to 4" as we used with pipes) you can only hardly reach (I mean amount of work, I certainly do not mean some kind of impossibility, ability or so) good result, especially in case that there is noone using it around you. So you cannot compare. BUT - since I know there are many flyers who went to electric from pipes, I am sure there are plenty of retired pipes so you can take and try. On picture you can see my set, if you want one of them, it is your 

Small hits for you:
- you will need extremaly long pipe if you use 6" prop (6" instead of 4" means 50% longer pipe)
- you will get better result with large wolume pipe (converts to large diameter) if you are beginner with pipes = another problems with installation
- those internals will define character of resonation and thus way how it goes ON and OFF ... or does not go at all ... all those pipes are designed for schnuerle porting with boldown higher that your motors use, so you will need to redesign them (will need experience and time) ... they are usually 2 (in form of baffles) with different ability of reflecting wave (number and size of holes)
- you will be disapointed with its sound, you will try to invent silencer - even more space in fuselage

[Ruslan Kurenkov]

Ruslan, I highly recommend to copy existing solution with all details from some working solution, then you can try to experiment and to see if it helps or not.
(And that is not only this problem, it is also about electric, models, airfoils etc.)

There are too many variables, if you start from "opposite side" (for example you try to use >6" pitch props instead 3.5" to 4" as we used with pipes) you can only hardly reach (I mean amount of work, I certainly do not mean some kind of impossibility, ability or so) good result, especially in case that there is noone using it around you. So you cannot compare. BUT - since I know there are many flyers who went to electric from pipes, I am sure there are plenty of retired pipes so you can take and try. On picture you can see my set, if you want one of them, it is your 

Small hits for you:
- you will need extremaly long pipe if you use 6" prop (6" instead of 4" means 50% longer pipe)
- you will get better result with large wolume pipe (converts to large diameter) if you are beginner with pipes = another problems with installation
- those internals will define character of resonation and thus way how it goes ON and OFF ... or does not go at all ... all those pipes are designed for schnuerle porting with boldown higher that your motors use, so you will need to redesign them (will need experience and time) ... they are usually 2 (in form of baffles) with different ability of reflecting wave (number and size of holes)
- you will be disapointed with its sound, you will try to invent silencer - even more space in fuselage

Hello Igor
Thanks for the professional advice. I will definitely take your advice.
Now for the option for production and testing I take the dimensions of the pipe for the PA 75 and for comparison the dimensions of the pipe for the RO-Jet engine.
The propeller with step 6, I choose only because of the working speed of the engine. Engine Stalker works only in the rev range from 7500 - 7800 on the ground to 8200 - 8500 revolutions in the air. This is due to the deflector on the piston, transverse (single-channel) purge and a large piston stroke. At revolutions of 9000 and higher, the speed of the piston increases greatly, the motor starts to warm up and holds the mode poorly. It's like a diesel engine to spin up to 7000 rpm and go in second gear. (if you install the propeller with a screw pitch 4 in.)
If you reduce the pitch of the propeller to 4 in - then you have to greatly increase the diameter.
I will try, I will experiment, in order not to make mistakes with the production of the pipe and listen to all the advice and comments, I opened this post here in the general forum.
In the photo I see that you had a lot of experience with pipes. Did you design them and make them yourself?
 I see you with electric motors for so long that I was even surprised, but then I remembered your OS MAX 46 with a side exhaust installed in a cylinder down and to the side by 225 degrees.
Regards
Ruslan

[Lauri Malila]

I don't remember well, but was it Igor & Co who used Retro .60 with some kind of minipipe (Bajer..?) at some point in end of last century? -97 or so.
L

[Igor Burger]

The propeller with step 6, I choose only because of the working speed of the engine. Engine Stalker works only in the rev range from 7500 - 7800 on the ground to 8200 - 8500 revolutions in the air. This is due to the deflector on the piston, transverse (single-channel) purge and a large piston stroke. At revolutions of 9000 and higher, the speed of the piston increases greatly, the motor starts to warm up and holds the mode poorly. ...

If you reduce the pitch of the propeller to 4 in - then you have to greatly increase the diameter.

In the photo I see that you had a lot of experience with pipes. Did you design them and make them yourself?

1/ Well, yes, they heat more, because they do more power probably at lower efficiency (because of regulation) that is vhy typical piped motors need much more fuel and thus much better internal cooling. You run them lean, it needs I guess 200ccm fuel, not 160. That is what I am telling you, you go from opposite side. So single suggestions from Brett wil probably not work for you. Therefore I suggest to take whole setup, make it working and only then to tweak it your way if you see you need it.

2/ I do not think, if you use smaller pitch, motor will make more RPM and you will need SMALLER diameter for the same power (and you will have MORE power). That is trick we use on electric - instead of using large and heavy 2 blade 13x7" prop, we nicely use 11x5 3 blade and it gives the same power and reduces all problems of large prop and even accelearte quicky when necessary (small moment of inertia).

3/ Most of those carbon pipes are from Randy Smith, 2 of those thick are from Aldrich and those alu are made here by my specification, but not by myself because I do not have machine for alu molding. However since electric is "somehow easier" I do not fly them anymore. Yes I had OS MAX LA .46 but not only that. I used piped Jetts from Aldrich, PAs from Smith, MVVS with rear intake etc. All now sleeping in boxes :-P

[Igor Burger]

I don't remember well, but was it Igor & Co who used Retro .60 with some kind of minipipe (Bajer..?) at some point in end of last century? -97 or so.
L

Yes Bajer tried it but also other guys around, however Bajer was of your nature, almost every day shawed some piece of metal from them (modified scavenging, baffles, ports, PDP etc), and every day another configuration, I think he stopped with C/L when there was nothing to remove anymore :-P

On picture is his model with side mounted Retro and alu pipe of type "magic muffler".

[Brett Buck]

1/ Well, yes, they heat more, because they do more power probably at lower efficiency (because of regulation) that is vhy typical piped motors need much more fuel and thus much better internal cooling. You run them lean, it needs I guess 200ccm fuel, not 160. That is what I am telling you, you go from opposite side. So single suggestions from Brett wil probably not work for you. Therefore I suggest to take whole setup, make it working and only then to tweak it your way if you see you need it.

    That's what I was trying to get at earlier (and in the off-line conversations about the same thing). I can't even give a theoretical number without knowing the goal. The 25" length will give a tuning peak at 8500 +- a bit for the variation of the speed of sound/temperature. It will have more torque/power at the tuned RPM and either side of it, reduced torque/power in other spots. Basically, you are creating a relative peak at the tuned RPM and valleys elsewhere.

   But what you do with that change - reduce the pitch (increase performance), increase the diameter (increase efficiency/decrease performance), run the same prop and make it more variable/peaky (with, presumably, a smaller venturi)?   I can't tell anyone what the length should be without having some idea how it will be used to increase the performance.

    What everyone else (in stunt) does with piped systems, whether they realize it or not, it create a relative peak below the normal operating RPM, and put the operating point on the downslope. It might even not make any more power at the normal level flight operating point than it would have otherwise. But, when maneuvering drags down the RPM, it climbs the peak, resisting further change better than it would otherwise, similarly, when the prop unloads, it falls further towards the valley, also resisting further RPM increase. That's what I would start out with for a low-rev system, too, lacking any better idea.

   But the reason we need a regulator in the first place is because we have extremely excessive power before we even start, and have to find some way to regulate it in a positive way. Once you have a regulator, you can take advantage of a very similar regulating effect in the prop efficiency (lower  efficiency = better speed stability) and have it operate in level flight at  an extreme level of inefficiency. Early systems like the 46VF/12-3.25 prop system Ted ran (and my similar 40VF system) was probably operating at 1.25-1.5 HP shaft power for the required .4ish HP the airplane needed/needs to fly around in level flight.

     We have gone a little away from that for a variety of reasons (the ear-splitting noise, for example), but we still have setups like David's, where he is flying an airplane we would have considered on the small side for an ST46 with a piped 75. No pipe, and a maybe 15-6 or 16-6 prop with muffler at low revs, and that would be a complete disaster - not because there is anything wrong with a PA75 (far from it...), but because you would have extreme difficulty dealing with even slight power variations. Not to mention you aren't cranking a 16" prop around a corner without extreme trim difficulty, no matter how you build it.

  This is not intended to be perjorative, but we are talking about a huge engine that currently turns a 13-6 at about the same speed as an ST60 in about the way an ST60 used to do it. This is not a lot of power or performance compared to what we have been running for the past 30 years or so. Run it with the same prop at the same RPM and it doesn't matter what engine it is, aside from how it reacts to maneuvering, you are getting the same power levels. In fact the bigger the engine, the more difficult the regulation issue becomes.  Compare that to what you could do with PA75 or Jett76 run the same way - same pitch, same RPM - and 2-3" (or more) diameter. But you can't (or don't want to) do that for a lot of other reasons, and you would have to deal with the efficiency increase somehow.

    *The prop* is what it all comes down to - if you are going to run the same prop, you are going to get about the same performance and the same nominal power level no matter what engine you have spinning it. All you are doing with other features or larger or bigger engines is changing how much the power varies in flight.

    Brett

[Ruslan Kurenkov]

1/ Well, yes, they heat more, because they do more power probably at lower efficiency (because of regulation) that is vhy typical piped motors need much more fuel and thus much better internal cooling. You run them lean, it needs I guess 200ccm fuel, not 160. That is what I am telling you, you go from opposite side. So single suggestions from Brett wil probably not work for you. Therefore I suggest to take whole setup, make it working and only then to tweak it your way if you see you need it.

2/ I do not think, if you use smaller pitch, motor will make more RPM and you will need SMALLER diameter for the same power (and you will have MORE power). That is trick we use on electric - instead of using large and heavy 2 blade 13x7" prop, we nicely use 11x5 3 blade and it gives the same power and reduces all problems of large prop and even accelearte quicky when necessary (small moment of inertia).

3/ Most of those carbon pipes are from Randy Smith, 2 of those thick are from Aldrich and those alu are made here by my specification, but not by myself because I do not have machine for alu molding. However since electric is "somehow easier" I do not fly them anymore. Yes I had OS MAX LA .46 but not only that. I used piped Jetts from Aldrich, PAs from Smith, MVVS with rear intake etc. All now sleeping in boxes :-P

Hi Igor
Thank you very much for your information.
My construction and testing of the resonant tube is based on reports from athletes who have already installed and are using different sized resonant tubes with Stalker engines. They all have different results, someone else has good results, and someone else has bad results. There is not one common pipe adjustment system for all - and therefore there is no stable good result. My task is to find the settings that will be guaranteed to work on the engine chosen by me. I decided that it was Stalker 76. Now I almost made a connector for connecting the engine to the pipe.
 Maybe I will not be able to achieve a good result. Then it will also be the result of which everyone will know and will not repeat my mistakes. Then they will take their own choices and go their own way.
Now we use engines Stalker 66 and less (61 and 51) in 4-2-4 mode. Engine Stalker 76 and 81 - only in mode 4, this is due to excess power. All of these engines take off with a highly open fuel supply (rich mode). With such a strong fuel supply, good fuel cooling occurs at 8200 rpm in air, and 100 ml (3.4 oz) of fuel with a propeller of 360 mm / 180 mm (14.2/7 in) are needed for 5 minutes of flight.
If you install a propeller with a reduced pitch of 360 mm / 160 mm(14.2/6.3 in), the increase in revolutions up to 8500 rpm occurs by reducing the fuel supply, which leads to a decrease in engine cooling with fuel, the engine heats up more and leads to more fuel consumption, the flight takes 5 minutes - 120 ml ( 4.1 oz) of fuel.
Using these long-term engine performance data obtained from more than 10 years of experience with a large number of flights, I believe that the optimal operating mode of the Stalker 76 engine is maximum speed - 8500 rpm. For these revolutions, the optimum three-blade propeller will be a 13.4 in / 6.5 in. Propeller. for cold air and 13.7 in / 6.5 in for hot and discharged air.

Regards
Ruslan

[Ruslan Kurenkov]

    That's what I was trying to get at earlier (and in the off-line conversations about the same thing). I can't even give a theoretical number without knowing the goal. The 25" length will give a tuning peak at 8500 +- a bit for the variation of the speed of sound/temperature. It will have more torque/power at the tuned RPM and either side of it, reduced torque/power in other spots. Basically, you are creating a relative peak at the tuned RPM and valleys elsewhere. 

    Brett

Hello Brett
I read your message very carefully several times. You wrote everything in detail and everything is correct with regard to the theory and practice of using customized exhaust for engines PA , Ro-Jet and OS - MAX. I think that these are the three main champion engines for aerobatic model aircraft in America. For these engines for dozens of years of operation a theory of use has been fully formed. This system consists of an engine, a resonant tube and a propeller.
My goal and my task now is to form, with your help, a similar working system using a high-tech piloting engine with a deflector on the piston and with a single-channel longitudinal blowing - Stalker 76 RE.
Acceptable revs for this engine using a conventional muffler - 8500 rpm
Perhaps it is even 9000 rpm in the air for the resonant tube, if you greatly increase the flow of air and fuel to the engine to increase the cooling of the fuel. For this, I will test the system with various pipes, various propellers and various air holes in the fuel supply system. Now I received a lot of useful information about the device of resonant tubes from my friends from America, Spain and Australia as a friendly help. To implement all these ideas will take a lot of time.
If you, as an athlete who is very well versed in the device of a resonant tube, will have constructive suggestions for various configuration options for a resonant tube for my engine, then I will be very happy to get any help from you.
Regards
Ruslan