Author Topic: What is happening in a piped setup? (Read 1790 times)

Curare · « **on:** March 15, 2018, 08:16:34 PM »

OK guys, it's a bit of an academic question, but I'm curious as to how a pipe helps in a stunt run.

Muy background in messing with pipes as been from Pattern, where initially it was get max rpm out of a screaming 61, and then over time it augmented to give boost a significantly lower rpm range (to counteract noise) and swing bigger props. Obviously speed regulation never came into it, you did that with your thumb!

So how is a piped stunt run different? Is it tuned to boost or to regulate the RPM? Also does it need to be a divergent cone style pipe or could you use a Heimholtz resonator, like a magic muffler?

Brett Buck · « **Reply #1 on:** March 15, 2018, 08:36:01 PM »

Quote from: Curare on March 15, 2018, 08:16:34 PM

OK guys, it's a bit of an academic question, but I'm curious as to how a pipe helps in a stunt run.

Muy background in messing with pipes as been from Pattern, where initially it was get max rpm out of a screaming 61, and then over time it augmented to give boost a significantly lower rpm range (to counteract noise) and swing bigger props. Obviously speed regulation never came into it, you did that with your thumb!

So how is a piped stunt run different? Is it tuned to boost or to regulate the RPM? Also does it need to be a divergent cone style pipe or could you use a Heimholtz resonator, like a magic muffler?

You run the engine faster than the peak of the resonance, when it slows down, it comes into resonance and increases the torque, limiting additional RPM loss.

Brett

Curare · « **Reply #2 on:** March 15, 2018, 08:55:45 PM »

Ahhh, so it's regulated, but gets boosted back to the original RPM, as opposed to switching to lower rpm, like a old school 4-2-4 break does!

So this brings me to my next question - you've mentioned "power" a lot in terms of a stunt run, I take it in a piped setup, this is it's ability to hold that rpm, through whatever load the propeller can apply to the engine, am I correct?

It's a bit off topic, but it does seem the ideal setup, why would you want to run a .76 flat four stroking instead of a VF on pipe?

Tim Wescott · « **Reply #3 on:** March 16, 2018, 01:39:44 PM »

Quote from: Curare on March 15, 2018, 08:55:45 PM

Ahhh, so it's regulated, but gets boosted back to the original RPM, as opposed to switching to lower rpm, like a old school 4-2-4 break does!

So this brings me to my next question - you've mentioned "power" a lot in terms of a stunt run, I take it in a piped setup, this is it's ability to hold that rpm, through whatever load the propeller can apply to the engine, am I correct?

It's a bit off topic, but it does seem the ideal setup, why would you want to run a .76 flat four stroking instead of a VF on pipe?

Brett mentions "power" with quotes. He's got some really fine rants about the misconceptions of the power needs of a stunt ship. You need power on tap, that comes on line when the engine slows, for speed regulation. You neither need nor want lots of continuous power at the limits of what the engine can do -- the name we use for that in stunt circles is "runaway".

Any two stroke (actually, I think it's any engine at all) tends to need a richer mixture the more its being loaded. When you set up a two-stroke glow fuel engine to run way richer than "normal" (note the quote marks) you're setting it up to deliver more power the slower it goes. This can happen as a 4-2 break, or with the engine running a deep four stroke, or with the engine running a wet two stroke.

I would want to run a 76 in a deep four stroke because it sounds really, really cool.

Allan Perret · « **Reply #4 on:** March 16, 2018, 09:38:58 PM »

Quote from: Brett Buck on March 15, 2018, 08:36:01 PM

You run the engine faster than the peak of the resonance, when it slows down, it comes into resonance and increases the torque, limiting additional RPM loss.

Brett

Brett:
Can you define resonance as relates to our piped 2-stroke stunt engines ?
Max torque occurs at resonance RPM ?
Is there an easy way to determine resonance RPM on the ground ?

Brett Buck · « **Reply #5 on:** March 17, 2018, 06:55:46 PM »

Quote from: Allan Perret on March 16, 2018, 09:38:58 PM

Brett:
Can you define resonance as relates to our piped 2-stroke stunt engines ?
Max torque occurs at resonance RPM ?
Is there an easy way to determine resonance RPM on the ground ?

Resonance occurs when the time it takes the compression/acoustic wave inside the pipe to travel from the exhaust port to the reflecting surface and back is the same time it takes from the exhaust port opening to it closing again. With a 145 degree exhaust duration, the time is the time it takes for the engine to rotate 145 degrees. Assuming 1300fps speed of sound and 18.5" pipe length (for total travel of 37 inches/3.08 feet), the wave takes about 2.37 milliseconds to go out and back. That corresponds to about 10,200 rpm, which sounds about right for a in-flight RPM of around 11,000.

It is *presumed* that the maximum torque occurs at this frequency, but the only sure way to tell is by testing, you could do a torque curve with and without the pipe, and see where the peak deviation occurs. There are many complicating factors, like the fact that most pipes have multiple reflecting surfaces and we traditionally measure by distance from the glow plug to the first baffle (which is just one of 3-4 possible reflecting surfaces. It might be a sharp "crack" but when the echo comes back, it's smeared out over a longer period.

I strongly suggest, stick with a known system to start with, then learn by doing.

Note that I am just talking about a pressure wave moving through the exhaust - the actual exhaust molecules flow one way, out the exhaust. The pressure wave causes them to bunch up and spread out (since a high pressure caused by the shock of the "crack" of the exhaust is followed by a low pressure) but they continue to move out towards the end.

The effect of the wave at the exhaust port is first to create a low pressure as the port continues to stay open, enhancing the scavenging of exhaust and thus the entrance of more charge as the cylinder pressure goes down, then the wave comes back, and pressurizes the charge right as the port closes, packing in a denser charge. Both of those things increase the fuel/air charge of the engine, make it fire more strongly.

This can have startling effects, but most famously for model airplanes, Bill Wisniewski and the US team used tuned exhausts to blow away the field in FAI at the 68 WC, beating everybody by 20 MPH, roughly a 50% increase in power all in one step.

Brett

Istvan Travnik · « **Reply #6 on:** March 17, 2018, 08:08:00 PM »

However not Bill Wisniewsky invented the tuned pipe, it was a brilliant idea of him to borrow it from racing motorcycles and glider racing boats.
From my kid years (5-6) I felt first time the smell of burning castor oil, as came out of screaming "slippers" at Danube river. It was round at 1960...

Brett Buck · « **Reply #7 on:** March 17, 2018, 08:22:18 PM »

Quote from: Istvan Travnik on March 17, 2018, 08:08:00 PM

However not Bill Wisniewsky invented the tuned pipe, it was a brilliant idea of him to borrow it from racing motorcycles and glider racing boats....

I am well aware of that. Wild Bill Netzeband had suggested the idea at least 10 years earlier, and he certainly didn't invent it. It goes back to at least the mid-late 30's.

Brett

Allan Perret · « **Reply #8 on:** March 18, 2018, 10:10:40 PM »

Quote from: Brett Buck on March 17, 2018, 06:55:46 PM

Resonance occurs when the time it takes the compression/acoustic wave inside the pipe to travel from the exhaust port to the reflecting surface and back is the same time it takes from the exhaust port opening to it closing again. With a 145 degree exhaust duration, the time is the time it takes for the engine to rotate 145 degrees. Assuming 1300fps speed of sound and 18.5" pipe length (for total travel of 37 inches/3.08 feet), the wave takes about 2.37 milliseconds to go out and back. That corresponds to about 10,200 rpm, which sounds about right for a in-flight RPM of around 11,000.

It is *presumed* that the maximum torque occurs at this frequency, but the only sure way to tell is by testing, you could do a torque curve with and without the pipe, and see where the peak deviation occurs. There are many complicating factors, like the fact that most pipes have multiple reflecting surfaces and we traditionally measure by distance from the glow plug to the first baffle (which is just one of 3-4 possible reflecting surfaces. It might be a sharp "crack" but when the echo comes back, it's smeared out over a longer period.

I strongly suggest, stick with a known system to start with, then learn by doing.

Note that I am just talking about a pressure wave moving through the exhaust - the actual exhaust molecules flow one way, out the exhaust. The pressure wave causes them to bunch up and spread out (since a high pressure caused by the shock of the "crack" of the exhaust is followed by a low pressure) but they continue to move out towards the end.

The effect of the wave at the exhaust port is first to create a low pressure as the port continues to stay open, enhancing the scavenging of exhaust and thus the entrance of more charge as the cylinder pressure goes down, then the wave comes back, and pressurizes the charge right as the port closes, packing in a denser charge. Both of those things increase the fuel/air charge of the engine, make it fire more strongly.

This can have startling effects, but most famously for model airplanes, Bill Wisniewski and the US team used tuned exhausts to blow away the field in FAI at the 68 WC, beating everybody by 20 MPH, roughly a 50% increase in power all in one step.

Brett

Thanks for the explanation..

What event was this ?
"Bill Wisniewski and the US team used tuned exhausts to blow away the field in FAI at the 68 WC, beating everybody by 20 MPH, roughly a 50% increase in power all in one step."

Brett Buck · « **Reply #9 on:** March 18, 2018, 10:18:15 PM »

Quote from: Allan Perret on March 18, 2018, 10:10:40 PM

Thanks for the explanation..

What event was this ?
"Bill Wisniewski and the US team used tuned exhausts to blow away the field in FAI at the 68 WC, beating everybody by 20 MPH, roughly a 50% increase in power all in one step."

1966 World Championships, a complete blowout. I had the wrong year above.

http://controlline.org.uk/microair/f2a/66WC.htm

http://www.modelenginenews.org/people/wiz/wiz_pipe_man.html

Brett

David Ruff · « **Reply #10 on:** April 13, 2018, 07:41:50 AM »

I am quite sure there is plenty of data on this that comes from the two stroke motorcycle world. Some manufacturers still produce two stroke bikes with expansion chambers. That is what we called them. Not pipes.

Brett Buck · « **Reply #11 on:** April 13, 2018, 09:05:48 AM »

Quote from: David Ruff on April 13, 2018, 07:41:50 AM

I am quite sure there is plenty of data on this that comes from the two stroke motorcycle world. Some manufacturers still produce two stroke bikes with expansion chambers. That is what we called them. Not pipes.

The most common reference is the "Two-Stroke Tuners Handbook":

http://www.amrca.com/tech/tuners.pdf

wherein many of the basic concepts are explained. But there has never been a topic more thoroughly and accurately discussed, at extraordinary length, in the history of stunt.

One thing that is wildly different about current engines and piped systems compared to ancient engines and techniques is that you can copy someone else's (already developed and tested) system and have a reasonable expectation that it will work the same with your engine and other parts, or at least close enough to work. So, the budding piped system owner can (and should) find an established standard system and copy it to start with. Many such systems have been published.

The other thing to know is if someone have an established system and it doesn't seem to work correctly, the VERY LAST THING you will need to adjust or modify is the pipe length. The problem almost certainly lies elsewhere, but budding piped engine operators tend to assume every problem they have is a pipe length issue. David Fitzgerald and I were out flying flight after flight from 8 AM to 8 pm at the 93 NATs, never touched anything, and if anything it was WAY too much practice to be effective. But bulletproof, one flight after another. A bunch of people wandered by and either asked for help or we saw struggling in adjacent circles. In *every single case*, they had an incorrect pipe length, AND, were wildly adjusting it from flight to flight in large increments - and in every case, jumping across the correct length.

Brett

Jim Kraft · « **Reply #12 on:** April 14, 2018, 12:44:19 PM »

Back in my R/C pattern days, I pretty much set my pipe up like the control line guys do. Long enough so that when I really needed the power as the rpm dropped, the pipe would come on. I saw way to many where when the engine falls off the pipe, you lose all that you might have gained where you need it. This was on a YS 60 short stroke on 12-9 APC props.

When you get to the top of a vertical climb, and pull over to level flight, the plane is at its slowest, and needs all the power at that point to make it easy. And at the same time in the double immelman you need to do a half roll. When you loose to much speed the torque begins to pull the plane off heading. The 120 four strokes changed all that. Gobs of power every where. But then they took away the displacement which put 2 strokes back in the game. R/C pattern changes all the time. Bigger planes with gobs of power and big bucks. For those with gobs of money and lots of spare time, it is fun.

I will say if it was not for R/C pattern we would not have all the goodies we have at reasonable prices. Just look at the modern stunt ship. Carbon push rods, turned pipes, ball link connectors, light weight wheels and spinners, and super engines. Now, electric. As I remember right it was an R/C pattern flyer that came up with the idea of turned pipes in stunt.

David Ruff · « **Reply #13 on:** April 14, 2018, 12:57:20 PM »

I am always grateful for all the great people on this site. It is an honor to be on this site and gain the knowledge and tips from the true greats and experts.

Thanks for being here.

Author Topic: What is happening in a piped setup? (Read 1790 times)

Curare

What is happening in a piped setup?

Brett Buck

Re: What is happening in a piped setup?

Curare

Re: What is happening in a piped setup?

Tim Wescott

Re: What is happening in a piped setup?

Allan Perret

Re: What is happening in a piped setup?

Brett Buck

Re: What is happening in a piped setup?

Istvan Travnik

Re: What is happening in a piped setup?

Brett Buck

Re: What is happening in a piped setup?

Allan Perret

Re: What is happening in a piped setup?

Brett Buck

Re: What is happening in a piped setup?

David Ruff

Re: What is happening in a piped setup?

Brett Buck

Re: What is happening in a piped setup?

Jim Kraft

Re: What is happening in a piped setup?

David Ruff

Re: What is happening in a piped setup?