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Author Topic: Piped Engine Run  (Read 806 times)

Offline G McClintic

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Piped Engine Run
« on: October 08, 2019, 05:46:01 PM »
Hello - I have been reading a lot on this forum, and I see much information about engine setup, especially the 4-2-4 break run. I ran a SuperTigre 46 that way for many years.

My question: Does a Piped engine run a 4-2-4 break as well? Or is it a steady 2 cycle with a low pitch prop? What is the advantage of using a Pipe? Does it try to stay "on-the-pipe" meaning it holds the rpm relatively constant with varying load?

Sorry if this has already been covered - maybe point me to a thread if one exists?

Thanks

Glenn

Online Brett Buck

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Re: Piped Engine Run
« Reply #1 on: October 08, 2019, 11:06:49 PM »
My question: Does a Piped engine run a 4-2-4 break as well? Or is it a steady 2 cycle with a low pitch prop?

   Either way, I run mine in a constant 4-stroke, others run with a break, and others run in a constant 2-stroke. The key is the low-pitch prop - a piped 61 has no problem running props like a 13-4 3-blade in a constant 4-stroke at a sufficient RPM.


Quote
What is the advantage of using a Pipe? Does it try to stay "on-the-pipe" meaning it holds the rpm relatively constant with varying load?

    It generally runs "over the top" of the pipe tuning, that is, past the tuned peak, so it regulates the airspeed  - slower, and it comes closer to the tuned peak, inhibiting further drop, and if it speeds up, it goes further out of tune, inhibiting further speed increase. The RPM moves up and down a lot, and the same thermal effect that causes a 4-2 break will cause speed overshoot and undershoot even if you don't change the misfire pattern.

    The real trick is that the basic engine, before the pipe, is so vastly more powerful than we ever had before that "getting enough power", which was always a struggle before, is a non-issue. But, for the most part, taking these ultra-powerful RC schneurle engines and trying to run them at 9000 RPM with no other factors is more-or-less impossible - and the pipe regulation is more to make the power manageable, rather than get more of it. Reducing the pitch, even with no other change, also gives vastly more speed stability.

   You now choose the engine based on how it responds, not how much raw power it has, because any engine from a piped 40 to an 88 has far more than enough power to fly the airplane. This arrangement has been the defacto baseline approach to competitive stunt since about 1990, only recently has electric been comparable.

   It has also been documented in as much detail and length as anything in stunt. Unfortunately for your purposes, most of it predates the internet, or is so well-known that it is hardly even discussed any more.

     
    Brett

Offline Peter in Fairfax, VA

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Re: Piped Engine Run
« Reply #2 on: October 09, 2019, 07:50:24 AM »
Brett,

Simply wanted to thank you for your clear writing on this topic. 

This being my first season of flying stunt since about 1974 when the Fox .35 / Nobler was relatively popular at the local level, I was curious on how the newer pipe technology was deployed. 

I'm interested in trying a pipe setup and will be on the lookout for a built stunter, pipe equipped.  There are some prolific builders out there who enjoy trimming / tuning, which is a good fit for my approach to the sport, which is to concentrate my efforts on practicing at the flying field every week, entering contests.

Peter

Offline G McClintic

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Re: Piped Engine Run
« Reply #3 on: October 09, 2019, 10:24:35 AM »
Thanks Brett, that's really informative. Exactly the information I was looking for.

I plan to build an Oriental Plus with a PA engine and pipe, and needed to understand about the setup.

Thanks again

Glenn

Online Brett Buck

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Re: Piped Engine Run
« Reply #4 on: October 09, 2019, 10:52:27 AM »
Simply wanted to thank you for your clear writing on this topic. 

  You are quite welcome but be aware that other people, Bob Hunt in particular, were at it a long time before I was!

    I would also add that tuned pipe systems completely transformed the way the event is flown. Paul Walker was the first to make full use of the performance and that pretty much reset the bar for being competitive. The fact that the engines are incredibly powerful and easy-to-deal with also raised the performance for everyone, and narrowed up the range of skills. Now everybody can have an adequate engine run, which certainly wasn't the case before. 

      I flew extensively before and after this big change (in the 1990-1994 time frame), so I have some perspective. It's a different world. Now, you have other solutions, too, that are just as good (electric with feedback control) and 4-strokes (which are at least adequate if you follow the directions to the letter) but nothing is tremendously better. Electric will eventually dominate, because piped systems are about as good as then are ever likely to be, but electric is already on a par with it, and has abundant room for improvement.

   BTW, you can get 95% of the same effect with conventional muffler engines with low-pitch props (like the OS 20FP or 25LA (new) with a 9-4 APC, OS 46LA, Magnum 36) that are more suitable for smaller airplanes originally intended for Fox 35s. You wouldn't seriously consider putting an OS40VF and pipe in a Green Box Nobler - it would work but it's nuts to put 20 ounces of propulsion system in an airframe that might weigh only 20 ounces by itself. 25LA (new) is an ideal solution.

     An interesting point about that - these engines run nearly perfectly for our purposes with all the stock parts, maybe just replacing the RC carb with an appropriate venturi. But, there are legions of people modifying them to run like Fox 35s, which generally causes loss of their superior characteristics and makes them no better than old crock engines like ST46s, etc. Randy is one of the few people I would trust for something like this, but frequently, it is not necessary and they work very well with absolutely *no* modifications.

    Brett

     

Online Brett Buck

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Re: Piped Engine Run
« Reply #5 on: October 09, 2019, 10:54:52 AM »
Thanks Brett, that's really informative. Exactly the information I was looking for.

I plan to build an Oriental Plus with a PA engine and pipe, and needed to understand about the setup.


  PA40UL, maybe? Randy should get you set up.  I will also inform Michael Scholtes about this thread when I see him, because his 40UL setup works much better than anyone else's, at least around here.

    Brett

Offline Peter in Fairfax, VA

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Re: Piped Engine Run
« Reply #6 on: October 22, 2019, 04:16:27 PM »
Further question:  for a given pipe, does varying the header length tune?

Offline RandySmith

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Re: Piped Engine Run
« Reply #7 on: October 22, 2019, 04:37:49 PM »
Further question:  for a given pipe, does varying the header length tune?

Peter
You want to use  the header  full length, and  trim  the front of the  CF Pipe, The pipe  will  tune off of the  first Baffle  in the  CF Stunt pipes, cutting the header  puts the pipe closer to the  hot  engine, and  you do not  pick up much in weight savings, as the  Aluminum tube is very light.
Typical lengths  run from  16.75  to 20 inches  for  40 to 75 or 82 size motors
Length will depend on  rPMs and  Exhaust timing,  a  chart  is  here  online for you

Randy

Online Brett Buck

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Re: Piped Engine Run
« Reply #8 on: October 23, 2019, 09:54:57 AM »
Further question:  for a given pipe, does varying the header length tune?

    The tuning depends on the distance from the engine to the reflecting surface, how it gets there makes little or no difference. I have had a severe resonance issue (in my professional life) that involved waves whacking their way through what looks like a cat hairball of tubing, the traditional shape for models is not required.

    A *very slight* difference is how much heat is lost, lowering the temperature, and thus lowering the speed of sound in the exhaust. That would tend to lower the tuned RPM. Probably, the aluminum conducts heat and radiates it faster than the carbon/epoxy, to it will be a little cooler overall.

     Randy suggests using the header full length, although almost no one does that. The advantage to that is that the exhaust always cools as it goes down the pipe, so the further from the engine, the cooler it is, so it has less tendency to collapse the front of the pipe. He also supplies an aluminum insert for the front of the pipe, that keeps it from collapsing from the pressure of the coupler. Before that, we used various things, like a ring of 1/16 copper tubing JB-Welded around the outside of the pipe. That has the effect of keeping it round, gives a smooth sealing surface for the coupler, and a flare to keep it from blowing off. I still do that, insert or not, just because sealing on the rough surface of the pipe is iffy.

   If it does it at all, it collapses under the coupler - the tension on the rubber squeezes it, and, the rubber insulates it so it gets much hotter under the coupler than anywhere else.

    Note that depending on which pipe you have, it can have multiple places at which you can reference the tuning. Almost all stunt tuned pipes contain baffles across the interior, in addition to a "tail cone" of some sort. The waves of exhaust reflect from all the baffles, and the tail cone, and since they are pretty widely separated, you get multiple return waves at different times. Flat surfaces return sharp "Cracks" over a very narrow RPM range, the tail cone returns more of a "thud", not as high a peak, but wider. Point being, you can tune it to any of these surfaces, or a combination of surfaces. So you might get similar results at "17 inches to the first baffle" as you do at "19.5 inches to the first baffle" - one you are using the tail cone return wave, the other, one of the baffles.   I have always measured the traditional way, "to the first baffle" but actually use the tail cone for most applications.

   The fact that there are multiple baffles also means that there are always multiple tuned RPMs and multiple "valleys" where the pipe tuning hurts the breathing instead of helps it.

   Randy's chart will get you a good starting point, it may not be optimum and you might want to deviate from it when you learn more, or for experiments sake.

    Brett

p.s. while I encourage people to do careful experiments, one thing that tends to happen with beginning pipe users is that they figure that every engine run problem they have is caused and fixed by changing the pipe length. Changing the length over small ranges has completely predictable effects, but once you have it working, the *pipe length is the very last thing you need to be adjusting on a day-to-day basis*. All the other things that effect engine runs on non-piped engines still happen and are still a bigger source of issues.

Offline Peter in Fairfax, VA

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Re: Piped Engine Run
« Reply #9 on: October 23, 2019, 10:53:26 AM »
Thanks Randy and Brett for the information.  Though I am not setting up a tuned pipe at this moment, the opportunity to learn is appreciated.

Two things.

First, the location of the first baffle is not obvious at this moment.  Are there markings or dimension supplied?  I've sold a number of tuned pipes on eBay for a friend, and never was able to determine what was inside them.

Second, there are a number of tuned pipe designs out there.  I was thinking that "double cone" was in use, but now understand that Randy's pipes are a different shape.  Interesting write-up here: http://www.rcuniverse.com/forum/glow-engines-114/255994-tuning-o-s-46-w-tuned-pipe.html

Peter

Offline RandySmith

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Re: Piped Engine Run
« Reply #10 on: October 23, 2019, 01:43:50 PM »
Thanks Randy and Brett for the information.  Though I am not setting up a tuned pipe at this moment, the opportunity to learn is appreciated.

Two things.

First, the location of the first baffle is not obvious at this moment.  Are there markings or dimension supplied?  I've sold a number of tuned pipes on eBay for a friend, and never was able to determine what was inside them.

Second, there are a number of tuned pipe designs out there.  I was thinking that "double cone" was in use, but now understand that Randy's pipes are a different shape.  Interesting write-up here: http://www.rcuniverse.com/forum/glow-engines-114/255994-tuning-o-s-46-w-tuned-pipe.html

Peter

Hi Peter

i assume  that if you want to  seriously try  the pipe setup for  stunt, you would use a  light  carbon fiber pipe, if  not  and  your using a  metal pipe, then please let us know, the  info will  change,  If you use  the  CF pipe, or  any pipe  with a baffle for muffling,  use  the  FIRST  baffle,  The problem  with using  second third baffle  of tailcone, in a  baffled pipe, is  the tuning will put the  wave from the  first baffle  back into the  engine  much earlier, this pushes a  huge amount of HEAT back into the engine, I have seen many  with the  INTAKE ports  black/brown  from the  heat, It will also push fuel out of the  venturi,  if  it is  set  too short. This can also shorten the life of the  engine.  So  I would not recommend you try to  tune off of  baffle  2 , 3  or  tailcone , cause much more trouble than it is worth

regards
Randy

Offline Mike Scholtes

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Re: Piped Engine Run
« Reply #11 on: October 23, 2019, 10:01:26 PM »
Hi Peter:

As Brett notes above, I have been flying an Oriental Plus with a PA40UL on pipe for about three years. It started as an RSM kit that I modified with a larger tail area and longer tail moment. Due partly to luck and partly to lots of help from Brett my PA40 does run very well. I see your comment that you don't intend to build this model quite yet, but if you send me a pm I can send you (or anyone else interested) pictures of the setup and info on pipe length, props, fuel, balance, etc.

The Oriental Plus would be just dandy built dead stock with an LA46 (stock!!!) and an APC 12.25 x 3.75 prop, or maybe 11.5 x 4 for a bit more ground clearance. This is also a whale of a lot less expensive; the PA40 and pipe setup will run you around $550, plus $50 prop, whereas the LA46S is about $80 and the prop about $5. Lots of LA46 stunt motors available used, or NIB. As you presumably know the OP shares nothing with the Classic-era Oriental except the name. It is a totally new design for piped 40 engines from ground up. Same designer, Dee Rice.


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