More and more these days, I read of people using big engines on pipes. I understand the rationale behind the classic 4-2-4 engine run. The use of say a 40 on a pipe is also a logical concept. The classic Dean Papas article helped out on that one. Quite why one should run a modern .75 on a pipe is a little less easy to get my head around. Some contributors seem to imply that there is little difference between running a .75RE on a pipe or a muffler.
  Now we are not seemingly scaling up the aeroplane, to run on these large motors. In fact the aircraft seem to be of relatively modest dimensions. I can only assume that the engines are running in a very deep fourstroke? Are people using the pipe as an expensive muffler and if so why? Can someone explain what is going on here? I seem to be missing the point.
   

 Seems to be. We don't use the pipes to increase the power. All of the schneurle engines, 40 on up, have had far more than abundant power to fly the airplane. What you are doing is controlling that power to get the kind of run you want. You need to control the power of a 76 far more tightly than you do a 40 so in a sense you need a pipe *more* the larger the engine gets. There are numerous examples of people who  said, hey, I have a 76 (or 90) so why do I need a pipe. They quickly found out that it was a very hair-trigger affair - 1% too much power, way too fast, 1%  too little power, and way too slow.

  The only other alternative is to make the airplane bigger, and we hit the practical limit on that in the 60's, as long as the line length is limited to 70'.

   Brett

Allan,

You might talk to Alan Resinger. He's an importer of Stalkers and I've seen him run everything from a .66 to an (I think) .82 on mufflers. Nice, wide power band. The point of a pipe is to regulate power, not produce it. With a PA75 and a pipe, you can set the power on the end to produce in a power band that is consistent and repeatable. The engine is so big that you are only using a portion of the available power, but it's easily controlled.

Sort of the same approach Ford used with the GT40. Build a really big engine then detune it and let it loaf. Makes for easily repeatable and consistent run.

"There must be a limit to this line of thought, as engines get bigger, they get heavier and the fuel consumption presumably goes up?"

Not really, a PA 65 is lighter than an OS VF40 , the PA 75 is about the same weight as an OS VF40. Also you have a much larger power "envelope" to work with with the more powerful motors, and they are also able to produce much power while still being very tame and docile, as you see many people only run them loafing  in a 4 cycle.
You will use more fuel but it really is not any deal breaker because it is not that much
I used almost 6 oz on a piped 40, and I use the same 6 oz on 51s and 61s, the 65 in my ships use 6.5 ounces with the 75 using just a tad over 7 ozs, so it is not a big differance

As far as the plane\airframe goes, ..example is my KATANA, it is a 676 sq in ship that weighs from the high 50s to the low 60 ounce range with a 40 thru a 75 installed in it...there is just not that much differance...   You see the same in Old Time and Classic ships with an Aero Tiger or some of the new light hi power stunt AAC stunt motors made today in the old airframes...huge power increase over the old motors but not so much in weight

Randy

  More and more these days, I read of people using big engines on pipes. I understand the rationale behind the classic 4-2-4 engine run. The use of say a 40 on a pipe is also a logical concept. The classic Dean Papas article helped out on that one. Quite why one should run a modern .75 on a pipe is a little less easy to get my head around. Some contributors seem to imply that there is little difference between running a .75RE on a pipe or a muffler.
  Now we are not seemingly scaling up the aeroplane, to run on these large motors. In fact the aircraft seem to be of relatively modest dimensions. I can only assume that the engines are running in a very deep fourstroke? Are people using the pipe as an expensive muffler and if so why? Can someone explain what is going on here? I seem to be missing the point.

Regards,

Andrew.
   

Hi Andrew

Another thing I consider, is when you have a larger motor with all things being equal in timing and the engine design the larger motor has always been more stable or stunt freindly, Even if the smaller ones are too. I have built many engines with the same timing , crank intake, and exhaust, same head shape Comp. ratio etc, and in every case so far the bigger motor has been easier to setup and had a wider envelope that it would run the same in.
There are a lot of people using big PA 65 75s and several other brand motors on  mufflers because they are giving them more of a wide range of condidtions that they run the same in. This is not to say the pipe helps them work even better ,as Brett has wrote, it does and he is exactly right.

Regards
Randy

Thanks Brett and Randy,
  What threw me was tha fact that it seems that quite a few people seem to be using a muffler on these large engines. One contributor even talked of pipes and mufflers as though there were no difference in the operation of these two set ups. That is what got me very confused! So the larger engines run in a 4 stroke and this gets you away from any 4-2 problems. I know the 40VF is heavy, but I didn't realise that the PAs and Ro Jetts were so light! I am very surprised ar your quoted fuel consumption. It just goes to show how one can have a prejudiced view of things without realising it! After years of building overweight, I am now able to get my weights down and I am having a somewhat over enthusiastic purge on anything that adds weight! Hence my earlier comments!

    The difference in the engine weights, all things considered, is minimal/negligible. I disregard that when it comes to choosing, they are all close enough that I pick them entirely on their run characteristics. You never want to pick an engine based on the weight. If there was a 16 ounce engine that I thought ran better, I would figure out how to build an airplane for it. I am set up to run either a RO-Jett, PA, and 40/46VF with pretty much no changes to the airplane, and while I haven't done it on the current version, I ran them back-to-back-to-back in the old airplane. Any of them would provide Nationals-quality performance, and in fact I did run a RO-Jett for qualifying and a PA for Top 20 day back in 2003.

    These sort of quality engines, and the associated understanding of how to set them up, have been a revolution in the event. Now *anyone* can follow the directions and get a near-perfect engine run every single time, without even knowing why it works or the underlying concept. That was certainly not true before about 25 years ago. It has made the event, and good performance, so much more accessible to so many more people that now you see 30 very competitive guys at a NATs, instead of 3-4. It's done far more to expand the event than any other aspect.

    This is also why I get so frustrated sometimes when people are talking about wanting a "conventional stunt motor" and then mention a Fox, ST46, etc. A Fox hasn't been a mainstream stunt engine for 40 years and no matter what you do to it, you aren't going to get significantly more performance out of it than they did 40 years ago. Same thing with ST. My buddies and I were the very last die-hard holdouts with the ST, as late as '89, but it took 1/4 lap with a 40VF to know that was over with.
 

Thanks to you both for straightening out my distorted views. It is very refreshing to have very experienced fliers take time out to put us "bottom feeders" on the straight and narrow. My sincere thanks

   You are quite welcome. But I don't recognize the concept of "bottom feeders". Randy and I (and several other posters) have been doing this for darn near ever. It's to be expected that we know a lot more than someone who is just starting, or who hasn't dedicated a large portion of their lives to pursuing every tiny little aspect for decades. But it's a mistake to assume that people will less experience are stupid or can't understand. If someone fails to understand it's because the explanation is not good enough.

    That all assumes that the recipient (and those lurking, or not directly participating) is genuinely interested in the information. There are a few people who post stuff just to tweak people, or approach it as if they were already "experts" or intentionally set up arguments between experts. These guys quickly get identified and essentially any time you have seen me get irritated with people or fail to answer is when it's one of those guys.

      Brett

Sometimes there just isn't any substitute for cubic inches! Case in point:  My Gee Bee "D" started out with an OS46LA.  Now it has a Stalker 51.  Still not enough power, probably because of all that draggy stuff hanging out in the breeze!  Don't know what will be next, although I do have a RO-Jett 61 available!

Floyd

Brett said: You are quite welcome. But I don't recognize the concept of "bottom feeders". Randy and I (and several other posters) have been doing this for darn near ever. It's to be expected that we know a lot more than someone who is just starting, or who hasn't dedicated a large portion of their lives to pursuing every tiny little aspect for decades. But it's a mistake to assume that people will less experience are stupid or can't understand. If someone fails to understand it's because the explanation is not good enough.

Hi Andrew,

It is guys like Brett, Randy, Paul, Ted, Dean, Bob, Billy, Dale, Derek, and others (I *steal* information form all! LOL!!), who are more than willing to share their knowledge and experience who make this event so much easier, if a person is willing to ask and learn.  I never had the time, in the past, to work things out.  With the profession I was in, practice time was very limited.  These guys would always answer my questions honestly, and all I had to do was follow instructions.  Randy has always been at our local meets (or at the end of the phone line) and he has always been there to help with problems.  With the modern engines, there has been so little in the way of "problems" that it is silly.  This has made my "stunt life" SO MUCH EASIER!  At least as far as engine runs go, I can go out and have a pretty good engine run all the time.  A touch of the needle, a slight tank shim, or a different prop, and things are "perfect".  I just wish there was the same "program" that would make my flying that much better! LOL!!  (really, the problem with flying is simply practicing more with a good coach!)

Bill