So, I have a plane with an OS40VF and Randy Smith pipe for same. I set it up as previously instructed (18.25" pipe length to start, fuel is Powermaster 10/22, Randy needle valve).

So, here's the problem. I keep dialing the thing up (turning in the needle) on the ground. With a 11.5 x 4 carbon prop, I started off with the needle set about 10,400. As soon as it hit the air it went into a 4 stroke. I kept turning in the needle until finally the launch rpm was about 11,200 and still, as soon as it hits the air, it goes to a 4 stroke and will 4-2-4 in maneuvers. I've used this setup before successfully. Historically, it runs in a solid if a bit rich 2 stroke throughout the flight at about 10,800 or 10,900. But for some reason, the thing keeps breaking to a 4 stroke right after launch.

I thought I had a leak somewhere (though it should go lean in such a case, I thought). I checked the pipe for leaks and it's fine. Changed out the connector to a new unit, resealed the header and changed out the needle valve (to another Randy unit). Same problem.

I figured next I'd change out the pipe for something else, but thought I would ask first.

Hi Brett,

It's a .265 now (sorry, should have said that). I've got a .260 I can try. And I'll push the pipe out a half inch.

Thanks Brett, I was going to go shorter on the pipe.

Hi Brett,

It's a .265 now (sorry, should have said that). I've got a .260 I can try. And I'll push the pipe out a half inch.

Thanks Brett, I was going to go shorter on the pipe.

    ?? If it's already backed off just to go the right speed, you don't want to add power capability by pushing it in!

    Pulling it out shifts the HP peak to a lower RPM, therefore you will have to run it harder, therefore it will need to be leaner. Which is what you want. In fact, try that first, and it may take care of the problem entirely.

    Brett

Randy
I am curious, why do you think a 4 cycle hi RPM run, beeping into 2 cycle is not good?  I watched Paul W use that 4\2 setup on his Impact at a NATs and team trials and did exceedingly well with that run. Or do you just want a 2 cycle all year long

Regards
Randy

     I can answer that one - because the boost/brake is too hard that way. With the it in a two continually, that is significantly reduced.  I have run it both ways, and I always ended up running it in a two all the time (and then ran all the bigger engines in a 4 all the time for the same reason). 

     Brett

Yes it can be with high nitro, or other setups, but NOT always hard...Paul's was not, he had a very very mild break into the 2. I have seen many others also with a very very very mild break-boost with a 40 VF, I have also seen a 40 VF wet 2 run that would slow and sag at the tops of some maneuvers. I have also seen just what you describe in some runs. There are more way to run these than just one.  And none of them are always bad or  always good.

Regards
Randy

Yes it can be with high nitro, or other setups, but NOT always hard...Paul's was not, he had a very very mild break into the 2. I have seen many others also with a very very very mild break-boost with a 40 VF, I have also seen a 40 VF wet 2 run that would slow and sag at the tops of some maneuvers.

   Mine was too abrupt even on 10% and everything else stock. It wasn't terrible that way, just not as good as some other ways.  I know how to mitigate it, of course, but in the end I found it superior the way we normally run it. I am not saying that it's the only way to go, or that everybody should do it that way.


    In this case, I can predict exactly what is going to happen without being physically present as long as Randy sticks to the canonical setup. Once he gets some reliable high-quality runs, then, maybe tweaking around it can get him something better. Given that he was probably mis-diagnosing the problem and wanted to shorten the pipe instead of lengthening it suggests that may there needs to be a baseline system first.

     BTW, apropos of this discussion, I have found a very consistent relationship between the gentle breaks and mild reactions and the exhaust timing. There are various versions of the Jett 61 BSE - those with 136 degree exhaust duration all tend to have a strong or abrupt break, and those with 140 less so, and 144 deg (like the special "Brett" version that was apparently created by accident) is just about ideal. The "Brett" version runs like a giant 40 or 46VF, hey, those are around 144 degrees as well. I think that is about half the trick.

   The shorter-duration versions, particularly the 136 degree version, seem to work a lot better at much lower revs that I would normally use, which makes some sense (and is what I think they are currently shooting for) but lower revs= more pitch and more diameter, which I want to avoid like the plague. Running them at the revs I want to run, they get too boosty/brakey for my tastes.

   There are certainly a lot of ways to skin the cat but I think you want to start with a known system running the way it was intended, then deviate from that only when you have a good baseline.

    Brett

"In this case, I can predict exactly what is going to happen without being physically present as long as Randy sticks to the canonical setup. Once he gets some reliable high-quality runs, then, maybe tweaking around it can get him something better. Given that he was probably mis-diagnosing the problem and wanted to shorten the pipe instead of lengthening it suggests that may there needs to be a baseline system first. "

Hi Brett

Agreed, I can do exactly the same with my setup,and I was among the very first to setup VFs  back in the 80s, and yes there always needs to be a baseline, and again I was curious as to why Randy felt that way, nothing wrong with that , it was just for my info.

Regards
Randy

   To first approximation it doesn't have much to do with the volume. The critical parameter is the distance from the port to the reflecting surface, not the volume. The volume has some effect on the temperature so it may have some effect on the speed of sound (larger=cooler, presumably) and the speed of sound is slower as the exhaust gets cooler. But I think this is a minor effect in this case.

     Brett

Correct, which is why the pretty much all tune at the same length, one of the differences you will see is in temps and also in how the motor responds,Too large and you loose some of the "hold back" ability of the pipe system in winds, too small and you increase temps and affect the engine run in a negative manner, this can be as simple as not cycling or going over lean and getting a lot of detonation. Lot more going on, but that needs another post

Regards
Randy

OK, so answer me this: I have a RO-Jett 65. With the Eather pipe I use, I have to set it at 17.5" to get a hard 4 stroke run at about 10,100 rpm (prop is a 12.5 x 4 two blade carbon job). with the Randy Smith "60" pipe I recently got, I have to set it at 18.5" to get the exact same run? The Eather pipe is larger in diameter and has a rather long inlet before expanding to the larger diameter. The Smith pipe has a much shorter inlet and probably has a larger volume overall. In each case, the length is measured from the plug center to the first baffle.

   Probably bouncing the wave off of different reflecting surfaces. At 18.5 I would guess that you are mostly getting the first baffle, with the Eather you are probably in the middle of the (closely-spaced) baffles, each of which returns a reflection. They all smear to together to create the effect of a conical reflecting surface. That's how he gets away with a perfectly flat tail.

    BTW, you might be able to get the same run with Randy's pipe with it set at ~16.75 - where the reflection will bounce off the tail cone instead of the first baffle. I did that for years. The only potential problem with that is that all the other reflections are shorter/higher RPM instead of longer/lower RPM - so there is some possibility that you can get revved up enough in flight to jump from the tail cone to the last baffle, and get a runaway. It worked OK with the VF because it didn't have enough poop to spin the prop I rand at the time that fast, but 65's might well be able to.

    Brett

OK, so answer me this: I have a RO-Jett 65. With the Eather pipe I use, I have to set it at 17.5" to get a hard 4 stroke run at about 10,100 rpm (prop is a 12.5 x 4 two blade carbon job). with the Randy Smith "60" pipe I recently got, I have to set it at 18.5" to get the exact same run? The Eather pipe is larger in diameter and has a rather long inlet before expanding to the larger diameter. The Smith pipe has a much shorter inlet and probably has a larger volume overall. In each case, the length is measured from the plug center to the first baffle.

Hi Randy

What is the exhaust timing of the motor? It is curious but you maybe tuning to the second baffle in my pipe, The diameter should not change the reflective wave coming back with the small amount of diameter your talking about, and the second baffle is exactly 1 inch longer. When did you buy the 65 pipe from me? Which one is it?

Regards
Randy

Randy,

I got the 65 pipe and a 40 pipe from you, what, last summer sometime as I remember. Both have very little time on them. I set it up as told with the pipe length set to about 18.5 to 18.75 (set somewhere in between right now, I'd have to measure to remember exactly. Runs great. I just thought it was weird that the Eather pipe I have has to be set at 17.5" to get the same run. That's with the same setup and prop and just switching pipes. Yours is predictable, it's the Eather job that was a mystery.

   Hopefully it's not too mysterious now. The critical factor is the distance from the exhaust outlet to the reflecting surface. It's not at all surprising that two pipes of radically different internal design might require different reference points.

     Brett

Yea, as Brett said, I just needed to mess with it to get a reference point. I tried the Eather pipe at 18.5, but couldn't get a steady run in a fast 4 stroke. I messed with that one quite a bit, moving the pipe around in the range I had (about 18" to about 19.25 or so). Then I used a somewhat shorter header that let me adjust it from about 17" up to about 18.25 or so. I started off long and kept shortening it a 1/4" a flight until I hit 17.5 and the system locked in with the run I wanted. When I got the new pipe from Randy, I put the old, longer header back on to see what I had and started off at about 18.5". It was right from the first flight, so I didn't mess with it. Just goes to show, it helps if you know what you are looking for, but you have to have a direction.

Oh, forgot to answer this: I would suggest trimming the prop down to 11.25, Is this a Bolly type CF prop?

I've been using a Steve Wilk copy of a Bolly 11.5 x 4 two blade. I went back and measured and it's actually cut down to whisper over 11.25. Maybe 11.3 or 11.35. That's from the original question about the 40VF.

Thanks guys.

OK, non-engineer Ted here so be gentle on me.

As the volume of a pipe gets larger doesn't it demand that the exhaust pulse expand at a greater rate (cooling its temperature, I would think) and when it expands why doesn't its velocity reduce? 

Because I'm simple I like to look at things in extremes and see if logic can't shed some light on an issue.  Of course, doing so doesn't mean your logic will prove definitive, but...If you went from the little "Grunt" Randy pipe that I ran on my .40s and .46s back in the olden days, which was very small in diameter/volume, to a Pipe that went from the header and expanded to the diameter of a garbage can but with a baffle or cone still at the same distance from the exhaust would the exhaust wave still travel at the speed of sound or would it simply dissipate in to the massive volume and the velocity drop off rapidly?

Apparently, based on this thread, that isn't correct so I'm curious what the physics involved are.

    Changing the volume doesn't change the temperature very much within the range of interest. If it was a simple expansion with no heat input, it would make a lot of difference (roughly equal to the ratio of expansion), but you are also flowing it through the pipe and thus adding heat at a prodigious rate. So the temperature (and thus the speed of sound) doesn't change all that much once you get it into the expansion portion. At least not enough that the other uncertainties (like the total flow) don't wash it out.  Frank W has a plot of it somewhere over the length of the pipe.

    Brett

It's a compression of the gas, which travels at the speed of sound relative to the gas, regardless of how fast the gas is moving.  Speed of sound in air in ft./sec. = 49.02 x the square root of the temperature in degrees Rankine, according to my wife's Pratt & Whitney handbook.  Temperature in degrees Rankine is temperature in degrees F + 459.7.

  I was merely pointing out that the temperature does not necessarily follow the rules of adiabatic compression because there is a very large and more-or-less continuous source of additional heat.

     Brett

I think the rate of change in volume makes some difference. If the volume changes radically and quickly - from a short inlet pipe to the main pipe body in a very short and steep expansion cone with a very long main section before the compression cone - it makes a difference in tuning versus a pipe that has a long inlet pipe and a very gradual expansion to a shorter main section with a longer, shallower compression cone.

Peter,

Well, whatever the ultimate reason, I can't get it to run with the current pipe. I've had pipe lengths from 18" to 19.5 and I get pretty much the same thing ... what appears to be just a very long muffler. With an 11.5 x 4 two blade prop, I can't get it to run in a rich 2 stroke until it hits about 11,800 rpm. If I set it at 10,800 (where I usually run it), it drops into a 4 stroke as soon as it hits the air and with 4-2-4 for the rest of the flight. Wouldn't mine so much but it  just charge out of corners, shows poor speed regulation in maneuvers and doesn't have the power or drive it usually does.

Oh well, I've given up on that pipe for now. I'm back to the 51/60 pipe I got from Randy awhile back (one of the colored pipes - a red one). It ran pretty well with that pipe previously with the pipe length set to 18.5". I had damaged the very front of it when the first RMD when in. I repaired it (cut a 1/2" off the front and reset the insert then checked for holes), but it was too short. Digging around in the drawer turned up a brand new header I forgot I had and that one was long enough to allow the use of this pipe. So I'll just go back an use it. Worked really well before.

Not sure why the new "40" pipe isn't working. I could experiment and just start cutting a half inch off of it, try it over the range it will adjust and if it still doesn't work, another half inch and try it until it starts working (reflecting off another surface), but I didn't want to cut the pipe. At least not yet. If it all works again with the old pipe, then I may start messing with the new one.

Gary,

Read this whole thread. Brett explains it pretty well (better than me, not being an engineer). It has to do with propagation of pressure waves and reflective surface. Volume of the pipe has something to do with, but as I was saying to Peter, it's more about shape than size (to an extent). I suspect that the problem with the current pipe is that the reflective surface is the wrong distance at the setting. With pipes, if you aren't in the correct range, it's just a long muffler.

Yep, I've seen that before. That's what made this so weird in the first place. Oh well, I'll get it figured out.

I think the rate of change in volume makes some difference. If the volume changes radically and quickly - from a short inlet pipe to the main pipe body in a very short and steep expansion cone with a very long main section before the compression cone - it makes a difference in tuning versus a pipe that has a long inlet pipe and a very gradual expansion to a shorter main section with a longer, shallower compression cone.

Randy  The biggest change it makes, is in the.... perceived... venturie size, The large volume pipes will make the venturie act larger, the small pipes will make the venturie ...seem... smaller. This change also occurs when the stinger is changed in size. For example inserting or adding a 8mm  and then a 10mm exhaust deflector, will make the venturies seem much different in size, with all being equal you will need to go down in venturie size to get the same run with the more open exhaust...and reverse with the closed exhaust

Randy