This might sound like a silly question here but what does an engine do when set up for stunt but suffers the use of a full length double cone tuned pipe - as in a speed or RC pattern pipe.

Obviously the rpm or port timing won't be there to get it up onto the pipe but does it really suffer from the reflective signal from the pipe arriving too late or early?

Thanks.

  I am not sure I understand the question, but I think you mean, "how does a conventional divergent/convergent pipe (with no baffles, etc) work on a stunt engine?".

   It's really no different - a compression wave goes down the pipe, reflects off the tail cone,  and comes back  (2*L/1450 ft/sec) seconds later.  For a convergent tail cone, the length is measured from the centroid of the tail cone instead of the baffle. Works the same, mostly. Several people have used those types of pipe with good success. One thing that was found is that the tail cone needs to be *very* shallow angle, otherwise the tuning range is too short. They also tend to be very noisy. Otherwise, it works just fine.

     Brett

Ah, a shallow angle on the tail cone - that makes sense now and you do get what I mean.

Its just that I have a 60 sized engine that was supplied with a look a like RC pattern alloy pipe that was called a "stunt pipe" but certainly doesn't look like one.
It is the same length as my Ro Jett pipe but of course has a very tapered tail cone, so (as I gather) its going to act as a shorter pipe than the Ro Jett one.

Ok, I have enough info on what I can expect now.

Thanks Brett.

Chris
can you please post a pic of the pipe you got please?

thanks

The pipes like the one you show will run "shorter"  overall looking from the outside than a regular baffled pipe, but the inside tuning range will be the same.
It just all depends on where the reflective wave comes from. I sometimes have to make pipes with the baffles in differant places for special needs and applications, so the tuning range will be where it works with particular planes. ie long chamber.. or short chamber pipes
Also the pipe you are showing is a speed pipe, and will not work as well as ones designed for stunt, or for that matter aluminum RC pipes that are baffled or have a much flatter end cone

Regards
Randy

   As photographed, it appears to be pretty close to the same. Eyeballing it, the centroid (1/3 point)of the tail cone measured starting from the peak looks to be near the place the pipe is cut to put in the baffle/baffles. It's very similar to the pipe Frank Williams was running.

     I actually have no idea how well the RO-Jett pipe works, either. I haven't ever run one. I still use the Billy version which has proven to be impressively leak-free.

    With no baffles or after-muffler, one thing I think we can all agree - the metal one going to be *very loud*.

   Brett

It depends, typically they will run shorter than my pipes, as I am making long chamber designs, the pipe shown looks like it would run maybe an inch shorter, and most all metal pipes would also run shorter unless  they are baffled, most of the metal pipes do not have a tail converging cone that is that steep of an angle, most of the ones people run look more like a metal pattern pipes, instead of the speed pipe shown.
Brett mentioned Frank Williams, I think the thing about that type of pipe he liked was it fell off the pipe very abruptly , and Frank turned more RPMs with it, which again makes it tune, and run shorter.

Randy

Brett mentioned Frank Williams, I think the thing about that type of pipe he liked was it fell off the pipe very abruptly , and Frank turned more RPMs with it, which again makes it tune, and run shorter.

  Right, falling off (or in our case, the RPM getting dragged below the peak) is the reason to want a shallow tail cone. For those not familiar with the notion, the Two-Stroke tuner handbook explains it pretty well. Functionally,  flat wave front hitting a flat wall returns all at once, a flat wave front diagonally into a corner returns a "smeared" wave from some of the echo coming back sooner than others as it reflected off the angles wall. One first first provides lots of boost  but only at a very narrow range of fixed RPM. The second provides less boost but it is spread over a wider RPM range. The tail cone angle on a conventional tuned pipe determines the degree of "smear" in the returned wave

   Baffled pipes return *multiple* reflections, one from each baffle and one from the tail cone, and it is entirely possible to see these on a dyno, and to jump from one peak to another, if you set it to operate in-between the peaks, and can have multiple stable points. That's why Paul ran his 40VF at 18.5 - 18.75" and I ran mine at 16.75" (same pipe) and it worked in either case. There's about 2" from the baffle to the centroid of the tail cone, so it turns out that science is real.

    Brett

 Right, falling off (or in our case, the RPM getting dragged below the peak) is the reason to want a shallow tail cone. For those not familiar with the notion, the Two-Stroke tuner handbook explains it pretty well. Functionally,  flat wave front hitting a flat wall returns all at once, a flat wave front diagonally into a corner returns a "smeared" wave from some of the echo coming back sooner than others as it reflected off the angles wall. One first first provides lots of boost  but only at a very narrow range of fixed RPM. The second provides less boost but it is spread over a wider RPM range. The tail cone angle on a conventional tuned pipe determines the degree of "smear" in the returned wave

   Baffled pipes return *multiple* reflections, one from each baffle and one from the tail cone, and it is entirely possible to see these on a dyno, and to jump from one peak to another, if you set it to operate in-between the peaks, and can have multiple stable points. That's why Paul ran his 40VF at 18.5 - 18.75" and I ran mine at 16.75" (same pipe) and it worked in either case. There's about 2" from the baffle to the centroid of the tail cone, so it turns out that science is real.

    Brett

Again Frank was doing something a little different, he was running much more "on the pipe" and higher RPMs, maybe that made his drop off faster, he also ran a small prop and he was doing this originally on a VF 40. His setup varied RPMs much more than the typical ones because of this.
 He also dropped one of these metal pipes out of the belly during a stunt pattern... Looked like a aluminum torpedo sliding across  the circle at Muncie one year.
The best and lightest aluminum one I have ever seen was Igor's that he brought to Muncie one year, it was the lightest metal pipe I have yet to see.
And yes if Frank had been running the way we were using pipes, he would have been better off with a steeper  angled pipe, but Frank is known for doing it differantly.

Randy

Randy

Chris

In my opinion,
Of the 2 pipes you have there, I would run the black one.
the OTHER pipe is mainly used for racing purposes.

Thanks

Carl

Yep  we did the exact same with the OPS 40 and the VF46 25 years ago, same 16.75 and 18.5, you can see them clearly on the plots I made, and you can also the same with the 2 or 3 baffle pipe. Running in  between the two tune ranges  was not something to do
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   Dave and I were flying flight-after-flight-after-flight at the 93 NATs, dead nuts repeatable. A well-known modeler about twice our age came up, and said he was having all sorts of problems with power, overruns, underruns, etc, and couldn't solve it. We asked him what pipe length he was using - 17.5". Almost in unison, Dave and I said "move it in to 17". But no, "so and so told me that if I changed the pipe length he would break my arm". But we could change absolutely anything else. A few minutes of this, and we told him, good luck. He went on to overrun on 3 successive flights, NATs over.

    Brett

and with any engine you run, no variations, all engines must use this exact fuel all the time, and it cannot be varied :-)

Absolutely right!  It does make life difficult when you break out the Drone-powered old-timer, but hey, life's a challenge.