Most of us are aware of the "Fox 35 burp" that occurs on outside maneuvers when that engine is mounted sideways.  That burp characteristic also exists on other engines.  For example, I have witnessed it on some Stalkers.  I guess you will not know until you fly a plane with an engine mounted sideways.

Does anyone have experience with side mounting a RO Jett 67?  Does this engine exhibit the outside maneuver burp?

How can the “Fox 35 burp” be related to Schnuerle porting when the Fox 35 is not Schnuerle ported?

I have several Fox 35's (profile mount) that all burp. It might be my imagination, but I run fuel pressure from the muffler, and it seems to help a bit.

For nostalgic engines I prefer the McCoy 35 over the Fox. All of my McCoys run great, and no burp! With uniflow tanks and muffler pressure, the McCoy 35 runs pretty much consistent throughout the entire flight, even during stunts. 

Just my 2 cents.

How can the “Fox 35 burp” be related to Schnuerle porting when the Fox 35 is not Schnuerle ported?

    The problem in either case is that the gas flow inside the engine is too slow and is affected by the acceleration to a disproportionate degree. That's why the Fox "burp" is conclusively solved by speeding up the flow by making the bypass area smaller. 

   "Engine experts" will tell you how this will kill the power by restricting the flow. They are wrong, the bypass cross-sectional area is about 5x larger than it needs to be, and restricting it to way less than half the stock dimensions has no effect on the power at all - sometimes it slightly *increases* the power.

  Schneurle engines are inherently more prone to changing the run characteristics due to acceleration, because their scavenging counts on the momentum of the flow to go where it is aimed by the ports. The baffle on a baffle-piston engine is there to force the incoming charge up towards the top of the cylinder, a schneurle counts on the ports being aimed that way slinging it up towards the top of the cylinder (and then displacing the exhaust out the exhaust port). It is inherently more prone because it counts entirely on the momentum of the charge to make everything go where it is supposed to.

    In either situation, running it faster helps by speeding up the flow. But you don't want or need it to run faster, so an alternate solution is to reduce the bypass and port size to speed up the flow and give the charge more momentum.

   So, for the Fox, the definitive solution is they bypass stuffer, then you can run any speed you want with no danger of a bump, and no power loss (or even a slight increase). The net bypass cross-section required to keep from restricting the flow is tiny. And alternate solution it to put various things in the head to at least keep raw charge from blurping itself onto the plug element.

  For schnuerle engines, the solution seems to be to make the ports small enough to support the *very low* power levels we run with adequate velocity. Bear in mind, a piped 75 *could* probably be made to pump out 4.5-5 horsepower, we are running at .6 hp, so making the ports and bypasses smaller would have no effect on the power and maybe (with extensive evidence of the fact) less tendency to run differently inside and out. ]

   Some smaller engines seem to have nearly no problem with it, only by virtue of running at near-peaked power levels where the port and bypass area are appropriate. You can't (or at least don't want to) run larger engines at the same high fractions of the capability - stunt planes absorb about .3-.6 hp to fly normal stunt speeds, any more and you will either have to figure out some way to waste the excess, or fly it correspondingly faster.

   Problem #1 - Fox burp - was discovered in antiquity, and definitively diagnosed and solved by Frank Williams in about 1994 by the bypass stuffer.

   Problem #2 - schneurle engine speed/mixture differences on inside and outside maneuvers, was discovered by Ted Fancher and company (probably among others) in the late 70's and has been since at least mitigated by building custom and semi-custom stunt engines with much smaller ports than a typical high powered RC engine of the same displacement.

    Of course there are other reasons it might run differently inside and outside, like the tank position, but there are at least these two that otherwise have proven intractable that are inherent functions of the engine and the internal ballistics.

   Of course both problems are avoided completely by running electric.

   Brett


 

Richard Oliver ran the 67 on a profile at the Bill Rutherford Memorial contest in Houston a few years ago. The 67 he used was side exhaust with clunk tank on muffler pressure.
I watched his flights behind the judge's station. I was amazed because it was the first time I saw a stunt plane fly at a constant speed. RWO, up hill, down hill where ever how ever, that plane (profile Maverick) did not speed up or slow down. I made that comment to Richard after his second flight. He likened it to driving his Kubota tractor in 1st gear.

Side-mounted Supertigre .35s would go rich in outside maneuvers.  Some were worse than others.  I could never figure out why, but now I'd guess that the later, wider-bypass cases were worse.  My G.15s did it, too, with more than 10% nitro.  An outside loop would shut them off. 

There is a bit of confusion which years the Fox .35 bypass was deepened, but was ultimately changed back. See the info in the link:

http://www.foxmodelmotors.com/engines/35/35-history.htm

Also interesting to note that Mohrbacher (and Duke) stated that the engines were made using gravity fed (not pressurized) permanent mold castings. None were sandcast. But the earlier castings looked different from the later pressure-fed die castings, so....

Dave

   In any case, the bypass area was *far larger than necessary for chugging along at stunt speeds with a 10-6*. As evidenced by the fact that cutting it down to <25% of the original area has either no effect on the power at the RPM required for stunt, or slightly increases it.

    I remind everyone - almost none of the engines we use for stunt were designed for low-rpm running, not even the Fox 35. And the Fox 35 was a pretty hot engine in 1949. As a result of trying to get the most power out of them possible, they almost all have bypasses and other port features intended to give the most power. That a lot of them work pretty well for our purposes, with just crank timing and liner changes, is more-or-less an accident.

      As far as I know only one of them has a custom-made case - the RO-Jett 61 BSE - and that one runs quite differently from the equivalent cast-case version. The difference is exactly as described here - the cast-case engine is very aggressive in the corners and has significant inside/outside speed differences, like the PA40 and early PA61/65s.

    The BSE version has nearly no difference inside/outside and has *very mild* boost/brake characteristics, much more like a 40/46VF. And the liner ports on the 61 look like mail slots. Note this is despite having "radio numbers" for timing/blowdown. In fact it runs even smoother with the QM40 crank timing as opposed to the "Stunt" crank timing. This is also the basis of the PA61/65 "Aussie Crank Mod" and like the original PA51 crank.

     Brett