Well, schnuerles do that worse than baffle engines... especially when run rich (hence the blocking of the boost port).
Not necessarily. Schnuerles with huge ports designed for R/C applications at 16,000 RPM sometimes have issues, for pretty obvious reasons. But not always. Schnuerles with relatively small ports don't necessarily have problems.
I am no particular advocate of blocking the boost ports. It worked OK in Dave's case because the lost power was more than compensated for by the fact that it was a 75 in an airplane that would have flown just dandy with an ST46. Right now, he is running a better, and more symmetrical, engine with multiple boost ports, not just one. My engines run dead steady inside and out, and it has the same 4 ports (2 regular, 1 boost, and 1 exhaust) everybody else's does.
I think the area, shape, and angles make *far* more difference than how many there are. It depends on the particular design - as always the details are what matters, not so much the gross characteristics.
Here's a conundrum not explained by your theory of boost ports - One engine with gross variation from inside to outside, the other with virtually none. Same very large ports, same number of ports, same compression, same plug, same fuel, the only difference was that one had a spigot venturi and the other did not. Oops! I misspoke, it's not just identical engines, it's *the same engine*, 10 minutes later after swapping venturis. How did the boost ports change between runs? I would conclude in this case that the problem was not caused by the presence of boost ports, nor cured by their absence.
BTW, the Big Jim ST 60's don't need that "plug shield". I have never seen an engine run more symmetrically than a good Big Jim ST 60. In fact, there is no more flawlessly running "stunt run" out there than Big Jim's engines (except for the McCoy 40 I suppose). The compression numbers used by Jim are used by most if not all of the "modern" stunt engines, whether they know it or not (of course, I think he got his head design from the Merco).
Huh? Compression of a PA61 is the same as Big Jim ST60? I was unaware of that improbable, uh, fact. And I certainly don't understand how engines ranging from 6:1 (GMA-Jett, early models) to 12:1 (40/46VF) can all match. Or are we now back to claiming the VF "isn't a stunt motor"!?
One of the things I *am* concerned with is that a lot of the 'blocked-boost-port-for-lucky-best-Stunt-Run' engines run nice and predictably, with reasonable symmetry - and absolutely no power. Same with several Iron Curtain "modern" stunt engines. I would think a currently-produced ABC 40, even a baffle-piston, should be able to out-do a Fox 35. But then I saw a bunch of them run, and they wouldn't. Stalker 40RE, run per directions, absolutely gutted out to the max lean, Top Flight 10-6, Nobler - 5.4 second laps, and absolutely nothing/sagging in the maneuvers. If I set a Fox like that, it would be going 4.6 right up to the point the crank broke. That's about 60% more power - from the Fox! Well, it's just that one engine, right? Well, no, the same guy got two more of them, pretty much identical results. So, switch to a Double Star 40 - slightly better, but still not even close power-wise. You can fly a Nobler with some success with a Veco 19 - but not a brand new "Stunt 40"? They're not all like that (the Rustler "Metamorph" is pretty strong) but a lot of them are.
A lot of the mod-jobs are even worse - 46LA "modified for stunt run" unable to get a Pathfinder ARF up to sufficient speed to attempt a wingover - even with a 10-7! At sea level and 50 degrees. A stock engine with 10-7 would fling that thing around at 80 mph, and with a 12.25-3.75 APC it makes the airplane sit up and talk. But the modified engine came up about 2000 RPM short.
If your point is that you would prefer a McCoy 40 to a super-gutless modified 46LA then I would not be inclined to argue with you.
Of course, not all of the modified engines are like that, but I have seen enough of them that it frustrates me mightily. I hate to see people continuing to live in the same mindset as 1985 when we were always desperate for more power and living with 6" of pitch because the engine isn't strong enough to do any better.
But it makes absolutely no difference to me - if you can get a McCoy 40 to run better than you can a PA75, have at it. The results will provide confirmation of your theories, or prove them wrong. I am certainly not wedded to any particular approach and will switch in a heartbeat if I think it will work better.
One of the biggest issues is asymmetry (I believe you had similar problems?).... breaking lean inverted and rich right side up (alsmot all engines do it when mounted inverted---even the 4 strokes). The Russians finally gave up and went sidewinder.
"Had" being the operative word. I have had no symmetry issues *at all* with the 40VF or the RO-Jett. Or the 5 flights I had with the 46VF (stock, I never ran the AAC version). I had a minor, but "close-enough-to-come-in-third" problem with that my Stage I PA61. It was symmetrical 95% of the time, but it was more powerful and provided better performance than almost everything else 100% of the time, so it was not a hard decision to run it. That appears to have been conclusively solved by the Stage III from 4-5 years ago, or the two-port, or 5-port PA61/65. I am pretty sure that I would be able to notice such a thing pretty easily on the 53-oz. Vector 40. Not a problem with the current PA75s that Dave runs, near as I can tell. I guarantee that if it *did* do that we would fix it or switch.
I think the mechanism is obvious - if the charge shoots through huge ports with low velocities, then the acceleration along the cylinder axis has some effect on it. In some cases, that causes different scavenging from inside to outside. In other, maybe most, cases, it doesn't seem to have any consequential effect. Note that the same problem can happen with baffle-piston engines - Fox burp being an absolutely hallmark example, the tendency of the ST60 to stumble on insides (as discovered by Frank, and diagnosed/corrected by the inboard cylinder) being another. Blocking the boost port, and thus cutting the port area by 33% clearly increases the charge velocity, and that's what changes or fixes the symmetry. Sometimes. However there is also abundant evidence that simply making the 3, or 4, or 5 ports smaller to get decent velocity will have essentially the same effect. As will running 4" of pitch instead of 6 (and thus increasing the shaft HP {far less efficient prop at normal level flight, same power into the airframe}, and increasing the flow velocity *that* way). The 4 vs. 6 part was what was missed by almost everyone during the schnuerle wars.
There are also clearly other factors that appear to have nothing to do with the port size, number, or placement - like the venturi change example above. Or the fact that it is a serious problem if you run a conventional muffler, and no issue when you run the same engine on a pipe.
Brett
Topic: Old stunters vs. new ones (Read 28044 times)