What field elevation do you guys fly at? What’s the difference in the conditions out west vs Texas vs Midwest vs southeast?
I don't know. But we have taken systems (not just the same settings, literally the same components with no changes to anything) that worked fine in the Southeast/Midwest, and were hopeless here. Our elevations at various sites, anywhere from 3 feet to maybe 100, typically dry. Also note that we have no real problem going from 50 degrees/fog to 105-110 and 15% humidity, don't even change the nitro.
Typical for inverted-mount schneurle engines, shim the tank to get equal upright/inverted speeds. In the round 8, it goes slightly rich in the inside, lean on outside, second time through the inside, even richer, second time through the outsides, screeching lean. Shim the tank to try to fix it, maybe 3/8", upright times 5.0 and inverted 5.8 to 6, get to the round 8, still goes rich on insides, still goes lean on outsides. That is hopeless.
As far as run asymmetry goes, the 75 (3 port and complete with Dave’s setup and NVA I’m still “renting” from him) I’ve run exclusively since 2016 always broke harder on outsides even though lap times were the same. It wasn’t until I hooked it up to pipe pressure last year that made the tank shim so out of whack that I needed to shim it a full 1/16”. Now it runs *slightly* richer on outside rounds but the break during the horizontal 8s and square 8s is perfect. I don’t have enough time on my other two to know if it’s just an issue with that particular motor or the other 75s.
Yes, that's a classic symptom. The other is to run nicely until you turn toward the ground on the outside part of the square 8, then have it break hard lean and pile-drive you into the ground. That was death with the PA40, and even in Muncie, I heard the same thing repeatedly. The 61 is much less prone to it, once we got the spigot venturi, would only do it about 5% of the time. Stock venturi, drilled out, "larger but curved", big problem.
David's engine occasionally briefly blips in to a 2-stroke on the top of the inside loop on the square 8. If yours is the one I think, that's the one that started going lean through the intersection at the 2009 TT, which led to an engine swap between qualifying and the finals and an after-dark test flight.
I also note that we may have found another thing to try that seemed to remove almost all of these issues in the few flights we have had. No unexpected blips anywhere on the 75, as good as the original 2-port engine, and *even steadier* on the Jett
My 61 was one of the most idiot proof setups I found when I put it in Dracula. I stuck the biggest Venturi I had in it (I would have to check to see what it is), set the pipe to 17.75” and stuck a 12.5” Bolly three blade on it. Went flying, shimmed the tank to make the run even and it runs in a very high pitched 4 cycle, hard pipe rattle everywhere it goes and a perfectly even run. I do plan to test it in the world championship plane and see if there’s any advantage to be had by running the smaller prop and the cornering ability vs the larger props the 75 requires
I would at least look at trying the smaller prop on the 75. That prop is nowhere near the max, it's already pretty light load, so slightly lighter might not make any difference. It also smooths out the run by allowing you to run deeper in a 4-stroke and reducing the feedback to the engine. Derek's point earlier is on point, no one said you had to have a 75.
Same thing for me - I run the 12.5-3.75 and the engine has no problem spinning the same prop you are running. It took two hands to get it out of level flight, however.
Brett
Topic: PA 40 UL settings (Read 1895 times)
