Thank you gentlemen for your help. What is your favorite stunt maneuver to check for shim adjustment.
Horizontal 8, but don't ignore it in other places.
Many engines, even most, will not run the same in positive and negative-G maneuvers at all g loading. So you can shim the tank for one condition, and it will not be right for the other conditions. This is sort of the bane of existence for a stunt engine, and some engines cannot be adjusted or modified to work correctly no matter what you do. I think that is more-or-less understood, at least to me, due to the way the scavenging works, and it seems to be worse on engines with large ports to "get more power" for RC, which is swell if you are running it at 2 hp at 15,000 rpm and not so swell when you are chugging around at 10,000 rpm and 1/2 horse in a stunt plane.
Even harder to resolve is when the difference gets progressively more marked as the maneuver goes along - rich on the first inside, lean on the first outside, richer still on the second inside, leaner still on the second outside. I had one engine that had something like 1/4" of shim to try to correct it in the horizontal 8, and in level flight, the same shim made it go 6 laps/sec in level inverted flight and 5 seconds upright - but still screeching lean in the second outside. This is particularly true of schneurles, and usually much less of an issue on baffle-piston engines.
The other factor that affects/effects this is the carburetion. This is both the venturi design AND the restrictions in the fuel feed system (like the bore of the spraybar and the fuel viscosity). "Fuel post"/"spigot" venturis sometime help over "flush inlet" systems, because the more consistent (although not necessarily increased) atomization and fuel draw, but the fuel flow restrictions seem to be nearly as important.
This has gotten several of us into pretty lengthy low-level arguments with various engine people over the years, because it seems to be much more noticeable in sea level dry air than anyplace else, particularly the midwest/southeast. So, in trying to resolve it, it doesn't necessarily repeat from place to place.
So, shim it to get it even in the horizontal 8, then check the lap times in level and inverted flight, and see how close you are. If it's within a 0.05-0.1 seconds of the same, that's probably OK, if not, then you might have to look into other solutions, depending on the engine and venturi type. A priori, I would use a diffuser over the venturi (air-filter or panty-hose diffuser), because I haven't seen anything negative about it and it usually helps. Beyond that, getting it closer to right makes it much easier to fly consistently.
The other thing to consider is that making the venturi larger to Get More Power tends to exacerbate the problem, so sometimes it is to your advantage to give up on the power to get more even runs. That's essentially why we are running piped 61's and 75's in 625-650 square inch airplanes. You can't use the full power of even most 40's in these airplanes, much less a 75, but you don't care, you can put a tiny venturi on a 75 and still get plenty of power and dead-steady runs. David won the WC running a PA75 with some port modifications from the standard at the time, and a venturi smaller than we run on the 20FP.
Brett
Topic: How thin of Shims used to adjust the tank height? (Read 2394 times)