WELL not quite how it works on my planes, it is not only just level and inverted flight, but the engine ALSO changes when I move the point where the fuel exits the venturi, and into the venturi bore , it will either breaks harder in one direction or goes richer in the other direction, For instance if I move the bar on my plane down, and touch nothing , the engine will go richer on insides, and leaner on outsides.
I have several engines /planes that I can move the venturi up and down very quick and easy, to tune the engine height vs tank, and it makes it very easy to see this
I haven't tried moving the NVA on purpose, although I have put engines with different needle valve positions without having to shim to get the tank position right. Any time I shim the tank, it has the effect you would expect, it's richer upright, and more rich in the inside maneuvers (compared to what it was before).
But there is much more going on with the internal ballistics and the "apparent mixture" than just the fuel delivery pressure. I have had engines were I shimmed the tank to try to correct the issue maneuvering that were completely unworkable the rest of the time. For example, a particular engine would run the same speed upright and inverted (and sound the same), run slightly rich on insides, and hard dead lean on outsides. I ended up shimming the tank to try to correct this, and even with something like 5/16" of shim and it running 5 seconds upright and 6 seconds inverted, it *still* ran hard lean on outsides and relatively rich on insides. Moreover, it exhibited Ted's classic example of the "schneurle effect", where (for a round 8, for example) the first inside would be almost OK, the first outside moderately lean, the second inside dead rich, and the second outside screeching lean. We have had this happen over and over, particularly with muffled schneurle engines. And for whatever reason, it is much more marked effect running around here at sea level than anywhere in the midwest or east. Mounting the engine at a different angle seemed to make a big difference, and it's generally *far worse* on schneurles than on baffle-piston engines
From this, and from Frank Williams' Fox Bypass stuffer, I cam up with the working hypothesis that the issue is that the acceleration along the cylinder axis has a significant effect on the flow inside the engine, and that if the flow velocity is slow, or can easily be disrupted, it's more prone to the problem. Schnuerles used for stunt, particularly on mufflers (like the 40FSR and other "schneurle of the month" engines) and run at low revs seem to be the worst case, because the flow velocity is slow (larger ports and bypasses to Get More Power, while we are trying to run the 14000 rpm engine at 8500), and because the scavenging depends entirely on the momentum of the internal charge being aimed in particular directions, sort of aimed up a the top of the cylinder in the hopes that the fresh charges run up the sides and bypass sides, displacing the exhaust down the middle and out. As opposed to a having a baffle on the piston to force it up one side while the exhaust is displaced out the other. It doesn't make baffle-piston engines immune to the issue, just much less prone to it.
I can't directly prove the hypothesis, but anything I have done to increase the momentum of the flow seems to make the problem better, the Fox bypass stuffing being another obvious example, but also, blocking the boost port (which greatly increases the velocity of the other two), or making the port sizes smaller and more appropriate for the amount of flow, seems to help. Same thing with the spigot venturi and venturi diffusers, which can also have a marked effect over side-inlet dribble holes. regardless on their effect on flow or pressure recovery. Or, the most obvious solution - don't try to run your 14000 rpm engine at 8500 rpm - us a 4" pitch prop.
The position of the incoming charge WRT the glow plug also seems to matter, but I am not at all convinced that this is the driving problem.
Note that on larger engines, there also seems to be something interesting about the fuel delivery, with interesting and beneficial effects from reducing the flow restrictions, either with thinner oil or bigger tubing and spraybars.
The "schneurle effect" is about the only thing I really care about with evaluating stunt engines any more - they all have enough power and they all last long enough. It's just a matter of how they respond in the maneuvers VS level flight.
Brett
Topic: NVA height in the venturi (Read 2849 times)
