I have read in several places about bypass ports being plugged to improve performance and/or eliminate the "Fox burp" or similar behavior.
What was the purpose of the bypass port in the first place, if the engines run better with them blocked off?
I think you are mixing some terms up. The boost port in a schneurle engine *greatly* increases the power capability of the engine. The problem is that we *don't want* and *can't use* the extra power in many cases. People seem to want to run 40FPs in Noblers, and if you run a 40FP the way it wants to run and near it's capability, the airplane is far too fast for stunt. If you somehow manage to run the engine weakly enough to fly the airplane at a reasonable speed/power output, the total bypass area is far, far, too large, leading to poor performance like a huge difference in the run characteristics from inside turns to outside turns, unstable runs (runaways), burps and other issues. All because the total flow of the gas charge is too small to get a decent flow velocity in the bypasses and ports.
Another, far better solution to the problem (in my opinion) is to use a smaller engine that can run nearer it's capability without generating too much power. A 40FP in a Twister is generally a big problem, because it's so powerful that you need to run it at 40% of it's capability. Get a 20FP, and it runs at 80%, the boost port and bypasses are properly sized for that amount of power, and you still have enough reserve to allow it to pick up in the maneuvers.
All case-induction 2-stroke engines need a bypass of some sort to transfer the charge from the case to the cylinder. A Fox has a single bypass, for example. A baffle on the top of the piston deflects the incoming charge to the top of the cylinder. As this goes up the side of the cylinder, it forces out the exhaust. This is commonly referred to as "loop scavenging" but that's probably not technically accurate. A Fox is a single-bypass engine, where the bypass is nonetheless still too big for the power generated, thus the burps and funnies when you run it on a profile and the charge is deflected into the sides of the bypass from acceleration. If it hits the liner (on inside turns), no problem, it's really hot and the charge just stays a gas and works normally. If it hits the other side of the bypass (outside turns), it's cold, it condenses, no charge, burp or quit.
A 40FP is a 3-bypass engine, two fore and aft , and a "boost" port. It counts on the angle and arrangement of the ports, and the momentum of the incoming charge, to run the charge to the top of the cylinder and force the exhaust out. If the charge comes in too slowly, acceleration of cornering alters where the charge winds up. That can cause the ignition to happen differently on inside and outside turns. At stunt power, this charge is very slow, since there is so little total flow compared to the volume and cross-section of the bypasses. Once it speeds up, it can also prefer that, pump better, which makes it speed up more, which it still likes better, faster still, etc, until it approaches its maximum output. In stunt, this is called a "runaway", in the rest of the world this is very desirable as the power output is higher. It's just running the way it was designed, the problem is that we want it to at half power or less.
Blocking the boost port does two things - it gives less bypass area so the same amount of charge goes through with more velocity, making the directional effects of the porting more consistent, AND, it cuts the maximum power and the RPM at which maximum power occurs to be closer to the power levels you might want out of a 40FP in a Nobler. If you put a 20 or 25 in the same airplane, it pretty well works without much problem with all three ports since you are generating the same amount of power as the 40, but the total bypass area is far smaller and the velocities far higher.
Brett
Topic: Bypass Ports (Read 2293 times)
