The diameter of the propeller directly proportional to the torque. The higher the torque, the larger the diameter of the propeller and the lower the revolutions. Someone likes to fly at high speeds and a small step of the propeller, and someone likes to fly at low speeds and a large step of the propeller. The speed of flight in a circle about the same. If you use a very powerful engine with a three-channel blowing, then somehow you will have to fly either at very low revs with a large propeller or at very high revolutions with a small step of the propeller. Both in the first and in the second case you will use only 50-60% of the power of your engine, otherwise your model will fly very very quickly. We specifically on Stalker engines do not use three-channel blowing, because there is a lot of power. At Stalker 66, we use the power to a maximum of 70 percent and on the Stalker 76 - a maximum of 60 percent of the possible. And we don’t fly at all on Stalker 81, we don’t know what to do with it, it turned out to be very powerful, we don’t need such power. On Stalker 76, we greatly increase the combustion chamber with additional gaskets to reduce the compression ratio and artificially greatly reduce power. That is why we do not use nitromethane. With nitromethane, we cannot reduce excess power. With such settings of the combustion chamber, the engine works like a 4-stroke engine, constantly in mode 4. If the motor is set incorrectly then in mode 2 the plane shoots like a cannon and flies like a mad upright and it doesn’t matter how much the plane weighs.
To quote a famous man "
I *love it* when a plan comes together!"
You are describing *exactly* the problem everybody else was having trying to make more powerful engines (like the OS 40/45 FSR, etc) work in a stunt plane in the late 70's/early 80s - right down the line. Excess power, excess power boost, excessive speed. Everybody tried your solutions - lower compression/nitro, adjusting the exhaust timing, etc. Windy (who was the last competitive flier trying to do it this way) was the last holdout. He also had the notion that "I have a 76 (or even a 91, as I recall), so why do I need a pipe?". We still have people trying to do that, half the posts here talk about throwing in 14 head gaskets and grinding the intake port to reduce the blowdown. That just makes it even more prone to running away.
The answer is as above - th emore powerful the engine, the more you need the pipe. The problem you have with excess power is solved by putting a power regulator on it, AND, figuring out a way to waste power in level flight, but having it still be effective and predictable in the maneuvers. The solution is:
reduced pitch and diameter along with whatever increase in RPM that is required to get the right speed - this has the effect of making the propellor less efficient, thus permitting more shaft power to generated without excess speed in level flight, but a dramatic increase in efficiency dependent *strictly* on the airspeed. And a reduced variation in the prop load, because there is so much power being absorbed by parasitic drag on the prop, the increased induced drag in the maneuvers has less over affect on the feedback load.
a pipe tuned for an RPM *lower* than the level flight operating point, to increase the slope of the power curve at the operating point. This is essentially entirely dependent on the RPM and nearly nothing else, so it can't make it run away and done correctly, the power falls off a cliff as the RPM increases. Effectively you have much more and much more adjustable control over the power curve. I think the tuned RPM of my PA61 system was around 9000-9200 RPM based on the curve - but it turns 10800 in level flight. The power curve is extremely steep around 10800, falling rapidly. When we unloaded it in the ground testing, we couldn't get it over about 11,500 even with tiny little props. It spun a 14-6 wood prop at something like 9500 RPM - without touching the needle from the normal launch settings - 140-150 in-ounces, MUCH more than a Saito 72. And that was a mere 61 with about 6 years of running on it. We now manage even better with an even bigger engine.
Note also- we had to figure it out on smaller engines (40's) because if you get it wrong with a 40, it only goes a little off the rails, if you get it wrong with an 88, you are in deep trouble almost immediately. That's *exactly* what happened to Windy and his Jett 91- 1% too much power, way too fast, 1% too little power, way too slow, a little too much power boost, it was like a rocket, and not enough, and it was dead. A smaller engine has less capability to do that, and it was necessary to get it really right on a 40 so you knew what to do with a 60 (and now a 75).
With these gigantic engines, you *cannot tolerate* the sort of prop effiiciency you get with high-pitch large diameter props, you will have (as you have noted) difficulty keeping the level flight speeds consistent in different conditions, and you will get either JATO-boosted corners or have it feel "flat" with very tiny (almost unmeasurable) changes in the air. What you are doing with the compression, no nitro, etc, is *detuning* the engine to reduce it's capability in order to make it easier. We all tried that, we think it's better otherwise. You are having the same sorts of problems with the giant engines that we had with Schnuerle 40s (since they have about the same sort of capabilities).
This is an *entirely solved* problem, we have it to the point we can run arbitrary-size engines in any size airplane, we pump the compression as high as we feel like and run as much nitro as we can carry - because we learned to control it.
The only thing "wrong" with it, from your perspective, is that the noise is much higher, no matter what you do, because all that horsepower you are wasting has to go somewhere, and it's going into churning up the air and making noise (although we have reduce the diameter to the point it's not as bad as it was, say, back in 1991 when we were running props like 13-3.25 at 12,000). It's not as bad as running open exhaust, but can be annoying to hear 40VFs monotonously whining away all afternoon for 4 days. That's a more-or-less irrelevant consideration here, but maybe not for you. If so, you still can't solve it your way (in my opinion) because you will give up too much performance. Fortunately, there is a thing called a brushless DC motor that also works better.
Obviously, you can do as you wish, and you will learn something regardless, but if you just want to solve the problem, I can assure you it has already been solved as described, your problems are essentially the same as the ones we had, we tried all these approaches, and this hasn't been an issue for competitive US fliers for at least 15 years, and more like 30.
Brett