Reading the "retracts" thread, which morphed into commentary/opinion of TC v. electric power.....good stuff there....
I have paid a lot of attention to prop selection by fliers of IC power....and have read the virtues of large v. smaller diameters and heard top fliers rail about proper prop pitch....
I have watched Windy fly back to back to back....flights changing the prop on each....
Prop pitch in 1/10th degree increments has been discussed.....
Yet, when power is from a battery, the instructions seem to be "just speed it up".......
IC engines are capable of being sped up as well, but that is poo-pooed.....
Just curious....
The difference with electric is that changing the pitch (or the load in other ways) doesn't have the same effect on electric that it does on an engine. With an engine you have to concern yourself with the feedback it provides, and where it puts the engine WRT the "setting"', for lack of a better word. Changing the speed on an electric is closer to changing the venturi diameter than it is the needle. You tweak the needle in, it goes faster but it also goes leaner, you add prop drag, it slows down and goes "leaner". Add venturi diameter, and it goes faster, and you hold the same mixture (more fuel, but also more air) but the effects of the changing prop drag in the maneuvers also gets more dramatic. If the prop drags the engine RPM down, your fuel suction goes down, and it generally goes leaner - which is about 3/4 the effect of the effect that causes a 4-2 break, also why you get a power change effect even if it goes from a medium 2 to a lean 2 like the 20FP.
Look at the effect of the Berringer-style venturi on a 4-stroke. Instead of trying to adjust it with the needle, you adjust the screw (which is like a narrow-range, very finely controlled throttle, without all the crudity and leaks of a conventional RC carb) and change the choke area. You would like to do the same thing on a PA75, if you didn't have to take it apart to change it.
Big Jim was telling people more-or-less the same thing 35 years ago - the needle is not a throttle. It's great for *small* adjustments because realistically, that's all you have on a conventional engine, but anything more than a small tweak, and you probably want to change the venturi, compression, or the nitro, which have similar effects for any big change.
The very best engine setup have the interesting effect of going "flat" when you get the needle a little lean, or getting more responsive if you are a little slow on the needle. Too fast/lean overall and the speed is fast, but it picks up less boost in the maneuvers, so it tends to minimize the feeling of speed in the maneuvers. Too slow/rich, and it's slow overall, but has more boost in the maneuvers to provide adequate line tension to get through. One of the reasons tuned pipe engines are so much more repeatable and insensitive to conditions is that you have to count much less on venturi and other feedback effects - because you have a very good regulator in the exhaust tuning, which works more like an electric RPM governor rather the complex relationships involved in 4-2 break engines.
Brett
Topic: IC v. Electric and props (Read 1552 times)