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ORIGINAL: HighPlains

Several interesting questions.

Long life on an engine has more to do with staying on the rich side and operating in a clean environment than RPM. If anything, low RPM is harder on the engine than higher. If you insist on lower RPM operation, use additional oil. Low RPM operation also makes the needle setting much more critical.

As to the prop size difference between 40 and 60. An 11" prop has 21% greater area than the 10", while at the same rpm has a 10% greater tip speed. When you increase speed by 10% it takes about 30% more power. Multiply the two factors (1.33 x 1.21 = 1.6). Not an exact equation and certainly not the actual equation, but close enough to give an idea of why.

Aircraft fly on horsepower. The rate of climb is directly related to hp, while the speed is related to a cubic function. The trick is to find a prop that allows the engine to develop the power while effectively transferring that power to thrust. At best a prop is only 85% efficient in transferring power, and can be lower. It is usually best to set up an engine for sport flying to operate at a RPM two to three thousand below where it is rated for peak power. This allows the engine to unload and operate closer to peak power.

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