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For example Rotax 912. We can meet this engine with 2 and 3 propellers mounted. Or compare airplanes equipped with engines with pretty the same power output i.e. C152 with Lycoming O-235 - 110 HP and 2-blade propeller versus AT3 with Rotax 912 - 100 HP and 3-blade propeller (also both engines have similar max propeller RPM (Rotax has reduction gearbox)). I know that increasing number of propeller blades alllow better power absorption, but what does it change if the engine has pretty low max power?

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  • $\begingroup$ I don't think so, because the engine power is the same or almost the same, but the number of propeller blades is changed. So what why it is changed, because no extra power is "added" to absorbed by more propeller blades. $\endgroup$ – Konrad Jan 30 at 14:19
  • $\begingroup$ This might very well be for noise reasons. Lowering the lift coefficient on the blades reduces noise a bit. The three-bladed propeller is slightly less efficient, but produces less noise. One or two decibels might mean the difference between being allowed to fly and being grounded in some airfields, especially in Europe. $\endgroup$ – Peter Kämpf Jan 30 at 15:55
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What it means in practical terms is if you replaced a 2 blade Warp Drive prop on a 100 HP engine with a 4 blade prop with double the total blade area, and if the blades of the 4 blade were set to the same pitch as the 2 blade, the torque requirement of the 4 blade would be significantly higher and you'd find the engine can't make the same static RPM it was making with the 2 blade at wide open throttle.

You would have to re-pitch the blades of the 4 blade prop to a lesser angle and retry until you regained the original static RPM. You're basically offloading the individual blades until the total torque demand matches the original prop. You might find you've lost a lot of efficiency from the blades now operating at less than their optimum blade angle of attack.

If the 4 blade prop had much smaller individual blades, so the total blade area was the same as the 2 blade, the difference in pitch would be minimal, although some adjustments in pitch would still need to be made to match static RPMs, and the static thrust you are getting at the target RPM would be reduced somewhat from the 2 blade because of the lower efficiency of the 4 blade.

If it was a constant speed propeller, the propeller governor would be doing all that for you.

If you are after the maximum thrust efficiency for a given horsepower, 2 blades are always better if the power can be absorbed and the target RPM can be achieved with 2 blades. What makes multiple blades attractive, as a conversion to an existing engine, is the desire to limit noise, or limitations on diameter due to tip speed limits or ground clearance requirements. For example, 3 blade propellers are popular on DeHavilland Beaver seaplanes because the noise levels are lower because the tips speeds are lower, and the shorter blades get a little bit less water spray erosion damage, benefits sufficient to live with a small reduction in prop efficiency compared to the 2 blade paddle prop it originally had.

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