Sport Pilot,

I can't point you to the data you want offhand, but I will make an observation. When you go to a 3 or 4 bladed prop from a 2 bladed one, a lot of things change. If you don't change the pitch and/or diameter, you will greatly increase load on the engine, and probably take it out of its ideal RPM range. If you reduce the diameter, you will definitely reduce efficiency. And, yes, all things being equal, more blades is less efficient. My suspicion is that people who see large performance reductions are not making the other adjustments to match the prop to the engine and plane, and that is a more visible problem than the efficiency. Just a guess.

banktoturn