What works in theory on paper doesnt always work the same in real life. Some people just cannot be convinced of that. I know plenty of guys who swear by the opinion that if the book says it, it must be true. It's hard to use an example of an aircraft designed in the 20's or 30's. Two blade props were used on most all aircraft back then because there weren't many three blades around. If you want to talk about racing aircraft, well, the fastest piston engined prop driven aircraft in the world, the Reno unlimited racers, ALL run multi bladed props. The Formula One and Biplane classes run two blade fixed pitch props only because the rules dictate that they have to. The T-6 class runs two blade props because they are required to be of the type that came on the aircraft (the two bladed Hamilton Standard prop on the T-6 is one of the most inefficient props ever made, by the way. The tips are supersonic at cruise power settings, that's why they are so loud.) You are correct however in saying that there is a big difference in FS and R/C in the fact that full scale three blade props are constant speed (alot of the two blades are also). that in itself opens another can of worms when you start talking about efficency. Believe me, I know what the aerodynamic laws of theory say about propeller efficiency, but there are many many real world examples to prove that it just doesnt apply in ALL cases, even though the books preach it as gospel.