But... on real aircraft rpm is lower than on our glow motors and also you have to account for variable pitch props on the real deals. The blade causes a vortex and on multi blade props the next blade enters the vortex thereby not pulling much air. A lower rpm allows the air to recover and also by varying the pitch creates a shorter term vortex.
We normally go down in diameter for every blade we add to counteract some of the above but still loose efficiency!