I was wondering how it is possible to determine what is the optimal number of blades in a helicopter rotor. I think that the length of the blade is involved as a longer blades would have to spin slower in order to avoid the extremes to overcome the speed of sound, and so one should be able to add more blades avoiding turbulences. Are there any other parameters? Is there a way to combine them together or we just rely on trials and empirical experience?
Looking around a bit I've found this question Wind generators – why so few blades? The physics case is just opposite, however that situation has a pretty different answer.
It seems to me that there is a correlation with the weight of the helicopter, for instance the CH-53 has 7 blades while the famous bell 206 just has two. Still comparing it to the case of wind generators, I cannot get an overview of what's going on.
Best Answer
I wont give you precise formulas, but one can calculate this. The most efficient theoretical rotor has only one blade. Obviously 1 blade would cause problems due to misplaced center of mass. That is why we use at least 2. Then to get more thrust you need long blades and you need to spin them very fast. There are two issues that would force you to have more blades. First is the blade tip must remain subsonic. Second is the practical size of the rotor. Very large rotors would require large areas to land and maneuvering would be more difficult. So basically more blades mean more power but with less efficiency and they take up less space. A particular helicopter design should use as little blades as possible given specific requirements.