There's an optimum number of blades for a rotor/propeller, depending on the airframe, engine, operating conditions etc etc.
Too many blades and you lose efficiency due to the propeller blades interfering with each other. More prop blades means you can have a larger pressure rise over the propeller disc, meaning that you can either have the propeller suck up more torque from the engines and put out more thrust, or you can have a smaller propeller disc. In the latter case, you can increase the RPM without having the tips go transonic and lose efficiency. Higher RPM means more power for the same torque, which means lighter prop shaft, gearbox etc. On the other hand, it means more noise.
And so on, ad nauseum.
Furthermore, one number of blades may be more ideal for one phase of flight, while another number is ideal for another phase.
|Quoting EMBQA (Reply 8):|
The Saab 2000 prop turns nearly 50% slower then the Saab 340, but the 2000 flies faster and burns less fuel.
The 2000 burns almost twice as much fuel per hour as the 340. Range, speed and payload is greater as well though.
Compromises, compromises... the mantra of the aeronautical engineer and the airline manager alike!
I thought I was doing good trying to avoid those airport hotels... and look at me now.