Wouldn't it be smarter to design VTOL rotorcraft with many smaller rotors than one big rotor? With more rotors you have more redundancy and smaller blade velocity (and perhaps better stabilisation) thus leading to a safer more reliable system. It seems like the problem with lots of rotors is that you lose all efficiency. I don't get this though. If a rotor prop produces x thrust at y efficiency, couldn't this be simply scaled up to produce the same thrust and efficiency ratio?
$\begingroup$ You should emphasize what information your didn't find in the answers of the question Why haven't quadcopters been scaled up yet? $\endgroup$– Manu HNov 13, 2016 at 16:39
2$\begingroup$ The answers are correct that multiple small rotors will be less efficient than one rotor, but the unspoken question of "why is this aircraft designed this way?" is that it's an electric tiltrotor--it uses differential thrusts and torques for control in vertical mode (mechanically simpler than traditional helicopter controls) and shuts off some of the motors in horizontal cruise. I don't know what the predicted overall system efficiency is, but it can be better for some mission profile than a main+tail rotor helicopter even if the prop vs. rotor efficiency is strictly lower. $\endgroup$– Erin AnneNov 13, 2016 at 22:53
3$\begingroup$ Related: Why do solar planes have many small propellers instead of fewer large ones? $\endgroup$– userNov 15, 2016 at 12:53
Rotors need something to power them and the current generation of aircraft (once you get bigger than model size) burn hydrocarbons to get that power. If you want twelve rotors, then you either need twelve engines or you need a smaller number of engines plus a bunch of heavy driveshafts and gearboxes.
I assume from the fact that most contemporary aircraft designs use a small number of engines (usually one or two; sometimes three or four) that having a few larger engines is generally more efficient than having more smaller engines. This seems to apply equally to jet engines, turboprops and piston engines.
It's possible that the tradeoffs will change if you move to electric motors. However, at the moment, batteries are so heavy that electric motors don't seem to be a practical option.
The problem with many propellers is that more of the propeller slipstream will flow around wings and tail. The higher local flow speed might be beneficial if you want blown flaps, but for regular flight and especially in hover it will increase viscous and pressure drag without any benefit. This will reduce overall efficiency when the number of propellers goes up.
Generally, one single, big item can be made more efficient than many small ones.
1$\begingroup$ So the solution for this problem is to keep the props above or below the wings? $\endgroup$ Nov 13, 2016 at 19:00
$\begingroup$ @DanielCaoili: No, the solution is to use fewer, larger props. $\endgroup$ Nov 13, 2016 at 21:31