dangermouse -
Your comment regarding the number of motors and reliability/failure rates is correct. Below is a typical fault tree for an electro-mechanical flight control actuation system. The threshold for primary flight control applications (like a helicopter tail rotor) is 1x10^-9 failures/flight hour. As you can see the least reliable parts of the system are the motor power electronics and controllers. A system like this would probably require 3 or more independent and isolated power/control circuits, and fault tolerant multi-phase motors. Fixed pitch blades are less efficient, but eliminate the need for very high reliability (and complex) actuators.
