You need to conteract the reaction torque of the prerotator!
So use two rotors! Synchropter see the Touringcopter; or, coaxial rotor see the Gyrocoax!
With the Synchropter, you can block the roll axe of each rotorhead, not to have the problem of the residual torque of the driving device...
Give feedback.
Bye.
André.