[going hot]

The rotors give opposite torque to the shaft which is connected to the frame, thus the tail rotor is needed to counteract the torque of the blades.

With two rotor discs, the tail rotor is not needed, as the net torque cancels.

I think if one set of rotors were to be given a little bit more of pitch, and the other set a little less, the bird would stay as same altitude, but the net torque to the shaft would no longer be neutral, thus causing the tail to come around.

I have not quit my current day job, so this is just speculation based on previous infor from another life. (!)

[Openurmind]

Thank you! SJSAMPLE explained the same principle. :)

[ProtectOurFreedom]

Wouldn’t it be hard to hold constant altitude, though, if you are generating enough torque to turn the helo around?