Multi-Rotor question
An argument came up in the pub yesterday between me and a mate which raised a question concerning helicopters which hopefully someone here can answer.
The basis of the argument was as follows (and this is very basic stuff, so please forgive me). On a normal helicopter there is a single rotor. When this revolves to provide lift it generates torque which will cause the fuselage to revolve in the opposite direction to the rotor. To counteract this a tail rotor is fitted to provide a counter-thrust to the fuselage so it doesn’t spin. However if a helicopter has two contra-rotating rotors the forces between the two balance out so there is no need for a tail rotor to stop the fuselage spinning.
But (and here the argument started) there is still a requirement to turn the fuselage (to face the direction of travel, saves getting a crick in the neck when flying backward). In the standard single-rotor machine this is, I believe, done by varying the thrust from the tail rotor so it provides either too much or too little (which will turn the fuselage). However how is it done on a multi-rotor machine?
I claimed it could be done, at least theoretically, by varying the properties between the rotors (different pitch or, less likely, different speed for the two would produce a torque imbalance which would turn the machine). He insisted that they must have some external thrust, either from a tail rotor or variable ducting for the exhaust (for the turbine powered machines) to turn them (I cited the Chinook as not having a rotor but he insisted it has ducted thrust).
Does anyone know if this is possible (or even has been done) or am I talking through my posterial orifice again? (And there is a pint riding on this).