Traditional helicopter designs use a main rotor for lift and thrust, and a tail rotor to counter the torque applied to the fuselage. Another design, known as coaxial rotors, uses a pair of helicopter rotors mounted one above the other to produce both lift and thrust. Sikorsky’s high speed technology demonstrator the X2 uses this design as well as many Russian helicopters.
To neutralize the torque, the rotors spin in opposite directions creating equal and opposite torques that cancel each other and eliminate the need for a tail rotor. Yaw control is achieved by increasing the collective pitch of one rotor and decreasing the collective pitch on the other. Coaxial rotors also reduce the effects of dissymmetry of lift. Because they spin in opposite directions, both sides of the rotor disc have a retreating blade and an advancing blade.
Another benefit of a coaxial design is a higher payload for the same engine power. A tail rotor consumes some of the available power produced by the engine. With a coaxial rotor design that extra power can be devoted to lift and thrust. Moreover, eliminating the tail rotor reduces noise, allows for a more compact design and increases safety on the ground.
The major disadvantage of the coaxial rotor design is the increased mechanical complexity of the rotor system. Two swash plates and their related linkages for both rotor systems need to be constructed on the same mast, which in itself is more complex because of the need to drive two rotors in opposite directions. This is offset somewhat by eliminating the intricacy of a tail rotor system. It would seem that the complexity of the rotor systems would increase the risk of a catastrophic failure. However, helicopters with this design have a good reliability record.