Tandem rotors Tandem rotor helicopters have two large horizontal rotor assemblies instead of one main assembly and a smaller tail rotor. Single rotor helicopters need a tail rotor to counter the twisting moment produced by the single large rotor. Tandem rotor helicopters, however, use contra-rotating rotors, with each canceling out the other's torque. This configuration also has the advantage of being able to hold more weight with shorter blades, since there are two sets. Also, all of the power from the engines can be used for lift, whereas a single rotor helicoptor only uses one main rotor for lift with the tail rotor only to counter the torque. Source: Wikipedia / posted by Flyboy