Tandem rotors

Tandem rotor helicopters have two large horizontal rotor assemblies mounted one in front of the other. Currently this configuration is mainly used for large cargo helicopters.

Single rotor helicopters need a mechanism to neutralize the yawing movement produced by the single large rotor. This is commonly accomplished by a tail rotor, coaxial rotors, and recently the NOTAR systems. Tandem rotor helicopters, however, use counter-rotating rotors, with each canceling out the other's torque. Therefore all of the power from the engines can be used for lift (force), whereas a single rotor helicopter uses some of the engine power to counter the torque. An alternative is to mount two rotors in a coaxial configuration.