A backpack helicopter is a helicopter motor and rotor and controls assembly that can be strapped to a person's back, so that he can walk about on the ground wearing it, and can use it to fly. Its harness, like a parachute harness, should have a strap between the legs, so that the pilot does not fall out of the harness during flight. Typically a backpack helicopter differs from a conventional helicopter in two ways. Firstly there is no tail rotor, and the main rotors are contra-rotating. Yaw is controlled by fine adjustment of a differential gear in the rotor drive transmission.