 Do you know why there are two rotors in any helicopter? This is to conserve the angular momentum of the helicopter. According to the law of conservation of angular momentum, in the absence of an external dog, the angular momentum of any object stays constant. The lift for the helicopter can be generated by a single rotor as well, but in that case, the helicopter body would start spinning in the opposite direction to conserve the angular momentum which was zero initially. The rotation of the second rotor balances the angular momentum generated by the first and thus, the helicopter stays stable. Food for thought for you. Can the angular momentum of the helicopter be conserved by using three rotors instead of two?