High power engines are necessary to drive rotating blades; this explains the delay in helicopter development after the Wright brothers' invention of the fixed-wing aircraft. The turboshaft engine, invented in the 1940s, matched the rotary wing power demands.
As the rotor creates lift, it also causes TORQUE in the helicopter body attached to it. This torque will spin the body in the opposite direction to the rotary movement if left uncontrolledNewton's law again. A small vertical propeller at the rear of the helicopter body, gear-connected to the main rotor, counteracts the torque force to keep the helicopter stable. In operation the rear propeller will often overcompensate for torque and cause the machine to drift. Controlling the rear rotor (propeller) to compensate for the drift is one of the operator's tasks.