The helicopter is a flight vehicle using two or more rotor blades fixed to a common drive shaft rotated by an engine. In contrast to fixed-wing aircraft, the helicopter can take off vertically, land vertically, and hover. It can move forward, backward, and sideways. The helicopter is designed for effective operation in hover for long periods of time, for high-speed cruise, and for high range and payload. Helicopter aerodynamics usually refers to rotor-blade aerodynamics. Proper design of the rotor blades is essential.