A rotor wing is a lifting rotor or wing which spins to provide aerodynamic lift. In general, a rotor may spin about an axis which is aligned substantially either vertically or side-to-side (spanwise). All three classes have been studied for use as lifting rotors and several variations have been flown on full-size aircraft, although only the vertical-axis rotary wing has become widespread on rotorcraft such as the helicopter.
Some types provide lift at zero forward airspeed, allowing for vertical takeoff and landing (VTOL), as in the helicopter. Others, especially unpowered free-spinning types, require forward airspeed in the same manner as a fixed-wing aircraft, as in the autogyro. Many can also provide forward thrust if required.