Heavier-than-air craft that supports itself
in the air with a rotary wing, or rotor.
The Spanish aviator Juan de la Cierva 
designed the first successful autogiro 1923.
The autogiro's rotor provides only lift and
not propulsion; it has been superseded by 
the helicopter, in which the rotor provides
both.  The autogiro is propelled by an 
orthodox propeller. The three- or 
four-bladed rotor on an autogiro spins in a
horizontal plane on top of the craft, and is
not driven by the engine.  The blades have
an aerofoil cross-section, as a plane's wings
do.  When the autogiro moves forward, the 
rotor starts to rotate by itself, a state 
known as autorotation.  When travelling fast
enough, the rotor develops enough lift from
its aerofoil blades to support the craft.

Subject by: Russell Webb