The technical challenges and accomplishments in the development of the autogiro are described. Exactly 80 yearsago, the autogiro was the first successful rotating-wing aircraft, and the first powered, heavier-than-air aircraft to fly other than an airplane. Unlike a helicopter, the rotor on an autogiro is not powered directly, but turns by the action of the relative airflow on the blades to produce a phenomenon known as autorotation. The aerodynamic principles of autorotation are explained and are combined with the historic technical insights of Juan de la Cierva, who used the principle to successfully develop and produce the autogiro. It is shown that although the autogiro encountered many technical hurdles its developers worked in a systematic, step-by-step approach to advance the state of knowledge. The autogiro did not have a long commercial or military life, but it was certainly a significant technical success. There were major scientific and engineering contributions from both practical and theoretical fronts. The most significant was the development of the articulated rotor hub with flapping and lead/lag hinges and later the complete and precise control of the aircraft by tilting the rotor plane using cyclic blade pitch (feathering). The era also accomplished the first scientific understanding of rotor behavior and the first mathematical theories of rotor aerodynamics, blade dynamics, structural dynamics, and aeroelasticity. The success of the autogiro also paved the way for the helicopter, but predating it by about 15 years, and providing fundamental technology that greatly accelerated its development.
