In the early 1900s, Juan de la Cierva, a Spanish aviator who built airplanes and gliders, unknowingly helped with the development of the helicopter. When one of his airplane prototypes crashed on its second flight during a low speed stall he decided to try to find a way to allow airplanes to fly slower. Windmills got him thinking that a rotating wing could produce lift without the need for forward airspeed. This led him to build the first autogyro (an aircraft that uses a propeller for thrust, but replaces the wing with a free-wheeling rotor for lift).
His first design lifted off the ground and immediately rolled over and crashed. He rebuilt the aircraft and tried again only to see the same result. This perplexed Cierva because the small model he built first as a proof-of-concept did not roll over. What was becoming clear to him was the concept of dissymmetry of lift – that is the difference in relative wind (and as a consequence lift) seen by the advancing and retreating sides of a rotor flown edgewise through the air. After much thought, the difference between his model and the full scale aircraft became clear. The model’s rotors were small and did not need supports which allowed them to flex, while the full scale rotors were heavy and required wire bracing making them stiff. The flexible rotors on the model could flap up and down which compensates for dissymmetry of lift. He then added hinges to his full scale aircraft to allow flapping and was able to proceed with development. The autogyro could not hover, but did meet his goal of slower flight. Over the next several years, various manufactures developed and sold autogyros.
The autogyro went on to achieve limited success until the 1930s, which saw the development of helicopters that could hover. As helicopter designs continued to mature, the autogyro faded out as a commercial aircraft. However, it was the autogyro that solved one of the biggest aerodynamic problems for rotary wing flight.