1923. First flight of an airplane of rotational wings: autogyro C-4.

The Cierva C.4 was an experimental autogyro built by Juan de la Cierva in Spain in 1922 which early the following year became the first autogyro to fly successfully. The failures of Cierva's previous designs, the C.2 and C.3, had led him to understand that he needed to overcome the problem of dissymmetry of lift in order to get an autogyro to fly without rolling over. He noted that the problems that he was experiencing with his full-size aircraft were not found in the models that he had successfully flown, and considered the difference between the full-size and small-scale rotors. Cierva noted that the rotors used on his model were made of bamboo, and were thus far more flexible than the ones on his full-size aircraft. This flexibility, he realised, absorbed the increased lift from the advancing blade in deforming the blade rather than imparting it to the aircraft. His new problem, then, was of how to create a flexible, full-size rotor blade with the materials available at the time. The solution suggested itself to Cierva during a night at the opera, where he noticed a stage prop of a windmill that had been fitted with blades that hinged near their hub. Rather than making the entire rotor blade flex as on his models, Cierva wondered if all he had to do was hinge the rotors in order to allow them to flap up and down as they progressed around the hub.

The C.4 used a fuselage taken from a Sommer monoplane (possibly recycled from the C.3) fitted with a four-bladed rotor, with the blades hinged at the hub. It was completed around April or May 1922, and was tested from June onwards by Jose Maria Espinosa Arias at Getafe. Success was not immediate, however, and Cierva undertook a long series of modifications and refinements to the design. Finally, in January the following year, the aircraft flew at Getafe, under the control of Alejandro Gomez Spencer, making a flight of some 180 metres (600 ft). Sources differ as to whether this event took place on 9 January or 17 January.

On 20 January, the engine failed in flight, and the C.4's nose pitched sharply upwards. Cierva's original interest in autogyros - that of air safety - was dramatically vindicated when the machine autorotated to the ground completely undamaged. Two days later, Cierva demonstrated the aircraft to military and aero club observers, including General Francisco Echagüe Santoyo, director of the army's air service, and Don Ricardo Ikuiz Ferry, president of the Royal Aero Club Commission. This led to a military demonstration at Cuatro Vientos on 31 January, where the C.4 made a circular flight of 4 km (2½ miles) in 3½ minutes, at an altitude of over 25 m (80 ft).

In July, Cierva built the C.5, an almost identical machine but for its three-bladed main rotor.