Cierva C.4 / C.6

The Cierva C.4 was an experimental autogiro built by Juan de la Cierva in Spain in 1922 which early the following year became the first autogyro to fly successfully. Failures of De la Cierva's attempts to compensate for dissymmetry of lift with the C.1, C.2, and C.3 autogiros, led him to consider alternative means of enabling an autogyro to fly without rolling over. He noted that the problems experienced with his full-size aircraft were not present in the models that he had successfully flown, and considered the difference between the full-size and small-scale rotors. The rotors used on his model were made of bamboo and were thus far more flexible than the ones on his full-size aircraft. While attending an opera he realized that the flexibility eliminated the moments acting on the hub and associated instability. Trained as a civil engineer, Cierva was aware that a cantilever structure hinged to its attachment point generated no moment, and he designed a rotor the blades of which were mounted to the hub through hinges to permit their vertical oscillation.