Cierva 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 De la 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. De la 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. While attending an opera he realized that the flexibility reduced the moments acting on the hub and eliminated instability. Trained as a civil engineer, Cierva was aware that an otherwise cantilever structure hinged to its attachment point also generated no moment, and he designed a rotor the blades of which were mounted to the hub through hinges to permit their vertical oscillation.