The Cierva C.6 was the sixth autogyro designed by engineer Juan de la Cierva, and the first one to travel a "major" distance. Cierva, the engineer responsible for the invention of the autogyro, had spent all his funds in the research and creation of his first five prototypes. So, in 1923, he turned to the Cuatro Vientos Aerodynamics Laboratory chief, Commander Emilio Herrera, who succeeded in persuading General Francisco Echagüe, the director of the Military Aviation Aeronautics Department, to take over the second stage in the research and development of Cierva's Autogyros.After several wind tunnel tests, Military Aviation built a Cierva C.6 autogyro in an Avro 504 frame. This machine, piloted by Captain Joaquín Loriga Taboada, performed three flights, all of them in March 1924. One of those flights, the eight minute trip from Cuatro Vientos airfield to Getafe airfield (10.5 km / 7 miles), was considered a giant's step and the "leap into glory" of Cierva's autogyros.
The Cierva C.6 prototype was fitted with ailerons mounted in two small wings, elevators and rudder. This complete three-axis control scheme was needed because the pilot had only limited control over the rotor (See Helicopter rotor). Only the front propeller was powered, so this aircraft could not hover, and could lose control at low speed. The vertical axis rotor spun freely; the faster the autogyro flew, the faster the rotor would spin and the greater lift it produced. A replica of the Cierva C.6 was built to be shown in Murcia pavilion in Seville Expo '92 World's Fair. That replica can be now be seen in Museo del Aire, Cuatro Vientos, Madrid, Spain.
Before the experimental Cierva C.19 Mk V, autogyros had been controlled in the same way as fixed wing aircraft, that is by deflecting the air flowing over moving surfaces such as ailerons, elevators and rudder. At the very low speeds encountered in autogyro flight, particularly in the landing phase, these controls became useless. The experimental machine showed that the way forward was to have a tiltable rotor hub and a control rod coming down from the hub to the pilot's cockpit with which he could change the rotor plane. This was known as "direct control" and was adopted by the C.30. The production variant, called C.30A in England was preceded by several development machines.
The first in the series was the C.30, a radial engined autogyro with a three blade, 37 ft (11.3 m) rotor mounted on an aft-leaning tripod, the control column reaching back to the rear of the two cockpits. The engine was the five-cylinder, 105 hp (78 kW) Armstrong Siddeley Genet Major I used in the C.19 series. The fabric covered fuselage carried an unbraced tailplane, without elevators but with turned up tips. The port side plane had an inverted aerofoil section to offset the roll-axis torque produced in forward flight by the advancing port side blades. As with most autogyros, a high vertical tail was precluded by the sagging resting rotor, so the dorsal fin was long and low, extending well aft of the tailplane like a fixed rudder and assisted by a ventral fin. A wide track undercarriage had a pair of single, wire braced legs; there was a small tail wheel. It flew in April 1933. It was followed by four improved machines designated C.30P (P here for pre-production) which differed in having a four-legged pyramidal rotor mounting and a reinforced undercarriage with three struts per side. The rotor could be folded rearwards for transport. The C.30P used the more powerful (140 hp, 104 kW) seven-cylinder Armstrong Siddeley Genet Major IA radial engine.
The production model, called the C.30A by Avro, was built under licence in Britain, France and Germany and was similar to the C.30P. The main alteration was a further increase in undercarriage track with revised strutting, the uppermost leg having a pronounced knee with wire bracing. There was additional bracing to the tailplane and both it and the fin carried small movable trimming surfaces. Each licensee used nationally built engines and used slightly different names. In all, 143 production C.30s were built, making it by far the commonest pre-war autogyro. Between 1933 and 1936, Cievra used one C.30A (G-ACWF) to perfect his last contribution to autogyro development before his death in a DC-2 (fixed wing) crash in late 1936. To enable the aircraft to take off without forward ground travel, he produced the "Autodynamic" rotor head, which allowed the rotor to be spun up by the engine in the usual way but to higher than take-off r.p.m at zero rotor incidence and then to reach operational positive pitch suddenly enough to jump some 20 ft (6 m) upwards. At least one of the RAF C.30As was in January 1935 on floats as a Sea Rota.
A full-scale flying reproduction was built in Spain in the mid-1990s. After a brief flying career a crosswind accident led to the damage to the rare rotor blades. The aircraft is now on display at the Museo del Aire, Cuatro Vientos, Madrid, Spain.