Juan de la Cierva (21 September 1895 – 9 December 1936) was a Spanish civil engineer and pilot. His most famous accomplishment was the invention in 1920 of the Autogiro, a type of aircraft that later came to be called an autogyro. After four years of experimentation, la Cierva developed the articulated rotor which resulted in the world's first successful flight of a stable rotary-wing aircraft in 1923 with his C.4 prototype.
De la Cierva was born in Murcia, Spain to a wealthy family. After several successful experiments with aviation as a boy, he eventually earned a civil engineering degree. He moved to England in 1925, where with the support of Scottish industrialist James G. Weir, he established the Cierva Autogiro Company.
At the outbreak of the Spanish Civil War, De la Cierva supported the forces of Francisco Franco, helping the rebels to obtain the De Havilland DH-89 'Dragon Rapide' which flew General Franco from the Canary Islands to Spanish Morocco. His brother was executed by the Republican army in Paracuellos del Jarama.
Juan de la Cierva started building aircraft as early as 1912, and in 1919 he started to consider the use of windmills or rotors as a means of sustaining lift at low speed, and possibly eliminating the risk of stalling altogether.
In order to achieve this, he utilised the ability of rotor to autorotate, whereby at a suitable pitch setting, a rotor will continue to rotate without power, sustained by the torque equilibrium of the lift and drag forces acting on the blades. This phenomenon was already known, and was available as a safety feature to allow controlled descent in the event of engine failure. With Cierva's Gyroplane, the rotor was drawn through the air by means of conventional propeller, with the result that the rotor generated sufficient lift to sustain level flight, climb and descent.
Before this could be satisfactorily achieved, Cierva experienced several failures primarily associated with the unbalanced rolling movement generated when attempting take-off, due to asymmetry of lift between the advancing and retreating blades. This major difficulty was resolved by the introduction of the flapping hinge. In 1923, Cierva's first successful "Autogyro" was flown in Spain by Lt Gomez Spencer.
All Cierva's pioneering work was carried out in his native Spain. In 1925 he brought his C.6 to England and demonstrated it to the Air Ministry at Farnborough. This machine had a four bladed rotor with flapping hinges but relied upon conventional aircraft controls for pitch, roll and yaw. It was based upon an Avro 504K fuselage, initial rotation of the rotor was achieved by the manual tension of a rope passed around stops on the undersides of the blades.
The Farnborough demonstration was a great success, and resulted in an invitation to continue the work in the UK. As a direct result, and with the assistance of the Scottish industrialist James G Wier, the Cierva Autogyro Company was formed the following year. From the outset Cierva concentrated upon the design and the manufacture of rotor systems, relying on other established aircraft manufacturers to produce the airframes, predominately the A.V. Roe Company.
The Avro built C.8 which was basically a refinement of the C.6, with the more powerful 180hp Lynx radial engine, and several C.8's were built. The C.8R incorporated drag hinges, as it was found that the presence of flapping hinges caused blade oscillation in azimuth; giving rise to high stresses with the risk of blade failure, this brought on other problems such as ground resonance for which friction type drag dampers were fitted.
The resolution of these fundamental rotor problems opened the way to progress, confidence built up rapidly, and after several impressive cross country flights a C.8L was entered for the 1928 Kings Cup Air Race and although it was forced to retire, it subsequently completed a 4,800Km (3000 miles) tour of the British Isles. Later that year it flew from London to Paris, extending the tour to include Berlin, Brussels and Amsterdam, thus becoming the first rotating wing aircraft to cross the English Channel.
A predominant problem with the gyroplane was concerned with achieving initial rotor rotation. Several methods were attempted in addition to the rope and drum system, which could take the rotor speed to 50% of that required, at which point movement along the ground to reach flying speed was necessary, while tilting the rotor to establish autorotation.
Another approach was to tilt the tail stabiliser to deflect engine slipstream up through the rotor. The most acceptable solution was finally achieved with the C.19, which was produced in some quantities; a direct drive from the engine to the rotor was fitted, through which the rotor could be accelerated up to speed. The system was then declutched for the commencement of take-off run.
As Cierva's autogiros achieved success and acceptance, others began to follow and with them came further innovation. Most important was the development of direct rotor control, which was achieved initially by tilting the rotor hub and subsequently by the application of cyclic pitch, causing the blades to rise or fall at appropriate points in their rotation, thereby effectively tilting the rotor in the required direction.
The introduction of jump take-off was another major improvement in capability. The rotor was accelerated in fine pitch until the rotor speed required for flight was achieved, and then declutched. The loss of torque caused the blades to swing forward on angled drag hinges with a resultant increase in collective pitch, causing the aircraft to leap into the air. With all the engine power now applied to the forward thrusting propeller, it was now possible to continue in forward flight with the rotor in autorotation.
All the above features were brought together in the C.30, which was produced in quantity for civil and military use, the autogiros of the 1930's were looked upon as a wonder of their time, but it must however be emphasised that they were not helicopters, and were not capable of vertical take-off, landing or hover in still air, albeit that they could maintain very low speed and accomplish a near vertical landing into wind.
Having successfully created and flown his gyroplane in Spain, Cierva established his company in Britain, comprising of a design office and sales organisation, calling upon established aircraft manufacturers to produce his designs. A number of well-known British firms produced aircraft for Cierva including Avro, Comper, De Havilland and Westland.
The importance of Cierva’s work on rotors and the effect upon the evolution of the helicopter cannot be overstated, and is indeed recognised throughout the industry. In the process of creating the Gyroplane (Autogyro), Cierva established an understanding of rotor dynamics and control, which was applicable to all rotorcraft, and undoubtedly led to the realisation of the helicopter.
It is interesting to note that although this work led the way to the helicopter, Cierva himself never set out to create such a machine. His primary interest was to produce an aircraft that could not stall. It is perhaps an ironic twist of fate that Cierva should die in an airliner accident near London at age 41.