A STORY FOR ROTORHEADS AND YOU
A lot of modelers fly RC helicopters today. And I assume most of them know that the first controllable helicopter was built by Igor Sikorsky and first demonstrated on September 14, 1939. But, how many know that Igor’s triumph was built on designs by a young Spanish engineer, Juan de la Cierva? Cierva is credited with designing and flying the first successful rotary wing aircraft, the Autogiro, or gyrocopter, in 1927. He had designed several fixed wing aircraft, and knew the wing must travel through the air to develop lift. He also wondered if the passengers must travel through the air at the same speed as the wing and would a rotary wing be possible to allow lift at near zero forward speed which would then allow vertical flight.
At the time there were other rotary wing experimenters, but no one had mastered the problem of torque induced by the engine rotating the rotor blades, which produced torque in the opposite direction spinning the fuselage uncontrollably. In 1920 Cierva wondered if the rotor were unpowered and spun by the wind like a windmill, would it reduce or eliminate adverse torque.
He found the unbalanced lift created by the advancing blade compared to the retreating blade in his fixed pitch solid hub design was uncontrollable and briefly experimented with a dual contra-rotating rotor head, but the top rotor downwash halved the torque of the lower rotor. The Aotogyro flopped over on its side also.
Cierva tried a cam arrangement on the rotor head to lower pitch on the advancing blade to balance the lift component. This aircraft tipped over before it left the ground also. So, small ailerons were added to small ‘winglets’ in the engine slipstream, and the aircraft made small hops, rather than controlled flight. Cierva was thrilled with the minor success and continued experimenting.
Small rubber powered models flew quite well, but the full-scale rotors did not. Why? Cierva reasoned that the model rotor blades were flexible and as the advancing blade created lift it bent upwards and as they did, the angle of attack of that blade was lowered, effectively reducing lift of that blade. The retreating blades did the opposite, increasing lift proportionately.
A rotor was constructed with pivoting hinges on each blade with cables attached to the top of the rotor mast to prevent the rotor blades from dropping too far. In 1923 this Autogiro made short, but controllable, flights. Stress analysis showed high loads on the rotor blades and Cierva decided to add a second set of hinges allowing the blades to advance or retreat balancing fore and aft loads. This was the birth of a controllable aircraft with all the design features used by Igor Sikorsky on his first helicopter design. Sikorsky’s tail rotor overcame the torque problems experienced by Cierva.
Performance of the later aircraft gave a cruise airspeed of 65 – 120 MPH and a vertical autorotation landing. Some models were equipped with clutch driven rotors to start rotation on the ground and when de-clutched the aircraft could ‘leap’ off the ground for some altitude, moving into forward flight as it slowed.
This success led to initial production of a number of Autogiros and licensing production in Britain, Germany, France, Russia, and Japan. In the U.S. Harold Pitcairn purchased production license and his Autogiros were used to deliver air mail between New York City and Miami Florida. Wallace Kellet and his brother Rod obtained a license and built a line of Autogiros too. In all, there were about 500 licensed Autogiros built.
With the ability to almost leap off the ground, and land in ridiculously short spaces the Autogiro became wildly popular. Amelia Earhart flew an autogiro to a world altitude record of 18,415 feet ASL. In 1931 a Pitcairn Autogiro made a landing on the South lawn of the White House to accept the Collier Aviation Award from President Herbert Hoover for “the greatest achievement in aviation, the value of which has been demonstrated by actual use during the preceding year.” Admiral Richard Byrd took an Autogiro on one of his Antarctic expeditions.
For their time, Autogiros were expensive costing around $15,000 each with the advent of the reliable helicopter; the Autogiro became a historical oddity. In the years since WWII the Autogiro became an oddity. But amateur builders have built numerous Autogiros. Some towed, some powered by a variety of engines pushing or pulling the generally single placed craft into the air. A few restored Autogiros are flown at air-shows and fly-ins around the world.
RC models have been available in sport and scale versions for model experimenters who want “something different”
Re: Air & space magazine – December 1989-Jan 1990 edition
Photos from Internet sources
Models/web articles/autogiro 2012 March