EAA Sport Aviation - January 2010

hile continuing his education as a civil engineer, Cierva designed a large tri-motor biplane bomber in 1918 for a military competition. On its first

flight, it stalled in a low-level turn and crashed. The pilot walked
away from the wreck, but Cierva decided to turn his efforts to
solving the problem of the stall.
And the concept of the Autogiro was born.

While many inventors at the time were trying to develop a helicopter, Cierva hit upon the idea of the unpowered rotor as a source of lift, using the concept of autorotation. Take four long skinny wings, anchor them at a common point, and you have a rotor that spins itself when subjected to airflow and will not stall, eliminating the stall/spin problem of fixed-wing aircraft.

It’s a great concept, but in practice there’s a lot more to it, as Cierva discovered in his first experiments in the early 1920s. With forward motion of the aircraft, the advancing blade creates more lift, which with a rigid rotor head causes the machine to roll toward the retreating blade. This was solved by adding “flapping” hinges, allowing the advancing blade to rise and effectively decrease its angle of attack to match its lift to the retreating (and descending) blade. Due to minor changes in the speed of each rotor blade during flight, fore-and-aft or lead-and-lag hinges were required (See illustration on pg. 25).

By the middle 1920s the design was considered a success,
and soon a number of “windmill planes” were built by licensees
of Cierva and flying around Europe under the trademarked
Autogiro name.
Enter Harold Pitcairn, American builder of airmail aircraft
and a man interested in the development of the helicopter.

Recognizing the potential of the Autogiro, Pitcairn persuaded Cierva to let him become the American representative and went on to build dozens of ’giros of varying types at his factory in Willow Grove, Pennsylvania. They were expensive, costing from $5,000 to $8,000 during the Depression when the average annual income was less than $1,000, but they found use as advertising vehicles for many companies like Coca-Cola, Beech-Nut (whose Autogiro was flown by Amelia Earhart), and the Champion Spark Plug Company, whose Pitcairn PCA- 2 Miss Champion is now in the EAA AirVenture Museum.

Both Pitcairn and Cierva continued to advance the design, Pitcairn coming up with the rotor spin-up drive using a unit on the back of the engine, and both contributed to advancements such as cyclic control to vary the angle of each blade individually, eliminating the need for flapping hinges, and the collective, which could vary the angle of all blades at once and which was used for jump takeoffs. These concepts were crucial to the development of the helicopter in the late 1930s. Ironically, with its ability to hover, the helicopter then drove the Autogiro into obscurity.