Cierva C.8W (C.8L Mk. IV)

Cierva C.8W

In 1928, Harold Pitcairn imported Juan de la Cierva's latest Autogiro, the C.8W (also known as the C.8 Mk.IV) to the United States as an experimental testbed for his own line of rotary-wing aircraft. This aircraft, as the first of its type in the United States, generated considerable interest in commercial and governmental circles. It validated Pitcairn's interest in the new category of aircraft and inspired other American pioneers to enter the field. The C.8W deserves recognition as the progenitor of the American gyroplane and as the first successful rotary-wing aircraft to fly in the United States.

The word "Autogiro" is a proprietary name coined by Juan de la Cierva. His designs were the first aircraft to fall in the gyroplane category. Nonetheless, nearly all gyroplanes built from the 1920s through to the end of World War Two became commonly known as "autogiros" (or the more generic "autogyros"), regardless of the manufacturer. A gyroplane is an aircraft that derives most, if not all, of its lift from the unpowered autorotation of a horizontally mounted rotor or rotors. Unlike a helicopter, an engine does not drive the rotor blades while the aircraft is in flight. Instead, the resultant of the lift and drag forces acts to pull the blade forward in rotation while also creating lift - the same effect that turns the sails on windmills. This state of autorotation is only possible with a sustained airflow through the rotor disc, with the air moving from below and in front of the rotor to above and behind it. The gyroplane requires some propulsive force to maintain sufficient speed to sustain autorotation and hold altitude. In the Cierva and Pitcairn Autogiros, an engine driving a tractor propeller supplied the necessary force. If the pilot reduced throttle while flying, the rotors would begin to slow and the autogiro would descend. The increased airflow of the descent allowed the rotors to continue in autorotation and maintain the blades in an unstalled condition - even without the forward pull of the propeller. Although the pilot still had to keep some forward motion for a landing flare-out, and to maintain airflow over the control surfaces, it allowed for unpowered and near vertical descents ending in a very short landing rollout. This was an excellent safety feature in case of engine failure. Nonetheless, until the advent of direct control gyroplanes, the diminished control effectiveness in slow speed flight required a highly experienced Autogiro pilot to perform minimal landing rollouts. Without a powered rotor, gyroplanes are incapable of hovering, though later designs did include rotor spin-up mechanisms that allowed near-vertical "jump" takeoffs.

Most of the early gyroplanes were similar in layout to single-engine low-winged monoplanes, with the exception of the rotor mounted on a pylon in front of the cockpit that provided the primary source of lift during slow-speed flight. They employed standard airplane-type control surfaces (elevator, aileron, and rudder) and the rotor blades all maintained a fixed pitch. The stubby monoplane wing did not serve primarily for the generation of lift. Rather, it was a convenient means of mounting the ailerons and providing stability. It also had the unintended benefit of making the aircraft appear more conventional to skeptical airplane operators who were doubtful about flying without fixed wings.

Cierva constructed his first Autogiro, the coaxial rotor C.1, in 1920. As the rotors on the C.1 autorotated at different speeds, rendering the aircraft incapable of controlled flight, he decided to switch to a single rotor design. However, the abortive first flight of the new aircraft revealed a problem that he had not considered. As the Autogiro began to gain speed during its takeoff roll, the rotor blade that was turning towards the front of the aircraft received the benefit of additional airspeed because of the forward motion of the Autogiro. However, the blade retreating towards the rear of the autogiro suffered a loss in its airspeed relative to the oncoming air for the same reason. The net effect was a difference in airspeeds of the two blades that naturally caused asymmetry of lift between the two sides of the rotor disc (as lift is a function of airspeed). In turn, this resulted in the Autogiro rolling into the retreating blade side. A subsequent Cierva Autogiro also suffered the same problem and failed to take off successfully.

In 1922, Cierva conceived an inspired solution to his problem. By incorporating a hinge that allowed each blade to "flap" independently at its root, he developed a rotor that equalized lift amongst all of the blades, regardless of whether the Autogiro was flying fast or slow. When the advancing blade generated additional lift because of its higher velocity, the flapping hinge allowed it to rise, which effectively reduced the angle of attack of the blade, thus reducing its lift. On the other side of the rotor, the flapping hinge allow the retreating blade to descend with its reduced lift, which effectively increased its angle of attack, thus generating more lift. This breakthrough was not only an essential component for the Autogiro - it was also necessary for the development of the practical helicopter.

Cierva's first successful Autogiro (and the first successful rotary-wing aircraft of any kind), the C.4, took flight on January 17, 1923 at Getafe airfield in Madrid, Spain. Over the next three years, Cierva made progressive improvements that resulted in the standard monoplane configuration for gyroplanes that remained in use until the mid-1930s. The greatest improvements came in the design of the rotor blades and the hinges, both of which would later prove essential for helicopters.

Cierva constructed his first C.8 model, the C.8V, in close association with A.V. Roe & Co. Ltd. (commonly known as Avro). The airframe was based on the fuselage of the Avro 552A, a variant of the venerable Avro 504 biplane. The most innovative component of the C.8V was its new four-bladed cable-braced rotor that incorporated drag hinges to reduce the stresses on the blades. The "V" in the model designation indicated the type of engine used in the variant - in this case, a Wolseley Viper. Cierva constructed six different C.8 configurations, all of which were experimental testbeds built to test improvements in autogiro technology before a production model, the C.19, was to appear in 1929. Given the limited knowledge of rotary wing aerodynamics of the time and the necessity of relying on trial-and-error methods, it is not surprising that Cierva constructed two dozen experimental autogiros before he completed a model worthy of production.

The most powerful of the six C.8s constructed was the two-seat C.8W fitted with a 220 horsepower Wright Whirlwind J-5. Cierva had equipped the aircraft with an American engine at the request of its buyer, Harold Pitcairn. As a teenager, Pitcairn had developed a fascination with the possibilities of rotary-wing aircraft and had avidly followed news of Cierva's progress. In 1924, he had established himself in the aircraft manufacturing industry by building rugged biplanes, one of which, the PA-5 Mailwing (see NASM Collection), was to gain a legendary reputation amongst pilots who flew the nascent airmail routes. Nonetheless, Pitcairn pursued his dream to enter the then highly dubious field of rotary-wing aviation. In 1925 and 1926 he traveled to England for discussions with Cierva about either license-building Autogiros or using them as the basis of his own helicopter designs. While there with his engineer, Agnew Larsen, Pitcairn viewed film footage of the aircraft in operation. They observed that, while the Autogiro was capable of landing in short distances within a confined space, it also required considerable amounts of high-speed taxiing to build up sufficient rotor rpm to take off. Clearly, the ability to land short was of little value without the capability to take off from the same terrain. Nonetheless, Pitcairn remained interested, and in the summer of 1928 he returned to England and flew in the C.8, which greatly impressed him. A rope sling pulled by a ground crew spun up the rotor of the C.8 to near takeoff rpm, which meant that less taxiing was required before a takeoff of only (100 ft). Pitcairn decided to purchase one of the experimental Cierva C.8s as a testbed for his own company's rotary wing program.

European engines turned counter-clockwise when viewed from the rear, while the American Wright Whirlwind J-5 turned clockwise, which caused Pitcairn some apprehension over its affects on rotor rpm. After Cierva confirmed that this would not be an issue, Pitcairn agreed to purchase a new C.8 with the Wright engine. On December 11, 1928, the C.8W arrived in the United States onboard the S.S. Aquitania. Pitcairn planned to have the Autogiro make its flight on the twenty-fifth anniversary of the Wright brother's first powered flight, on December 17, at his Bryn Athyn, Pennsylvania airfield. However, he was conservative when dealing with the experimental testing of the newly reassembled aircraft and was extremely anxious that all should go well on the first flight. Thus, not until December 18, would the C.8W perform the first flight of a gyroplane in the United States, with Cierva test pilot, Arthur "Dizzy" Rawson at the controls. Pitcairn himself flew the aircraft the following day. Whatever doubts Pitcairn had concerning the potential of rotary-wing aircraft evaporated as he completely reorganized his company to support autogiro production. His first step was to acquire the American patent rights to Cierva's innovations and to manage and license them under the direction of the Pitcairn-Cierva Autogiro Company of America. This enterprise, later renamed the Autogiro Company of America, would remain separate from the production side of Pitcairn Aircraft, which would become the Pitcairn Autogiro Company, Inc. in 1933.

Testing of the C.8W proceeded well, but it was immediately apparent to Pitcairn and his staff that the aircraft required substantial improvements in performance before it could hope to become a commercial success. One problem was that rotor flapping was unchecked, so that at speeds greater than 161 kph (100 mph) it was possible for the rotor to strike the fuselage - with catastrophic results. By the end of 1929, a three-degree reduction in the angle of incidence of the autogiro's fixed wing reduced this hazard significantly by altering the attitude of the aircraft in flight. Shortly after Pitcairn began testing the C.8W, Cierva contacted him and suggested that he add upturned wingtips with a 45-degree dihedral to improve its stability. This addition, tried initially on Cierva's C.12, increased the wingspan by an additional 0.73 meters (2 ft 4 in). The modification worked extremely well, and was a staple on almost all winged autogiros until the advent of wingless, direct-control versions.

Pitcairn focused on developing his own, more powerful, design, with an improved mechanism for bringing the rotor up to speed so that the aircraft would possess true short takeoff and landing capabilities. His engineering team experimented with new tail designs for the C.8W, developed by Cierva for the C.19, which deflected propwash upward into the aft section of the rotor disc. This helped to bring the rotors into a state of autorotation in preparation for a minimal takeoff run.

At the end of the 1920s, the JN-4 Jennys of the barnstorming era had given way to larger and faster aircraft that required airfields more substantial than a simple cow pasture. The minimal takeoff and landing rolls of Pitcairn's proposed Autogiros promised that the well-to-do could operate from their lawns, while conventional aircraft were becoming increasingly tied to the fixed infrastructure of the local airfields. Unfortunately, Pitcairn's timing was technically fortuitous but commercially disastrous as his new autogiro venture coincided with the advent of the Great Depression. Despite the best efforts of numerous entrepreneurs, general aviation was to remain the purview of the wealthy throughout this period and the company struggled to survive. Most of the company's Autogiro sales were to demonstration pilots who made their living performing at airshows and using the aircraft as a highly visible flying billboard, either on its own or towing banners.

Nonetheless, Pitcairn did succeed in improving on the C.8W and placing a number of variants into limited production with slightly fewer than 100 airframes constructed over a ten-year period. The C.8W did much to publicize the Autogiro during the development of Pitcairn's initial design, the PCA-1 (see NASM Collection). By the spring of 1929, Pitcairn was under considerable pressure from the National Advisory Committee for Aeronautics (NACA - the forerunner of NASA) to make the C.8W available for viewing. No one in the United States other than Pitcairn and his staff were well versed in the engineering challenges posed by the autogiro, and the aviation industry was beginning to grow anxious for the opportunity to examine the novel craft. Pitcairn undertook the first rotary-wing cross-country flight in the United States on May 13, 1929 when he flew the C.8W more than 805-kilometers (500-miles) from Bryn Athyn to Langley, Virginia, for the annual NACA conference. He flew via Philadelphia, Baltimore, Washington and Richmond, attracting considerable crowds at each of his stops. After impressing notables at the conference, including Orville Wright, he flew to Norfolk to demonstrate the Autogiro to Navy officials, who were naturally intrigued about an aircraft with the potential to operate easily from naval vessels. Shortly after making the return flight, Pitcairn flew the C.8W to Washington for a demonstration before Congress and members of President Hoover's administration. Also in the audience was the Spanish ambassador, who had come to see one of his nation's greatest aeronautical contributions in action. Two years later, Pitcairn's first production model, the PCA-2, would land on the South Lawn of the White House.

The development of the PCA-1 proceeded steadily, but slowly, as the challenge of scaling-up the C.8W was much more significant than a mere increase in the dimensions. One innovation was the use of a steel spar instead of the wooden one employed on the C.8W. Pitcairn's continuing frustration over the rotor spin-up problem led to his incorporation of an important autogiro development - a clutch that allowed the engine to bring the rotor rpm up before flight. This was not possible in flight because there would have been no means of counteracting torque - something that the friction of the tires accomplished while the aircraft remained on the ground. However, this important development did set the stage for the jump-takeoff autogiros that appeared in the late 1930s.

In 1930, the National Aeronautic Association awarded Pitcairn the prestigious Collier Trophy because of his pioneering flights in the C.8W and the successful Americanization of the Autogiro in the form of the PCA-1. After the C.8W finished its useful life as a testbed upon completion of the PCA-1 series of experimental Autogiros, it lay dormant for a brief time. Pitcairn then decided to donate it to the Smithsonian Institution's repository of historic aircraft at the urging of Paul Garber, who attended to the Smithsonian's aeronautics collection. The last flight of the C.8W occurred on July 22, 1931, when Pitcairn test pilot Jim Ray landed the aircraft on the National Mall in front of the Smithsonian "Castle" after a 209-kilometer (130-mile) flight from Willow Grove, Pennsylvania. Dr. Charles Abbott, the Secretary of the Smithsonian Institution, personally accepted it into the national collection. The aircraft currently awaits restoration and display. Harold Pitcairn stated at the presentation that "the time is fast approaching when the air will hold a vast number of similar but more perfected machines, all engaged in the everyday social and commercial activities of our people." The gyroplane was unquestionably a safe and economical means of solving the short takeoff and landing problem that had troubled aeronautical engineers for decades. However, the type's limited payload and low airspeeds usually made it a poor substitute for conventional airplanes in the eyes of most operators. Unfortunately, the Autogiro failed to become a widespread commercial success. The advent of the commercial helicopter in the 1940s, with its unique ability to hover, quickly displaced the gyroplane as the predominant rotary-wing aircraft.

In terms of Autogiro design, the C.8W was largely unremarkable. However, the publicity generated by the aircraft, as well as the technical knowledge gained by Pitcairn during its use, guarantees its status as a true pioneer of rotary-wing flight. Its legacy lives on, as the practical helicopter was only possible through the introduction of rotor hinges and effective blade designs that Pitcairn and Cierva had perfected on their Autogiros.

Rotor Diameter:11.97 m (38 ft 8 in)

Wingspan:8.64 m (28 ft 4 in)

Length:8.64 m (28 ft 4 in)

Height:3.84 m (13 ft 1 in)

Weight:Empty, 787 kg (1,735 lb)

Gross, 1,134 kg (2,500 lb)

Engine:Wright Whirlwind J-5 Radial, 220 hp

Crew:1 pilot, 1 passenger

References and Further Reading:

Brooks, Peter W. Cierva Autogiros: The Development of Rotary-Wing Flight.

Washington: Smithsonian Institution Press, 1988.

Smith, Frank Kingston. Legacy of Wings: The Harold F. Pitcairn Story. Lafayette Hill, \

Pa: T.D. Associates, 1981.

Townson, George. Autogiro: The Story of "the Windmill Plane." Destin, Fl: Aviation

Heritage, Inc., 1985.

Cierva C.8W curatorial file, Aeronautics Division, National Air and Space Museum

R. Connor