Not Quite a Helicopter

The Spanish engineer Juan de la Cierva had built and flown another type of rotating – wing aircraft as early as 1923 – see Juan de la Cierva (1926, 1930). This aircraft looked like a hybrid between a fixed-wing airplane and a helicopter, with a set of conventional wings and a tail, but with a rotor mounted on a vertical shaft above the fuselage. However, unlike a helicopter, this rotor was not powered directly and was completely free to turn on its shaft. Cierva found that when the rotor disk was inclined backward at a small angle of attack and as the machine was pulled forward by a propeller, the rotor was turned by the action of the airflow on the blades. This aerodynamic self-rotation of the rotor is called “autorotation,” and had been understood by Crocco and Yuriev before 1910. Yet the idea of pulling the rotor horizontally through the air to generate lift was clearly that of Cierva. He called his rotating-wing aircraft an “Autogiro.” The name “Autogiro” (with an “A” and an “i”) was later to be coined by Cierva as a proprietary trademarked name for his machines, but when spelled starting with a small “a” and using a “y” it is normally used as a generic name for this class of aircraft. Today, “gyroplane” is the official term used to describe such an aircraft, although the names “autogiro,” “autogyro,” and “gyroplane” are often used synonymously.

Cierva’s first Autogiro was a coaxial design, the airframe being from a converted WW1 fixed-wing aircraft. The problem of asymmetric lift was well known to Cierva and his first idea of using a contrarotating coaxial design was that the lower rotor would counteract the asymmetry of lift produced on the upper rotor, thereby balancing the net rolling moment on the autogiro. It is the subject of Spanish patent No. 74,322 “Nuevo Aparato de Aviacion” of June 1920. However, the aerodynamic interference produced between the rotors resulted in different rotor speeds, spoiling the required aerodynamic roil balance ana the aircraft crashed. Undeterred, Cierva conducted basic wind tunnel experiments on model rotors, and he was one of the first to establish a scientific understanding of their aerodynamic behav­ior, especially in autorotation. He built two more full-scale machines with single rotors before he achieved final success in January 1923 with the C-4. Based on his tests with small models, this fourth machine incorporated blades with mechanical “flapping” hinges at the root, which Cierva used as a means of equalizing the lift on the two sides of the rotor in forward flight – see Cierva & Rose (1931). This novel solution allowed the blades to flap up or down about these hinges, responding to the changing airloads during each revolution of the blades and providing a means of equalizing the lift between the advancing and retreating sides of the rotor disk. The development of the flapping hinge was to be one key to the future success of the helicopter.

The principle of flapping blades had actually first been suggested for the application to propellers [see Rosen (1984)] apparently by Charles Renard, but the idea of hinged blades was formally patented by Louis Breguet in 1908 and then by Max Bartha & Josef Madzer in 1913 – see Liberatore (1998) for details. Juan de la Cierva, however, must be credited with the first successful practical application of the flapping hinge to a rotor. Cierva noticed that the incorporation of the flapping hinge eliminated any adverse gyroscopic effects and also allowed the lift forces on the two sides of the rotor to become more equalized in forward flight. Metal ties with tumbuckles located near the mid-span linked each of the blades together in an attempt to reduce the in-plane or lead-lag motion of the blades (see Chapters 4 and 12 for details). Cierva’s initial avoidance of using a lead-lag hinge to alleviate the in­plane blade drag and Coriolis forces (resulting from the flapping motion) was an oversight that was rectified on later models of his Autogiro.

In all of Cierva’s Autogiros, the engine drove only the propeller. Clearly this makes the autogiro mechanically simpler than a shaft driven helicopter because the engine gearbox and rotor transmission can be dispensed with. Furthermore, it is not necessary to develop a separate means of countering torque reaction, as on the helicopter. This all significantly reduces weight and also reduces design, production, and capital costs. Starting the rotor, however, required a team of helpers to pull a rope wound around the rotor shaft, or taxiing around on the ground could also get the rotor spinning. Later models used a mechanical prerotator – see Section 12.6. Thereafter, the pilot opened the throttle and the thrusting propeller pulled the machine forward until it quickly lifted off into flight. The freely spinning rotor turned relatively slowly in flight compared to a helicopter, about 100-150 rpm.

With the success of his Autogiros, in 1925 Cierva was invited to Britain by the Weir Company. His C-6 Autogiro was demonstrated at the Royal Aircraft Establishment (RAE) and these flights stimulated early theoretical work on rotating-wing aerodynamics by H. Glauert and C. Lock in Britain, and M. Munk in the United States. Cierva was to write two books for the fledgling rotorcraft industry, albeit formally unpublished – see page 701. In later models of his Autogiro, the first of which was the C-8 (Fig. 1.16), Cierva added a lag hinge to each of the blades, which alleviated stresses caused by in-plane drag and Coriolis forces and completed the development of the articulated rotor hub. In later models, a control stick was connected to the rotor hub, which allowed the rotor disk to be tilted for control purposes (orientable direct rotor control). While this allowed the ailerons to be dispensed with, the rudder and elevator on the machine were retained. The Cierva Autogiro Company and its licensees went on to build many more versions of the Autogiro through 1938.

Although the autogiro was still not a direct-lift machine and could not hover, it required only minimal forward airspeed to maintain flight. Cierva proved that his autogiros were very safe and because of their low-speed handling capability, they could be landed in confined areas. Takeoffs required a short runway, but this problem was solved with the advent of the “jump” and “towering” takeoff techniques, although this required a lot of piloting skill – see Section 12.14. For a jump takeoff the blades are set to flat pitch and the rotor rpm is increased above the normal flight rpm using the engine. This is followed by the rapid application of collective blade pitch, while simultaneously declutching the rotor and thereby avoiding any torque reaction on the fuselage. This technique lifts the aircraft rapidly off the ground, powered only by the stored kinetic energy in the rotor system. As forward speed builds, the rotor settles into its normal autorotative state – see Prewitt (1938) for a technical description. To achieve jump takeoffs, in 1935 Cierva introduced a pitch change mechanism into the rotor desigp, which was installed in later production versions of the

C-30. These machines saw some service with the British Royal Air Force during WW2 for radar calibration missions.

About the same time, Raul Hafner introduced the “spider” cyclic pitch control system to his design of autogiro – see Fay (1987) and Hobbs (1984). This provided a means of increasing collective pitch and also tilting the rotor disk without tilting the rotor shaft with a control stick as in Cierva’s direct control system, giving the pilot better control with lower control forces. Hafner used this design in his AR. III autogiro, which flew in 1935. About the same time, another autogiro using lateral cyclic control was built by W. R. Kay in Scotland. With its nearly vertical towering takeoff capability, low handling speeds and short landing run, this form of the autogiro was to closely rival the soon to be successful helicopter in terms of its performance capability.

Several other British companies, including Weir, A. V. Roe (Avro), Pamall, de Havilland and Westland went on to build variants of the Cierva Autogiro designs. The first Weir designs were developments of Cierva’s models and used the orientable direct rotor control system. The Weir W-l through W-4 models were all autogiros and were some of the first machines to use a prerotator to help bring up the rotor rpm prior to takeoff (see Section 12.6). The de Havilland and Westland company built a few larger prototype autogiros. The Westland C-29 was a five-seat cabin autogiro built in 1934. The aircraft was never flown because it exhibited serious ground resonance problems and the project was canceled with the untimely death of Juan de la Cierva in 1936. However, Cierva’s work was carried on by designers from Weir and another Westland designed autogiro called the CL-20 was flown just before WW2 although it did not show sufficient performance – see Mondey (1982).

The Kellett and Pitcairn companies entered into licensing agreements with Cierva, re­sulting in the first flight of an autogiro in the United States in 1928. Harold Pitcairn went on

to design and patent many improvements into the Cierva rotor system [see Smith (1985)], but it became clear that it was a true helicopter with power delivered to the rotor shaft that was required. The autogiro was extensively tested in the United States by the NACA. Gustafson (1971) gives an authoritative account of the early NACA technical work on autogiros and helicopters.[7] In Russia, the TsAGI built autogiros derived from the Cierva designs. Kuznetsov and Mil built the 2-EA, which was derived from the Cierva C-19 – see Everett-Heath (1986). Later developments of this design led to the first Russian helicopters built with the assistance of Vittorio Isacco, who had earlier led basic helicopter develop­ments in Italy, Spain, and France during the 1920s. The Japanese even made copies of the Cierva and Kellett autogiro designs, combining some of their best attributes and used them as submarine spotters during WW2 – see Gablehouse (1967).

During WW2 the Focke Achelis company built the Fa 330 Kite. This aircraft was a pure autogiro with a relatively simple lightweight construction and was designed as observation platform for one man while being towed behind a surfaced submarine. It was attached to the submarine by a steel cable wound out from a winch on the deck. When returning to the submarine, the winch wound in the cable and pulled the Fa 330 onto the deck. The Hafner Rotachute [see Everett-Heath (1986)] was a skeletal autogiro designed to replace a conventional parachute, but it never saw operational use. The simplicity of both these platforms later formed the inspiration for inexpensive amateur homebuilt autogiros, many of which are still popular today.

While the autogiro did see some commercial success, mainly in the United States, it was never on a large scale. During the 1930s and 1940s it was used by the US Post Office for regular mail service between Washington DC and Philadelphia, as well as in other cities, including Chicago and New Orleans. The 1920s and 1930s were an exciting and adventurous time for aviation despite the Great Depression and the autogiro was widely popularized as a super-safe, easy-to-fly aircraft, which it was for the most part. The autogiro subsequently found its way into the private market, where it gained good popularity with pilots and some level of public acceptance for a somewhat unusual aircraft. It was also used for aerial photography and advertising, the latter role giving it good public exposure. The Buhl Aircraft Corporation of Detroit, Michigan, was another company involved in autogiros. They designed and built a small two-seater autogiro with a pusher propeller, the first of its kind, which had no fixed aerodynamic surfaces other than a tail.

Despite the ultimate demise of the autogiro in the post-WW2 era, it got the public, government and the aircraft industry used to seeing a practical rotorcraft. When serious helicopter proposals started to appear in the early 1940s, they were not rejected out of hand but instead received more respect and consideration than most nascent aircraft technologies receive at that early stage of development. See also Chapter 12 for further details on the technical development of autogiros and modem gyroplanes.