Improving Performance: Jump and Towering Takeoffs

In a lecture given in Italy in 1929, Cierva first alluded to the idea of a giving his Autogiros a vertical takeoff capability. Initially introduced on a version of the Weir W-3 autogiro, the first advance made was that the rotor could be clutched to the engine through a lightweight transmission and spun up to a higher than normal rpm when the autogiro was on the ground. The second advance made was in the hub design to give it a blade pitch – change capability. While this hub went through many different changes, finally an ingenious kinematic blade pitch—lag coupling device was used to give a change in blade pitch between a low (zero thrust) setting and the normal flight pitch. This design was eventually to become the Cierva—Weir autodynamic rotor. Bennett (1961) explains how fifteen different hinge assemblies were tried before success was achieved.

The idea was that while the rotor rpm was over-sped on the ground up to a higher than normal flight rpm, the drag on the blades caused them to lag onto mechanical stops. Under this condition, the blade pitch was reduced to the low value by the coupling device. The rotor produced little lift in this condition. When the rotor was declutched from the engine, the blades snapped out of the stops and back to the original pitch. The resulting sudden surge of rotor lift caused the autogiro to “jump” into the air and climb rapidly off the ground (Fig. 12.16). While this jump takeoff capability comes about partially a result of the excess thrust produced by stored kinetic energy in the rotor system at the higher rpm, there are also aerodynamic benefits of a rapid change in blade pitch. This benefit gives a lag in the inflow development through the rotor and so a sudden thrust overshoot is produced — see Carpenter & Friedovich (1953) and discussion in Section 10.9. This excess or “dynamic”

Figure 12.16 The jump (a) and towering (b) takeoff capability gave the autogiro a capa­bility rivaling a helicopter.

thrust, however, decreases quickly (within a few rotor revolutions). Applying full engine power, as the machine climbs after the initial jump, accelerates it into forward flight, after which the rotor rpm decays to its lower flight value and the autogiro flies away normally.

However, there was always some loss of altitude with this jump takeoff technique (Fig. 12.16), the drag from the unpowered blades slows down the rotor rpm and con­tributes to a decrease in the initial dynamic thrust. Because the rotor also had a substantial backward tilt to aid in normal autorotation, there was a tendency for it to jump off slightly backwards. This increased the overall difficulty of the maneuver for the pilot. Good piloting technique, however, made the vertical jump takeoff technique consistently successful and the autogiro often rose quickly to more than 30 ft (10 m) into the air. The autodynamic rotor system was installed on a modified C-30 and was demonstrated successfully in 1935. By this time the Breguet-Dorand helicopter had made its first flights (see Chapter 1) and this significant advance in the performance of the autogiro received less attention than it might have done otherwise.

Raoul Hafner was to develop another type of jump takeoff system on his AR. III autogiro in 1935, which is described by Hafner (1938). This fixed hub, three-armed “spider” blade control system allowed the pitch of the blades to be continuously variable and to have both cyclic and collective capability. The spider sat atop the rotor and was actuated by a control rod supported on a spherical bearing inside the hollow rotor shaft. This design was technically superior to the Cierva-Weir “binary” blade pitch change system. To perform a takeoff with Hafner’s system, the pilot first over-sped the rotor with the blades in flat pitch while holding the machine firmly on the brakes (as with the Cierva jump takeoff system), and then rapidly applied pitch with a collective lever while simultaneously de-clutching the rotor, gunning the engine and applying forward cyclic. Hafner’s “spider” control system allowed the pilot to make continuous pitch control adjustments, allowing precise “towering” takeoffs with no loss in altitude as rotor rpm decayed (Fig. 12.16). Combined with a somewhat more exacting landing capability that was now possible with the “spider” hub, this version of the autogiro had a takeoff and landing flight capability that was to match future helicopters. The autogiro still could not hover, despite being able to make nearly vertical takeoffs and landings. However, Hafner’s spider blade pitch control system was soon to become the standard for early British-designed helicopters – see Hafner (1954, 1963).

The Americans too were also introducing the ideas of blade pitch control to autogiros to enhance its capabilities, with the Autogiro Company of America (its licensees were Pitcairn and Kellett) developing a jump takeoff system starting in 1933. Pitcairn had designed and patented many improvements into the Cierva rotor system [see Smith (1985)], and in 1931

Harold Pitcairn received the highly prized Collier Trophy for his technical contributions to aviation. See Pitcairn (1930) for a summary of activities, and Prewitt (1938) gives a technical discussion of the jump takeoff technique. The jump takeoff was also studied experimentally by Wheatley & Bioletti (1936) at NACA using model rotors and later by means of aerodynamic theory by Carpenter & Friedovich (1953). The mechanical systems used by the Americans were different to those of Cierva-Weir, but used the same principle of producing a jump takeoff by increasing rotor rpm followed by a sudden change in blade pitch. The American system was first tested on the PA-22 test bed and later on the enclosed cabin PA-36 with good success. Slowly, as the pilots gained experience, the jumps became higher and eventually Pitcairn pilots demonstrated an ability for the PA-36 to take off, translate into forward flight, and clear a 30-foot high obstacle without losing any altitude. Both Pitcairn and Kellett were later to compete for funding under the Dorsey-Logan Act of 1937 [see Smith (1985)] that specified a “rotary-wing military aircraft” capable of a vertical takeoff to clear a 50-foot obstacle. This goal was too unrealistic for the PA-36 to meet and it lost out on funding to LePage’s XR-1 side-by-side rotor helicopter (page 32), a machine that itself proved ultimately to be unsuccessful. Pitcairn’s PA-36 later made many successful jump takeoff demonstration flights to military and government officials, however, taking off and landing vertically and doing nearly everything a helicopter could do except hover. The jump takeoff worked, but it was never easy on the pilot or the aircraft.