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Further Reading

XH-51

The XH-51A and XH-51A Compound were extremely valuable technology testbeds, and many of the systems pioneered or refined in these aircraft were later incorporated in such advanced helicopters as the AH-56A Cheyenne. The XH-51As themselves were finally retired from service only in the late 1960s.

The Lockheed Model 186 was designed in response to a 1960 joint Army/Navy requirement for a high-speed, highly manoeuvrable research helicopter. It demonstrated the smooth handling qualities available from a gyro-stabilized rigid rotor system. The gyro-stabilization system relied on a spinning mass, which attached to the mast by gimbals, to act as a servo on the rotor pitch links and compensate for control feedback caused by blade flexing. In addition to stabilizing the aircraft, this system eliminated most of the vibration that made piloting a helicopter a challenging and exhausting experience. The military was very interested in the rigid rotor system, which promised substantially higher maximum airspeed.

The onset of retreating-blade stall limits the maximum forward speed of helicopters, but the absence of hinges on the rigid rotor increased the speed at which this condition occurred. The rigid rotor also allowed aerobatic maneuvers that were well beyond the capabilities of most other helicopters. The Langley Transonic Dynamics Tunnel (TDT) was used for helicopter testbeds testing. The first was built by Lockheed Aircraft Company and was used for testing of hingeless rotor configurations in support of the XH-51 research helicopter development program.

Two examples were ordered in early 1962, with the first of the two making its maiden flight in November of that year. The aircraft, designated XH-51A, were operated by both Army and Navy pilots.

The XH-51A's impressive performance significantly improved, following the second prototype's 1964 conversion into a compound rotorcraft. The conversion of 151263 included the addition of a pod-mounted Pratt & Whitney J60 turbojet engine, short stub wings fixed to the lower fuselage sides, an enlarged horizontal stabilizer, and other detail changes.

The XH-51A research compound helicopter has a 35-foot-diameter, four-bladed, gyro-controlled rigid rotor, wings for unloading the rotor lift at high forward speeds, a jet engine mounted close to the fuselage on the left wing for propulsion, horizontal and vertical tail surfaces and a two-blade teetering rotor for anti-torque and directional control. The aircraft requires no fixed wing dynamic control surfaces since the rigid rotor provides sufficient pitch and roll control moment through application of cyclic pitcb at any rotor thrust level.

In the event of a shaft engine failure while operating at high speed with the rotor unloaded, it is necessary to load the rotor to develop an autorotation condition. This cannot be accomplished merely by increasing rotor angle of attack since the wing lift increases also. To overcome this problem in the XH-51A compound helicopter, wing spoilers are installed and an emergency procedure has been established and found to be satisfactory in flight test. When a shaft engine failure ir detected, the wing spoilers are deflected to unload the wing.

The reworked XH-51A Compound made its first flight in September 1964, and in June 1967 set an unofficial helicopter world speed record of 487 kph. In high-speed flight with the rotor unloaded, the effects of retreating blade stall on performance and control capability were negligible. Retreating blade stall occurs only in the reverse flow region where local velocities are low. Where a control moment must be generated with differential lift between the 90-degree and 270-degree azimuth locations, the high velocity condition on the advancing blade easily provides the mechanism for sufficient differential lift.

Previous research efforts on various compound helicopters had been directed largely toward speed gains and transient load factors. Although these programs were successful, their scope was limited in one important area. This was the area of maneuverability and agility over the entire speed range. With rapidly approaching compound helicopter applications, additional maneuverability and agility information and accompanying quantitative data on dynamic stresses and hand.ling characteristics are needed to assist designers of future compound helicopters.

A high-speed extension flight test program was conducted by the Lockheed-California Company on the rigid-rotor XH-51A compound helicopter during May 1965 under Contract DA 44-177-144C-150(2). The objective of this program was to investigate the flight characteristics of the compound helUcopter with special emphasis on the areas of flying qualities, performance, structural loads, vibration, and maneuverability in the speed range of 200 to 230 KTAS. This objective was met and a maximum level flight speed of 236 KTAS was demonstrated.