Win the adventure of a lifetime!

UNITED24 - Make a charitable donation in support of Ukraine!


Lockheed Model 186 / CL-575 / XH-51 / XH-51N

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.

Lockheed’s inability to successfully scale up its XH-51 by a factor of five to its AH-56 led the Army to cancel the Cheyenne production program in 1969 because of unresolved technical problems.


In early 1965 NASA acquired the 3rd XH-51 produced, and the aircraft was flight tested at Langley and at the RAE in England through 1970. The XH-51N was instrumented for flapwise and chordwise bending, and the mast and pitch links were also strain gaged. Likewise, the tail rotor was strain gaged for flapwise and chordwise bending. Several components of the control system were instrumented for loads and position. Accelerometers, rate gyros and vibration pickups were also utilized. Unlike the other 4 aircraft produced, the XH-51N maintained the original 3-bladed configuration, while the others were built or modified for 4 blades, including the XH-51A Compound Helicopter tested under an Army program.

The aircraft had a number of unique features including the hingeless rotor. The aircraft employed a mechanical gyro in the control system such that the pilot did not control the rotor directly, but provided force inputs to the gyro shown in figure 18; the gyro then provided control inputs to the rotor based on inputs from the pilot or from rotor feedback provided by the forward sweep of the blades. This control system was the fore runner of the control system utilized on the AH-56 Cheyenne that resulted in severely limiting problems for that aircraft.

Another unique feature of the XH-51N was the cabin isolation system which was utilized to control cabin vibration. Another vibration control device employed after the fact on the XH-51N were the blade mounted masses as illustrated in figure 20 which were utilized to detune the 2nd flap bending frequency of the rotor. During the research flying with the XH-51 N, the aircraft was flown both with and without the cabin isolation system and the blade masses. Both the rotor loads and the flight dynamics of hingeless rotor configurations in maneuvering flight were investigated during the flight investigations with the aircraft. The XH-51N had even more severe chordwise and flapwise rotor bending problems than the H-13 hingeless rotor helicopter. Both the flapwise and chordwise bending moments consistently exceeded the endurance limit for the measured hub plate during maneuvers, and loads were always monitored in real time utilizing telemetry.

One of the last experiments to be run on the XH-51N was the investigation of an active cabin isolation system to replace the passive spring utilized to isolate the cabin. Excessive cabin vibration levels were experienced with the isolation system locked out and the blade mass removed. This is typical of hingeless rotor helicopters with high effective hinge offset and one of the problems still to be faced with the newer bearingless rotor systems. Reduction of the effective hinge offset, as has been achieved with some of the newer designs, can help allieviate the vibration problem.

Another idea that was stimulated by the work on the XH-51N and its high vibratory loads was the concept of reducing the strength of the tip vortex through the use of the "ogee" tip. The "ogee" tip was conceived by John Ward and initial tests of the tip along with conventional tips were conducted in a small scale smoke tunnel. These preliminary tests indicated a reduction in vorticity of as much as 40% over a conventional square tip and were encouraging enough to initiate work on a full scale evaluation utilizing a UH-1H which was acquired at Langley in the early 70s as a test bed for the "ogee" tip rotor.

Join the mailing list

Page last modified: 16-02-2014 18:28:10 ZULU