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The Royal Aircraft Establishment at Bedford

Scout XP191 & Lockheed XH51
Rigid Rotor Research

During the 1960s, there was considerable interest in the benefits of helicopter rotor systems which did not employ blade hinging. This approach promised handling performance improvements and a reduction in the rotor mechanical complexity which in turn implied reduced operating costs. In the USA, a number of experimental research helicopters had been built and, within the Anglo-French collaboration programme which also produced the Gazelle and Puma, the Lynx was intended to be a UK hingeless contribution.

Scout XP191

In support of the Lynx development, two Army Scout helicopters (XP189 and XP191, above) were given modified rotor heads. These aircraft were intended to be used by Westland Helicopters Ltd (XP189) and RAE (XP191) to prove the concepts, assess benefits, and to reduce the technical risk in moving to rigid technology for the production Lynx itself. These rotor heads allowed the blades to pitch, necessary for power and control, while they were fixed in flapping and lagging modes (which would normally be hinged). A small amount of lag and pitch was provided by bending of the root fixing components (amusingly known as ‘cutlet’ and ‘dogbone’) hence the rotors were actually semi, rather than fully, rigid. This approach allowed the forces on the blade roots and the main shaft to be kept to acceptable mechanical limits without compromising the flight performance.

Scout XP189 & Lockheed XH51

In point of fact, the preparation of the Scouts – which had to be equipped quite comprehensively for flight research – took longer than anticipated and there was increased pressure to obtain experience in this type of technology. Of the two British aircraft, XP189 was completed first, at the former Fairey factory at Hayes.

However, in the late summer of 1970, prior to the delivery to RAE of XP191, and under a bilateral agreement, NASA offered the loan, to RAE Aero Flight, of one of their semi-rigid research aircraft – the Lockheed XH51-N. This aircraft had completed a programme of flight research at Langley Air Force Base in Virginia.

Toward the end of that year, XH51 was shipped to RAE Bedford and began a programme of trials which included comparison flying with the Westland owned semi rigid Scout helicopter, XP189 – the only British aircraft having similar rotor technology at the time. The two helicopters are seen together here, flying over the airfield at Bedford during comparison handling trials.

XP191 was delivered to Bedford in 1971 and began a programme of research into flight dynamics intended to explore the characteristics of the aircraft at speed where hingeless helicopters are increasingly less stable. Also, in support of anticipated Naval Lynx, landing on sloping ground to investigate blade ‘sailing’ issues and, using the Naval Air Deptartment Rolling Platform, possible problems in the ship operation of this rotor type.

Specialised instrumentation was fitted to XP191 to allow the measurement of the many critical flight loads in the rotor and transmission, together with a 30 channel telemetry system for safety monitoring and the provision of a ground based trials management and direction capability. In view of the specialised behaviour of the rotating components, a high speed cine camera was also fitted to the head to allow the complex blade motions to be recorded on 16mm film during flight.

The handling performance benefits of these rotors in comparison with conventional fully hinged types was generally not seen to be overwhelmingly significant and the Lynx is the only production helicopter so fitted. A valuable comparison between the Scout and the XH51 was made, although the Lockheed aircraft was fitted with a rotor head gyro system which made its performance slightly dissimilar to the Scout and more difficult to analyse. However, a great deal of valuable experience, of significance in determining UK helicopter development policy, was gained from these programmes.

Scout XP191 Rigid Rotor Research

Where are they now?

The XH51 was returned to NASA Langley Field at the completion of trials. It was last seen in 1995 – sadly in a derelict state – at the US Army Helicopter museum at Fort Rucker, Alabama.

In 1976 Scout XP191 was restored to standard and returned to service with the Army Air Corps. In 2001 it was at Bramley Army base near Basingstoke where it had been used as a test aircraft for airframe repairs. Later it was bought as scrap by Xray-Tango Helicopter Club to support the restoration of another Scout in their possession. This project later failed to be completed and the remains of XP191 (by then just the remains of the main cabin) were last to be found at Prenton in the Wirral.


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