Hawker Siddeley P.1127 Kestrel
Progress in the development of a practical and capable vertical takeoff and landing (VTOL) fighter has been exceedingly slow over the years, largely due to propulsion limitations and related problems. In more general terms, the VTOL is thought of as a conventional type of aircraft with special features added to enable it to rise vertically during takeoff and to land from a vertical descent. The successful and imaginative employment by the military of the first VTOL type, the helicopter, is now part of aviation history.
The real breakthrough in jet VTOL operations came with the development of the vectored-thrust turbojet designed specifically for VTOL aircraft installation. The vectored thrust principle was originated by the French designer, Michel Wibault, who conceived the idea of deflecting the thrust from centrifugal compressors, driven by the Bristol Orion engine. This idea was further developed by Dr. Stanley Hooker and resulted in the first vectored-thrust turbofan, the Bristol Siddeley 53 Pegasus 5. This engine is basically a turbojet driving a ducted fan. A part of the relatively cool compressed air of the ducted fan is expelled through the front pair of cascaded nozzles; the rest of the air is passed on to the compressor of the turbine. After combustion the exhaust gases are expelled through the aft pair of nozzles.
The British-built Kestrel was designed with vertical/short takeoff and landing capabilities, making it possible to operate from grass or semi-prepared surfaces offering great operational flexibility. Four adjustable exhaust nozzles beneath the wing roots could be rotated to provide thrust for vertical, backward or hovering flight as well as conventional forward movement.
The 1957 design for the P.1127 was based on a French engine concept, adopted and improved upon by the British. The engine utilized four swiveling nozzles to redirect the engine thrust for vertical or forward flight. The project was funded by the British Bristol Engine Co. and by the US Government through the Mutual Weapons Development Program. With the basic configuration of the engine largely determined and with development work under way, Hawker Aircraft Ltd. engineers directed their attention to designing a V/STOL aircraft that would use the engine. Without government/military customer support, they produced a single-engine attack-reconnaissance design that was as simple a V/STOL aircraft as could be devised. Other than the engine's swivelling nozzles, the reaction control system was the only complication in the effort to provide V/STOL capability.
As Hawker proceeded in the engineering development of the P.1127 from 1959 to 1960, numerous critical issues arose. These critical issues included the design of the flight control system; whether artificial stabilization was required; the lifting capability of the aircraft in ground effect; and the stability, control, and performance of the P.1127 in conventional flight. Perhaps the most daunting question was whether the aircraft could satisfactorily perform the transition from hovering flight (supported by the vertically directed engine thrust) to conventional wing-borne flight.
The initial P.1127 was rolled out in the summer of 1960, by which time RAF interest in the aircraft had finally resulted in funding by the British Government for the two prototypes. First hovers in the fall were made with a severely stripped airplane. This was due to the fact that the first Pegasus engines were cleared for flight at just over 11,000 pounds thrust. With potential NATO and other foreign interest in the P.1127, four additional airplanes were ordered to continue development.
The first Kestrel began conventional flight trials on March 13, 1961, in Britain. Despite the success of the P.1127 flight program, the British Royal Air Force did not consider the aircraft as a serious strike aircraft, citing an unacceptably small payload capability and low engine thrust. Aggravating the lack of interest, in March 1961 NATO requested proposals for a new V/STOL close-support fighter with supersonic speed capability. The Hawker design team responded with the P.1154, a configuration with twice the thrust, twice the speed, twice the weight, and twice the performance of the P.1127 [hence 27 = 2 x 54]. While pursuing the P.1154, Hawker continued demonstrations of the subsonic P.1127 and kept the program alive. The Labour government in the United Kingdom cancelled the P.1154 program and instructed the frustrated Royal Air Force to accept an upgraded version of the subsonic P.1127 - the Harrier.
As the project proceeded into the early sixties international interest in V/STOL tactical aircraft led to an agreement to conduct a tripartite operation, with the United Kingdom, West Germany and the United States sharing equally in development and evaluation. In 1962 the governments of the United States, Britain and the Federal Republic of Germany ordered nine aircraft for combined testing by those countries' representatives. They formed an evaluation squadron that conducted Kestrel trials between April and September 1965. By the mid-1960s resurgent interest in the VTOL fighter was in evidence. The aircraft industries of some six or more major countries were active in the testing, development, or production of VTOL aircraft.
Nine P.1127s were ordered and designated Kestrel F.G.A. 1s in the RAF name system. A number of major configuration changes were incorporated in it although the basic concept remained unchanged. Changes made to the P.1127 to upgrade it into the Kestrel included a new engine with increased thrust, a new swept wing with more fuel capacity than the P.1127 wing, a drooped horizontal tail, and improved reaction controls. The flight-test evaluations began in 1965.
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