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Further Reading

AV-8B Harrier History

However clean the Harrier's lines may be, the tracing of its lineage is a remarkably complex business. During its life span, the Harrier's parent company has changed names from Hawker to Hawker-Siddeley to British Aerospace, with McDonnell Douglas acquiring stepparent status for the coproduced American AV-8B derivative, a responsibility later passed on to Boeing with that company's acquition of McDonnell Douglas.

The Harrier and its immediate predecessors, the P-1127 and Kestrel, have been known by no less than eight names: The concept that led to the Harrier was initially assigned the Hawker project designation P-1127, under which it flew as a prototype and concept demonstration vehicle. The Kestrel, the ensuing service test version, was named for a species of small European falcon noted for its habit of turning into the wind and hovering over a fixed spot while looking for its prey.

The designation XV-6A was applied to nine Hawker Siddeley (P.1127), Kestrels (FY-Serial: 64-18262 - 64-18270), (RAF Serial: XS688 - XS696), only 7 of which were delivered. Following completion of the operational evaluation in the United Kingdom, six of the Kestrels were shipped to the United States in 1966, designated XV-6As. The designation VZ-12 was applied to a pair of Hawker Siddeley P.1127 (FY-Serial: 62-4507 - 62-4508), which were never delivered.

An improved version, known as the Harrier, became the world's first operational V/STOL fighter when it entered Royal Air Force service in 1969. The definitive Royal Air Force production derivative was named Harrier after a genus of highly maneuverable, low-flying hawks that build their nests on the ground. Sea Harrier was subsequently-and logically-applied to the navalized version. The initial Marine Corps variant was assigned the colorless AV-8A designation.

The Harrier today is one of the truly unique and most widely known of military aircraft. It is unique as the only fixed wing V/STOL aircraft in the free world. It also is unusual in the international nature of its development, which brought the design from the first British P.1127 prototype to the AV-8B Harrier II of today. When the Harrier II was first flown in the fall of 1981, 21 years had elapsed since the original Hawker P.1127 first hovered in untethered flight. This basic design, only one of many promising concepts of the time, has weathered its growing up period and reached maturity in the AV-8B.

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.

Initial US involvement with the Harrier began in 1957 when Hawker's revolutionary design was met with disinterest by the British government and a lack of government funding to proceed into development. By that time, the US had conducted extensive research on numerous competitive concepts for V/STOL flight, including aircraft-tilting (tail sitters), thrust-tilting (tilt rotors), thrust-deflection (deflected slipstream), and dual-propulsion (lift-cruise engines) concepts. The simplicity and elegance of the rotatable nozzle vectored-thrust concept of the P.1127 so impressed NASA Langley management and researchers that a formal agreement for cooperative testing was initiated with Hawker under the Mutual Weapons Development Program of NATO.

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.

Following completion of the operational evaluation in the United Kingdom, six of these Kestrels were shipped to the United States in 1966, designated XV-6As. Here they underwent national trials, including shipboard tests and additional testing of V/STOL fighter techniques. Two subsequently served in a research role with NASA. Within the United States it was a tri-service venture (Army, Navy, Air Force) with the Army functioning as the lead service. However, the final interservice agreement later transferred responsibility for this category of aircraft to the Air Force.

The Hawker-Siddeley Kestrel (XV-6A) is a single-place, prototype, vectored-thrust, V/STOL strikereconnaissance aircraft. A single Rolls Royce Pegasus Mark 5 engine powers the Kestrel. The Pegasus is an axial-flow vectored-thrust turbofan engine with an uninstalled sea-level static thrust rating of 69 000 newtons (15 500 Ib). Thrust is vectored through two pairs of controllable engine exhaust nozzles and is equally distributed between the forward nozzles which exhaust cool air from the fan and the aft nozzles which exhaust turbine air. The nozzles are mechanically interconnected and can be rotated, at rates up to 90°/sec, to any position from fully aft (O: = 0°) to 5° forward of vertically downward (9* = 95° Y Nozzle angle is controlled by a single lever located inboard on the throttle quadrant which is the only additional control required for thrust vectoring in the Kestrel.

Control moments during nonvectored flight are provided by conventional aerodynamic surfaces. The ailerons and tail plane are powered by tandem hydraulic systems; the rudder is unpowered. Lateral control forces are provided by a nonlinear spring unit and longitudinal forces by a q-feel unit supplemented with a feel spring. A bobweight in the control run increases longitudinal maneuvering forces by 8.9 N/g (2 Ib/g), and 4.9 N/rad/sec2 (1.1 Ib/rad/sec2) for pitch acceleration.

During vectored flight, reaction control moments are added to those produced by the normal aerodynamic surfaces. Reaction control shutter valves, located at the nose, tail, and wing tips, are mechanically connected to their corresponding aerodynamic control surface and receive high-pressure engine bleed air as a function of engine nozzle angle. Full reaction control is provided at engine nozzle angles greater than 300. No stability augmentation system (SAS) is provided. However, during flight at low dynamic pressures where the pilot does not get feedback to the control stick from forces on the control surfaces, an artificial-feel system is provided. Lateral feel is provided by a nonlinear spring unit and longitudinal forces are provided by a g-feel unit supplemented with a feel spring.

Analytical and simulator studies of the flight and handling qualities of aircraft require that accurate estimates of the aerodynamic parameters be used if the results are to be valid. One of the more accurate methods of obtaining aerodynamic parameters is from data obtained during flight tests. To provide aerodynamics for analytical and simulator studies, and also to provide numerical values for comparison with wind-tunnel data and theoretical estimates, parameters have been extracted from flight data for many years. Flight-test data were used to extract the longitudinal aerodynamic parameters of the Kestrel aircraft. The aircraft configurations included thrust-jet angles of 0°, 15°, and 30°, and Mach numbers of 0.43, 0.62, and 0.82. The results show that deflecting the thrust past 15° has an effect on the pitching-moment derivatives. Deflecting the thrust downward decreases the longitudinal static stability parameter -Cm and generally decreases the damping-in-pitch parameter -(Cm + Cm .\ for trim normal-force coefficient -Cz 0 values greater than 0.2. The trend toward reduction in the longitudinal stability parameter also had been noted by the pilots during flights of the Kestrel.

While the Kestrel operation trials were being completed and the six aircraft were headed for the United States, the RAF ordered an updated version, the P.1127 (RAF), subsequently given the designation Harrier GR 1. Retaining its basic concept, Hawker-Siddley extensively redesigned the P.1127 for production.

In 1969 the concept was developed of using thrust vectoring on P.1127-type aircraft to enhance the maneuverability of fighters in air combat. Although the application of vectoring in forward flight (VIFF) was fundamentally attractive, considerable engineering concern existed over potential control requirements, stability characteristics, and the physical well being of the engine in such maneuvers. A joint VIFF program between NASA and the Royal Aircraft Establishment was initiated in 1972, and flying in the United Kingdom continued through 1976. Results obtained in flight evaluations against a variety of high-performance adversary aircraft and analyses of evasive maneuvers provided by VIFF against enemy ground-to-air and air-to-air missiles resulted in overwhelming support for VIFF as a valuable tool for the AV-8 pilot.

By the mid-1960s, naval gunfire was increasingly hard to obtain for Marine Corps amphibious landings. The Navy began employing missile-equipped ships and naval air support had decreased as they fielded fewer aircraft with smaller bomb loads. In 1968, during the height of the Vietnam War, Colonels "Tom" Miller and Bud Baker went to England to attend the Farnborough Air Show. While there, they flew the Hawker Siddeley Harrier and returned to the States convinced that they had the answer to the fire support problem. After successfully briefing the Commandant, Colonel Miller became the key man in a campaign to get the aircraft for the Marines. With a mixture of political skill, hard work, and sheer enthusiasm, he overcame the odds and convinced the Navy, the aircraft industry, and Congress that the Harrier was a "must" for the Corps.