AV-8B Harrier Design

The Rolls-Royce (Bristol division) Pegasus turbofan engine is the key to the great versatility of the Harrier. Unlike other jet engines with only one jet-exhaust nozzle, the Pegasus has four exhaust nozzles; two are located on each side of the engine. The two front nozzles discharge unheated air compressed by the fan, and the rear nozzles discharge the hot jet exhaust. A rotating cascade of vanes is used in each nozzle to vector the thrust from a horizontal direction for high-speed flight to a vertical direction for hovering and vertical takeoff and landing. Intermediate positions are used for short takeoff and landing (STOL) and for maneuvering in combat situations. (This latter technique is referred to as VIFF, vectoring in forward flight.) The use of VIFF to enhance aircraft maneuverability and hence combat effectiveness was pioneered in flight studies at the Langley Research Center in the late 1960's and early 1970's. For rapid deceleration, the nozzles can actually be rotated past the vertical position to about 98°.

Another key element in the Harrier concept is the method for controlling the aircraft. When operated as a conventional airplane, the usual ailerons, rudder, and horizontal tail are used to generate aerodynamic control moments about the roll, yaw, and pitch axes, respectively. In hovering flight and at low forward speeds, however, the aerodynamic controls are ineffective, and reaction jets are used to provide the necessary control moments. At intermediate speeds, both reaction jets and aerodynamic controls are used. Pitch jets are located at the nose and tail of the fuselage, a roll jet is at each wingtip, and a yaw jet is located behind the tail. The reaction jets utilize compressed air from the high-pressure engine compressor and respond in a proportional fashion to conventional movements of the control stick and rudder pedals. The control jets come into operation automatically when the thrust-vectoring nozzles are rotated to any angle in excess of 20°. Control of the thrust-vectoring nozzles is exercised by a lever in the cockpit located alongside the throttle.

Although the engine and reaction control system are the key elements that give unique operational capability to the Harrier, the airframe itself exhibits several interesting features. With 12° anhedral (negative dihedral), the 34° sweptback wing is mounted on top of the fuselage; like the wing, the all-moving horizontal tail has a large anhedral angle (15°). The anhedral angles of the wing and horizontal tail are intended to minimize the aircraft rolling moments due to sideslip. Even so, at certain combinations of low speed and high angle of attack, aerodynamic rolling moments greater than the combined aerodynamic and reaction control power may occur if the angle of sideslip is allowed to exceed a prescribed value. To assist the pilot in maintaining the angle of sideslip within acceptable limits, a small yaw vane that provides a visual indication of sideslip angle is mounted on the fuselage just ahead of the windshield.

The unusual landing gear of the Harrier is designed to avoid interference with the engine and thrust-vectoring nozzles. A single twowheel bogie is located in the fuselage behind the engine, and a single steerable nose-wheel is in front of the engine. Balancing outrigger wheels mounted at the wingtips retract into the reaction control fairings. The wing anhedral angle minimizes the length of the outrigger landing-gear struts. Also evident in the figure are the large side-mounted subsonic inlets that supply air to the 21 500-poundthrust engine.

The fighter version of the aircraft is manned by a single pilot; a two-seat trainer with the full military capability of the single seater is also available. As with so many modern jet fighters, the Harrier is equipped with zero-zero ejection seats; that is, crew escape is possible on the runway at zero altitude and zero speed.