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Aviation History
1964
1964 - 2321.PDF
328 FLIGHT International, 27 August 1964 Bristol Siddeley Developments IN THE V/STOL POWERPLANT FIELD By R. A. Bishop* SIGNIFICANT progress has now been made in this country inthe development of direct-lift and lift/thrust jet engines.Bristol Siddeley have concentrated on the vectored-thrust engine, but derivation development is being carried out on a direct- lift turbofan. The former concept has lead to numerous V/STOL strike fighter and transport projects. It has been adopted for the Focke Wulf FW1262 strike aircraft which will replace the Fiat G.91 in Germany and Italy. Bristol Siddeley's proposal for this important project is the BS.94 lift/thrust engine based on the BS.75 turbofan, developed privately for short-range airliners. Foundation of such work is the Pegasus twin-spool turbofan (Fig 1). Approximately 60 per cent of the air from the fan is ducted through the forward (cold) nozzles, and the remainder passes through the h-p compressor and turbines to exit through the rear (hot) nozzles. Separate two-stage turbines drive the fan and h-p compressor in opposite directions to eliminate gyroscopic couples. The four nozzles are rotatable through at least 100° under the control of the pilot's nozzle lever, and the thrust by the throttle lever. The nozzles are driven by a dual Plessey air motor through a simple mechanical system, and in the Hawker Siddeley P. 1127 the nozzle lever is the only additional control in the cockpit. Air for reaction control systems is fed from the combustion-chamber casing. The bleed valve opens automatically as nozzles are rotated down from the horizontal, no additional control being required. The P. 1127 (Fig 2) was designed as a tactical strike fighter. It has a transonic speed capability and, aided by a thrust/weight ratio in excess of unity, formidable rate of climb; and it takes off vertically, without autostabilization and with only the simplest control system. Reaction controls are at the nose, tail and wing tips, each control valve being linked mechanically to its related aerodynamic surface. To take-off vertically the pilot starts and taxies with nozzles horizontal. The nozzles are then turned to the vertical and the throttle opened fully. If hovering is desired, position is maintained by small natural movements of stick and rudder; the nozzles remain vertical, and height is maintained by small movements of the throttle. Transition from the hover is achieved simply by moving the nozzles progressively aft. The aircraft accelerates rapidly, the increasing aerodynamic lift compensating for decreasing jet lift. Speeds well in excess of stalling are obtained with only 45° nozzle angle, and the only evidence of a completed transition is the rapidly increasing airspeed and the nozzle-angle indicator at zero. De- celerating transitions are obviously carried out by moving the nozzles down and increasing jet lift as aerodynamic lift is reduced. Deceleration of 0.5g can be achieved by selecting nozzles 10° forward giving braking thrust and intake drag. The nozzles are moved back to the hover position just before zero speed is reached, otherwise the aircraft hovers 10° nose-down. Variable nozzle angle was used to advantage during carrier trials; the aircraft hovered 5° nose-down due to wind speed over the deck, but returned hori- zontal when the nozzles were selected 5C aft. * Project Department, Bristol Siddeley Engines. Fig I Pegasus twin-spool turbofan, the engine on which the programme has been based Fig 2 The first of nine Hawker Siddelej P.I 127 V/STOL fighters which wi« equip a tripartite evaluation squadron
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