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Aviation History
1972
1972 - 2776.PDF
534 DESIGNING THE PEGASUS the fan or core of an engine conventionally situated in the rear fuselage would merely create an unbalanced moment tending to pitch the nose down. From the begin ning it was obvious that the Pegasus would be a two-spool engine in order to meet the performance requirement. There was immediately the possibility of distributing the lift thrust between the low-pressure and high-pressure systems. This was convenient because it enabled the nozzle size to be kept down (four small ducts compared with two large ones). The bypass ratio, about 1-35:1, was settled by the need to ensure approximately equal thrust from the front (cold) and rear (hot) nozzles. This meant keeping the products of the mass flow and gas velocities comparable. Since the velocity through the fan is lower than through the hot section, its mass flow has to be greater, and hence the deviation from what, at first sight, should be a 1:1 ratio. The pressure ratio, 13:1, was settled by the need for maxi mum range. A plot of range versus pressure ratio shows a shallow curve with range initially increasing but eventu ally levelling off and then falling. The reason for this characteristic is that, while increasing pressure ratio makes for better efficiency and decreasing s.f.c, the effect of the extra weight needed to counter the greater internal engine stresses eventually becomes predominant. Upgrading of the engine means more than simply increasing the hot-end thrust by running the turbines hotter, since this would eventually unbalance the thrust centre from the e.g. of the aircraft. De-coupling the fan exhaust from the turbine efflux has the advantage that changes or modifications to one spool can be made without significant effects upon the other. Improvement pro grammes can be directed independently at the front or rear of the engine, subject to the need to keep the thrust centre substantially unchanged. And power increases— more than with any other class of aeroplane—are always needed to keep pace with increases in weight and demanded performance; in V/Stol there is hardly such a thing as excess thrust. This approach to thrust improve ment does mean, however, that drastic measures, e.g. the introduction of a zero-stage compressor, cannot be made without badly disturbing the thrust centre. The Pegasus 6 was the first production engine and differs FLIGHT International, 19 October 1972 This view of the Pegasus 6 shows the large-diameter titanium fan — the casing is 48in across—and the snubber ring near the tips of the blades to damp out vibration in these thin components. The three stages are overhung from the low-pressure shaft. Below, a "Flight" study of a two-seat Harrier (this one has an extra-long tailboom housing an anti- spin parachute, for test purposes) from the -5 chiefly in having an all-titanium fan. It also has a water-injection system for thrust restoration in hot-and- high conditions. The interim Pegasus 10 was, it seems, a rather unexpected half-way step to the 11 necessitated by fairly tight financing by the British Government. The goal of the Pegasus 10 was the development of the turbine, leaving the fan improvement as the object of the Pegasus 11. The first moved the thrust centre somewhat aft, the second restored it to the original position. The Mk 103, Pegasus 11 has a re-designed fan to permit a greater mass flow, and a slightly increased pres sure ratio. The spools rotate in opposite directions and their resultant gyroscopic forces very largely cancel one
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