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Aviation History
1955
1955 - 0273.PDF
4 March 1955 273 tion gear. Following roller-cage failures and track breakdown onthe satellite unit (then a large-bore gear with an aluminium cage) a smaller bore (to increase stiffness, and so reduce deflection underload), and a strengthened steel cage were introduced, and an improvement in manufacturing accuracy enabled undercuts ontracks to be deleted. To eliminate pick-up on the steel cages, a 0.020in nickel cadmium liner was plated on to the inner face. The first production Double Mamba underwent final tests andwas delivered in January 1953. Gannets were handed over to the Navy in April 1954 for intensive flying trials, and it wasduring these trials that the danger of compressor-stalling (under certain conditions), which led to all Gannets being grounded forthree months, was discovered. The nature of the problem encountered can best be understoodin relation to the wider questions of turboprop control in general. A simple single-lever control would be that in which the fuelcontrol unit and the airscrew speed control unit are directly con- nected to the pilot's throttle. This would be effective providedthe pilot's lever were moved very slowly: in the case of more rapid movement, however, engine and airscrew inertia would causeengine speed to lag behind the selected speed, and their effect on airscrew pitch would cause oscillations about the required enginespeed before the engine stabilized at the new selected conditions. For the rapid-acceleration case, this system has three majordisadvantages. The first response to throttle-opening is in the Above, tht introduc- tion of photpkat'mg, a smaller satellite-gear bon, and cadmium-plated steel cages, Mow, the original (upper) and present production types of combustion chamber on the ASMD.l. The ASMD3 has an annular chamber. tn the Double Mamba production assembly shop at Ansty, individual Mamba "hahes" are prepared for final assembly. wrong sense (as pitch and thrust momentarily decrease instead ofincreasing); there is an appreciable time-lag before the thrust does increase; and some time elapses before the engine stabilizes andruns steadily at the new r.p.m. and thrust. To avoid these defects, Armstrong Siddeley's single-lever con-trol system incorporated a hydraulic servo between the pilot's lever and the c.s.u. This servo provided a differential rate ofmovement between the pilot's lever (still directly coupled to the fuel control unit) and the speed-selecting lever on the c.s.u. Theeffect of this device was to cause the selected r.p.m. to lag behind the movement of the pilot's lever, resulting in the increased thrustbeing achieved more rapidly, and in the elimination of the violent oscillation of engine speed and thrust. Apart from the small lag in the response of thrust to movementof the pilot's lever, this system was found to give satisfactory engine-control and handling under all normal flight conditions.For a carrier-based aircraft, however, immediate response is neces- sary during the landing approach: thrust must fall to zero instantlywhen the pilot cuts the throttle, while full thrust must be achieved very rapidly in the event of a wave-off. In any system in which the r.p.m. are changed at the same timeas the power, however, the inertia of engine and airscrew during any sudden increase or decrease of power must inevitably causea lag in response of airscrew thrust to throttle movement. Thus a logical step to improve the speed of this response is to operatethe engine at constant speed over the power range.
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