GB877717A - Improvements in or relating to barometric pressure controls for regulating the fuel supply of aircraft engines - Google Patents

Abstract

877,717. Fluid-pressure servomotor-control systems. BRISTOL SIDDELEY-ENGINES Ltd. Nov. 28, 1957 [Aug. 29, 1956], No. 26442/56. Class 135. [Also in Group XXVI] In a barometric pressure control for regulating the fuel supply of aircraft gas-turbine engines, of the kind which is responsive to the total or ram air pressure at the engine air intake (referred to as nacelle pressure) in opposition to a pressure derived from the fuel pump delivery, the control is arranged so that the fuel pump delivery pressure nacelle pressurecontrolling characteristic assumes the form of two connected straight lines of different slope which match closely the curved characteristic obtained under constant speed conditions of the engine. As shown in Fig. 3, the variable delivery pump 10 supplying fuel to duplex burners 12 of the engine is servo-controlled in known manner by the variable leak valve 25 actuated by the pivoted beam 22 which is responsive in one direction to nacelle pressure tending to compress a capsule 24 and in the other direction to a pressure derived from the pump delivery which acts on a piston 38. The pressure applied to this piston is derived through pipe 28 from the downstream side of an orifice 13 connected on its upstream side to the pump delivery, the downstream side of the orifice being controlled by a needle valve 15 which is spring-loaded in the opening direction and subject on opposite sides of a piston 16 respectively to the derived pressure and to the pump delivery pressure through a pipe 19, the outlet from the needle valve being opposed by a second orifice 14 which returns fluid to the pump suction. The parts are proportioned so that at lower altitudes the needle valve 15 is operative to maintain a substantially constant difference between the pump pressure and the derived pressure, i.e. on opposite sides of piston 16, whilst at higher altitudes the needle valve 15 remains fully open and the derived pressure in pipe 28 (being the pressure from between the two orifices 13 and 14) is in a constant ratio with the pump pressure. Fig. 1 shows the pump pressure (Pp) -nacelle pressure (P 1 ) characteristic corresponding to Fig. 4 in full line, matched against the curved engine constant R.P.M. line, " Y " being the transition point at which the needle valve 15 becomes fully open. The second orifice 14 may be omited and be replaced in effect by the orifice 15b of the needle valve 15. In a further modification (Fig. 4, shown in part), adapted to give a graph in which the respective slopes on either side of the transition point " Y " are reversed for operating with simplex burners, the needle valve 15 is replaced by a needle valve 31 which is spring-loaded in the closing direction and subject on pistons 33, 34 of different diameter to the derived pressure and fuel pump delivery pressure respectively, the space between the pistons being vented and the second orifice 14 being omitted. In operation, for example, during a climb from sea level at constant R.P.M. the needle valve 31 which is initially fully open gradually closes until it becomes fully closed and remains so at and above the altitude corresponding to the transition point Y.