Patent GB-605780-A (GB)
Title: Improvements in or relating to control systems for aircraft power plant
IPC: F02D29/00
Filed: 1944-12-15
Abstract: 605,780. Fluid-pressure servomotors. STRATOS CORPORATION. Dec. 15, 1944, No. 25207. Convention date, Oct. 12, 1943. [Class 135] [Also in Groups XXVII, XXXIII, XXXV and XXXVII] In a control system for an aircraft power plant comprising contra-rotating variable-pitch propellers 2, 3, (see Group XXXIII), the pitch of propeller 3 is controlled by a fluid-pressure servomotor to vary the speed of a supercharger 4 geared to the transmission mechanism of the propellers, and the pitch of propeller 2 is controlled by a speed governor 21 and another fluid-pressure servomotor, so that the pressure of gases delivered by the supercharger and the speed of the engine are maintained in accordance with the predetermined power and speed requirements of the plant. A servomotor 30 also operates a gate 31 to restrict the flow of air through the inlet pipe 6 of the supercharger at low throttle settings, and a regulator 24 comprising a fluidactuated valve is provided in the fuel system. The control of the pitch of propeller 3 is effected by means of a regulator 8 (shown diagrammatically in Fig. 1 as operating a switch in an electrical control system (see Group XXXV)) comprising opposed bellows 69, 71, bellows 71 being acted on by pressure in the delivery pipe 7 of the supercharger and bellows 69,by pressure in a line 11 supplied by a pump 12. The pressure in line 11 is varied in accordance with the setting of a throttle lever 9 by means of a spring-loaded relief valve 14, the loading of which is controlled by plunger 18 having a roller 17 engaging a cam 16 on the lever. A regulator for use with propellers having hydraulic pitch-control mechanism is shown in Fig. 5 and comprises three concentric bellows 91, 92, 93, each secured at one end to the end wall 88 of a casing Sb and at the other end to a plate 94. The space enclosed by bellows 91 communicates with pipe 11 through a fitting 87, the annular chamber between bellows 91 and 92 is evacuated and the annular chamber between bellows 92 and 93 communicates with the interior of casing 8b through restricted orifices 94a. The end wall 90 of casing 8b is provided with a fitting 89 for connection to delivery pipe 7. Unbalance of the pressures acting on plate 94 operates the piston 96 of a pilot valve controlling flow of fluid through a port 98 connected ,to a source of fluid pressure (not shown), a port 99 connected to the pitchcontrol mechanism and a discharge port 100. Piston 96 is connected to plate 94 by a rod 95 having a bearing 116 at one end to permit rotation of the piston and a joint at the other end to allow for lack of alignment between the centre of the plate and the axis of the piston. Plate 94 is centred by means of springs 117. Ports 98, 99 are in communication with annular chambers 101, 102 respectively and port 100 with a longitudinal passage 103 in the valve casing, passage 103 communicating at one end with a chamber 104 surrounding the end of piston 96 and at the other end with a chamber 105 defined by bellows 106, to equalize the pressures acting on the ends of the piston. Piston 96 is formed with longitudinal passages 107, 108 and three sets of radial passages 109, 110 and 111, passages 109, 110 communicating with passage 108 and passage 111 with passage 107. Piston 96 is shown in the neutral position in which fluid is neither admitted to nor discharged from the pitch control mechanism. If plate 94 is moved to the left as a result of either an increase in pressure in line 11 controlled bv throttle lever 9 or a decrease in engine manifold pressure, fluid flows through port 98, chamber 101, passages 109, 108 and 110, chamber 102 and port 99 to decrease the pitch of propeller 3 and increase the speed of supercharger 4 until the balance of pressures is restored. Conversely, movement of plate 94 to the right allows discharge of fluid from the pitch-control mechanism through port 99, chamber 102, passages 111, 107, chamber 104, passage 103 and port 100. To reduce the frictional resistance to movement of piston 96, helical channels 114, 115 are provided which allow a small leakage of fluid from chamber 101 to a chamber 112 and to chamber 105 respectively to cause a continuous rotation of the piston. Due to the restricted orifices 94a, the pressure in the chamber between bellows 92, 93 will differ from that in casing 8b on movement of plate 94 and a restoring force is thus provided which prevents " hunting " of the regulator. The fuel regulator 24, Fig. 6, comprises a spring-loaded valve 120 controlling a byepass in the fuel delivery pipe 26 and operated by a bellows 123 which communicates with line 11 through a passage 127. An overriding manual control is provided and consists of a pulley 130 which is rotated by cords 133 to raise or lower a screw-threaded member 132 and vary the compression of a spring 129 engaging the upper end of the valve spindle 128. The servomotor 30, Fig. 7, operating the gate 31 in the inlet pipe 6, comprises a casing 30a open to atmosphere at 134 and concentric bellows 136, 137 connected between an end wall 135 and a movable plate 138. Bellows 136 communicates with pipe 11 through a fitting 140, and the annular chamber between. bellows 136, 137 communicates with the interior of casing 30a through restricted orifices 139 to prevent " hunting." Plate 138 is connected to a valve 141 which controls admission of fluid to and discharge from chambers above and below a piston 148 which operates gate 31 by rack and pinion gearing 151, 152. The shaft 150 of gate 31 carries a cam 153 which operates through roller 154, plunger 155 and spring 156 to return valve 141 to port-closing position after the required movement of the gate has taken place. Normally, the gate is maintained fully open, but, if the pressure in line 11 drops below a predetermined value, the valve 141 is operated to move the gate to a partially closed position. The tension of the spring of governor 21, Fig. 1, is variable by means of a speed-control lever 23 and, if the engine speed varies from that appropriate to the setting of the lever, the governor operates through a pilot valve similar to that of Fig. 5 to alter the pitch of propeller 2 and bring the engine speed back to the correct value. Specification 604,756, [Group XXVII], is referred to.