Patent GB-820709-A (GB)
Title: Improvements relating to fuel supply systems for propeller turbine engines
Inventor: STANTON FRANCIS PATRICK
IPC: F02C9/16
Filed: 1955-07-01
Abstract: 820,709. Gas-turbine plant. BRISTOL AEROPLANE CO. Ltd., and DOWTY FUEL SYSTEMS Ltd. June 26, 1956 [July 1, 1955], No. 19128/55. Class 110 (3). [Also in Groups XXIX and XXXV] Apparatus to control the supply of fuel to an aircraft propeller turbine engine to hold constant the torque exerted by the engine on the propeller, the torque held constant being adjustable by hand and in response to changes in atmoshperic air pressure, wherein a control device responsive to changes in atmospheric air pressure is arranged to have no action on a fuel flow controlling member through a lower working range of altitudes so that in this range the torque held constant by a governor circuit is always equal to that selected by the hand control, and to act upon the fuel flow controlling member through a higher working range of altitudes to reduce proportionately to increase in altitude the ratio maintained between the torque selected by the hand control and the working torque held constant by the apparatus, is shown in Fig. 1. The gas turbine comprises a high-pressure turbine 6 coupled to a highpressure compressor 5 by a sleeve 7 surrounding a shaft 4 connecting a low-pressure turbine 3 to a low-pressure compressor 2, which in turn drives a propeller 9 through an epicyclic reduction gear 8 having an annular gear 19 which is rotatably mounted but is prevented from rotating by a rod 21 of a torque sensing unit 22 which transmits fluid-pressure signals, proportional to torque, through a pipe 25 to the torque selector mechanism 67. The propeller is controlled by a constant-speed unit 26 which varies the pitch so as to maintain the speed at a value selected by the manual control 27. A bevel-gear drive 28 extends from the high-pressure compressor 5 to a gear pump 29, which draws fuel from a tank via pipe 36 and filter 37 and delivers it through a pressurizing valve 39 and pipe 34 to a gallery 33 fitted with a number of spill burners 13 for injecting fuel into combustion chambers 12; an extension of this drive drives a highpressure compressor overspeed governor 31. The spill flow from the burners returns via pipe 35 to the tank side of the fuel pump and is controlled by a valve assembly 42 comprising an axially movable valve member 43 operated by a lever 51 coupled to a remote manual regulator 55 and a spring-closed valve member 44 having an actuating piston 56 whose upper face is exposed via pipe 41 to fuel supply pressure and whose lower face is exposed to the same pressure via restriction 58 so that its position is controlled by the amount of fuel bled from the lower chamber 57 through pipes 59, 61. Fuel is only bled through pipe 59 when the h.p. compressor overspeeds and causes the flyweights 66 of the governor 31 to displace the arm 64 carrying the half ball-valve 63 against the action of spring 65. The bleed through pipe 61 is controlled by a half-ball valve 68 carried by a lever 72 pivotally mounted in the torque selector mechanism at 73, and is returned to the fuel tank from chamber 71 by pipe 75, a seal 74 preventing egress of fuel into a chamber 76 which has pipe 77 connected to the inlet passage 11 of the gas turbine and has an outlet restriction 78 so that the chamber 76 is always filled with air at the pressure and temperature of air entering the gas turbine. The chamber 76 contains a continuation of lever 72 having a straight portion 79 and an angled extension 81, the straight portion being contacted by a roller 82 carried at the free end of a lever 83 which is pivotally mounted at the end of a slidably mounted rack 84 engaged by a pinion 85 mounted for rotation with a lever 86 which is connected to the manual regulator 55. The lever 83 is engaged by a roller 88 carried by a plunger 89 mounted in the wall of chamber and subject to the liquid pressure of pipe 25 connected to the torque-sensing mechanism 22. Thus the force exerted by plunger 89 is transmitted in part only to roller 82, the proportion depending on the position at which roller 88 engages lever 83; the moment exerted by roller 82 on lever 79 is also variable by reason of its moment along lever 79, relative to the fulcrum 73, caused by rotating lever 86. The moment exerted on lever 79 varies, therefore, with engine torque sensed through pipe 25 and position of the lever 86 controlled by regulator 55. The lever extension 81 is acted upon by a compression-spring 92 and a rod 93 in opposition thereto; the compression of spring 92 being adjustable by means of a rod 94 axially movable by an electric motor 95 in response to a signal received from a magnetic amplifier 96 controlled by the temperature sensed by a thermocouple 97 at the turbine inlet 6, a hand lever 98 in turn being used to select the maximum permissible control temperature at the turbine inlet 6; the force transmitted by the rod 93 to the extension 81 being obtained on expansion of a closed capsule 99, which is filled with air at a predetermined low pressure and acts in opposition to compression spring 101 which is located against stops 102 forming part of the chamber 76, adjustment of the combined capsule 99 and spring 101 being effected by screw 103 such that at low altitudes the air pressure in chamber 76 maintains capsule 99 compressed to the extent that rod 93 cannot press against lever extension 81. The manual regulator 55 includes upper and lower limit stops 104 and 105 which provide, in particular, the upper limit of torque that may be selected. In operation, when the engine is running at low altitude and the manual regulator 55 is set at a position intermediate upper and lower stops 104 and 105 the roller 82 and lever 83 will take up a position as shown in Fig. 1. The constant-speed unit is also set for a given speed, and it may be assumed that the turbine inlet entry temperature remains below the predetermined maximum. Any tendency of the propeller to change speed will be sensed by the constant speed unit and appropriate alteration of the propeller pitch will result to keep the propeller speed constant. Variation of propeller pitch may result in temporary variation of torque, in which case adjustment of the balance of lever 72, 79 will take place to keep the torque constant by adjusting fuel flow, the final adjustment giving a constant selected propeller speed at a constant selected driving torque. As the aircraft climbs the propeller pitch becomes coarser and the air speed consequently higher by maintaining the same torque and engine speed, at about 20,000 ft., the capsule 99 expands sufficiently to overcome the compression of spring 101 and rod 93 presses against lever 81 causing more fuel to be spilled to reduce the selected torque in order to achieve fuel economy. With increase in altitude, for a selected torque, the turbine inlet temperature will increase; further reason for reducing the maximum selectable torque above 20,000 ft. (to prevent this temperature becoming excessive). Should this temperature become excessive a signal generated in the magnetic amplifier 96 causes the motor 95 to take action or reduce the load on spring 92 to cause more fuel to be spilled. When the aircraft is on the ground with the engine idling the regulator 55 is moved back to stop 105 and the torque exerted will not be sufficient to provide a moment on lever 72, 79 which would lift it to control fuel flow, thus slow idling speeds are controlled by the valve member 43 alone, the propeller being set at a fine pitch by the speed control. Also described is an hydraulic apparatus for carrying out the function of the torqueselecting mechanism 67.