GB833496A - Improvements in reheat control systems for gas turbine engines - Google Patents

Abstract

833,496. Gas-turbine jet-propulsion plant. HOBSON Ltd., H. M. Aug. 27, 1957 [June 6, 1956], No. 17510/56. Class 110 (3). [Also in Group XXIX] An aircraft gas-turbine engine having a variable area exhaust nozzle and means for burning fuel in the turbine exhaust has a device responsive to overall pressure ratio or to Mach No. arranged, when the overall pressure ratio or Mach No. exceeds a predetermined value to move the exhaust nozzle to a divergent position. A variable area nozzle 20 is movable from the fully closed position A to a position B of maximum effective area under the control of a turbine pressure ratio control constituted by a micro-jet 13 comprising a diaphragm 14 mounted in a chamber 15. The diaphragm 14 is subject on one side through a passage 16 to the turbine exhaust pressure and on the other side to a pressure proportional to the compressor delivery pressure through a line 80, a normally open valve 66 and an orifice controlled by a needle 62. When the reheat is turned on the turbine exhaust pressure will increase and cause the diaphragm 14 to operate the regulating valve 17 controlling the supply of air to a motor 18 driving a screw jack 19 controlling the nozzle 20. The amount of opening of the nozzle 20 will depend upon the amount of reheat fuel injected as selected by the pilot's control lever. When the nozzle reaches its maximum area position B, a cam 52A connected to the jack 19 opens a spill valve 59 and lowers the pressure on the upper surface of the diaphragm 60. This causes the needle 62 to be raised resulting in an increased pressure on the left-hand side of the diaphragm 14. This prevents the valve 17 and jack 19 moving the nozzle further in the opening direction. The nozzle may be moved from the maximum effective area position B into the divergent position C by an overall pressure ratio control 63. This comprises a pair of diaphragms 30 subject to compressor discharge pressure through a conduit 80 and to ambient air pressure through an inlet 181. When the overall pressure ratio, i.e. the ratio of compressor discharge pressure to the ambient pressure exceeds a limiting value, the diaphragms 30 move down with a snap action to move the valve 66 towards the closed position thereby reducing the pressure on the left-hand side of the diaphragm 14 which acts through the valve 17 to open the nozzle 20. A control valve 65 is closed when the exhaust nozzle 20 is in the position B but in a position less open the valve 65 is open as shown. This valve supplies air at the compressor discharge pressure to the diaphragm 14 in parallel with that controlled by the valve 66 so that closure of the valve 66 will not cause movement of the nozzle 20 to the divergent position unless the nozzle is at its maximum effective area. If desired, the turbine pressure ratio control may be modified as a function of compressor pressure ratio by means of a device 283. The device 283 comprises a pair of diaphragms 84 subject to the pressures at the inlet and outlet of the compressor. The diaphragms 84 control a needle 86 which varies the proportion of the compressor delivery pressure applied to the lefthand side of the diaphragm 14. In a modification, Fig. 2 (not shown), the nozzle 20 is actuated by an hydraulic ram controlled by a servomotor under the action of turbine pressure ratio control 13. In this arrangement the nozzle 20 is moved to the divergent position C by a device responsive to Mach No. This device is similar to the device 63 except that ram pressure is substituted for the compressor discharge pressure. This device is inoperative until the pilot moves his control lever to a position selecting 80% of maximum reheat fuel when a cam operated by the lever opens a valve and allows the ram pressure to be applied to the device in place of the ambient pressure. The device operates a valve which renders the operation of the limit valve 59 ineffective. Specification 833,493 is referred to.