Patent GB-958881-A (GB)
Title: Improvements relating to control apparatus for turbine engines
IPC: B64C27/32
Filed: 1963-03-06
Abstract: 958,881. Gas turbine plant. UNITED AIRCRAFT CORPORATION. March 6,1963 [March 30,1962], No. 8993/63. Headings F1G and F1Q. [Also in Divisions B7 and G3] A fuel control system for an engine having a free turbine driven by a gas generator in which the turbine drives variable pitch blades, such as on a helicopter or turboprop engine, comprises controlling the fuel flow in response to the lower fuel scheduled required by the generator or turbine speed, and modifying the effect of the speed signal with changing pitch of the blades to compensate for speed changes which would normally be caused by the droop characteristics of the governor systems. The nominal speed of the gas generator is set manually by a cam 96 to position a roller 102 of a lever 98 which rotates a lever 108 to position a roller 112 forming the pivot point of a lever 114 which is rotated by a speed governor 162 through a control valve 164 and cam 174. The lever 108 is shown out of contact with the roller 112, this being the case when the fuel flow is scheduled in response to the free turbine speed. Movement of the cam 174 also acts through follower 178 and lever 180 to modify the .output characteristic of the cam in known manner and the cam can also act on an arm 182 to limit fuel flow during acceleration. Movement of the lever 114 in response to speed changes rotates a cam 122. The free turbine speed is nominally set manually by a cam 186 to position a pivot 195 through a lever 190 and link 194. A free turbine speed governor 208 acts through a control valve 210 and piston 214 on a lever 196 and link 204 fixed to a shaft 124 which shaft also carries the cam 122. The link 204 has a slot 202 such that when the lever 196 engages the bottom of the slot (as shown) the fuel schedule is in response to free turbine speed but when the lever abuts the top of the slot the fuel schedule is in response to gas generator speed and the lever 114 contacts roller 112. When the pilot changes the pitch of the blades lever 51 moves the cam 96 to reposition the rollers 102, 112 and also rotates a cam 220 which through lever 222 repositions the pivot 195, thus counteracting the droop characteristics of governors 162, 208 when the load on the engine varies. Rotation of the cam 122 in response to either speed signals acts on a bell crank lever 128 to adjust a multiplying bar 134 carrying rollers 148 and a lever 146, subjected to compressor discharge pressure acting on a bellows 138, causes rotation of a lever 150 to actuate a control valve 154 controlling the pressure in a chamber 158 of the fuel flow valve 68. Movement of the valve 68 adjusts the fuel flow from an inlet 50 to an outlet 54 and also acts on the lever 150 through a feedback spring 162. The pressure drop across the fuel valve 68 is kept constant by a relief valve 76 subjected to a constant servo pressure in chamber 87 and the pressure acting on the opposite side of the valve which is controlled by a pilot valve 74 subjected at opposite ends to the pressures upstream and downstream of the fuel valve 68. Flow through pipes 82, 84 under the control of pilot valve 74 takes place through a normally open valve 92 which is closed by connection 218 if the free turbine over speeds and thus moves the piston 214 through an excessive movement. The closure of valve 92 also results in the closing of a valve 72 to stop fuel flow to the engine. The valve 92 can only be reopened manually. The invention may be applied to two engines driving one rotor where in the event of failure of one free turbine, the control will react to deliver a greater fuel flow and the other engine will react to the increased speed to reduce its output. Specification 753,970 is referred to.