Patent GB-724196-A (GB)
Title: Improved control system for an aircraft power plant
Filed: 1951-10-23
Abstract: 724,196. Automatic speed-control systems. GENERAL MOTORS CORPORATION. Oct. 23, 1951 [Nov. 8, 1950], No. 24664/51. Class 38 (4). [Also in Groups XXVI and XXXIII] In a control system for an aircraft power plant wherein two power units may each be clutched to a variable-pitch propeller, each power unit is controlled by a manually operated lever 49 to operate a cam 113 controlling the simultaneous movements of contacts 127a and 128a over potentiometers 127 and 128 energized from a bus-bar 2. The levers 49 are moved from a " stop " position to an intermediate position during which the propeller blades are directly adjusted in pitch from a feathered position, continued movement of the lever from the intermediate position bringing a governor device into operation for controlling the speed of the propeller. When the lever 49 is moved from the " stop " position signals are transmitted by contacts 127a and 128a, that from 128a passing through a closed throttle-operated switch T12, contact PRa of a discriminator relay PR, line 248, a follow-up relay PF and line 251 to contact 249a of a potentiometer 249, energized directly from bus-bar 2. The movable contact of relay PR is biased either on to contact PRa or PRb by coils PR1 or PR2 energized by the signal from contacts 127a so that the signal from the contact 128a farthest advanced from the "stop" position will be transmitted to the contact 249a. The movable contact of relay PF is biased to the open position. The contact 249a is mechanically coupled to the armature 76a of a blade pitch changing actuator 76, the circuit for which is from bus-bar 2, line 252, contact PFd, line 253, limit switches 76n and 76p in series, line 254, contacts of a feather relay PE, line 256, contacts PI1 of an " increase pitch " relay PI, the coil of a " decrease pitch " relay PD and line 257 to earth. The front contacts of relay PD close a circuit from bus-bar 2 through line 258, front-contacts of relay PD, line 259, armature 76a and a brake release coil 76r, line 261, back contacts PI2 and line 257 to earth. When the lever is at the " stop " position the actuator motor is at one limit of its travel so that movement of the lever 49 causes a current flow from contact 128a to 249a which biases relay PF to close on contact PFd to energize the motor 76 to unfeather the blades of the propeller. When the actuator 76 has moved contact 249a to the position at which the potential across relay PF balances, the relay will open the circuit to the motor and when the actuator 76 reaches the position corresponding to the blade angle for minimum governed speed of the propeller, limit switch 76p opens and thereafter blade pitch is controlled automatically by governor 81. The armature 76a is mechanically connected at 275 to contact 275 of a potentiometer 276 energized from the governor, the pick-off from 275 being fed at 277 to the governor which balances it against the input from a propeller-driven alternator 84, the discrepancy signal being fed through lines 283 and 284 to the solenoid coils HI and HD in the propeller which control a hydraulic valve controlling the transmission of liquid from a pump. The signal from the governor 81 adjusts the valve until the propeller speed corresponds to that called for by the position of the lever(s) 49. Specifications 713,280 and 724,255 are referred to.