746,903. Variable-pitch propellers. GENERAL MOTORS CORPORATION. Feb. 23, 1953 [Feb. 23, 1952], No. 4981/53. Class 114. [Also in Group XXVI] A control system for an aircraft power plant comprising two engines coupled to a common variable-pitch propeller assembly comprises a single throttle lever which acts through a coordinator unit to control, for each engine, a throttle valve and the setting of an overspeed governor controlling a fuel valve and the setting of the variable-pitch propeller assembly. A power plant, Fig. 1, comprises gas turbine engines A, B, a reduction gear assembly C and a variable-pitch propeller assembly D as described in Specification 720,049. Each engine is coupled to the reduction gear through a fluidoperated clutch as described in Specification 690,592. The propeller assembly comprises contra-rotating variable-pitch propellers which can be feathered, set to a desired blade angle or in a governing phase in which the pitch is automatically varied by a speed governor. Each engine has a fuel control 40 comprising a fuelcontrol lover 41 and a lever 42 by means of which the setting of an overspeed governor controlling a fuel valve may be predetermined. The control 40 also includes an overriding control to decrease automatically the fuel supply if the temperature of the engine becomes excessive. The levers 41, 42 are connected by links 52, 53 to levers 49, 51, Figs. 3 and 4, of a manually operated power control device 43 for coordinating control of fuel supply, overspeed governor setting and propeller pitch. This device also has a lever 44 coupled by a link 46 to a pilot's control lever, a lever 54 acting through a linkage system as described in Specification 746,907 to control the pitch of the propellers and a lever 47 connected by linkage 48 to a pilot's feather lever for the sole purpose of feathering or unfeathering the propellers. A locking device 58 at the propeller must be released by a flexible cable arranged in a tube 59 before feathering can be effected. The coordinating control device 43 comprises a casing, Figs. 3, 4 and 5, partly filled with oil for lubrication and carrying shafts 201, 202, 239, 243 and 244 on which are mounted respectively levers 49, 51, feather input arm 238 and levers 54, 44. A pointer 248 is clamped on shaft 244. A cam disc 251 splined to shaft 244 has cam slots 252, 253, Fig. 8, in which are arranged followers 259 mounted in arms 256 of yokes 254 carried by the shafts 201, 202. The shafts 201, 202 are connected by shafts 207, 208, Fig. 4, to the fuel controls of the other unit. The shaft 243 is interconnected with shafts 244, 239 by discs 270, 271, Fig. 5, on the latter shafts and an arm 272 integral with shaft 243 which pivotally carries a coupling member 273 having arms 274, 276 adapted to engage notches in the peripheries of the discs 270, 271. The arrangement is such that the shaft 243 is coupled to the disc which is farthest advanced from its zero position. A driving wheel 320 splined on shaft 239 is adapted to reciprocate cable 321 to release the locking device 58 to enable the propeller to be feathered. The angular movement of the feather shaft is limited by a stop 342 engaging a slot in one face of the disc 270. Switches S2, S6 adapted to cut off the fuel supply to the engines and to declutch them from the propeller are operated by cams 333, 334, on a spool-like body keyed to the shaft 239. The cam disc 251 is formed with a notch 346, Fig. 8, which may be engaged by a slidable detent plug to lock the disc in a datum position for the alignment of the controls. The fuel supply system comprises a pump 39, Fig. 23, driven from the engine shaft 410 which discharges through a by-pass valve 454 and a throttle valve 453 including a plunger 457 shown schematically as operated by a cam 462, corresponding to cam slot 252 of the coordinating control, from the pilot's control lever 45. The valve 453 discharges to a valve 455 comprising a plunger 467 shown schematically as operated by a cam 473, corresponding to cam slot 253 of the co-ordinating control and a sleeve 464 controlled according to engine speed by governor 466. The fuel passes from valve 455 through a motor-operated shut-off valve 484, a drip valve 489 and pipe 491 to the engine. The air supply to a starter motor 414 is controlled by a solenoid operated valve 423. By means of solenoids 422, 422<SP>1</SP>, the motor may be clutched to operate either shaft 411 or shaft 417 to start engine A or engine B. The electrical control circuits are principally energized from a bus-bar 401, Fig. 22, fed from a D.C. supply 400. Stop relays RS, RS<SP>1</SP> are energized when run-stop switches S, S<SP>1</SP> are closed by the pilot or when feather relay RF is energized by closure of switch S2 by operation of the pilot's feather control lever. The starter relay 406 is in a circuit comprising a starter button S3 and back contacts RF3 of feather relay RF. The solenoids 422, 422<SP>1</SP> which determine which engine shall be clutched to the starter motor are arranged in parallel and energized with the coil 406 in circuits containing the back contacts RSI, RSI<SP>1</SP> of the stop relays RS, RS<SP>1</SP>. The ignition coil 430 of engine A is fed from an A.C. bus-bar 431 through back contacts RI<SP>1</SP> of an ignition relay RI which is energized to break the ignition circuit through amplifier 436 and relay TR when thermocouples 435 sense that normal combustion is established. The ignition relay is energized at starting through the normally closed contact 438 of speed-responsive switch S4 which is opened by governor 443, Fig. 23, when the engine reaches 15 per cent of rated speed. At this speed the relay RI is deenergized and the ignition circuit energized. Concurrently therewith drip valve solenoid 497 is energized through back contacts RI2 of ignition relay to close the drip valve. When speed switch S4 breaks contact 438 it moves to close contact 448 to energize motor 500 through limit switch 504 and back contacts RS2 of the stop relay RS to open fuel valve 484, the limit switch breaking the circuit when the valve is open. The starter button is held closed by the pilot until the engine speed reaches a self-sustaining value. The reversible motor 538 for operating the valves 521, 534 controlling the supply from the engine-driven pump 518 for clutch actuation comprises an armature 539 in series with a brake coil 540 and filed windings 541, 542. Limit switches 543, 544 and 545 driven by the motor cause the latter to stop in the extreme or intermediate positions. The clutch position is indicated by lights 550, 559, 554. Clutch actuation is controlled by the pilot-operated three-position switch S5 which'is shown in the " disengaged " position. Movement of the clutch is controlled by the valve 521, the valve 534 controls the.supply of cooling oil to the clutch. The engine is clutched to the propeller by moving switch S5 to the other extreme position. When the propeller has been brought up to speed the switch is moved to the intermediate position to cut off the supply of cooling oil. The engine B may be started in the same way as engine A. Alternatively it may be started by clutching it to the rotating propeller. The engines may be cut out by direct operation of the stop switches SI, SI<SP>1</SP> or by feathering the propeller and thus closing switch S2 to energize the relay RF and consequently stop relays RS, RS<SP>1</SP> through back contacts RF1, RF2. The pitch of the propeller blades is changed by hydraulic motors energized through a control valve system from a propeller driven pump. Unfeathering or feathering when the propeller is not rotating is effected by a " feather " pump driven by a three-phase motor 600 in the propeller coupled by slip rings to three-phase bus-bars 601, 602, 603 through contacts RT3, RT2, RT1 of a propeller transfer relay RT. Operation of the pilot's feather lever to unfeather the propeller moves.switch S6 to contact 609 to energize relay RT to cause operation of the " feather " pump and to energize heating coil 613 of a bimetallic switch RD which after a time interval closes a circuit through a relay RL to open the circuit of relay RT. The relay RL is held in the open position by a latch 617 engaging a dog 616 on the relay until the switch S6 is again moved to contact 619 to feather the propeller. In this movement it contacts 621 to energize solenoid 618 to withdraw the latch 617. If the propeller speed falls to a value too low for effectively operating the pump for the hydraulic system centrifugal switch S7 is operated to energize the " feather " pump 600. In the de-energized position of relay RT, the back contacts RT1, RT2 make circuits by which the propeller governor 630 transmits signals to the regulating valve by which the pitch is adjusted to maintain the desired propeller speed. These control signals are transmitted to the centrifugal switch S7 over lines 632, 633 through back contacts RG1, RG2 of a governor disconnect relay RG, lines 634, 636 and back contacts RT1, RT2 of relay RT. When this switch is closed the signals pass to solenoids 639, 640 of the regulating valve. The solenoids could be field coils of a reversible motor for changing pitch or could actuate clutches for this purpose. Relay RG can be energized under the control of a switch actuated by the pilot's control lever 45, Fig. 23, through the cam 650. The control lever is arranged in a gate with offsets to prevent inadvertent movement from the flight range into the taxi range or from the positive thrust part of the latter into the negative thrust part. In a modlication, reverse pitch is not avaialble. The propeller governor comes into action at the operational idle position.