808,559. Variable pitch propellers. UNITED AIRCRAFT CORPORATION. June 8, 1955 [June 17, 1954], No. 16445/55. Class 114. Pumps. A pump unit 10, Fig. 1, comprising a pair of tandem pumps 14, 16 is actuated from the aircraft propeller driving gear 19, one pump 14 drawing fluid from an atmospheric tank 12 and discharging it into a pressurized tank 15 from which it is sucked by the other pump 16 and discharged into a line 17 ; the latter is also connected to a similar pump unit 20 that is operated, when the propeller is feathered or during pitch changing or whilst the propeller is not rotated within the normal speed range, by an electric motor 24 which may be manually and/or automatically energized. Power device. Downstream of a check valve 25 in the line 17 the fluid passes into two conduits 26, 27, the latter leading to a feathering valve 30 whilst the former is connected to both a relief valve 28, the outlet from which opens into the tank 15, and to one chamber 33 of an axially movable cylinder 31 of the power device, the piston 32 of which is stationary ; the cylinder 31 embodies teeth 35 that are engaged by a pinion 36 on a shaft 37 connected to the pitch changing mechanism, not shown. The power device incorporates high and low pitch stops 38 and 40, respectively, the latter being arranged on an arm of a bell-crank lever 42 which is pivotable to permit movement of the cylinder 31 into reverse pitch ; a reverse pitch stop 46 is associated with the stop 40. The effective area of the chamber 33 is less, one-half for example, than that of the chamber 34 on the other side of the piston 32 and the power device is operated by varying the pressure in the chamber 34, which variations are controlled by flow-control valves 48, 50 and pressure-regulating valves 52,54. Pressure regulating valves. The high-pressure valve 52 comprises a valve member 57 with a through passage 61 and a loading spring 58 in a lower space 59 connected by a channel 56 with the chamber 34 so that the pressure therein and the spring force move a member 57 upwardly until passage 61 is aligned with a conduit 63, connected via the feathering valve 30 with the conduit 27, whereupon the pump pressure, which is always higher than the pressure in chamber 34, is admitted into an upper space 60 and thrusts the member 57 downwards until the passage 61 is no longer in communication with the conduit 63 and the pressure in space 60 will equal that in space 59 together with the pressure set up by the spring 58 so that space 60 provides a source of fluid at a pressure which is a fixed amount, for example 30 p.s.i., greater than that in chamber 34 ; the fixed amount is equal to the pressure exerted by the spring 58. The space 60 is connected by a duct 64 to the top port of the centrifugal flow control valve 48. The low pressure valve 54, which is similar to valve 52 but inversely disposed, comprises a valve member 67 and upper and lower spaces 69, 73, the former housing a spring 68 and being connected to the lower most port of valves 48 by a line 70, and space 73 being open to the chamber 34 via the channel 56 ; a conduit 74 opening into the valve leads to the tank 15 to drain the pressure in space 69 until the valve member 67 again assumes the position shown. The arrangement is such that the pressure in space 69 will be a fixed amount, for example 30 p.s.i., less than that in chamber 34, which amount is equal to the pressure exerted by the spring 68. Flow-control valves. The centrifugal flowcontrol valve 48, which is sensitive to over or under-speeding of the propeller, normally remains in the position shown and enables fluid from the valves 52, 54 to flow to the normal control valve 50. The valve member 75 of the valve 50 has a projecting portion comprising the armature of a solenoid 77 having one or more coils connected to an electronic governor, not shown, which operates the valve under all normal running conditions ; the member 75 is raised or lowered when the propeller speed rises or falls above or below, respectively, the set value to connect the channel 56 and hence the chamber 34 to the high or low-pressure valves 52 or 54. Should the valve 50 fail, its function is performed by the valve 48 which comprises a valve member 80 attached to a governor mechanism driven from the gear 19 and at normal propeller speeds duct 64 and line 70 communicate through the valve 48 with conduits 66 and 72 that are connected to the upper and lower ports, respectively, of the valve 50. Should excessive overspeeding occur the member 80 is moved upwardly and closes conduit 66 whilst opening channel 56 and conduit 72 to line 70 whereby the chambers 34 is connected to the low-pressure valve 54 through both valves 48, 50 so that the chamber 34 is drained and permits leftward movement of the cylinder 34 in the high-pitch direction and highpressure fluid is prevented from flowing through the valve 50 to the chamber 34. When the valve member 80 is thrust downwards due to excessive under-speeding it closes communication between conduit 72 and line 70 and opens the duct 64 from the high-pressure valve 52 to the channel 56 and conduit 66 whereby the chamber 34 is supplied with highpressure fluid from both valves 48, 50 so that the cylinder 31 moves rightward in the lowpitch direction. Feathering. Upon depressing a plunger 100 the conduit 27 is disconnected from the inlet conduit 63 of the high-pressure valve 52 so that low-pitch movement of the power device is prevented; by the same means a supply of fluid is no longer available to a solenoidoperated reversing-valve 104 in consequence of which a piston 45 cannot be moved and the low-pitch stop 40 is inoperative. When the feathering-valve member 101 is thrust down the system is conditioned to prevent low-pitch movement of the cylinder ; at the same time a lever 107 is urged counterclockwise by the member 101 thereby lifting the valve member 80 and giving rise to the same conditions of the valve 48 as occur during excessive over-spending. Upon the propeller finally ceasing to rotate when the pitch angle of the blades has reached a given value the motor 24 is energized to drive the pump unit 20 which then supplies the system until the propeller is fully feathered. When the plunger 100 is released a spring 108 forces the member 101 to the position shown and a speeder spring 83 urges the valve member 80 downwardly as during excessive over-speeding ; the system is supplied with fluid by pump unit 20 and the solenoid 77 may be energised at the same time as the plunger 100 is depressed. The feathering operation may also be effected by forcing the valve member 101 down by fluid pressure under the control of a solenoidoperated valve 109 which also directs fluid into a piston 117 to assist the counterclockwise movement of the lever 107 ; the solenoid 77 is energized at the same time as the valve 109. Reversing. The reversing operation is done by energizing the solenoid 122 of the reversing valve 104 and admitting fluid to move the piston 45 and pivot the stop 40 ; concomitantly it admits fluid to a chamber 127 and thrusts upwardly a piston 128 to pivot a lever 129 clockwise whilst the piston 117 is lowered to pivot the lever 107 counterclockwise, the levers 107, 129 holding the valve member 80 in the centred position. The solenoid 77 is also energized and the valve 50 moved downwards to cause fluid to enter the chamber 34 and move the cylinder 31 into the reverse-pitch range. Upon returning to the positive range of pitch angles the valve 104 remains energized whilst the valve 50 receives a signal corresponding to a propeller over-speeding condition after which the valve 104 is de-energized. Specification 692,119 is referred to.