Patent GB-725181-A (GB)
Title: Improvements relating to control systems for variable pitch propellers
IPC: B64C11/42
Filed: 1953-03-13
Abstract: 725,181. Variable pitch propellers. GENERAL MOTORS CORPORATION. March 13, 1953 [March 15, 1952], No. 6968/53. Class 114. In a control system for a propeller the pitch of which can be varied by a fluidpressure operated motor and is locked when the fluid pressure fails, means is provided for rendering the lock ineffective, e.g. when, after the propeller stops rotating, it is desired to vary the pitch. Pressure fluid supplied continuously by a pump 1 flows along a main line 10 past a distributer valve 8 along lines 12 or 14 to a motor 9 actuating the variable pitch mechanism of a propeller blade 27. The actual pressure in the main 10 is determined by a control unit 5 connected thereto by a line 4 and comprising a relief valve 54 for controlling the maximum pressure in the system and an equal area valve 52 responsive on the one side to the mains pressure and on the other side to the combined pressures of centrifugal force (acting upwardly), a valve spring 62 and the pressure actuating the motor 9. The motor 9 comprises increase and decrease pitch chambers 13, 19 respectively separated by a piston 17 and it is the chamber at higher pressure which communicates with the underside of the equal area valve 52 by displacing a shuttle valve 56 across a passage 72 leading from the valve 52, the valve opening up either lines 68, 14 to the decrease or lines 70, 12, a pitch lock valve 11 and a line 120 to the increase chamber 13. Excess fluid from the pressure regulating valve 5 flows down a passage 118 and a line 42 to the upper side of a plunger 34 of a pitch lock pilot valve 7 and by depressing the plunger, with the aid of a spring 32 and against centrifugal force, opens or keeps open to the main line 10 a line 16 leading to the pitch lock valve 11. On failure of the excess flow due to a drop in the main pressure, the centrifugal force raises the plunger, cutting off the line 16 from the'line 10 and connecting it to a waste outlet 40. Pressure fluid from the line 16 normally enters a chamber 57 in the pitch lock valve 11 and forces a piston 39 to the right opening communication between a chamber 55 on the right of the piston and a chamber 33 closed off by a second piston 49 integral with the piston 39. Pressure fluid, under control of the distributing valve 8 is then free to flow along the line 12 chambers 55, 53 and line 120 to the increase pitch chamber 13 of the motor 9 or along the line 14 to the decrease pitch chamber 19, the open valves 7, 11 allowing the piston 17 freedom of movement. If the main pressure falls, closure of the valves 7, 11, the latter under the influence of a spring 47, provides an hydraulic lock for the piston 17. While the propeller continues to rotate, centrifugal force maintains the pilot valve 7 in position to cut off the line 16 from the main 10; when the propeller is stationary the spring 32 re-establishes the communication. If it is now desired to vary the pitch of the blades, closure of an electric switch 65 starts up an auxiliary or feathering pump 59 creating a supply of fluid at a pressure of 500 lbs./sq. in. This opens a check valve 67 and flows into the main 10 re-establishing sufficient pressure to move the piston 39 of the pitch lock valve to open that valve and permit movement of the piston 17 of the pitch motor by the pressure available. The variation of the pitch is controlled by a hand lever 112 which moves the fulcrum of a lever 104 pivoted at its remote end to the plunger 88 of the distributing valve 8; this varies the leverage by which a spring 108 opposes the upward thrust of centrifugal force on the plunger. Specifications 544,580, 545,520 and 706,389 are referred to.