Patent GB-517696-A (GB)
Title: Improvements in and relating to variable pitch airscrews
IPC: B64C11/42
Filed: 1938-08-04
Abstract: 517,696. Screw propellers. MILNER, H. L., and ROTOL AIR SCREWS, Ltd. Aug. 4, 1938. Nos. 23124 and 32108. [Class 114] In a variable - pitch propeller with a reciprocating hydraulic motor for varying the pitch, the motor is prevented normally by a valve control of a fluid conduit from moving the blades beyond a predetermined setting and means is provided for by-passing the valve so that the motor is made responsive to a pressure higher than that normally employed for the motor. The predetermined setting may be a coarse pitch beyond which the blades are feathered, a fine pitch beyond which the blades are moved reversed or a " flying pitch beyond which the blades are moved to fine-pitch position. As shown, a hollow shaft 12 carries a piston 23 in a longitudinally movable cylinder 18 attached by a link 16 to a frame 15. Movement of the cylinder is transmitted to a block 14 through which passes a pin 13 on the blade root 11. The piston rotates with the propeller but does not move longitudinally. A spring 22 in a cup-shaped housing assists movement to coarse pitch position and resists centrifugal force tending to turn blades to fine pitch position. The shaft 12 is formed with conduits 27 leading directly to the front of the cylinder and conduits 25, 26 leading to the rear of the cylinder through respective automatic valves 28, 30. A radial port 29 may bring the rear space of the cylinder in communication with the conduit 26. A stationary bush 35 surrounds the shaft 12 and has annular passages 36 37 connected to corresponding pipes 38, 39 leading to ports 40, 41 in a casing 42 containing a piston valve 55. The latter is pressed in one direction by a spring 63 and in the other by the action of rotating pivoted weights 61. Hands 70, 71 on the valve 55 control communication between a pressure port 72 and one of the ports 40, 41, and between the other port and drain-ports 73. If the pilot moves a lever 68 to lower the speed, the valve 55 is moved to the left by the centrifugal device and pressure liquid passes from the port 72 and pipe 38 to the conduit 27 and the front of the cylinder. The liquid from the rear passes along the conduit 26 and pipe 39 to the drain port 73. The consequent movement of the cylinder increases the propeller, pitch and lowers the speed. A pump 76 which lubricates the engine, also delivers oil by a branch pipe 75 to a pump 74, which raises the pressure for operating the cylinder 18. A relief valve 78 and piston valve 87 control respectively the pressure and the liquid supply to the valve casing 42. The valve 87 is in a casing 86 connected at one end to a pipe 91 leading to a relief valve 94 and to an auxiliary pump 92 driven by an electric motor 93. When the pump 92 is working the piston valve 87 is operated by pressure from the pipe 91 and closes connection between the pump 74 and its delivery pipe 81, higher pressure liquid then passing along a branch pipe 98 to the pressure port 72. A bleed hole 100, formed in the. piston 19, gradually equalizes pressure on both sides after adjustment, thereby stabilizing at the adjusted position. Movement of the lever 68 to the position 68c, brings the propeller to feathered position. The pipes 38, 81 connect up with the front of the cylinder 18. Thereupon, the electric motor 93 is switched on, and oil is supplied at high pressure to the pipe 91 and thereby to the front end of the cylinder. The valve 30 is forced back and allows oil trapped in the rear of the cylinder to pass along the conduit 26. For bringing back the propeller to working range, the lever is moved to a position between 68a and 68b and the electric motor 93 is switched on to move the cylinder. In a modification, the blades may be moved into reversed position or to a position where the pitch is adjusted beyond the fine-pitch end of range. Specification 517,372 is referred to.