Patent GB-775780-A (GB)
Title: Improvements in or relating to variable pitch airscrews
Inventor: GODDEN THOMAS EDWARD
IPC: B64C11/40
Filed: 1954-09-24
Abstract: 775,780. Variable pitch propellers. ROTOL, Ltd. Sept. 19, 1955 [Sept. 24, 1954], No. 27716/54. Class 114. In a variable pitch airscrew having a pitch change motor comprising two elements relatively movable by hydraulic pressure, a pitch lock mechanism is normally maintained inoperative, at least during controlled pitch fining movements, by fluid pressure dependent always on predetermined minimum pressure on the fine pitch side of the motor so that the lock becomes operative when the fluid pressure falls below the predetermined minimum pressure. An hydraulic ram 12 effecting pitch change and shown in the feathering position carries a forward extension 19 with a V-notched inner surface disposed with clearance about a fixed barrel 15 on which the ram slides. The forwardly facing sides of the notches are steeper than the rearwardly facing ones. Within the barrel and projecting forwardly from it, a movable sleeve 67 carries a ring 83 supporting spring fingers 16 having stops 17 at their free ends to limit movement of the ram 12 while they are on a step 61 of a stepped ring 60 supported by an annular member 58. This member is solid with a piston 56 operating in the forward portion of a generally cylindrical member 64 secured to the cylinder 13 in which the ram 12 moves. A spring 63 between the stepped ring 60 and a flange 66 on the sleeve 67 urges them apart. A conical core 31 in the barrel 15 has a hollow axial extension to the nose of the hub, the extension having a bore 32 housing at its forward end a tube 34 surrounded by a spring 44 which opposes forward movement of a fluted valve member 40 opposite to a second fluted valve member 41 urged forwards by a spring 45 weaker than the spring 44. Integral with the cone 31 is a radial flange 46 having an axial skirt 47 forming the working chamber for a piston 48 the skirt 49 of which engages the annular member 58. The hub portion 11 has a central passage 21 for coarse pitch fluid surrounded by an annular passage 24 for fine pitch fluid, an outer passage 30 acting as a drain. To turn the blades to fine pitch, fluid pumped along the annular passage 24 flows through a chamber 26 and passages 27 to a port 29 leading to the space behind the ram 12. Simultaneously it. flows through a passage 71 to a space between the valves 40, 41 displacing the latter to permit fluid to flow through passages 72 to a chamber 73 behind the piston 48 moving the piston forward to displace the annular member 58 until a radial projection 59 on the piston engages a step 67a in the sleeve 67. The movement displaces the stepped ring 60 from beneath the stops 17 on the spring fingers 16 which move inwardly giving the ram 12 and sleeve 19 free movement until projections 17a on the stops 17 engage the last of the V-grooves in the sleeve 19. Should the fluid pressure fail, the piston 48, 49 becomes inoperative and the spring 63 moves the stepped ring 60 to force the stops 17 into the path of the ram 12. On landing, a higher pressure is applied to the fine pitch fluid which overcomes the pressure of the spring 45 and feeds fluid to the front of the piston 56 which moves the stepped ring rearwardly whereupon inward movement of the stops 17 permits the ram to move beyond them. In the reverse direction, the space behind the ram 12 is opened to drain, and coarse pitch fluid pumped along the central passage 21, through passages 22 and annular chamber 23 about the core 31 to spaces between the fingers 16 and through a gap 18 in the barrel 15 accommodating the stops 17 to the space in front of the ram 12. From there it flows through radial grooves 81 in the front of the cylindrical member 64 to the front of the piston 56 and thence through passages 83 to the tube 34 and so past flutings on the valve 40, ports 37, 54 to a chamber 52 whence ports 52a and 57 lead to the back of the piston 56. The projections 17a on the stops 17 being engaged in the grooves of the sleeve 19, the fingers 16 and sleeve 67 move with the ram compressing the spring 63. When the stops 17 move off the step 61 of the ring 60, the spring 63 snaps the projections 17a on the stops over one of the ridges in the sleeve 19 and the stops ride up on to the step 61 again, this " ratchet " action continuing until the stops are clear of the sleeve 19. In a modification the slope of the V-notches is reversed and a single valve replaces the valves 40, 41.