GB790226A - Improvements in or relating to variable pitch airscrews - Google Patents

Abstract

790,226. Variable pitch airscrews. ROTOL, Ltd. Dec. 6, 1955 [Dec. 30, 1954], No. 37700/54. Class 114. In a hydraulic variable pitch propeller comprising pitch change mechanism, a fine pitch stop, hydraulic means for retracting the stop, and means for initiating a coarsening of the pitch, a first detecting means detects conditions, e.g., touching down under which the stop should be in its operative position and a second detecting means for detecting when the propeller has fined off to a predetermined pitch setting which is finer than that set by the stop, the two means cooperating to operate the pitch coarsening means when the second detecting means becomes operative. The propeller hub 11 supports a cylinder 16 housing a ram 15 movement of which under hydraulic pressure alters the pitch of propeller blades 12. The ram moves forwards to fine the pitch until, normally, the chamfered end 30 of an annulus 29 secured to the ram engages fine pitch stops 19 on resilient fingers 20, the stops projecting through openings in a stationary sleeve 18 on which the ram moves. Normally the stops are held in their operative position by the wider part of a stepped ring 21 carried by a piston 22 urged forwardly by a spring 26. A narrower part 21b of the stepped ring 21 permits the stops 19 to move out of the way of the chamfered end 30 but to engage the front edge of a ring 28 within and behind the annulus 29 to provide a superfine pitch for starting or for touching down. Pressure fluid to operate the ram 15 is controlled by a valve 69, Fig. 3, actuated by a centrifugal governor 76 against the pressure of a spring 86. Increased speed raises the valve to draw a land 70 thereon away from a passage 31, leading to the cylinder 16 in front of the ram 15, and thereby open the passage to an inlet 103 supplied with pressure fluid by a constant speed pump 97 : reduced speed lowers the valve keeping the passage 31 isolated and opening up a passage 140 to the rear of the ram 15 to fine the pitch. The loading on the spring 86 determines the propeller speed and is adjusted manually through a gear wheel 85 moving a cylinder 82 against one end of which the spring abuts. The valve 69 engages with lost motion a stepped cylindrical member 89 carrying a stepped piston 90. When it is desired to coarsen the pitch of the blades, the solenoid 111 of a solenoid valve 105 is excited and moves a double valve 112 opening a passage 104, always open to the delivery of the pump 97 to a passage 118 leading to the underside of the piston 90. The piston rises, lifting the valve 69 and opening the passage 31 to the pump delivery. When a superfine pitch is desired when airborne, engine speed is adjusted to move the blades to the fine pitch stop setting, the chamfer 30 engaging firmly the stops 19. Simultaneously a cam 51 on the blade root rocks a lever 52 to bridge a pair of slip rings 56, 57 and excite the solenoid 111, preventing further fining movement. If the stops do not function, the blade pitch oscillates about the fine stop position as the cam 51 moves on and off the lever 52. On the ground, either for braking or taking off, the stops 19 are removed from the path of the annulus 29 by movement of the piston 22. Switches closed by the compression of the undercarriage, both main and nose wheel, excite relays which open the circuit of the solenoid valve 105 and close a circuit for a similar valve 107 supplying pressure fluid through a passage 108 to the end of a valve 109 to move a piston 145 which closes a valve 149 and opens another 150 permitting pressure fluid to flow from the high pressure passage 104 through passages 106, 108 to the chamber 147 housing the valve 150 and thence through a passage 42 to the chamber in front of the piston 22 where pressure builds up until it can overcome the spring 26. The solenoid valve 107 cannot be actuated unless the engine throttle is set for idling.