Patent GB-817207-A (GB)
Title: Gas turbine aircraft engine coupled to drive a variable pitch propeller
IPC: B64C11/34
Filed: 1958-05-21
Abstract: 817,207. Controlling variable pitch airscrews. GENERAL MOTORS CORPORATION. May 21, 1958 [May 28, 1957], No. 16304/58. Class 4. [Also in Group XXIV] In a multi-engine aircraft each engine E drives a variable-pitch propeller P through torque-responsive epicyclic reduction gearing G comprising a driving sun 20, a driven planet carrier 22 and a rotatable annular reaction gear 26 which during normal flight is held by pairs of springs 44 centrally between two stop positions determined by loose splines 28. If the torque passing from the engine to the propeller, or vice versa, exceeds that exerted by the springs, the gear 26 is rotated towards one or other of its stop positions and shifts a push rod 64 controlling a link 66 which varies the pitch of the propeller blades by electrical, mechanical, hydraulic or pneumatic means (not shown). The rod 64 controls the pitchchanging link 66 in two ways-firstly by mechanical means effective should the propeller windmill during normal flight, the rod 64 then causing adjustment of the pitch to limit the drag on the aircraft to predetermined value; and secondly by electro-magnetic means, including a switch 76, effective should the engine fail during take-off, the rod 64 then automatically causing feathering of the propeller and cutting off of the fuel supply. Limiting drag during flight. In the event of the propeller windmilling during normal flight or landing-due, for example, to air gusts, fuel interruption, or partial power loss- and causing more than a predetermined drag, the negative torque imposed on the epicyclic gearing causes the springs 44 to yield, and the gear 26 is rotated from its normal central position towards its anticlockwise stop. This causes a projection 116 on the rod 64 to engage a lever 118 and shift the link 66 to increase the propeller pitch and hence reduce the drag until the springs 44 can return the gear 26 to its central position. The value of the drag permitted during windmilling is thus determined by the springs 44. Means not shown may be provided to disarm the mechanism and allow reverse thrust from the propeller, such as after landing. Automatic feathering during take-off. During take-off, when maximum power is being transmitted, the gear 26 is normally moved clockwise to the position shown by the large positive torque imposed on the gearing. The automatic feathering mechanism is rendered effective by closing a manual arming switch 88 and moving a fuel supply lever 92 to its maximum power or take-off position so as to close a switch 90. Should the engine fail during the take-off the springs 44 return the gear 26 to its central position and thus cause a cam 74 on the rod 64 to close the switch 76 and energises a solenoid 100 which moves the link 66 to a fullyfeathered position. Simultaneously a solenoid 108 is actuated to cut off the fuel supply to the engine, and a hold-down solenoid 104 associated with a battery 114, is energised to maintain the energisation of the other solenoids and thus maintain the propeller feathered even if the switch 76 is opened. Three normallyclosed switches 84, one for each of the other engines, are provided to ensure that if any of these engines have previously failed the automatic feather circuit shown is inoperative. Irrespective of the position of the switches 84, however, the propeller P may be manually feathered by depressing a button 120. The switch 88 is normally opened after completing the take-off to prevent any subsequent automatic feathering. Each pair of centring springs 44 is mounted on a rod 38 movable in a fixed cylindrical housing 39, the rod 38 being coupled to a tooth 34 on the gear 26 by means of a pair of lands 40. Each spring 44 presses apart a pair of spring seats loosely mounted on the rod 38 and engaging an inner abutment and an outer snap ring on the rod. The movement of the spring seats is limited by an inner fixed abutment 50 and an outer snap ring 52 in the housing 39. For convenience Fig. 1 shows only one pair of springs 44 mounted externally of the gear 26. In the actual construction, Figs. 2, 4 (not shown), the gear 26 had a number of teeth 34 which project inwardly of the gear and co-operate with a number of pairs of springs 44 mounted inwardly of the gear. One or more of the springs may be removed to decrease the torque to which the gear responds, and if it is desired to make the negative torque response different from the positive torque response one or more the spring seats may be removed.