GB615757A - Improvements relating to variable pitch propellers - Google Patents

Abstract

615,757. Fluid-pressure servomotor-control systems. GENERAL MOTORS CORPORA. TION. July 10, 1946, No. 20558. Convention date, Aug. 30, 1945. [Class 135] [Also in Group XXXIII] The pitch of the blades of a variable-pitch propeller is varied by hydraulic pistons in the blade roots, moving on helical splines, the pressure of the liquid being normally under the control of a centrifugal governor. Means are provided for blade control independent of the governor when engine starting, idling, when " braking " the aircraft by the propeller, when taxi-ing, and when the governor fails, to avoid exceeding a maximum pitch, and for " feathering." The adjustment of the governor valve 315 is made by hand (lever 500, Fig. 40, lever 152, Fig. 1), this controlling the pressure applied to the piston 378 by pump 165. Movement of this piston moves cam 4061 through differential 392<1>, 394<1>, 395<1> to open distributer valve 109 and allow fluid from pump 168 to move pistons P in the blade roots. These pistons reset valve 190 through the differential. For engine starting, when fluid pressure is low, movement of piston 378 by the spring 386 is limited by the cam surface 381a contacting roller 357 so that a minimum pitch is maintained. This limit also operates when the engine is idling. For braking, roller 357 is moved to contact cam surface 381b allowing cam 406<1> to move up and admitting pressure fluid to reduce blade pitch to negative values. The simultaneous movement of roller 325 puts the governor 315 out of action. A similar position of the control is used during taxi-ing. For feathering, control 500 is moved to make roller 357 contact cam 381c, thus causing valve 190 to move to the right. Simultaneous movement of roller 325 puts the governor valve 315 out of action, the cam surface 381c acting to move the piston 378 instead of fluid pressure. Valves 432, 456 are provided to allow the accumulator 59 to complete the feathering movement after the pressure from pump 168 has fallen below a given minimum. Flow control valve unit 240 operates to bring into play the output of the two auxiliary pumps 166, 167 when the pitch movement required is large, the fall of pressure in the main pipe 239 being great. Specification 545,520, [Group XXXIII], is referred to.