Patent GB-713280-A (GB)
Title: Improved control system for variable pitch aircraft propellers
Inventor: TRESEDER ROBERT C
IPC: B64C11/30
Filed: 1952-05-01
Abstract: 713,280. Fluid-pressure servomotor-control systems. GENERAL MOTORS CORPORATION. May 1, 1952, No. 10954/52. Class 135. [Also in Group XXXV] The variable pitch propeller control system of Specification 676,390 is modified to control contrarotating variable pitch propeller units. The propeller unit houses centrifugal / hydraulic upper and lower speed limit governors; an electronic governor, as in Specification 676,390 operates between the upper and lower limits and an overriding manual control is provided. The hydraulic system for pitch control, Fig. 7, is all mounted in the hub, and the outer propeller is arranged to follow the pitch variations of the inboard propeller. Fig. 7 combines the flow-diagram for the inboard propeller, Fig. 4 (not shown) and the outboard propeller, Fig. 5 (not shown), and the electrical connections for both propellers, Fig. 6 (not shown), the same references being used. Inboard propeller system.-Considering the inboard propeller 26, pressure is applied through non-return valves to the HP line 224 (shown as a heavy black line, Fig. 7) by a main pump 220 and an auxiliary pump 222 driven from the propeller shaft and supplemented at low speeds by an electrically driven " feather pump" 494. There are only five electrical slip ring connections to the propeller hub, two and a "common" or shared line being utilized for the blade heaters. The remaining two are normally used for the electric speed control system, but at low speed a snap-action centri ugal switch 202 changes these connections within the hub to the feather-pump motor, enabling the latter to be energized externally from a three-phase supply (with the common line), to supplement the mechanical pumps at slow speeds, or to replace them when it is desired to set the pitch with the propellers stationary (on the ground). A spring- loaded control valve 232 short-circuits the feather-pump when the main pump pressure is adequate. A pressure control unit 294 transmits pressure to (i) the auxiliary-pump control 286 which short-circuits the auxiliary pump 222 when the pressure is adequately maintained by the main pump 220; and (ii) to a relief valve 308 which vents excess pressure from the main pump to the sump. The underside of the valving piston is subject to pressure in the blade servo via a shuttle valve. A hand control lever 38 enables automatic or manually controlled positions to be selected. Automatic control.-With the spring-biased selector valve 434 as shown a solenoid-operated shuttle-valve 172 controls the blade-servomotor 276 in accordance with signals from the electric propeller speed control system, within the limits defined by centrifugally-operated over-speed and underspeed governor valves 238, 242. These governor valves are connected to the pressureline and sump on one side, and on the other, through selector valve 434 to the blade servo whereby they provide an over-riding control at their preset limits. Hand control.-Lever 38 in the manual position actuates selector-control valve 250 admitting pressure to the operating piston of the selectorvalve 434 which disconnects the overspeed and underspeed governors, and connects the bladeangle distributer valve 246 to the blade servomotor 276. The distributer valve is controlled by camming action of a floating lever 422 operated by hand lever 38 and a positional feed-back link 538 from the blade 26. When the pitch decreases below the governed limits the positional feed-back also applies wedges 542, 544 under the pistons of the centrifugal governors to force them to the increase-blade-angleposition. In the " feather" position, the hand control lever switch is operated to energize the pump motor. Outboard propeller system.-The outboard propeller 22 has its own main, auxiliary and feather pumps with associated control valves. Corresponding parts bear the same reference numbers as those for the inboard units, suffixed with a nought. The blade-servomotor 2760 is controlled only by a blade-angle-distributer valve 2460 operated by camming action of a lever 4220 linked to the inboard propeller blade position and to a positional feedback from its own propeller. Construction.-Detailed construction of the hub unit is described with reference to Figs. 8 ... 25 (not shown). Figs. 8 ... 11 and 16 show the arrangement of the pumps around the propeller shaft, and the fluid pressure passages. Details of the electrical ring connections are given in Figs. 13 ... 15 and the blade heater connections at the root of the propeller in Figs. 17 .. 21. The blade servomotor is housed in a bore in the root of the propeller blade and comprises a piston with internally and externally splined skirt, engaging internally with screwed splines and externally with splines in the propeller blade, Fig. 25 (not shown).