Patent GB-702135-A (GB)
Title: Improvements in full power hydraulic control systems for operating airplane control surfaces
IPC: B64C13/00
Filed: 1950-05-11
Abstract: 702,135. Fluid-pressure servomotor-control systems; tanks. NORTHROP AIRCRAFT, Inc. May 11, 1950 [Jan. 9, 1950], No. 11832/50. Classes 69 (1) and 135 In a hydraulic servomotor control system for the full power operation of aircraft control surfaces having two hydraulic power units associated with each surface for its operation and in which each unit is independently supplied with pressure liquid so that in the event of failure of one unit the surface is capable of operation by the other, each unit comprises a piston working in a cylinder wherein the piston has a predetermined stroke so that it bottoms at either end of its stroke, whereby the operation of the control surface is rendered insensitive to air trapped in the system and such air is expelled from the system, and a control valve which in its neutral inoperative position permits a restricted flow of pressure liquid to and from both sides of the piston. As shown, pumps 9, 9a, Fig. 2, driven by separate aircraft engines receive liquid through pipes 12, 12a from reservoirs 11, 11a respectively and supply it through pipes 7, 7a to hydraulic power units 5, 6, 5a, 6a to control the aircraft ailerons; the pump 9 and its associated parts supplying the outer unit on each aileron and the pump 9a and its associated parts supplying the inner unit on each aileron. Each unit comprises a piston 24, Fig. 4, having a rod 15 fixedly but pivotally connected to the aircraft structure, a cylinder 14 slidable on the piston and pivotally connected to operate the aileron and a control valve 21 of the kind described in Specification 701,513, in which the valve casing 20 is connected to the cylinder 14 to move with it and the valve spool 30 slidable in a ported sleeve 29 fixed to the casing is operable by the pilot through the usual cable and linkage system. When the spool 30 is in its neutral position relative to the sleeve 29, pressure liquid from the valve inlet 35 passes through the partially closed inner metering holes 31 to either side of the piston 24 and from either side of the piston through the partially closed outer metering holes 31 to the valve return port 37 and thence through pipes 8, 8a to the reservoirs 11, 11a. This restricted flow of pressure liquid to and from either side of the piston preloads the piston and prevents control surface loads being transmitted back through the cable and linkage system. To ensure the availability of liquid at the pump inlets and the system being always filled with liquid, the reservoirs 11, 11a are pressurized by supplying them with air at 10-12 p.s.i. gauge pressure through non-return check valves 65, Fig. 2. Baffles 66, Fig. 5, enclosing the inlet and outlet pipes 8, 12 respectively, to and from the reservoirs ensure that liquid is available to the pumps when an inverted flight manoeuvre is carried out. By arranging the piston 24 to bottom on the cylinder at the end of each stroke very little space is available for the trapping of air and any air so trapped is expelled from the system due to the restricted leakage path through the control valve maintaining pressure on both sides of the piston. Check valves 77, Fig. 2, are fitted in the pressure lines 12, 12a adjacent the power units so as to eliminate any movement of the aileron a wind gust might cause when the control valve is open and there is no pressure in the system, e.g. when the aircraft is parked. Relief valves 76 are fitted between the pressure and return lines to maintain the pressure in the system at the desired value, e.g. 3000 p.s.i., in which case the preload pressure acting on the piston is 1500 p.s.i. In the event of failure of both pumps, e.g. due to the failure of the engines driving them, an electrically driven emergency pump 70 is switched on to withdraw liquid from the reservoir 11a to supply the inner power units. Specification 701,504 also is referred to.