GB1103811A - Improvements in or relating to aircraft hydraulic systems - Google Patents

Abstract

1,103,811. Auxiliary power installations in aircraft. PLESSEY-UK Ltd. 23 June, 1965 [22 April, 1964], No. 16665/64. Heading B7G. [Also in Divisions F1 and G3] The invention relates to an aircraft hydraulic system which comprises a fixed-displacement pump which is driven by a ram air turbine, the pump outlet being connected to an hydraulic system which includes an accumulator, the pump being equipped with automatic unloading means which allows liquid from the pump delivery to return freely to a low-pressure point. Control means for the flow of air through the ram air turbine are provided being jointly controlled by a pressure-sensitive element that is acted upon by pump delivery pressure in opposition to the accumulator pressure so as to provide a coarse control, and which is additionally responsive to the hydrodynamic pressure drop in a restriction through which the whole delivery, or a fixed portion of the delivery, of the pump is arranged to pass so as to provide a fine control of air flow through the ram air turbine. In the embodiment shown in Fig. 1, ram air is supplied through duct 3 to drive the turbine rotor 5 which drives the hydraulic system gear pump 13. The pump 13 takes its section from a tank 14 and delivers through line 15, and off-loading valve 21 and non-return valve 17 to the high pressure line 18 of the system which includes an accumulator 19. A low pressure return line 20 leads back to the tank 14. The ram air intake 7 is formed with a spill port 8 which is controlled by a shutter 9 pivotably mounted at 10, movement of the shutter being controlled by a coarse control unit 30 and a fine control unit 34. The coarse control unit 30 comprises a piston and cylinder, hydraulic fluid at pump delivery pressure being supplied through line 31 to the lower side of the piston, and fluid at the pressure of the accumulator 19 being supplied through line 32 to the upper side of the piston. In normal operation the piston is therefore moved to its upper position and the floating lever 29 acts through linkage 28, 27 to move the shutter 9 to the position in which it closes off the spill port 8. The off-loading valve 21 is provided so that when the system pressure in line 18 reaches a certain value the pump is off-loaded. The pressure in line 18 acting at the upper side of the piston 30 then moves the piston downwardly and the shutter is moved to open the spill port 8 and at the same time to restrict the flow area of the duct 3. The fine control 33 comprises a piston 34 in a cylinder 35, Fig. 2, high-pressure fluid from the line 18 being supplied through line 44 under the control of spool valve 39, 40 to either the upper or lower sides of the piston 34. The position of the spool valve itself is controlled by a piston 46 formed integrally therewith. High-pressure fluid from the pump discharge line 15 is supplied through line 37 to the upper side of the piston while lower pressure fluid from the throat of the Venturi 16 in the discharge line 15 is supplied through line 38 to the lower side of the piston 46 which is also acted upon by a spring 48. Thus, whenever the speed of the pump and so the pressure difference at the inlet and throat of the Venturi is greater than a desired value the piston 46 and spool valve 39, 40 move downwardly to allow fluid to pass to the lower side of piston 34 which thus moves upwardly and acting through floating lever 29 causes the shutter 9 to move so as to open the spill port 8 whereby the turbine speed is restored to its desired value. In a further embodiment, Fig. 3, not shown, the piston and cylinder assemblies of the coarse and fine controls are incorporated in a single unit.