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Aviation History
1950
1950 - 1546.PDF
192 FLIGHT, 17 August 1950 PRESSURE IN MAN LINE 5O H>/sq.m. One of the fundamental principles of hydraulics is that load equals piston area multiplied by pressure and, therefore, by the simple expedient of varying piston area, it is possible to move a number of dissimilar loads by a constant input pressure. HYDRAULICS . . . the first aircraft in Europe (and, possibly, in the world) to be equipped with a hydraulicaUy retracted under- carriage. As the biplane gave way to the monoplane and all-metal construction became more general, it was natural that loadings should increase and, with them, speeds and powers. But the advance in performance brought with it the necessity for such things as retractable under- carriages, flaps, cooling gills, radiator shutters arid variable- pitch airscrews, and the advent of these several elements emphasized the necessity for some simple, efficient means of operating them: such a means was offered by hydraulics. As time went on, and aircraft began to take on a shape not much different from that we know to-day, the demands of war brought additional concomitant requirements for the actuation of bomb-doors, gun-turrets and dive-brakes on military aircraft, plus wing-folding and arrester-hook snubbing on naval types. That services which are hydraulically actuated are, in general, moved by a jack which is fed with oil from a pump is common knowledge ; but the detail of the hydraulic system is, perhaps, not so widely known. In order to rectify this, it might be as well briefly to run over the essentials of a typical modern system as applied, for example, to a multi-engined bomber. Although at first sight a complex thing, this is basically quite simple. The bare essentials for any hydraulic system are a reservoir of oil, a pump, selector valves, suitable pipe-lines and jacks to operate the individual services. These are primary requirements and although there is, in practice, a host of subsidiary units involved, to give a comprehensive description of all the components which are to be found in most aircraft hydraulic systems would need a sizeable volume, and as their purpose is more often than not clearly signified by their titles, no attempt will be made in this survey to cover anything other than basic principles and some of the later developments. We have referred to the fact that a single input force is capable of moving a variety of loads, and this is achieved by so arranging that the area of the jack piston measured in square inches, multiplied by the load to be moved in pounds, is equivalent to the pressure exerted through the fluid in pounds per square inch. Thus, if the load in actuating service A is, say, six times greater than that in actuating service B, the jack piston area will need to be six times greater for A than for B. Speed of operation is also dependent upon piston area, the smaller the area (for a given flow rate) the greater the speed of operation. In an accompanying illustration is shown a simplified diagrammatic layout for a modern bomber's hydraulic system ; this serves to show the relative disposition and interconnection between the several basic units but, it should be emphasized, it is not intended to represent an actual installation. It is normal practice with multi- engined types to employ two pumps, each driven from a separate engine. In most systems, irrespective of size, a hand-pump is also embodied to permit emergency actuation in the event of engine failure and, in addition, to enable routine ground-testing to be carried out without the neces- sity of running the engine(s); alternatively, ground test- rigs can be coupled into the system through the ground test connections. Both pumps jointly derive oil from the reservoir (which, nowadays, is often pressurized for high-altitude operation) and deliver it via non-return valves and a filter to the automatic cut-out. This unit functions as a type of hydraulic clutch ; since the pumps are delivering oil the whole time that the engines are running—despite the fact that no service may be in operation—some form of de- clutching effect must be provided. When no service is being actuated, the automatic cut-out diverts the pump delivery back to reservoir, so forming an idling circuit ; but immediately a service is selected, the cut-out reacts and delivers the full flow from the pumps into the main pressure line, from which the various service control units are fed. If pumps of variable-delivery, or automatic off- loading, type are used the automatic cut-out is not neces- sary since its function is embodied in the pump itself. The whole of the system is completely filled with oil— any condition of aero-embolism is rigorously eliminated by bleeding—and when a selection is made, and pressure forces a piston along its jack cylinder, the oil on the opposite side of the piston is expelled back through the selector unit into the return line to reservoir. As may be seen from the diagram, the main pressure and return lines cater for flow in one direction only, as do the branch pipes to and from the selector units, whereas the pipes between the jacks and their selector units carry flow in either direc- tion according to whether the jacks are being extended or contracted. The hydraulic accumulator which is connected into the main pressure line from the automatic cut-out is incor- porated for two purposes, (i) it accommodates some of the oil which is fed to the system at a high rate and would otherwise operate the smaller jacks too quickly and, (ii) it serves as a shock-absorber ; for example, were no accumu- lator fitted, the sudden reaction of the automatic cut-out and the resulting immediate large-volume delivery of oil would impose a shock load on the system. The best- known form of hydraulic accumulator consists essentially of a cylinder divided into two chambers by a free-floating piston, one chamber containing compressed air and the The British Messier electro^felector embodies a unit motor which, througka leadscrew and cart! system, operates a pilot valve ; this provides a servo-action used, in turn, to operate the main valves.
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