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Aviation History
1950
1950 - 1551.PDF
17 August 1950 to a constant-work characteristic, and is suitable for system pressures up to 4,000 lb/sq in. Among the aircraft hydraulic pumps, that produced by British Messier, Ltd., is unusual in its employment of a swashplate for actuation of the seven piston /cylinder assemblies. These last named incorporate uniflow valve action, and a feature of the pump is its high volumetric efficiency at its rated pressure of 4,000 lb/sq in. A hydraulic component of relatively recent origin is the flow-divider, Hie purpose of which, as its name suggests, is to divide the flow in one pipe into two equal flows, irre- spective of variations in back-pressure which may exist in the two outlet pipes. Such a device is of great use in such circuits as those serving flaps and dive brakes, where synchronization is a primary requirement. Before the advent of the flow-divider, sympathetic action of port and starboard services was most usually obtained by means of mechanical interconnection, but with the plethora of equipment carried by the modern aircraft, plus the aggravating circumstances of reduced stowage availability, mechanical interconnection is vastly difficult to arrange, and thus the flow-divider is a most convenient alternative. It does, in addition, permit some considerable weight- saving over a mechanical cross-linkage. The general adoption of nosewheel undercarriages led to a further addition to the family of hyoVaulically actuated 195 services, viz.^ jiosewheel steering. As in most other hydraulic applications, some considerable diversity exists in the designs offered by the various companies. That devised by the Automotive Products Co. is representative of the best modern practice; use is made of a special steering jack of the follow-up type which, in addition to its steering function, provides (by hydraulic means) positive control of shimmy, and serves to limit the torque trans- mitted to the aircraft structure in towing conditions. Steering control is designed to be light and sensitive to the pilot's actions, and with an available power-steering angle of up to 120 deg, the mechanism is arranged auto- matically to centralize when the power is switched off— or in the event of power failure. In this condition, the nosewheel assembly becomes self-centring and castering. The ordinary hydraulic jack has changed little in basic design during the years. It has become somewhat slimmer and lighter, as a result of the increase in operating pressures, and it is now equipped with vastly more efficient glands and sealing media than were its predecessors of even a decade ago. To-day, the inherent hydraulic lock, by means of trap- ping the oil, is scarcely ever relied upon. The great in- crease in loadings, the pressure loss if a pipe is fractured or a gland leaks, the necessity for pressure relief as a result of thermal expansion, the complicated geometry which is often unavoidable in, for example, the retraction of an undercarriage—all these factors militate against reliance on the hydraulic lock. As a result, positive latch- locking is most often used in connection with under- carriages, and, often enough, in other applications as well. A system which, however, offers many attractions is the collet-type internal lock, which was devised by the French Messier company. This is now used by British Messier, Ltd., by Electro-Hydraulics, Ltd., and, in variations on the original theme, by some other companies. The com- ponent parts of the collet lock as fitted in an Electro- Hydraulics jack are shown in the accompanying illustra- tion, together with a schematic diagram defining the operation of the lock. The lock tongue, or collet, is attached directly to the jack piston, and,.the lock sleeve is an integral shoulder formed in the iack cylinder bore. The lock bolt is a stepped diaphragm, bearing against a coil spring abutting the end-wall of the jack cylinder. When oil pressure is directed to the underside of the piston (and the oil volume above the piston is free to return to reservoir) the piston The Dowty flap lever-switch (left) and drum-switch (right) complement each other. When the pilot makes a selection on the lever switch,current is passed through the drum-switch to energize the selector unit of the flap circuit ; and as the drum-switch is mechanically linked to the flaps its contacts are broken when the latter have reached the desired setting, so de-energizing the selector and closing the circuit. CONTACTS CORRESPONDING TO LEVER SWITCH CONNECTION
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