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Aviation History
1951
1951 - 0559.PDF
SOLD 345 auto- WIRE matic cleaning devices.X HYDRAULIC SERVICEABILITY , normally opened when the aircraft was in flight and the under-carriage and so on was retracted, and when hydraulic selection was required, the by-pass cock had first to be closed. A variation ofthis system was the so-called "open centre" circuit where the by- pass valve was contained within the main selector, the lever ofwhich returned to a neutral position after completion of a selected operation. Due to the possibility of failure to close the by-passvalve and of the selected operation not taking place, this system fell into disrepute. Automatic systems were introduced about 1938- There had now been a partial return to the earlier system oncertain civil aircraft by the introduction of'an off-loading valve with automatic cut-out so that the pump and system were notdamaged if the pilot forgot to open the valve after take-off. On transport aircraft it was preferable to off-load the system to zeropressure, rather than to leave the pump cutting in and out to main- tain the system at full pressure. Properly supported pipes shouldnot fail if they were not under pressure. The off-loading system, therefore, appeared of considerable benefit. Whether such an off-loading system could be considered fora military aircraft was an interesting point. The authors believed that it might be considered for bomber and transport types of air-craft but that it would not be practicable for fitment to a fighter or other short-range aircraft. It was obviously not applicable to thepart of the hydraulic system which might operate powered flying controls. Another important point in design was to provide for easymaintenance at low temperatures. A simple operation might take five minutes at normal temperature, but 15 or 20 minutes at—30 deg C. Intricate operations such as fitting locking wire might be almost impossible to perform with cold, gloved hands. Regarding hydraulic pumps, it was the author's opinion thattype test requirements were severe. While not suggesting that the present 100-hr type-test schedule should be altered, pump designersshould aim at a minimum life of 1,000 hr for a good hydraulic pump. A type of pump which could be considered of variablevolume was one which achieved this feature by the incorporation as a unit of a constant volume pump and a cut-out. This type ofpump had advantages over the variable-volume kind, but when two or four pumps were used in parallel, the normal type with a singlecut-out might be preferred. The cylindrical piston type of hydraulic accumulator had longbeen used in Great Britain and was now coming into service in the U.S.A., the "bag" or diaphragm type having been superseded onaccount of difficulties with the rubber diaphragm at low tempera- tures. The mounting of accumulators in a vertical position with theair chamber on top was recommended. The reliability of the piston glands could be improved if a small amount of oil wasadded to the top of the accumulator (air chamber), this acting as a lubricant to the air seal. The practice of chromium plating thecylinder bores was one which was believed to be desirable. The practice of fitting hydraulic jacks with internal locks wasnow well established, and the locking devices (of the collet type) were most reliable. The fitting of satisfactory indicators to theselocks was difficult, particularly as the seals involved were of small dimensions. An additional seal would prevent loss of oil fromthe system. (See illustration, right). Hydraulic jacks could be improved by (a) the addition of built-in nitration, and (2>) by hard-chromium-plating the cylinders and piston rods. Micro-switches should be arranged so that wiring required littleor no movement when a jack was removed. A robust adjustment for switch position was essential. Selectors and other types of valve normally relied on ball orpoppet valves, or the cylindrical-slide type of element. The earlier practice of using unguided balls had now been largely abandoned,as this led to indentation of the ball, permitting leakage. Modern practice was to use properly guided poppet valves, or a hardenedsteel ball trapped in a guiding member. Plastic valve elements had been used successfully. The designand manufacture of cylindrical slide-valves for use at the pressures common to-day was not easy. The clearances involved (0.0001 too.ooo2in) created problems of metal stabilization and distortion under pressure. Partial locking and increase of friction occurredwith time, and were aggravated by "micronic" particles of swarf or dirt. To ensure adequate valve-life, endurance testing up to100,000 cycles was now regarded as essential. Material now in use for piping was steel or proprietary productssuch as Tungum; these could be considered reliable. The suit- Improved pipe joints: a suggested internal-cone type, and above It, a flareless type with rubber sealing rings. ability of the material to stand flaring was a critical factor affectingreliability. Improved flaring technique to avoid sharp edges greatly increased reliability. The weight of piping on modern aircraft represented as much as 30 per cent of the total hydraulic system weight, and research into the possibility of using pipes of reduced wall-thickness in lighter materials was important. High-tensile steel tubing, bench- formed and welded at "T" or cross connections, might be used. The authors went on to discuss various details such as the spacing of pipe-supporting clips; pipe connections and flexible hose. The flared pipe connections now used in Britain and the United States were, they considered, satisfactory, but in France compression-oliye-type connections were still used. The British A.G.S. connection was heavy, costly and somewhat difficult to flare, and the adoption in this country of the new unified thread system created an opportunity for a new type of connection (two suggestions are illustrated on this page). It would be of interest to British engineers, said the authors, to know if the American use of tapered pipe threads to replace copper or equivalent washers on adaptors was to be recommended. On the subject of nitration, good modern practice was to provide (a) gauze protecting-filters in the reservoir or in the pump suction- line connection; (b) a very fine filter in the inlet connection to the automatic cut-out or the connection between the pump and the selectors; (c) gauze protective filters in the pipelines; (d) built-in filtration in components such as jacks. Components having small orifices, such as restriction valves, should be fitted with built-in filters or alternatively be provided with automatic cleaning devices of some simple type (see illustration, top left). The authors did not foresee much development in hydraulic fluids except in the introduction of some non-inflammable fluid or one with improved low-temperature performance. Under the heading "Development Technique" the authors stated that modern practice in the development of an individual hydraulic component was to make many tests of the unit per- formance, culminating in an official series of tests in order to obtain the necessary official flight approval and so-called design clearance. Tests on a typical hydraulic selector, for example, would include the following: (a) pressure testing; (b) leakage testing; (c) endurance testing—operating, say, 100,000 times Details of hydraulic-jack construction, with special features annotated. SECONDARY SEAL OPERATEIF PRIMARY SEAL FAILS BUILT-IN FILTRATION ONE-WAY BREATHERTO PREVENT ENTRY OF MOISTURE
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