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Aviation History
1954
1954 - 0966.PDF
434 FLIGHT A production Avon RAJ on the test stand at the works of the Standard Motor Company, Banner Lane, Coventry. The engine is exhausting into a Detuner silencer. The Quest for Power ... power from a given size of engine and, at the same time, improved combustion efficiency. The split or two-spool compressor opens the way to increased output and efficiency through high pressure-ratio, and is altogether more flexible in its characteristics in that the two sets of rotating assemblies match themselves automatically when running. Al though of increased mechanical complexity, such engines are in many ways easier to develop. As an example of this, it is evident that a two-spool engine can achieve a given high pressure-ratio with not much more than half the number of axial com pressor stages that are needed by a single-shaft engine, and reduction in engine length is perhaps as important as reduction in diameter. But more than the usual number of bearings is needed. Bearings, invariably of the ball or roller variety, may be obtainable "off the shelf." With modern speeds and loads, however, it is likely that the engine manufacturer will have to supply the bearing company (more than one as a rule) with data from which suitable bearings can be developed. In a short time, the parts begin to come through for the first run of experimental engines. As soon as possible, one set is married up and the first prototype engine assembled and inspected. From now on, the engineers really have something tangible to work with, and it is largely a matter of making and breaking engines until the job is done. The engine is mounted on a static test stand in a suitable cell—several cells are shown in accompanying pictures—and all connections for fuel, oil, air and electric power are made. The engine is then slowly motored over and, if all goes well, is run up to self-sustaining speed and the fuel turned on and ignited. Running is initially kept to low speeds with an oversize jet nozzle to maintain internal pressures and temperatures within moderate limits. After a short time, possibly only a few minutes to allow readings to be taken, the speed will be increased slightly. Succeeding runs will be of longer duration and at higher speeds until, if no trouble is met, full r.p.m. will be achieved. By this time, which may be a week or several months from the first run, a standard tailpipe will have been fitted and the engine will then be giving its design thrust. But all kinds of trouble may have been met (although it is only extremely rarely that anything catastrophic occurs, for the mechanical properties of the engine should have been fully resolved long before this). At the first sign of trouble, or probably before, the engine will be "torn down" and subjected to careful scrutiny, initially visually or with gauges and micrometers, and later with X-rays, magnetic/fluid or fluorescent equipment, microscopes and any other inspection tools which the company may employ. Much of the re-assembled engine—or more likely the second engine by this time— will be instrumented so that its behaviour can be closely watched. Dozens of thermo couples will be installed to measure bearing or gas temperatures, pitot traverses will indicate airflow, and strain-gauges will record mechanical behaviour, even on such inaccessible parts as the hot, high-speed turbine blading. The fully instrumented engine is then run under every condition that the establishment can reproduce. The results obtained will be the first to give a true picture of the engine, for it is impossible to produce accurate conditions on a test rig. A long period of performance testing then begins, largely in runs of about two hours' duration. With luck, the design performance may be obtained straight off, but ENG.129 pump, used on the Sap phire, has been retained for the American version of the engine, and is licence-produced by Thompson Products, Inc. Drilube Co., 723 West Broadway, Glendale 4, California, market a new film lubricant "Drilube" for use where bearing pressures or tempera tures are very high. It is a dry, flexible solid surface, bonded to almost any suitable prepared metal. Capable of use at 500 deg F, or 700 for short periods, it is employed in various "missile engines." Dunlop Rubber Co., Ltd., Aviation Division, Foleshill, Coventry, manu facture a wide range of engine-acces sory equipment. Electromatic valves and rams for piston-engine tempera ture control are well known, and operate at up to 3,500 p.s.i. For turbojets, Dunlop are important as suppliers of complete pneumatic actuating systems for afterburner eyelids, the Avon RA.7R, for example, having two Dunlop rams to each eyelid, with reducing valve, filter and two-piston, solenoid-oper ated valve. All these units can operate at 400 deg C. A wide variety of fuel-system flexible pipes is another Dunlop speciality, as is the provision of electrically heated de- icing equipment for engine intakes. Such de-icers are fitted to the Gannet and Viscount and, with an ice-detec tor, can be made quite automatic in operation, with cyclic or continuous heating. Dunlop are also marketing a number of grades of silicone rub ber, most of which have been speci ally developed to meet the needs of the aircraft industry. E.N.V. Engineering, Ltd., Hythe Road, Willesden, London, N.W.10, have precision gears and other specialized components in all types of piston and turbine power unit. English Steel Corporation, Ltd., Sheffield, produce various gas- turbine steels with high temperature- and creep-resistance. From these steels, E.S.C. drep-forge turbine discs and shafts, which might be considered the most vital parts of any aircraft. Equator Wheels and Sections, Ltd., Raglan Street, Wolverhampton, have turned out many thousands of stain less-steel stiffening bands for gas- turbine jet pipes. Rolls-Royce are among their principal customers. Esso Petroleum Co., Ltd., 36 Queen Anne's Gate, London, S.W.I. At first glance the difficulties facing the producer of modern aero-engine lubricants may not be appreciated. Trie oil must remain pumpable at very low temperatures so that Arctic or high-altitude starts can be readily accomplished. It must not break down under enormous bearing loads —particularly in turboprop gear- teeth—and it must retain excellent properties under extremely high temperature conditions, and not oxidize or boil away. The whole set of requirements is so formidable that only one oil is currently cleared for use with the latest type of British gas-turbine—Esso Aviation Turbo Oil 35, which meets D.E.R.D. 2486. A parallel development is Esso A.T.O. 15 for American use. Fel Electric, Ltd., 41 Sidney Street, Sheffield 1, make electro-magnetic crack-detection installations. Firth-Derihon Stampings, Ltd., Tinsley, Sheffield, make drop- forged turbine discs, which go into such important engines as the Armstrong Siddeley Sapphire. Firth-Vickers Stainless Steels, Ltd., Sheffield, supply the valuable 44S ferritic, stainless creep-resisting steel which is used for the turbine discs of the Avon, Sapphire and Olympus. Including fractions of vanadium, molybdenum and nio bium, 448 steel has three times the rupture-strength required by the S.62 specification at 600/630 deg C, and 300-hr life. Thos. Firth and John Brown, Ltd., Sheffield, are major suppliers of alloy and special steels for gas-tur bine production. i i
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