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Aviation History
1954
1954 - 1666.PDF
4 June 1954 745 WHERE WEIGHT IS WANTED Recent Developments in G.E.C. Heavy Alloy: Aircraft Applications INCREASING demand for G.E.C. Heavy Alloy has recently dictated the transfer of its manufacture from a laboratory pilot plant to full-scale production in the Osram-G.E.C. metals factory at Wembley. Improved tung sten supplies have made it possible to lift restrictions on the use of Heavy Alloy (of which tungsten is the major con stituent), and a new series of alloys are now being developed with densities exceeding 18 g/c.c. and thus approaching the theoretical density of tungsten. These new alloys are fully machinable and show mechanical properties comparable with those of the existing Heavy Alloy. G.E.C. are also develop ing a high-density alloy characterized by higher elongation without corresponding decrease in tensile strength; it will be available in the form of sheet of limited dimensions. Briefly, the production of G.E.C. Heavy Alloy calls initially for the use of tungsten, nickel and other constituent powders to stringent specifications for chemical and physical properties. These powders are mechanically mixed, placed in a die and compressed. Heating of the compact at a carefully controlled low temperature then brings it to what is known as the "green" state, in which it can be given further shaping to the final contours required. It is finally placed in an electric furnace in a hydrogen atmosphere and sintered at white heat, approximately 1,450 deg C. During this process the compact shrinks by nearly 20 per cent in volume, with a corresponding increase in density of from 10-12 g/c.c. to the final density required. Since this decrease in (Left) The flywheel of the Dunlop Maxaret brake, machined from G.E.C. Heavy Alloy. (Above) Sperry electric gyro-horizon assembly, with squirrel- cage rotor Araldite-tonded to interior of Heavy Alloy cup. volume is closely calculable the original dies can be designed with precision for many shapes or sizes of sintered product. G.E.C. Heavy Alloy is resistant to atmospheric corrosion, but it can be given any of die usual plated finishes if required. Aver age properties of material having a density of from 16.5 to 17 are quoted as follows (tested in accordance with BA.19 : 1950): — Specific gravity* ... ... ... ... 16.5 to 17 Approximate weight ... ... ... 0.6 lb/cu in Tensile strength 42 tons/sq in Yield point 38 tons/sq in Brinell hardness number 290 Coefficient of expansion at 20-420 deg C ... 5.6 X 10-6 •The specific gravity of lead is 11.4. G.E.C. Heavy Alloy is finding increased applications in many fields, notably that of radiology. In aircraft construction, of course, it has been known for many years, its principal use being for the mass-balancing of control surfaces. In most situations the compacmess of the material makes it possible for the mass to be completely contained within the structure; but where it has to be an exterior fitting it offers the minimum amount of frontal resist ance in comparison with other materials. This application, and that for dynamic balancing in aero engines, is a familiar one. Latterly, however, it has found a number of new uses in specialized aeronautical equipment. For instance, what might be termed the "heart" of the Dunlop Maxaret braking system is the flywheel which monitors the pressure supply. In many undercarriages, particularly of the bogie type, the space available for the flywheel is severely limited, so it is not surprising that Dunlop Aviation Division use the G.E.C. material. They obtain it in the form of a blank made to close limits, machine it all over, grind a ball race in it and machine away the thin central A compact before (right) and after sintering, show ing the volume reduction which takes place. web to leave a single diametral spoke. The alloy is amenable to all these operations and is machined to a tolerance of 0.003in. It is sufficiently hard to form a race for 3/32in dia. steel balls on each side of the central web. Rotational speeds may reach 8,000 r.p.m. Instrumentation is another sphere in which the G.E.C. material finds a use. The rotating assembly of the Sperry electric gyro- horizon, for example, is a squirrel-cage rotor which, on a vertical axis, spins around the stator windings; and the cage is housed in a cup of Heavy Alloy, to which it is bonded with Araldite. The weight is thus concentrated as near the periphery as possible, pro viding maximum inertia. The rotor must undergo a speed test at 30,000 r.p.m., so high tensile-strength is essential. The material finds a similar use—as a tyre on a rotor—in Smith's air-driven artificial horizons and gyro direction-indicators. Normal operating speeds of the rotors are 15,000 r.p.m. and 18,000 r.p.m. respectively. Dynamic balancing is carried out on the rotors by drilling out portions of the Heavy Alloy as required; balance of the order of 1 mg/cm is obtained in this way. The alloy finds another application in a g-restrictor developed by British Messier, Ltd. It is used for the 2 lb weight which, acting against a spring, closes a hydraulic valve and operates a jack that produces a control-force in opposition to that exerted by the pilot. As the size of the chamber containing the weight is a critical factor in the design it was important to employ as dense a material as possible; and lead would have been subject to electrolytic action with the hydraulic fluid. Vinten cameras are well known in aeronautical test and research work, and the H.S.300 high-speed model can operate at no less than 275 frames a second. In these circumstances, perfect balance of the motion is essential for a steady picture; G.E.C. Heavy Alloy, as the only material to give sufficient weight in the available space, is employed for the crankshaft counterweights. TANK Hydraulic circuit of the British Messier g-restrictor for reducing excessive vertical accelerations by automatically loading the control column in the opposite sense. A critical vertical acceleration causes the 2 lb Heavy Alloy weight (at left) to compress the spring on which it rests and thus close a valve to divert hydraulic pressure to the loading jack. AEROMAGNETIC SURVEY OF BRITAIN I N a statement in the House of Commons on May 25th Mr. Bevins, Parliamentary Secretary, Ministry of Works, said it was hoped to make an aeromagnetic survey of the British Isles. At present the survey was only in the project stage and Government financial support was still awaited; about £^m would be required. A preliminary survey is to be made during the next year or 18 months in order to discover the degree of sensitivity and accuracy which would be required in the readings obtained. No decision has yet been made as to which of the available instruments or techniques is to be used. The project is under control of the Department of Scientific and Industrial Research.
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