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Aviation History
1950
1950 - 1933.PDF
428 FLIGHT, 16 November 1950 TWO-EDGED BLADES . . . be planned for a still higher figure to allow for replacements and spares and also, in view of the high standards of pre- cision required, a reasonable percentage of rejects. The magnitude of these production requirements is more than a little disturbing.. It cannot be blandly put aside by talk of shadow factories, Ministerial backing, priorities, drafted labour and pooling of resources. These and similar agencies and influences can achieve remarkable results; but they cannot produce miracles. Of course, compressor rotor blades of light alloy are simpler to produce than the turbine rotor blades of special heat-resisting alloys. Nevertheless, such accuracy of size and form is required that they present no easy method of production. Any attempt to standardize or compromise on the sections in order to facilitate pro- duction is likely to defeat its ends as the blades will be relatively less efficient and more stages will be required to achieve the same effect. Stator blades for the compressor are less highly stressed and can be precision-cast by a variety of methods. All will call for extensive tooling and equip- ment to produce the large quantities required. Turbine nozzle blades in nickel-chrome alloys can be cast by the lost-wax method and possibly in groups of three or four blades instead of individually. The production of the highly stressed turbine rotor blades cannot, by present methods, be materially speeded up and output can be increased only by extending the facilities. If machining is to be eliminated, precision forging to an exceptionally high standard must be adopted and, owing to the nature of the material, may call for as many as ten reheats if excessive die wear is to be avoided. Should these blades be machined from solid bar stock, as many as twelve different operations will be necessary. In total, the produc- tion problem is staggering. Of course, other methods of production are at present the subject of experiment. Some blades can be produced by the powder-metallurgy process, by fabricating from sheet material or by a rolling process using techniques long em- ployed in the cutlery industry. These methods cannot, as yet, be regarded as offering either a solution or a substantial easement of the problem. Nor can it be expected, for in- stance, that America can come to our aid. That country, notwithstanding its much-vaunted genius for mass pro- duction, has its own problem of blade production. Accord- ing to a responsible manufacturer, American requirements would probably be 20,000 gas-turbine units per month, 75 per cent of which would be of the axial-flow type. This estimate, modestly based on 1,900 blades per unit, would necessitate 25,000,000 compressor rotor and stator blades, 2,000,000 turbine rotor blades and 1,500,000 turbine nozzle blades—a total of 28,500,000 blades per month. It may be cited that America is now committed to axial- type units and must have carefully assessed the production problem before scheduling them for many of her latest aircraft. It may also be remembered that Germany con- centrated on axial types in the last war and largely sur- mounted the production difficulty. It can scarcely be in- ferred that the problem would be too great for solution by Britain but, unless studied in time, it might constitute a serious and persistent bottleneck. Would it be the wisest use of our resources? Unless operational requirements make the type vitally necessary some portion of the work would constitute wasted effort. The question to-day is whether, with a sharp eye to the future, our aims, efforts and resources are most advantage- ously proportioned. Is too great a part of our resources devoted to the development of the axial compressor and too little to the centrifugal type? Is too much thought expended on improving the performance of the axial com- pressor and too little devoted to simplifying its production? Is the uncomfortable impression that the large-scale pro- duction problem is being pushed to one side as a bother- some distraction justified or not? The hackneyed policy of "We'll cross that river when we come to it" staves off awkward questions and doubts and gives a morale- uplifting impression of stout-hearted confidence. So long as the river is successfully crossed it is a perfectly good policy; but it can be a disastrous one if, when the bank is reached, the equipment provided is found to be of unsuitable type or is not available in sufficient quantity. LOW-TEMPERATURE OPERATION —and its Effects on Aircraft Materials : Major Teed's R.Ae.S. Lecture ON Tuesday, November 7th, Major P. L. Teed,F.R.Ae.S. (who is deputy chief of research anddevelopment, Vickers-Armstrongs, Aircraft Section), presented before the Royal Aeronautical Society a paper entitled Aircraft Metallic Materials under t^ow-Tempera- ture Conditions. (The occasion was one of periodical meet- ings at which R.Ae.S. " main " lectures are given at a pro- vincial centre, in this case Belfast.) The lecturer pointed out that the ubiquity of aircraft (in being or yet to be), whether civil or military, manned or un- manned, made them liable to exposure to wide extremes of atmospheric conditions. The range of temperature to which they were liable to be subjected might possibly be from plus 90 deg to minus 90 deg C.; that of pressure ranged from one atmosphere to something approximating to one-tenth of this amount; and water content (aqueous vapour plus water in suspension, e.g., in a very dense tropical cloud) could, on occasion, be as high as 2.5 per cent by weight and, at strato- spheric heights, at least as low as 0.001 per cent. Such variations were not without chemical and physical repercussions. In the first part of his lecture, Major Teed dealt with the chemical aspect, and concerned himself first with the effects of high-altitude flight on the wear of commutators and brushes, and secondly with the formation of dew on the interior surfaces of air-conditioned fuselages. In reference to the first problem, he suggested that the extreme dryness of stratospheric air caused rapid wear of commutator brushes ; this effect could be reduced by the use of special brushes or -could, alternatively, be largely mitigated by conditioning the atmosphere intmediately surround- ing the commutator. On the second problem, the condensation of moisture could cause corrosion, but adequate drainage and suitable protectives could eliminate, or reduce, this effect. In the second part of the lecture, attention was devoted to the physical consequences of low temperatures. Both on the ground and at high altitudes, very low temperatures might be experienced. It was not at present known which of these would have the greater effect but, on possibly very rare occasions, the aircraft in whole or in part might have to function at tem- peratures of the order of —72 to -92 deg C. Having touched on the nature of the engineer's interest in the consequences of such low temperatures, the lecturer gave a warning as to the sometimes doubtful value of results obtained from tests carried out at low temperatures on metals in the form of conventional test-pieces. He thereafter dealt with the great variation in the coefficients of expansion (and contraction) of different common aircraft metals, and went on to discuss the fundamental physical and metallographic structures of the common metals, and the main changes in their mechanical properties arising from decreases in temperatures. The third and largest part of the paper was devoted to the engineering significance of the effects of low temperatures. After dealing at some length with the change in tensile properties of iron and carbon steels, Major Teed went on to refer to various methods of lowering the transition temperature of ferritic steels; he then turned his attention to those non-ferrous alloys whose impact-resistance decreased widi temperature. In summing up, the lecturer stated that what he had done was to warn the designer; it was left to him (the designer) to surmount the dangers. This, in the main, he could readily do, sometimes by making concessions to the low-temperature deficiencies of a particular metal, sometimes by refraining from using it There was one bar, however, to making clear-cut decisions. There was a doubt as to whether the adverse shift in the transition temperature due to cyclic stresses below the fatigue limit in S.A.E. 1020 steel applied to other ferritic steels. Until this question was clearly answered, it remained uncertain whether or not aircraft components made of these most useful alloys would, with use, suffer a progressive decrease in low- temperature resistance to triaxial stresses. 1
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