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Aviation History
1926
1926 - 0056.PDF
SUPPLEMENT TO FLIGHT JANUARY 28, 1926THE AIRCRAFT ENGINEER by careful thought. There is a tendency to produce a complicated fitting of light-gauge material, bent up and reinforced with plates and washers, riveted and brazed on, and lightened in a rather elaborate manner. Much work can be avoided if two simple flat plates of thicker gauge are used instead. The increase in weight is infinitesimal, but the saving in manufacturing cost is considerable. The flat plates can be nibbled out and filed up to template by unskilled labour, but the bent-up clip necessitates the use of bending blocks, skilled labour, and naturally takes a longer time to inspect. These instances can, generally speaking, be employed throughout the whole of the designing office. Simplicity is the kevnote of successful design, and I am of the opinion that a certain amount of weight can be sacrificed without impairing efficiency to any appreciable degree, yet at the same time increasing ease of manufacture. Riveting can be, and often is, carried to such an extent as to become an uneconomical process : lightening holes may seem trivial, but a saving can be made by careful design. Avoidance of complicated shapes and numerous small holes in hand-made fittings should be made a rule. Welding and brazing processes are useful up to a certain point, but it must not be overlooked that normalising must follow these opera- tions, causing delay as apart from expense. The use of drop stampings, where quantities warrant the expense of dies, is a practice to be commended. Where a complicated profile is required on a machined fitting, a drop stamping will save considerable machining operations and simplify jigs and tools, beside conserving raw material. High-tensile Materials. The materials field affords many opportunities for the designer to assist the manufacturing side. I am not entirely satisfied with the use of high-tensile material. Undoubtedly, it possesses many advantages, and the designer, beset by the weight fetish, is 'sorely tempted to use it on every conceivable occasion, but in many instances the benefits to be derived are open to argument as to whether they are worth the expenditure. It is more difficult to work, and the heat treatment, apart from cost, necessitates test pieces and records, and careful following of batches through to shops to avoid confusion. With metal construction looming large in design, the use of high-tensile material is increasing, and, whilst certain features make it attractive, I am not convinced of its absolute superiority over other forms of metal construction. In the Hawker " Heron" I have refrained from using this steel, and have employed a mild- steel tubular construction throughout. I am not at liberty to enter into details, but I can say that the results from the cost and manufacturing point of view are startling, whilst the weight, strength, and performance figures are fully in accordance with expectations. Duralumin. The use of Duralumin to the best advantage is also open to question. Here again is another material which offers many attractions to the designer, but which presents manu- facturing difficulties which detract somewhat from its good qualities. Our knowledge of the qualities of this material is not as complete as could be desired, and its use should be restricted wherever possible to plain fittings, where the minimum of working is required. Complicated bent fittings are to be avoided, as wherever the material is worked to any extent the heat treatment required is such that, apart from expense, it causes delay and often disorganisation, owing to the fact that it has to be repeated before the fitting is completed. In considering the non-ferrous metals, I think the designer is slightly handicapped by the necessity for adherence to specifications, but it is sometimes possible to obtain a concession to diverge. Constructors have experi- enced great difficulties in the past with sand castings, owing to blow-holes and porosity, and quantities have again not been sufficient to warrant the manufacture of dies. Intne majority of cases the cause has possibly been due to inherent foundry trouble, but I have seen many instances where design has been at fault. The advantage of foundry advice when designing for castings is inestimable, and will result in a considerable saving. Where I have had difficulties with gun-metal and bronze castings as regards porosity, I have utilised hot brass stampings as being cheaper than die castings, and more reliable than sand castings, and even on small quantities a saving is shown, as scrap is practically nil. A material which the designer rather neglects is malleable iron, which is extremely useful on secondary structures. I recognise its limitations, but I have utilised castings with great success on gun-mountings and under-carriage fittings. It is cheaper and stronger than aluminium, and for replacing made-up plate fittings in certain structures, it is much better, the scrap being negligible. A point, seemingly small, but one which I have found responsible for many irritating delays, is the fact that the designer, when considering such things as tanks, cowling, &c, does not give due regard to the standard size sheet of material available, and it is necessary to obtain specially large sheets, which as often as not, apart from delay in procuring, results in large scrap pieces being left over from each sheet, a wasteful proceeding which could be eliminated by a little forethought in the drawing office. It is also advisable for the designer to keep to standard gauges of thickness wherever possible and to avoid deviations from makers' usual stock sizes. As regards the question of screw threads, the designer must consider each case individually, as this is not so much a difficulty as regards manufacture, but is rather a question of general economy. On these grounds it is better to concentrate on standard sizes, as every new thread introduced means an increase, not only in screwing tackle, but in gauges for both works and inspection departments. On the timber side, there is not so much scope for assistance from designer to manufacturer, but the former must keep constantly in mind the question of quality and sizes of planks available. Economy is more a matter for judicious selection by the works rather than consideration by the designer. In conclusion I regret that I can only touch on the fringe of what must be generally regarded as a most important question. The growing popularity of the aeroplane, the increasing tendency towards economy, the world-wide demand which is coming sooner or later for the best in aviation—British aircraft—makes closer co-operation between the designer, manufacturer and inspector a matter of para- mount importance. No one realises more than I the necessity for research and progress, and no one welcomes with more enthusiasm each innovation which furthers our knowledge, but the application of such knowledge must be tempered with that consideration for our manufacturing facilities which will enable us to utilise it in the most efficient and economic manner. DURALUMIN AS A MATERIAL FOR AIRCRAFT CON- STRUCTION. By OSWALD SHORT. For the purpose of all-metal aircraft construction, we have at the present time a choice of two materials, viz., steel, and the aluminium alloy known as Duralumin. Other light alloys are in course of development, but as they are still in a more or less experimental stage, they need not be considered in this article. It is proposed under this heading to give the results of experience in the use of Duralumin for aircraft construction, and in some places to make comparisons between this metal and steel as regards their suitability for certain work. The use of Duralumin in this country as the main material of construction for aeroplanes, etc., as distinct from lighter- than-air craft, dates from the years 1918-19, when Short Bros, produced an all-metal aeroplane in which this material largely predominated. Briefly, this machine had a monocoque fuselage made entirely of Duralumin, and plane ribs and covering of the same material. The spars, struts, and bracing wires, and the chassis struts were of steel. Previous to the building of this machine, no serious attempt had been made in this counry to employ Duralumin for the construction of heavier-than-air craft, and this position was
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