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Aviation History
1957
1957 - 0171.PDF
FLIGHT, 8 February 1957 173 Fig. 5 (left). Parts moulded in nylon or polystyrene. This process has proved invalu- able for aircraft and missile electrical details. Fig. 6 (below, left). Impact extrusions; these parts would otherwise have been machined from bar. Fig. 7 (right). An invest- ment-cast catch of a type formerly welded from machined parts. DESIGN FOR PRODUCTION . . . to understand the weights supplied by the shops. Since targetweights have been estimated from design grouping this leads to a considerable amount of difficulty. Finally, a numbering systemincorporating the fewest possible letters and digits results in a significant saving in clerical time when it is realized that one partnumber is likely to appear on design, planning, ordering, manu- facturing and inspection documents totalling at least 50 per partnumber (ignoring copies) and that something like 20,000 differently designed parts are needed to make an aeroplane. In general the smaller the unit size the better, giving reductionof "in-jig" times. This may demand the use of joints which might not otherwise have been necessary and design penalties fromweight and fatigue must be carefully watched. The S.B.A.C. has issued an excellent pamphlet entitled The Financial Aspect ofWeight Saving which is of great value in deciding just how far the D.O. can meet the shops. One of the fundamental requirements for efficient production isthat the operation cycle times should be as short as possible. The shorter the cycle time the more efficient the operator(s) can becomeand similarly the organization surrounding the operator becomes more efficient. In general, the higher the production rate, thegreater the amount of sub-assembly is required. Ideally, cycle times and jig times should be whole fractions of the shift time;unfinished cycles at the end of a working day are highly incon- venient and particularly embarrassing in double-shift working.It can be shown that in order to achieve a production cycle of one per week either two main jigs and four (much smaller) sub-assembly jigs can be used instead of six much more complicated jigs to build up the structure complete. The latter method hasseveral disadvantages which all add up to higher production costs. Great advantages accrue from arranging joints in systems, con-trols, etc., to correspond with unit sizes, enabling each unit to be completed, equipped and tested before the final assembly stage.Finally, unit sizes have to be carefully watched, not only from the point of view of major repairs but they often have to be transportedfrom one factory to another. This thorny subject probably represents one of the most seriousareas of conflict between the prime necessity of saving weight and ease of production There can be no doubt that wider tolerancesreduce costs of production whether it be in the machine shop or rolling mill. There can be no relenting here from the D.O. po.i. rof view and the shops must be urged to do the best they possibly can to keep to nominal sizes. The problem is very largely one ofpsychology and, recognizing this, much thought has been given to statistical methods of strength assessment whereby the generalshop performance is used instead of the most adverse drawing tolerances. Consider a non-fitting part on which the shops usually demand+ O.OlOin. As the regulations stand at the moment it is mandatory to stress this part to — O.OlOin on an outside diameter, whereas inthe majority of cases the shops produce the part to + O.OlOin. Assuming an average ruling dimension of lin this represents anexcess of weight of 4 per cent. Taking this as a basis and assuming that there are 5 per cent of such parts in the structure weight ofan aeroplane a weight penalty of 0.2 per cent ensues or 0.07 per cent increase in take-off weight. Alternatively, this could result ina 0.2 per cent reduction in payload of a medium-size civil aircraft reducing it by 50 lb in 25,000 lb, causing an annual loss in revenueof £2,500. At this point Mr. Keen turned his attention to a brief discussionof four advanced methods of parts-forming in which finished forms were produced with a minimum of machining. He alsocommented on the advantages which accrued from the handing of parts; i.e., from making port and starboard units interchangeable.A wing-tip made by Armstrong Whitworth was originally welded from two drop-hammered halves; later it was stretchedon a Hufford machine, with savings in cost and time, and with better pre-assembly properties. Fig. 5 showed some mouldedplastic (nylon and polystyrene) parts, which would otherwise have been machined from solid. Fig. 6 depicted examples of impactextrusions, which were cheaper than fabricated or machined parts and also had better grain flow. Fig. 7 was an example of invest-ment casting, the subject being a catch formed from close-tolerance castings requiring no machining. In conclusion, Mr. Keen re-emphasized his belief that earlyand co-operative planning was essential. For final success, a spirit of mutual co-operation had to pervade the whole organization. ALTIMETER-STANDARD REVISION A REVISED British Standard for sensitive altimeters (B.S.**• 2G 115:1956) has been issued, principally on account of the introduction of the I.C.A.O. "Standard Atmosphere," whichsupersedes the I.C.A.N. relationship between static pressure and height, to which these instruments were formerly calibrated. This standard is applicable to mechanical instruments operateddirectly by an aneroid barometer movement. The perform- ance requirements are based on two ranges, — 1,000ft to 35,000ftand — 1,000ft to 50,000ft respectively. The tabulated pressure- height tables have been revised and barometric data are quotedin accordance with B.S. 2520, Barometer Conventions and Tables. However, because test barometers with scales to standardsother than B.S. 2520 are likely to remain in use for some years, Appendix A of B.S. G 115 :1949, containing instructions for thecorrection of barometers with inch scales related to London laboratory conditions, is retained in the present revision asAppendix B with appropriately amended pressure-height tables. The table of fiducial temperatures for various latitudes andheights above m.s.l. has been modified as a result of changes in the formula for establishing gravity corrections. Comparable;xiformation for barometers in accordance with B.S.2520 is given in that specification. Copies of 2G 115:1956, price 3s 6d, are obtainable from theBritish Standards Institution, Sales Branch, 2 Park Street, London, W.I.
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