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Aviation History
1953
1953 - 1653.PDF
FLIGHT, 18 December 1953 807 INTEGRAL EXPERIMENT Short Brothers' Spar-box Project Described in an R.Ae.S. Section Lecture THE section lecture at the Royal Aeronautical Society on December 1st, entitled Integral Construction: A Survey and an Experiment, was given by Mr. K. L. C. Legg, D.CAe., B.Sc.(Eng.), who is assistant designer (structures) at Short Brothers and Harland, Ltd. The experi ment referred to was the manufacture of a spar- box structure employing integral construction (on show at Farnborough this year), and Mr. Legg's description is reproduced in slightly abridged form below, together with a brief summary of the remainder of his paper. In the first part of his lecture, Mr. Legg outlined some major advantages of integral construction. These were an inherent stability in compression due to the intrinsic solidity of forged, extruded or machined sections; a considerable reduction in the number of parts and the amount of paper work, with an overall saving in cost; a weight saving of up to 4 per cenr of the aircraft total weight; good resistance to fatigue; high surface-finish; and suitability for integral fuel tanks. Against these factors had to be weighed the considerable manufacturing difficulties to be overcome, and overall economic considerations. The lecturer went on to discuss in detail the various methods of producing integral construction, dealing in turn with forging, extruding, machining, casting and rolling. Forging methods appeared to offer the best design advantages but were the greatest distance away. Extrusions might provide an interim usefulness, while extensive machining methods existed already. From the economic point of view, the high initial financial outlay was justified by the complementary needs of optimum military aircraft performance and economy in civil aircraft production. After discussing limiting size requirements, Mr. Legg described the problems, achievement and future requirements of die forging, extruding and machining processes, and then referred to optimum structural efficiency, and design trends and die relation to practical forms of cross-section. It appeared that integral construction showed a worthwhile advantage over riveted or glued skin-stringer construction for the type of high-performance aircraft being built or designed and mat this advantage became greater as wing-depth decreased and speeds increased. There was a structural trend from skin/stringer construction to stiffened skin and hence to a simple thick skin involving the use of multi-webs. In order to investigate the advantages and the difficulties of the use of integral construction, it was decided to manufacture a spar-box structure using this method, to be compared with the existing spar-box of the Short S.B.5. To complete the exercise a strength test under representative fully factored aircraft loads was conducted, so that the project provided a direct comparison with an existing riveted skin/stringer spar-box. The upper and lower surfaces of the test specimen were integrally machined, and conventional spar webs and ribs were employed. Fig. 1. The spar-box project, incorporating integrally machined upper and lower surfaces, and (above) the con ventional riveted skin-stringer box of the S.B.5. The reduction in the number of parts is apparent. Unflanged stiffeners were adopted, in view of the required spanwise taper of the skin thickness and stringer thickness, height and spacing. As the depth of the wing was small, this method was not structurally inefficient. The box was 120in long and 22in wide at the root, tapering to 13.5in at the tip. The overall depth was constant at lOin and Weight Item Upper-surface:— Skin Stiffeners, channels Reinforcing brackets Nuts, bolts, rivets, etc Total—upper surface Lower-surface:— Skin Stiffeners, channels Reinforcing brackets Nuts, bolts, rivets, etc Total—lower surface Ribs Front and rear webs Total (complete box) Weight lb Riveted 33.97 42.08 2.09 13.32 91.46 33.97 33.59 1.48 8.76 76.80 11.52 31.60 211.38 Integral 57.50 0.60 58.10 50.90 0.60 51.50 13.78 36.26 159.64 Number of Parts STRENGTH OF SPAR IN RIVETED TYPE OF CONSTRUCTION g a. o o Item Upper skin Lower skin Stiffeners and channels Reinforcing brackets and attachment angles Spars—flanged web sheet Web stiffeners ... Ribs Rib-stiffening and attachment angles Bolts, nuts, washers Rivets Total Riveted 2 2 44 348 6 22 13 67 3,009 7,064 10,577 Integral 1 1 2 36 11 233 2,851 3,135 Estimated Cost of Production ROOT SPAN Fig. 2. A comparison of spar strengths using riveted and integral con struction. One source of weight-saving in the integral method is obtained by conforming more closely to minimum sectional requirements (straight tapers being used to facilitate machining). Item Assembly of spar box Upper surface Lower surface Ribs Front spar Rear spar Details Totals Estimated total cost Riv Labour cost £ 700 800 800 220 62 62 77 £2,721 eted Material cost £ 8 46 46 5 6 6 3 £120 £2,841 lnt< Labour cost £ 460 360 360 195 56 56 £1,487 •gral Material cost £ 5 95 95 18 6 6 £225 £1,712 Summary of savings (per cent): Weight, 24.6; number of detail parts, 43.7;. number of attachments, 71.7; cost, 39.8.
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