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Aviation History
1937
1937 - 0325.PDF
FEBRUARY 4, 1937. FLIGHT. 123 3.SOO z 2.SOO- o S00 SHEAR STRENGTHS OF ETCHED AND UNTREATED JOINTS BETWEEN AEROLITE MATERIAL HAVINC 80 THREADS PER INCH .IN WARP AND WEFT. CASEIN CEMENT HAND CLAMPED AT ROOM TEMPERATURE METHYLMETHACRYLATE CEMENT PRESSED AT ^ TON PER SO INCH 5 MINS AT 110' C CREA FORMALDEHYDE CE»E(U PRESSED AT ^TON/IN' FOR IOMINS IT »O"C ETCHED ETCHED HAND CLAMPED 5 MI.NS. AT IIO'C , ' I ETCHED CONTROLS CONTROLS CON How the etching of Aerolite affects the strength of glued joints. break up the phenol formaldehyde structure but leaving the cotton reinforcement untouched appeared to be the most promising. " We have not yet," Dr. de Bruyne said, " had time to carry out any extensive investigations, but the results, such as they are, lead us to believe that the problem of mak ing cemented joiuts and of building up structures in situ can be regarded as solved in principle." They had also obtained good results by sandblasting, and had constructed a simple device by which the fabric could easily be exposed along the directions in which the glued joint was to be made. It was comforting to learn from the paper that when bolted joints are used with Aerolite the bearing strength is over five times greater than that of spruce along the grain, and about 40 times greater than spruce across the grain. This meant that bolts could be spaced much closer than in wood. Applications of Aerolite After having dealt with the properties of Aerolite, Dr. de Bruyne, in his printed paper, gives some suggested applica tions in aircraft structures. Following are extracts from this part of the paper:— Monoplane Wing Root Joints.—The most troublesome joints in an aircraft structure are the attachments of the outer wings to the centre section. In a cantilever machine the bending moments at the wing roots are extremely high and necessitate the use of heavy fittings with numerous bolts. As an example we may consider an aeroplane of one ton gross weight. The factored bending moment at the front spar joint after allow ing for all relief loads, might be 285,000 lb./in. with a shear load of 3,400 lb. Airworthiness requirements stipulate that a t a load of 62.5 per cent, of the fuel load there shall be no permanent set. The distance between the top' and bottom attachments might be 12 in., resulting in a factored load in the top and bottom flanges of 23,750 lb. (io£ tons). Assuming the spar to be 3 in. wide we should require, say, eight f in. bolts in each flange; thus in the centre section front spar there PLYWOOD WEBS •** THICK B - VJI BRICHT MILO STEEL BOLTS MILD STEEL PLATE % THICK (COMMERCIAL) Joint in wooden front spar of centre-section. will be 32 bolts. Taking in account the rear spar and the outer wings we shall need 128 bolts. These bolts could be lightened with -fV in. holes, but even so the weight of the front and rear spar centre section bolts alone amounts to 7.76 lb. A saving in weight can be obtained by using Aerolite for the spar ends, not only through the reduction made possible in the number of bolts and the size of the fittings, but also through the reduction in the sizes of packing blocks. • These Aerolite ends could be glued with a spliced joint to the wood ; but for the centre section, which might be only 3 ft. wide, it would hardly be worth while making such glued joints and it would be better to use Aerolite entirely for the centre so f 40-0-0-0-- O) —p I I— isV •HQ-e-e-o-'. Q)- VYL6S OF 45'MATERIAL )*»" THICK 4 - V MILD 5T1F.L BOLTS SPEC. A.l. M.T. STttL. PLATES W THICK Joint in Aerolite front spar of centre-section. tion spar. The spar width could then be reduced to 1 in., thus reducing the length of each bolt to one-third. Due to the more uniform bearing pressure along the length of the bolts the fittings could simply be held on to each side of the spar instead of in saw cuts as in the wooden centre section. The reduction in flange area also makes possible an increase in the distance between top and bottom attachments with a consequent reduction in the forces produced by the applied bending moment. Sketches of the different methods of construction are shown. (To be Concluded.)
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