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Aviation History
1938
1938 - 0925.PDF
MARCH 31, 19 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT 332c rib loads leads back to the fully " stressed skin " type of the aforementioned '' ideal wing.'' Type "i" (Fig. 1) represents the tubular spar wing, where the wing skin serves only as a covering and does not carry any loads. The load-carrying member—the tubular spar—is a pure shell which carries bending loads as well as torsion. Statically it is similar to the Stieger- Monospar wing and can be regarded as its natural develop- ment ; it is a real single-spar wing, where the torsion-carry- ing fibres are combined with the bending member. In contrast to the stressed-skin wing as represented by types " b " and " c " the tubular spar wing goes one step further by concentrating the stressed material to such an extent that special skin stiffeners are generally unnecessary.* Fig. 2 shows such a tubular wing spar of riveted dural sheets, built for the outboard wing panels of a four- engined seaplane, to which all the other pictures also refer. Stiffening members, as would be necessary for con- struction type " d," can be omitted since the curvature of the shell is sufficient to allow considerably higher com- pression stresses. Regarding the tubular spar as a beam in bending shows—compared with a similar beam of dif- ferent construction—that it develops a considerably higher stress. The advantages of the tubular spar present themselves Fig. 4. The catapult socket is welded on to the spar of the Ha.139. at the very outset for a new project; since it works as a bending as well as torsion member its exact location within the wing contour is of minor importance. There is no objection to locating the engines partly within the leading edge; on very large aeroplanes there is even a chance to locate the engines entirely inside the wing, easily accessible and light in weight by mounting them directly against the spar. Engineering work is also highly simplified and time is saved ; even on wings tapered in thickness it is relatively easy to calculate the exact spar diameter, the location and shape of connecting fittings, etc., and since the spar can also take rather heavy local loads, attachment fit- tings become small, light, and simple in construction. Fig. 2, which shows the rib attachment fittings and the flange coupling, clearly demonstrates this fact. A careful weight estimate has shown the possibility of using steel for the construction of the spar at points of highly con- centrated loads, as in the case of the centre-section of a wing. This method of construction is relatively inexpen- sive from the standpoint of material as well as manufac- ture, and experience has proved the practicability of keeping the tolerances of dimensions well within the allow- able limits, depending, of course, on suitable shop methods. Fig. 3 shows part of a centre-section spar of this type. Another advantage of the tubular steel spar is the possi- bility of introducing highly concentrated local loads. Fig. * Curfouslv enough, Herr Vogt makes no reference to the Duncanson tubular srwr used in tlio Blackburn H.S.T.io. Details and methods of stressing were published in"'" Ancmfi F.nginrer of September 27, 1934.—Kd. Fig- 5- The very simple float-strut fitting is welded on to the 4, for instance, shows a welded-on catapult fitting designed to withstand a load of 110,000 lb. This example also gives an idea of the advantages of the tubular spar against a stressed-skin wing in regard to saving space, material and amount of riveting. Fig. 5 shows the attachment fitting for a main float. The total loads (110,000 lb. compression and 508,000 1b. bending moment) are concentrated in a single support tubing and connected to the spar by means of a flange coupling. Spar Used as Jig Construction of the tubular spar wing is relatively simple. To assure correct fairing of the wing surface, the most practical way is to build a wing with one-piece ribs, slipped over the spar tubing. The spar itself can serve as an axial base for the assembly jig and once the ribs are attached it can be mounted swivelling around its axis to give maximum accessibility for the attachment of the wing covering (Fig. 6). This procedure calls for a mini- mum of jigs and working hours, and is extremely well suited for mass production. On account of its sturdiness the tubular-spar wing is very reliable in actual service. Replacement of damaged ribs does not present any prob- lem and even heavy dents in the spar itself can easily be hammered out and reinforced by gusset plates if necessary, whether the structural material is dural or steel. As previously mentioned, the centre-section spar can economically be built of steel due to the heavy loads en- countered, and the thickness of the sheets is heavy enough to warrant the use of the welding process for assembling the different parts. This in turn offers the possibility of utilising part or the whole volume of the large tube as an Fig. 6. An outer wing portion in skeleton. The flap bracketsare carried on a false spar, but the ribs transmit the loads to the main tubular spar.
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