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Aviation History
1938
1938 - 0924.PDF
SUPPLEMENT TO FLIGHT 332'' 18 THE AIRCRAFT ENGINEER MARCH 31, 1938 Fig. 1. Diagrammatic representation of the different types ofwing construction discussed by the author. Construction type ' b " (Fig. 1) which uses only the middle part of the wing contour for load-carrying pur- poses eliminates some of the difficulties encountered in the fully skin stressed wings and was originated by Rohrbach. By concentrating the loads upon a narrower strip of the wing surface, the skin thickness naturally increases and the amount of material necessary to render the skin stiff enough for compression loads is proportionally smaller than in case " a." Now since leading and trailing edges do not represent any load-carrying members as far as our discussion is concerned, they can be made detachable and offer favourable conditions for the installation of engine controls, flying controls and the like. Even the mounting of the engines themselves within the wing contour is facilitated. Separate L.E. and T.E. On the other hand, the difficulties of this type of con- struction are numerous. From the engineering depart- ment it calls for a very accurate layout to determine the exact location and shape of the leading and trailing edge connections in order to produce a well-faired wing con- tour. For the same reason the different wing parts have to be assembled in costly jigs, since it is absolutely neces- sary to avoid steps and ridges along the wing span, and special difficulties arise in keeping the joints watertight. Construction type "c" (Fig. 1) originates from the building of sailplanes. In principle it is similar to the method just discussed; the difference is that only the nose part of the wing is used as the load-carrying member. The advantage of concentrating the loads upon the small section of the wing contour is maintained, but so are the difficulties, and it is impossible to locate the engines within the wing nose. This precludes its adoption for multi- engined aeroplanes The practical development of this type of construction led to type " d " where the load-carrying skin is continued further back from the leading edge and a special spar has been added. The closed skin section thus constitutes a pure torsion member and the spar has to carry the bend- ing and shear loads, thus offering conditions for simple computations. In most cases it will also mean a saving in weight.; due to the large cross-sectional area of the torsion member, the torsion stresses of the pure '' skin- stressed '' construction and the amount of skin-stiffening members will be considerably smaller. But the difficulties • mentioned above are still present. All types of construction discussed so far have in com- Fig. 2 is a portion of the riveted tubular spar in Duraluminfor the Ha 139. Note the simple rib fittings and the flanged end-coupling. mon a certain amount of utilisation of the wing covering for load-carrying purposes. When discussing wing structures with load-carrying members located entirely inside the wing we have to mention first the Stieger-Monospar wing; all bending loads are carried here by a single spar which is braced by a strut - and-wire system to take the torsion stresses. This type of construction (e) can be accurately stressed and so assures a saving in structural weight. From a military standpoint it is advantageous to have a multitude of primarily stressed parts distributed over a large part of the wing volume. By using a statically indeterminate and thus safer stTHC- ture, the system loses its original value of simplicity. -•?-•' The two-spar system (type " f," Fig. 1) consists of two beams for bending and shear loads; for torsional loads and drag forces the spars have to be so connected as to Fig. 3. Centre-section " cranked " spar in welded steel. Thespar is inherently leakproof and serves as fuel tank. -• form a complete structure. This is done by heavy centre ribs and utilising the wing skin as a shear plate, or by special wire bracing in the plane of the upper and lower spar flanges. Both types are standard practice in wooden wings as well as in metal construction. On large aeroplanes the three-spar system (type "g") is sometimes used ; in principle it is similar to the two-spar system and combines a better distribution of flange loads with a better protection against gun fire action. A still further distribution of flange loads is attained by the Junkers multispar construction (type "h")- I*s statically indeterminate structure of interconnected spars combined with the corrugated wing covering to take the
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