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Aviation History
1953
1953 - 0984.PDF
i38 FLIGHT THE TENSIONED SKIN... *00^: In their passage through a factory orthodox skins frequently suffer damage; this tensioned-skin panel is Jess vulnerable, and can readily be transported and stored. the adoption of the system in several types of aircraft, including fighters, but there is no doubt that it is particularly applicable to large transports. The fuselage—of circular section—is divided longitudinally into a number of identical strips running from nose to tail; each strip then becomes a regular panel with both edges of common curvature. Since these panels are identical in profile, their developed shape can be cut out from flat sheet very simply. In the aircraft now being built, the skin is of 26 gauge D.T.D. 610, and the fuselage panels, of which there are ten, are each some 42ft in length and of about 100 sq ft in area. All the skins for these ten panels are cut together in the flat beneath a common template. A simple rip-saw is used, and all that remains to be done is to file the edges lightly to remove burrs. Each panel is then placed on a fiat worktable and stiffened with edge members of the type shown in the accompanying drawings. These edge members are attached with dimpled Bif rivets, but Mr. Heal points out that spot or seam welding or Reduxing is quite practicable; if a bonding process is adopted, only one reversible heater press is necessary, for the work is flat and the curvature of all panel edges constant. Transverse curvature across each panel is then introduced by the attachment of the framelets, and the panel is finished by the addition of a reinforcing bridge-plate or gusset at each end. The photograph at the top of this page shows a completed panel of the original fuselage to Provost dimensions. A completed panel is next placed over a wooden assembly jig and firmly attached by ties at the end of the larger diameter. Then comes the most surprising part of the procedure; as shown in an accompanying diagram, the other end of the panel can now be bent down manually until the panel is lying tightly against the jig in its completed double-curvature condition, the tensioned skin con forming exacdy to profile. The 100 sq ft panels have edge members of 24 gauge D.T.D. 610, some 3^in in depth; yet the writer can confirm that they can readily be bent to their final shape in about a minute without the use of any tools whatsoever. When the panel is in position and secured at both ends, a second panel is placed adjacent to it and also bent round and secured at the other end. Other panels are added until the com plete jig for half a fuselage is covered by, in this case, five panels. Finally, the half-fuselage is completed by occasional through-bolt ing of each panel to its neighbour, the bolts passing through a frame turn-up, an edge member, the adjacent edge members and another frame turn-up in that order. Temporary half-frames are then bolted on the structure-ends and struts or ties are secured at suitable locations along the half fuselage to hold it to shape. The half-fuselage is then lifted off its jig and any further cutting and riveting that may be required can be carried out with free access to both sides of the structure. When two such halves have been mated to form a completed shell, the ties and end frames may be removed. At this stage of construction the flooring and local reinforcement may be added, after which door and window openings may be cut. The design of the panel-edge stiffeners ensures that the neutral axis lies beneath the skin at the maximum width of the panel. The maximum tension therefore occurs along the axis of symmetry In this diagrammatic side eleva tion the amount of longitudinal curvature involved is apparent. The framelets are attached to the skin ot such an angle that, when the panel is bent to its final profile, they lie at right angles to the fuselage longitudinal axis. PANEL ASSUMES THIS POSITION down the centre of each panel, while the stiffening edge members are in corres ponding compression. One advantage of this tensioning over the main area of skin is that an almost mirror-like surface is obtained even with very thin gauges of material. In fact, a visitor inspecting a fuselage made by this means typically quotes the skin gauge as being considerably heavier than it really is. Again, since panels can carry a much greater stress in tension than in compression, it is clear that pre-tensioning the skins is advantageous for all normal conditions of loading, for the parts placed under a compressive load have to be brought back to zero stress before any compressive stress occurs. The complete list of advantages claimed for Mr. Heal's construc tional method as applied to a circular-section fuselage reads: 1. A complete set of outer skins, being of similar shape, can all be cut and pre-drilled together for all common rivet holes whilst temporarily attached to a template. 2. The attachment of the edge members can be accomplished by riveting, welding or cementing. For the latter method, a single heater press only is required. These methods can be carried out with the skins in flat condition. 3. Material for the skins may be used in the as-received condi tion. Since there is no stretch work involved, no heat treatment is necessary, and D.T.D. 610 and also D.T.D. 687 and higher- grade materials have been so employed. EDGE STIFFENER RUNNING LONGITUDINALLY VERY THIN SKIN TRANSVERSE FRAMELET Constructional details : the main edge members are joined back- to-back and riveted along the edges of the webs; the framelets are through-bolted across their turned-up ends. 4. The erection or forming jig is extremely simple, intended merely to control the longitudinal profile of the panel when bending. 5. Double curvature may be produced by the simple act of manually deforming the individual panels longitudinally on a suitable forming jig. This method has the advantages of intro ducing tension in the skin which resists the onset of buckling under load besides reducing the bad effects introduced under the effects of pressure differential at altitude; replacing normal quilting of skins, due to riveting, with a surface of exceptional smoothness; elimination of all expensive stretching operations and equipment normally required to obtain double curvature; and reduction of manhours required to trim and fit skins, replacing the normal hand riveting on the final assembly jig with unobstructed machine riveting in the flat. 6. With the present 42ft panels of almost 100 sq ft area, the ALL LONGITUDINAL CURVES OF CONSTANT RADIUS PANELS LOCATED HERE UNDER ITS OWN WEIGHT WITH TIES AND STRAINERS
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