Fuselage Erection.
As the fuselage constitutes the nucleus of the aeroplane, accuracy of alignment in this component is essential, and the degree of accuracy obtained in the complete erection depends largely on the correctness or otherwise of the fuselage. In different individual designs the methods employed for the construction of the body will be found to vary considerably. The process of erection adopted in many instances is to assemble the sides first, upon a table or bench upon which the correct disposition of the various parts have previously been set out. The wires are adjusted until the sides conform to the setting out, which are then packed up on a pair of trestles and the cross-struts attached. It now remains to align the body so that it is perfectly symmetrical in plan; and this is accomplished by marking the centre of each cross-strut, preferably before insertion in the fuselage, and then adjusting the plan-wires until a cord stretched from the stern-post to the nose covers each centre line. The cross or sectional bracing-wires are then tensioned until each diagonal coincides absolutely in length. This procedure answers very well for a small fuselage of simple construction, and of the wire-braced fabric-covered type; but where the forward portion is covered with ply-wood, and the top rails of the body are horizontal, viewed in side elevation, it is usual to true up on a bench. This consists of a wooden structure built up of strong sides, with legs at short intervals, the whole being well braced. The top surface, on which the body lays, is composed of boards placed wherever a plan-strut occurs. The bench should be rigidly fixed to a concrete floor, the top planed until it is level both longitudinally and transversely, and a centre line marked on each board, while these lines, checked with a fine steel wire stretched from end to end, should be in exact agreement with it. The fuselage, having been previously assembled, with the wires inserted and the plan struts accurately centred, is placed on it in an inverted position. All wires should be then slacked off, and the top, which is now underneath, should be wired until the centre on each strut coincides with the centres on the bench. The side wires are then tensioned until the stern post is vertical, or until various fixed points, such as wing-spar attachments, are in agreement with points marked on the bench and squared or lined up, and also until the longerons are touching every board. The sectional wires are then tightened and adjusted so that each diagonal is of the same length; and this will ensure the centre lines on the cross-struts connecting the bottom rails being plumb or vertical over the centre lines of the cross-struts connecting the top rails. Where the top rails of the fuselage are not parallel to the line of flight, but slope down towards the tail, it would be necessary, if the bench method is used, to construct it so that the boards conform to the slope. With the wire-braced fuselage minor adjustments to the wing-spar attachments, which predetermine the angle of incidence of the main planes, can be subsequently made. A type of fuselage which precludes this operation, and which demands extreme accuracy in construction, is that in which the bracing of the forward portion is effected by three-ply, all wiring in a vertical dimension being eliminated, this system being described in [Chapter VIII]. and illustrated by [Fig. 66].
With this construction points such as the wing spar attachments are fixed, and cannot be altered after the fuselage is built, so that meticulous care must be taken in the setting of the short wing spars across the body, or the fittings to which the wing roots are anchored.
Where a joint occurs in the fuselage it is usual to build the tail separate from the front portion, and occasionally the two sections are trued up independently. This does not give such good results as when the two portions, although separately built, are joined together and trued up complete.