Aeroplane construction

by Sydney Camm

Fabric Covering of Planes.

The evenness and correct tautness of the fabric covering contributes largely to the trueness of the plane. Should the covering be stretched unevenly or too tightly, the application of the dope will cause distortion of the framework, which can only be obviated by re-covering. The bad effects of this is more noticeable with regard to the ailerons, elevators and rudder, which, being of very slender construction, are more liable to deformation. Twisted or warped control surfaces should never be used, as such surfaces not only offer increased resistance, but also interfere with the balance of the machine in flight.

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.

Checking of Fuselage.

To check the fuselage for alignment it should be placed on a pair of trestles, one underneath the forward undercarriage strut fixing and the other under a vertical strut a short distance from the stern post. The body should then be levelled up longitudinally by a straightedge placed on two short straightedges of exactly similar widths, one being placed at the front and the other towards the tail. It should then be packed up on the trestles until the top longerons are dead level across. At this point, if the body is in correct alignment, the engine-bearers would be level both longitudinally and transversely, the incidence of the main spar attachments should be correct and the stern post perfectly vertical in all directions. Other tests should include the placing of a straightedge at the nose, and another placed at the points where struts occur, should, when sighted across the top edges, be “out of wind,” that is in agreement. A point which should be carefully levelled is that portion of the fuselage towards the stern post to which is attached the fixed tail plane. Any inaccuracy here will result in the tail being twisted in relation to the main planes. Each fitting or attachment should also be equidistant from the stern post, and the effect of variation here will be evidenced by the tail plane being out of square with the centre line of the fuselage. Where the type of machine is such that the engine is supported on bearers of wood, it is usual to drill the holes for the accommodation of the holding-down bolts to jig before the bearer is built in the structure. In this case care should be taken to ensure that the corresponding bolt holes in each bearer are square with the centre line. Any deviation will result in the axis of the engine forming an angle with the centre line.

Alignment of Complete Machine.

In this connection it will be better to consider the alignment of a type of machine in common use: a tractor-biplane in which the upper plane is composed of two outer planes and a centre section, and the lower plane in two sections, each abutting against the side of the fuselage, this arrangement being shown in front elevation by [Fig. 122]. The first operation is the levelling of the fuselage transversely by placing the level across the engine bearers, and the attachment of the centre section, which is mounted upon four struts which have been previously cut to dead length and tested by jig. This, considered in front elevation, should be centrally placed over the body, and this is assured by adjustments in the wires A-A1. This can be checked by dropping a plumb-line from the centre plane spar ends and measuring the distance from the line to the side of the body, the distances on either side should, of course, coincide. The next point is to brace the outer sections to the correct dihedral. One method of accomplishing this, as shown by [Fig. 122], is by the use of a dihedral board, this being prepared perfectly straight on one edge, the other being tapered to the desired angle. The wires are then adjusted until the straightedge is level. Another method is to use an ordinary straightedge placed along the top surface of the plane, the angle being measured with a protractor or clinometer, the latter instrument being most accurate. To check the dihedral a line can be stretched between points immediately above the top interplane struts on each side and then measuring to the centre section, but it would be difficult to detect differences in the angles of each wing. With regard to the undercarriage, the distances between lines dropped from the fuselage sides and the wheel centres should coincide.