From a survey of the plane diagram, [Fig. 26], it will be noticed that the chief components, in addition to the main spars, are the ribs, box-ribs, stringers, and leading and trailing edges.
Fig. 28.—Construction of ribs.
The ribs, which is the term applied to the very light framework built over the spars to maintain the correct curvature, are variously constructed; one of the most popular methods in vogue is that shown by [Fig. 28]. The central portion, or web, which includes the nose and trailing edge formers, may be cut from either spruce, whitewood, cotton wood, which can be bent to a surprising degree without fracture, and three-ply. Three-ply, while excellent for some items, is hardly suited for this purpose, as the laminations have a tendency to come apart, especially in the lower grades, which is aggravated by the screws or brads necessary for the attachment of the flange. A rib, fretted out as in [Fig. 28], with the web of cotton wood and a spruce flange, can be made extremely light. A rib for a chord of from 4 ft. 6 in. to 5 ft. would weigh about 5½ oz. As it is very necessary that every rib should correspond, these parts should be made to a metal jig, which is about the only way to ensure exactitude. This should be made from mild sheet steel, about 16 B.W.G., and need only be shaped to the outer curve, as the lightening holes are of but secondary importance, these being usually marked out in the saw mill, and cut to the line with a fine jig saw. For production in quantity a box jig, between which a dozen ribs might be clamped and shaped, is preferable. Templates of wood are of doubtful accuracy, for not only do corners wear, but gradual shrinkage soon renders them useless. The incorrect shaping of the most insignificant piece of wood may have far-reaching effects when assembled, and any extra trouble taken in the preparation of parts is more than repaid by the subsequent ease and precision of erection.
While the method of rib building previously described constitutes general practice, there are, of course, other arrangements in vogue. [Fig. 29] illustrates a system in which the front spar forms the leading edge, a procedure which is somewhat rare now, owing to the features of modern wing sections, but at one time quite common. In this case the web is of three-ply lightened with a series of graduated holes, according to the width of the web, and the flanges of spruce.
The rib assembly, [Fig. 30], is extremely simple and light, as in this case the web proper is superseded by thin strips of three-ply, glued and bradded each side of the spruce flange. The amount of woodwork between the spars is reduced to a minimum, although one can hardly imagine such a system answering for a chord over five feet. Even then the wing curvature would require to be fairly simple, as a pronounced curve would flatten out. As a point of fact, this assembly is rarely used for chords exceeding 4 ft. 6 in. In another arrangement as shown in [Fig. 31], the connection between the top and bottom flanges is formed by blocks, a method which is certainly economical of material.
Fig. 29.
Fig. 30.