Aeroplane construction

Of the various components which comprise the complete machine, the wings, aerofoils, or planes, as these items are variously designated, may be said to contribute the greater part of the ultimate success of the complete machine. The aerodynamical properties of a wing are now fairly well determined, and have been the subject of a great number of experiments, resulting in the clearing away of many hazy ideas and notions, so that the actual design of the wing section for machines of given purpose is almost standardized. From this it might be deduced that the methods of construction were equally well determined, and although absolute uniformity of practice does not exist, the wing construction of most machines is similar, as far as the main assembly is concerned.

Effects of Standardization.

Incidentally, one may point out the detrimental effects of undue standardization as applied to an industry in its preliminary stages. These effects are well exemplified by certain machines, in which standardization has been studied to an almost meticulous extent, with the logical result that their performance is considerably inferior to that of other machines of contemporary design, but in which desirable improvements are incorporated as they occur. Although at present one cannot give actual figures, the average performance of modern British aircraft in range of speeds, rate and extent of climb is superior to the products of any other country, and one certainly cannot cite the construction of the average British machine as an example of standardization. Seeing that, as a typical instance, wing sections are frequently altered in minor detail, the impracticability of standardization is apparent, for this would entail, to a firm wishing to keep pace with developments, a considerable loss, through scrapping of jigs, etc., consequent upon the new design. When the principles of aeroplane design are as well defined as those pertaining to internal combustion engines, one may expect the various manufacturers to produce one type of machine per year, and the various improvements adduced from the year’s experience would be incorporated in the type of the succeeding year.

However, leaving the realms of vaticination for the more prosaic subject of wing construction, it will be realized that the process of producing the full-sized wing, accurately conforming to the measurements, etc., deduced from experiment, and so constructed that the chief characteristic of the section will permanently remain, is of importance. As one or two of the spar sections in use were dealt with in the first chapter, it will be unnecessary again to consider them in detail.

Fig. 26 shows diagrammatically the plan view of a wing assembly typical of modern practice, so far as the disposition of the various components is concerned.

Shaping of Main Spars.

Taking in greater detail the different parts, it is apparent that the spars form the nucleus of the general arrangement. There are two methods of shaping the spar longitudinally, and, as shown by Fig. 27, the one consists of leaving it parallel for the greater part of its length, while the end forming the tip of the wing is gradually tapered to a comparatively fine edge. This may be said to constitute prevailing practice. The other method which is illustrative of monoplane practice is not used to anything like the same extent, and differs in that it is constantly tapering from root to tip. The advantage of this spar construction is the improved distribution of the material for the stresses involved, and also that a wing built with this spar may possibly possess a greater degree of lateral stability owing to the weight of the complete wing being located nearer the centre of gravity. Against this one must balance the fact that each rib must necessarily be different in contour, entailing a greater number of jigs, an increase in the time taken in building, with a consequent increase in cost. In addition, all strut fittings would differ in size, so that, taking all things into consideration, this construction is hardly justified. It will be noted that at the point of attachment of the interplane strut fittings, or, in the case of the monoplane wing, the anchorage for the wires, the spar is left solid. It is possible to channel the spar right through, from root to tip, and to glue blocks where fittings occur; and although there is a possible saving of labour thereby, it hardly conforms to the standards of modern workshop practice.

Defects of Glue in Wing Spars.

Although gluing is a most necessary operation in modern wing construction, it is not what one would call an engineering proposition. It has a tendency to deteriorate with time, especially if exposed to a humid atmosphere. A great deal depends on the method of making the joint, and an operation such as gluing a laminated wing spar is usually carried out in a special room of certain temperature. Such spars are generally additionally fixed by rivets, bolts, or screws through the flanges. The material should always be dry, and as straight and close-grained as can be procured. The straightness and closeness of grain affect the strength to a remarkable degree; and here it may be remarked that the use of the best material is a most important factor for ensuring sound construction, and one that in the end pays. If a spar should happen to be cut from a wet log, it may in the interval between its finishing as a part and subsequent assembly in the wing cast or warp, which may cause trouble in assembling, and is more likely to result in eventually being sawn up as scrap. The resultant section of any wing is really dependent upon the spar being of correct section, and should the spar be out of “truth,” the section will vary at different points. This may not be eradicated even in the erection of the machine, so that finally the actual flying properties of the machine will be affected—another illustration of the importance of thorough construction in ensuring a good and lasting performance. To secure uniformity and interchangeability the wing spars are set out for the wing positions, and the necessary holes for the fittings drilled to jig, before being handed over to the wing erectors.

Arrangement of Planes.

The usual arrangement on machines of the scout type is for the lower plane to butt against the lower members of the fuselage, and the top planes being the same span, the width of the body is made up by a centre plane. Another method is to make the top plane in two portions only, thus obviating the centre plane; and occasionally the spars of the top plane run through, from wing-tip to wing-tip, although this is only possible in machines of small span. Apart from the fact that such a wing requires extra room, it is difficult to procure timber of length exceeding 20 ft. sufficiently straight in the grain; and a minor detail would be the difficulty of repair, as a damaged wing-tip would practically entail a new spar, as splicing, although permissible in some parts of the machine, should not be tolerated as a means of repairing wing spars.

The difficulty of obtaining timber will necessitate the wings of large machines being made in sections; and there are several instances where this form of construction has been adopted, in one case the sections being only five feet in length. This construction seems eminently suited to the post-war sporting machine, as chance damage would be confined to a smaller area, transport simplified, and, providing the joints are well made, no appreciable loss in efficiency should ensue.

Types of Wing Ribs in Use.

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.

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.

An interesting form of rib design is that shown by Fig. 32, and in this instance the fretting is specially designed to prevent any flattening out of the camber. The rib section is shown at A, Fig. 32, and it will be noticed that the flange of chamfered section is grooved to take the three-ply web. The vertical parts of the web are stiffened by small semi-circular fillets.

Ribs under Compression.

For those ribs contiguous to the interstrut joints, a different construction is necessary to withstand the tension of the cross-bracing of the planes and, to a lesser degree, the internal plane wiring, so that at this point the rib performs two functions, that of maintaining the wing curve, and also taking the strains due to compression. Where such provision is not made, the tension of the wiring will result in either or possibly both of the following: (1) the rib will buckle laterally; (2) the camber will increase to an extent varying with the pressure on the wires, both results being extremely detrimental to efficiency. In this respect the old box-kites of varying origin used to offer some interesting studies in variable camber, and when it is remembered that the wing ribs were commonly composed of a single ash lath, steamed to shape, and the fabric attached on the top side only, the wonder is that extended flying was possible at all. For all that, some comparatively classic cross-country flights were accomplished. One popular system is to incorporate a box-rib at these points, sometimes made by placing two ordinary ribs close together and connecting them with three-ply or thin spruce, so that, although the overall width of the finished box-rib would be approximately 2 in., it is exceptionally rigid and withal light.

Another solution is to use a solid web, lightly channelled out, as in Fig. 33.

In some wing structures the ribs are uniform throughout, a strut of either steel tube or wood being inserted and to which the internal wiring is attached. This latter method is possibly more desirable, that is, if the joint between the compression strut and spar can be combined with the interstrut fitting. This may necessitate a little extra work in the latter, but this is preferable to the use of a separate fitting, involving additional piercing of the spar.

Importance of Even Contour.

Whilst on the subject of rib building, one cannot over emphasize the desirability of even contour, and the template, illustrated by Fig. 34, serves as an admirable check. It is cut from very dry material to the outside curve of the section, and if this is tried on as each rib is fixed, one may be sure of comparative uniformity. The root rib is generally of stouter construction, and usually follows the same lines as the compression ribs. At this point the pull of the fabric has to be contended with, which is not infrequently a considerable strain. The same conditions prevail at the wing tip, which is one reason against excessive reduction of material at this point. Instances occur where the tension of the fabric after doping has considerably deformed the tip curve, which is at least unsightly, and may entail reconstruction.

Wing Tip Details.

The actual shape of the wing tip varies with the make of machine, and forms one of the distinctive features of the complete assembly. There is a general tendency to rake the ends, making the back spar longer than the front, on the score that increased efficiency due to reduction of end losses is attained. While this is somewhat problematic, seeing that several notable machines have square tips, and some actually constructed with the longest edge leading, it undoubtedly imparts a pleasing and distinctive appearance.

The actual construction is largely a matter for individual preference, as there are several ways of forming it. For instance, a single piece of ash may be bent to shape, or it may be cut out in sections from spruce boards and glued together with a long splice, while in another instance oval steel tube is the material. This small section steel tubing seems admirably suited for such items as wing tips, trailing edges, and the various components of the empennage, such as the fixed stabilizer, elevators, fin, and rudder.

Another method of construction used for the wing tips of some machines consists of a number of strips, about six for a wing tip 1 in. wide by ¼ in. thick, the joints between which are disposed vertically, forming a laminated wing tip. In manufacture, each piece is bent round bending jigs or blocks of the required shape, the edges of the strips having previously been glued. It is apparent that the smaller the section of strip used, the easier it can be bent, and with this arrangement quite sharp bends can be successfully formed in spruce. The alternative method of steaming a solid piece is often wasteful, apart from the fact that it enforces the use of ash.