Shaping of Main Spars.
Fig. 27.—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.