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.
CHAPTER V.
DETAILS OF PLANE CONSTRUCTION.
The tendency to lose lift, pronounced in some machines, hardly noticeable in others, may be directly traced and attributed to the manner in which the wings are built, which is largely dependent upon the design. In the preliminary stages of design it is usual to take as a basis the figures for lift and drift of a known tested section, that is if facilities are not available for testing an exact scale model of the section it is intended to use. Anyway, the whole design is dependent upon these figures, in respect of both the maximum and minimum speeds, and also the rate of climb, and the extent to which the actual performance of the machine complies with these calculations is determined solely by the exactitude and precision with which the full-size wing conforms to the scale model. By this means only is it possible to design with any degree of accuracy.
The Sagging of Fabric.
The sagging of the fabric between the ribs is one of the principal reasons for the failure of the finished machine to satisfy expectation and also of the tendency to lose lift. One or two causes contribute to this result. One is the spacing of the ribs, which in some cases is not nearly close enough. A rough average spacing is from 10 ins. to 1 ft., but in modern high-speed machines, loaded to anything from 5 lbs. to 8 lbs. per square foot, the spacing should be much closer. In addition, the ribs near the wing root should be closer than those at the tip, for at this point the stresses are greater, a certain amount of vibration from the engine having to be contended with, in addition to the effects of the slip-stream of the air-screw. Particularly noticeable is the tendency for the fabric to sag down on the top surface of the leading edge, a feature which imparts to the machine, especially when viewed from the front, a not unpleasing corrugated appearance. At this part of the section the curve is somewhat sharp, and naturally the fabric tends to conform to the definition of the shortest distance between two points, a straight line. This, of course, is aggravated in flight, when the planes are under load, and by far the greatest amount of pressure is located at the front portion, or leading edge, of the wing.
False Ribs.
FALSE RIBS
Fig. 35.—Arrangement of ribs at leading edge.
In some wing constructions the forces are minimized by the provision of subsidiary or false nose-ribs, [Fig. 35], which extend usually from the leading edge as far back as the front spar and occasionally to the longitudinal stringer. While this prevents, to a certain extent, the sagging in of the fabric, it does not entirely eradicate it. The only successful way in which the characteristics of the wing contour may be preserved is by covering the leading edge with thin veneer, spruce, or, still better, three-ply, as [Fig. 36]. Despite the great advantages attending this constructional feature, its use cannot be said to be really extended.