Fig. 36.—Three-ply covering for leading edge.
Pressure at Leading Edge.
The pressure at the leading edge produced by the enormous speed at which the modern machines fly (and the maximum diving speed of which, owing to the reduction of resistance, is correspondingly increased) must be abnormal, and calls for different methods of construction from those which at present obtain. There is at least one case on record where the fabric has burst at this point with fatal results. It is interesting to note that in the report of the N.P.L. for the year 1916–17 mention is made of the deformation of the wing form, due to the sagging of the fabric, which has been reproduced in model form, so that the allowances to be made and the resultant effects have been determined.
Effect of Lateral Control.
The system adopted for the lateral control is a decisive factor in deciding the general lines of construction. The arrangement of plane warping, whereby the wing was twisted or warped from root to tip, or the outer section only, has given place to the almost universal use of aileron control. With the old warping system the ribs, spars, and the whole wing collectively was subjected to a torsional strain, which could only have had a deleterious effect upon it. This fact was almost entirely responsible for the practice of using steel tube for wing spars, for by its use it was a fairly easy matter to arrange the ribs to slide or hinge upon the tube, which, at least, relieved some of the torsional stress.
Leading and Trailing Edges.
Fig. 37.—Leading edges.
The average practice concerning the formation of the leading and trailing edges is shown by [Figs. 37] and [38]. Where the section in use requires a bluff entry the spindled-out nose-piece is applicable, while for a sharp entry a fillet let into the nose-formers suffices. As previously mentioned, steel tubing makes a satisfactory trailing edge, although somewhat heavier than the spruce strip, while an extremely fine leading edge can be formed by steel wire. The edge, under pressure of the fabric, assumes a variegated shape, a distinctive feature of some types, but, nevertheless, a wire trailing edge is somewhat flabby and undulating, and as a method is obsolescent. Longitudinal stringers are employed to preserve the wing contour and also for a stiffening medium for the ribs in a lateral direction. About the only variation of the small spruce strip for the purpose is linen tape, crossed alternately.