Aeroplane 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.

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.

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.

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.

Efficiency of the Raked Wing Tip.

In the previous chapter mention was made of the probable gain in efficiency resulting from the raked wing tip, and that this has some foundation in fact will be apparent from a consideration of Fig. 39, which illustrates the flow of air across a plane, as generally accepted. Where the plane surface is continuous from wing tip to wing tip, the provision of the shaped tip would appear to compensate for any slight loss, but there are instances where the extent of the pilot’s range of view is of the utmost importance, and this may necessitate the cutting away of a portion of the centre section (which sometimes affords the only means of ingress and egress), or the root of the lower plane, as in Fig. 40.

Wing Baffles.

An attempt to prevent air leakage caused by this is occasionally observed in the employment of vertical vanes, or wing baffles. In the case of a machine with the lower plane abutting against the side of the fuselage, these would not be necessary, the fuselage acting in the same manner. The baffles are usually of three-ply or spruce, and shaped to project above the top and bottom surfaces, this projection rarely exceeding six inches. A typical arrangement is illustrated by Fig. 41, which also shows the exposed spars streamlined with a fairing of three-ply. It is typical of the varied opinions which still exist, that on some machines the wing roots are merely washed out somewhat abruptly. If this air leakage is of any moment, it is apparent that it must detrimentally affect the lift-drift ratio. As a proof of the existence of pressure at the openings in the wing, the writer remembers the case of a well-known seaplane, where the wing baffles on the centre section were made of somewhat thin three-ply. In flight it was noticed by the pilot that these were being forced away from the wing, and subsequently these were replaced by baffles of stouter construction.

Metal Wing Construction.

Of two machines, equal in air performance, the one which can be most easily produced has an obvious and, especially at the present time, a very important superiority. Rapidity of production is a most cogent argument in favour of metal construction, for once the necessary machines are set up, and the jigs and dies made, and given a constant supply of material, output is only limited by the speed of the machine. In addition, there are the very exacting demands of interchangeability. Now, it is infinitely more easy to obtain exactitude in metal than in wood, and, moreover, assuming that it is possible to produce woodwork to the nearest ·01 of an inch, what preventive is there against shrinkage, which occurs even when using the dryest of timber. By the more extensive use of metal there should be a considerably reduced proportion of scrapped parts, and erection would be accelerated. It is significant that the planes of some of the most recent German machines are constructed largely of steel tubing, which is at present the most practicable form in which steel can be used. Of course, steel tube spars are quite an old detail, although the more general English practice is to core them with spruce or ash, as in Fig. 42. One remembers a

monoplane, built some time before the war, in which the spars and ribs were of steel and the covering of thin aluminium sheet. In flight this machine was particularly fast, which may be accounted for by the reduction of skin friction, which a smooth surface such as aluminium would afford. In addition, the tendency of a fabric covering to sag was also obviated. Another example of metal construction is afforded by the Clement-Bayard monoplane, exhibited at Olympia in 1914. The plane construction of this machine, as shown by Fig. 43, consisted of channel steel spars, steel leading and trailing edges, and thin steel strips replacing the usual wooden stringers. However, steel construction in modern English machines is restricted to the various organs of the empennage, and occasionally one finds ailerons so built. There seems no valid reason for the continued use of wood as the material for the construction of such items as the fin, rudder, and elevators, as a considerable saving of labour and time can be effected by using the various forms of steel tubing; moreover, the tendency which most controlling organs built of wood have to warp and twist with variations in temperature is prevented by the steel frame. One frequently sees such items as the ailerons and elevators distorted, which must result in excessive drift, if not erratic flying. At the present time it is difficult to obtain aluminium alloy in any large quantity, and this, in conjunction with the present high prices, precludes its extensive use. When this material is procurable in quantity, and when design is reasonably standardized, rolled or lattice spars and stamped ribs may come into vogue.

Fabric Attachment.

Fabric and its attachment is a matter requiring considerable attention, with the great pressure to which modern wings are subjected. In the old days any fabric which was light with a moderate degree of strength was utilized. Nowadays, it is required to stand a certain strain in warp and weft, and rightly so, since the bursting of fabric in flight can only have one result. It is interesting to note that the fabric used on the Deperdussin hydro-monoplane was specially woven with threads running at right angles, forming innumerable squares. The purpose of this was that, should a bullet or any object pierce any one of the squares, damage would be confined to that square, and thereby prevented from developing; but the writer cannot recall any instance of its use to-day.

In covering, the fabric should be tightly and evenly stretched from end to end of the wing, and only comparatively lightly pulled from leading to trailing edge. If too much strain is applied to the fabric crosswise it will result in undulations between each rib. The tendency of fabric to sag between the ribs is accentuated by this, and, of course, matters are not improved upon the application of the dope. It should be remembered that the efficiency of any machine is greatly dependent upon the tautness of the fabric. It should not be stretched too tightly, as the application of the specified coats of dope may result in the fibres or threads of the material being overstrained.

With regard to the actual attachment of the covering to the wing framework modern practice is restricted to two methods. The older method is illustrated by Fig. 44, and consists of strips of spruce, or more usually cane, tacked or screwed to the ribs. It is usual, and certainly preferable, to affix this beading to every rib of those sections of the planes adjacent to the fuselage, as the fabric on these portions is subjected to the slip stream of the propeller, which meets it in a succession of small blows. The fabric in the outer sections need only be affixed to alternate ribs. The alternate method is shown by Fig. 45. In this case the fabric is sewn to the

ribs with twine or cord, the stitches occurring about every three inches. It will be noted that every loop or stitch is locked with a species of half-hitch knot. This stitching is then covered with bands of fabric, the edges being frayed to ensure perfect adhesion and doped to the main cover. It is largely a matter of opinion which system ensures the most even wing contour, although it would seem that the drift or resistance is slightly lessened by the sewing method. An obsolete method is that in which the fabric was tacked to the ribs with brass pins and taped with linen tape. All sewn joints in wing covers should be, and generally are, of the double lapped variety (Fig. 46), and arranged to run diagonally across the wing. A minor and somewhat insignificant detail of wing

covering is the provision of small eyelet holes in the under surface of the trailing edge, allowing water accumulated through condensation to drain away, and although not general practice, would appear to be necessary. A refinement which may be necessary on the post-war sporting machine is the attachment of small blocks, or “domes of silence,” to the leading edge, as a protection for the fabric against wear. When planes are dissembled more often than not they are stacked leading edge downwards on a concrete floor, and any movement or friction is likely to result in the rubbing away of the fabric, which, if unnoticed, may result in the bursting of the covering. Such fitments would hardly constitute an innovation, as the writer has distinct recollections of seeing such fittings on the D.F.W. biplane at Brooklands just prior to the outbreak of war. These consisted of brass balls, free to rotate in a socket, screwed to the leading edge. A narrow strip of aluminium screwed along the entering edge would be quite sufficient, and would not add appreciably to the weight.