Aeroplane construction

by Sydney Camm

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.

CANE STRIPS SCREWED TO RIBS

Fig. 44.—Attachment of fabric to ribs by cane strips.

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

Fig. 45.—Fabric sewn to ribs.

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

Fig. 46.—Double-lapped joint in fabric.

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.

CHAPTER VI.
INTERPLANE STRUT CONNECTIONS.

It may be taken as fairly conclusive that for war purposes the biplane has proved its superiority, and it appears also that for the commercial requirements of the future it is suited still better, and therefore, in view of the huge possibilities thus opened up, is likely to maintain this predominance.

As the arrangement of planes in a biplane forms the extremely simple yet enormously efficient box-girder, it is generally considered superior in strength to weight requirements, although for monoplanes of small span it is doubtful if this is so, which affords some indication of the possibilities of the small monoplane as the sporting machine of the days to come. Seeing that the principal difference between the biplane and monoplane consists essentially in the type of truss employed, the arrangement and attachment of the various members peculiar to the biplane truss becomes of interest, certainly of importance. It is intended to deal with the various trusses in a later chapter, confining the present remarks to the interplane strut fittings in use, and commencing by detailing the chief requirements and desirable features. The most desirable requirement is that the attachment of the fitting to the wing spars does not involve the drilling of the spar. In practice this is most difficult of accomplishment, for while no great trouble would be experienced in making a fitting fulfilling this requirement, it would be quite another matter to keep it in place under the tension of the bracing wires, and in the case of the outer strut fitting, to which any strain is ultimately transmitted, practically impossible. In spite of this, it must be remembered that the machine may occasionally, when landing or getting off, pitch over on to the wing-tip skid, and if severe, the shock transmitted to the spar may cause a fracture to develop which, starting at the hole due to the strut fitting, and owing to the fabric covering, would be difficult to detect. One or two similar mishaps, with a consequent increase in the extent of the fracture, give distinct possibilities of collapse in the air. Although one cannot give specific instances, it is a feasible contingency, and one that should be eliminated from the region of possibility.

Additional important features are the provision for rapid assembly and detachment, ease of manufacture, and the absence of brazing, welding and soldering as mediums for forming connections, at least for those parts subject to any stress.

The qualities of strong construction and good design are paramount considerations in the manufacture of these fittings, as the purpose of an interstrut joint is not merely to form a connection between the upper and lower planes, but also to distribute the intricate stresses encountered in flight.