Aeroplane construction

by Sydney Camm

Types in Use.

The sketches, explained in detail hereafter, are illustrative of some of the many systems in use, and taken collectively fall under two categories: (1) those in which attachment to the longeron involves drilling, and (2) those in which the fitting clips or encircles the longeron, friction only keeping it in position. The first method permits of a clip of comparatively simple design, but it has the serious disadvantage of weakening the material, and assuming the longeron section is sufficient to account for this, then clearly a fitting which is attached without the use of bolts would allow a reduction in the size of a longeron (which means a saving in weight) without depreciating the factor of safety. In the second method the attachment is usually accomplished by the pressure of bolts, with practically no weakening effect; but in this case the disadvantage lies in the fact that at each point of attachment a differently dimensioned clip will be necessary, this being due, as explained in the previous chapter, to the longeron tapering towards the stern post. (A method of reducing the number of different clips by suitably shaping the longeron was also dealt with.) It is evident that most fittings must inevitably form a compromise between the demands of production and design, although it must be admitted that in some cases the fittings collectively very successfully evade the requirements of both.

Fig. 76.—Fuselage fitting.

The clip indicated by [Fig. 76] is the particular form of construction associated with the various versions of the Bleriot monoplane, and favoured by the early pioneers generally. It was retained in the Bleriot construction until some time after the outbreak of the war—as a matter of fact, until the type was deleted for war purposes.

It is composed, as will be seen from the sketch, of simply two U-bolts, the attachment to the longeron involving the drilling of four holes, which constitutes the chief objection to this particular form of clip, and has been the subject of criticism from the time of its first appearance as an aircraft detail. The struts are slotted over the bolt, and although this does not conform to the best principles, it is simple, and may have been sufficient for a lightly loaded machine. A point about this clip, which undoubtedly was the cause of its popularity amongst the pioneers, with whom economy was an evil necessity, is that the wires can be strained or tensioned by an adjustment of the nuts on the longerons, thus rendering

Fig. 77.—Fuselage fitting.

turnbuckles unnecessary. [Fig. 77] shows the form of clip used on the Hanriot monoplanes, and is a good example of the class of fitting bent up from sheet metal. This is usually made to be slightly smaller than the longeron, the pressure resulting from the tightening of the bolts on the ends forming the cross-bracing lug, keeping it in place. The defect of this arrangement is that any slight shrinkage of the longeron will permit movement, and for this reason provision should be made for the subsequent adjustment of the bolts. The struts are taken by the lugs punched up from the body of the clip. This leaves very little material to resist the tension of the bracing wires or tierods, but a modification of this clip surmounts this difficulty.

Fig. 78.—Fuselage fitting.

A similar clip is used by the German Aviatik firm, but it is certainly inferior owing to the very poor connection of the struts. Instead of the four lugs gripping the sides of the struts, they are punched up to form a square or tenon, over which the cross-struts are mortised. In any case it would not satisfy the standards maintained by our leading constructors, and certainly not the technical advisers to the Air Board. The clip indicated in [Fig. 78] does not encircle the longeron, but abuts against the two inner sides of the longeron only. The body of this clip is a stamping, bent to a right angle, to which the square sockets for the struts are welded. Attachment to the longeron is effected by an eyebolt, which passes diagonally through it, this also providing the anchorage for the cross-bracing wire. A form of this clip has been used on a certain make of machine for a considerable period, so that it has advantages that are not readily apparent. One outstanding defect is existent in that the pull of the wires would tend to lift the socket on the side opposite to the eye bolt, and this in turn would cause distortion of the struts. A connection favoured by an American firm is shown by [Fig. 79], and possesses the merit of extreme simplicity. The longerons are not drilled, the attachment being through the agency of an L bolt, which also provides the anchorage for the cross-bracing wire. To prevent movement the clip is additionally fixed to the longeron by wood screws.

Fig. 79.—Fuselage fitting.

Fig. 80.—Fuselage fitting.

The method shown by [Fig. 80] is that used on the Deperdussin monoplanes, being patented by that firm as far back as 1912, and consists of two cast aluminium sockets, bolted to the longeron. The struts, in this case oval in section, are fastened in place by steel bushes, which are driven through in the form of steel tube, and expanded and burred round the socket, at the same time forming the anchorage for the wires. This system has been used in the construction of a fast scout of comparatively recent origin, but it embodies the same defects as the Bleriot clip, i.e. four holes are needed in the longeron for every joint; but it has the advantage over the latter in that a better terminal is provided for the struts.

Fig. 81.—Fuselage fitting.

[Fig. 81] indicates the arrangement on the German L.V.G. (Luft-Verkehrs-Gesellschaft) fighting biplane. This is an aluminium alloy casting, fastened to the longeron by screws, and as it is not affected by the taper of the longeron, all the fittings, or at any rate, those in the tail portion, can be of the same dimensions. A point which is often overlooked when using a fitting of this type is that any strain on the wires is transmitted to the longeron by the fastening screws only, or, in other words, the tendency of the wires when tensioned to pull the fitting from the longeron is resisted by the screws only. This does not impress one as being well suited to perform the functions demanded of the average joint, and about the only detail upon which its existence is justified is its ease of production. In the writer’s opinion the clip, [Fig. 82], is by far the finest connection yet devised, and one that should be standardized. Its attachment is accomplished without objectionable drilling; it provides an excellent housing for the cross-struts; can be tightened up should shrinkage occur in the longerons; and can be produced at an absurdly low figure. This clip has been used on machines which have accomplished some meteoric performances during the war, and, moreover, was designed and in use a considerable period before the war.

Fig. 82.—Fuselage fitting.

Fig. 83.—Fuselage fitting.

The clip, [Fig. 83], is simple and quite easily manufactured, being stamped out of sheet metal, and bent up to shape. The lugs forming the anchorage for the wires would have a tendency to straighten out at the bends; but the amount of this, whether serious or otherwise, in the absence of actual experience, is largely conjectural. However, a fitting of this kind was used in the construction of the nacelle of a seaplane exhibited at Olympia in 1914.