Aeroplane construction

by Sydney Camm

The design and type of fitting employed for connecting the longerons, cross and vertical struts of the fuselage, varies greatly, being usually one of the distinctive constructional details of a machine. This position renders uniformity of practice a comparatively unattainable quantity, which, in view of present requirements, and the absolute need of rapidity of output (which must commence as soon as possible after a successful design is produced), can only be considered as regrettable. This diversity of design is mainly the result of the desire for originality of each individual designer, and however commendable from this standpoint, is a position which is almost certain to disappear with the progress of the industry. Take as a hypothesis the case of, say, ten makes of scouting biplanes in use, each with approximately the same arrangement of longerons and struts, and with a similar overall size of fuselage. Each of these machines will incorporate a different fuselage clip, which means that somewhere highly skilled labour is being unnecessarily expended in the making of jigs and press tools, whereas a suitably standardized clip for all scout machines of certain dimensions would involve the making of one set of press and bending tools only for the machines of the one type built. Another aspect, quite as important, is the simplification of the supply of spares. Acceleration of aircraft output, if achieved only through the medium of small part production, is one of the most important contributary factors towards ultimate success in the field.

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.