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.
Brazing and Welding.
It is somewhat amazing that brazing as an essential operation in the making of a joint should still be employed, as it is difficult to imagine anything less suited to the conditions under which aircraft operate. The advantages of a uniform high-grade steel possessing a high ultimate tensile strength are dissipated by the intense heat necessary for the action of brazing, resulting in the strength of the finished joint becoming an extremely problematic quantity, indeed this is rendered the more so by the individuality of the workmen.
Welding properly performed is less objectionable, indeed, its use may be said to be constantly increasing, although it is well to recognize its limitations. It should not be used for parts subject to any great tensile stress, such as the fittings forming the subject of this chapter. The efficiency of any welded joint is hard to determine, as apparent soundness on the surface is no indication of the internal nature of the weld. Regarded from the aphoristic “maximum strength for minimum weight” view point, and taking into account the advantages in this direction which can be obtained by the use of a high-grade steel, brazing and welding are not to be commended.
The operation of soft soldering, requiring only a moderate heat, does not weaken the material to any great extent, and for some items a properly pegged and soldered joint is superior to the two methods of jointing previously described.
Connections in Use.
The illustrations given indicate the varying degrees of practice, taking as the standard for comparison the early Wright socket, Fig. 47. Although somewhat crude it was quite suitable for the purpose, especially as the wing warping system in the Wright machines necessitated a fair amount of flexibility in the joints. It serves also to illustrate that some advancement has been made in constructional work. The advantages of rapid erection and dismantling have been realized and provided for in most machines since the early days of the industry, and it is not surprising, therefore, that the salient characteristic of the joint (Fig. 48) used by S. F. Cody on his famous biplane was portability. The interstrut terminates in a kind of fork, which in turn is pinned to the head of a special bolt slotted to receive it. The fact that the wiring lugs were improvised from chain links is interesting.
The method of packing the wings for transport consisted in detaching the two outer cellules from the central structure, when the removal of one set of wires enabled the planes to be folded one against the other. It is possibly of interest to record the fact that in the military trials of 1912 this machine was taken down and re-erected in 51 minutes, quite a good performance taking into account its large dimensions. Although this attribute is scarcely necessary at the present time, it will be undoubtedly required by the sporting owner of the future with limited storage facilities. The fitting shown
by Fig. 49 is only suitable for machines with light wing loading. The plate forming the anchorage for the wires is pressed out, the lugs bent to the different angles, and then attached to the spar by an eyebolt, to which is fixed the plane strut, the ends of the latter being capped with steel tube of streamline section. A similar arrangement is that shown by
Fig. 50, the lug plate being pressed out and bent, but in this example the strut terminates in a socket of oval steel tube welded to the plate. It is connected to the spar by a bolt passing through the centre of the socket, the strut end fitting over this.
The practice of anchoring wires to eyebolts, as in Fig. 51, forms the nucleus of many strut connections, but as a method cannot be recommended. Continual strain on the wire has a resultant in the bending over of the head of the eyebolt as in Fig. 52. As a point of fact the use of the eyebolt is distinctly elementary, and gives the impression of a makeshift. The fitting illustrated by Fig. 53 constitutes an advance on the previous arrangements dealt with, and is also indicative of modern practice.
The main body of this clip is a stamping from heavy sheet-steel, bent up to the section of the spar, the bolts, it will be noticed, passing horizontally through it. The anchorage for the wires is formed by lugs, which have a direct pull on the bolts, and is so arranged that a slight clearance exists between lug and spar.
The plane-strut is shod with steel tubing, and connected to the fitting by a bolt, as shown. Of the strut connections described so far, hardly one can be said to conform to the leading principle of the ideal fitting, i.e. the secure attachment to the spar without piercing the latter for bolts. Fig. 54 gives a fitting which is as good a solution of the problem as is constructionally possible. The basis of this connection is the lug-plate, to which is welded the strut-socket, the whole being fastened to the spar by four bolts, which are let in the flange of the spar just half their diameter, and tighten on a washer-plate on the opposite side. Lateral movement along the spars is thus adequately prevented, although the outer strut-socket might conveniently be bolted right through the spar, without materially reducing the strength thereof. This is made possible by the fact that the wing spars, disregarding the small wash-out at the extreme tip, are generally parallel in depth from root to tip, the amount of material at the point of intersection of the plane-strut being in excess of that necessary for the stresses concerned. Another attachment achieving similar results is shown in the diagram (Fig. 55), forming an example of the fitting employed on the pre-war Avro biplane. It will be noticed that in this case two bolts only are used for the connection, the pull of the flying or lift-wires being counteracted by the duplicated wires taken from the washer-plate to a fitting located on the single central skid of the undercarriage.
Head Resistance of Strut Sockets.
A point calling for comment is the apparent oversight or neglect of the amount of head resistance offered by the average strut fitting, although great care is taken to ensure the strut and wing sections being of correct form. It seems probable that some difference must occur, especially at the high speeds now prevalent, between the air flow across the plane and that which meets the strut terminal. Anyway, some discontinuity of flow exists, and whether or no the aggregate resistance of all the fittings is of any great moment provides matter for discussion. It is quite possible to fair off any irregularities in air-flow due to the strut connections by the attachment of sheet-aluminium fairings, which could be beaten, pressed, or spun with little difficulty. Although examples of this practice are very little in evidence, the writer inclines to the belief that the additional weight would be negligible compared with the ensuing reduction in head resistance.
The foregoing examples cannot be said to constitute the latest practice, nor is it possible under present conditions to give such details, but sufficient has been said to indicate the progress and trend of design.