Aeroplane construction

In all aeroplanes the question of wires and the terminal connections associated therewith is a matter of some importance, and while this may vary in degree, there is little doubt that the efficiency of modern wiring systems is largely responsible for the structural efficiency of the aeroplane as a whole.

Aeroplane construction consists almost exclusively of a framework of wood braced by wires, a condition of things which has obtained since the inception of flight; as may be judged by the various engravings of Henson’s projected monoplane of 1842. This machine incorporated an arrangement of king-posts and wires approximating very closely to modern practice, and the natural sequence of improvements have tended towards the gradual elimination of exposed wiring.

Various Wires used.

The various wires used in construction may be classified into four distinct types: the solid wire stay, the straining cord or cable used for stay wires, the extra flexible cable used for controls, and the swaged tie rods in plain or streamline form. The earliest form of bracing was of the solid piano wire variety, this having been used on most aeroplanes from the days of the Wrights onward. From the view-point of the early pioneers, this wire was eminently satisfactory, being cheap (a vital consideration) and simple to attach and replace. Although the tensile strength of this wire cannot probably be excelled, its hardness renders somewhat difficult the forming of the end loop without fracture of the wire. For this reason piano wire gradually gave place to a softer grade of wire which, while being strong, was tough and ductile, enabling bends to be made with a lesser danger of fracture. The original connection used for the piano wire stay is shown by Fig. 103, this consisting of a loop or eye, the free end being turned round a ferrule of soft copper tube, this being sometimes varied by the use of a flat strip of tinned iron, wrapped round and soldered. While this was fairly satisfactory for short stays, it was hardly suitable for the main lift wires of the interplane bracing, owing to the comparative ease with which, under load, the free end pulled or cut through the ferrule, so that after a while the oval spring-wire ferrule, Fig. 104, came into use. This is made of the same gauge wire as the stay, and is from seven to nine convolutions in length. The eye should be formed as an easy bend, and not kinked, the ferrule being pushed tight against the shoulders, and the free end turned back.

Result of Tests.

Tests undertaken at the instance of the American Advisory Committee for Aeronautics showed that 80 per cent. of the wires tested failed by the free end pulling through the ferrule, the remaining 20 per cent. failing by fracture, the stays possessing an average efficiency of 68 per cent. of the maximum strength of the wire. Although various modifications, such as tying the free end to the ferrule with fine wire, as in Fig. 105, resulted in an increase in total efficiency, average European practice consists of that shown by Fig. 104. At the present time the solid wire stay of the form dealt with is used mainly for the bracing of the fuselage frame, and the internal wiring of the tail planes.

Stranded Cable.

The gradual increase in engine power and total weight of aeroplanes led to the adoption of stranded cable for all important loaded wires, this being made in two distinct ways.

The cable employed for interplane bracing is composed of a number of fine wires, varying from nineteen to thirty-seven according to the different diameters, the end section being indicated by Fig. 106.

Where extra flexibility is required, such as for control wires running round pulleys, the cable is composed of a number of strands, generally seven, which in turn consists of a number of fine wires, usually nineteen, the end section being shown by Fig. 107. English practice designates this form of cable as extra flexible, and the single rope of nineteen wires as straining cord. American classification is practically the reverse, in that the single rope is known as stranded cable, and the multi-strand as cord. Although the factor of strength is an important one it does not entirely govern the selection of a wire, as other considerations, such as flexibility and fatigue strain, influence greatly the efficiency of a stay under active service conditions. Under test the solid wire possesses the greatest ultimate breaking weight, the next best being the single rope. It must be understood that in flight a wire is subjected to constant and intensive vibration, which must have a deleterious effect on the material, and for this reason a flaw or slight fracture in a solid wire may escape notice until complete failure in the air; whereas the cable, by the unstranding of the damaged wires, would give warning of wear. Chiefly owing to the difficulty of forming a satisfactory splice in the single-strand cable, modern practice inclines toward the use of the multi-strand cable for all purposes, as the construction of this wire lends itself to the forming of a successful splice.

Cable Connections.

The earliest form of terminal connection for stranded cable consisted of a loop, the free end being bound to the main part of the wire and soldered. With the addition of a binding or serving of wire round the loop to prevent injury, due to contact with the wiring lug, or strainer eye, this wire, in a recent test, gave an efficiency of 100 per cent. for all diameters up to ¼ in.

This result, considering the elementary nature of the joint, is surprising. Unfortunately the effect of corrosion due to acid and solder is a somewhat doubtful quantity; moreover, the appearance of the joint is far from neat. An attachment which at one time achieved some popularity is shown by Fig. 108, and is especially suitable for the single-strand wire. This consists of a cone-shaped forked end with a taper hole, into which the cable is inserted, the free end being unstranded, spread out and soldered. The attachment has been used on what was at one time one of our best products. The efficiency obtained with this fitting is in the neighbourhood of 100 per cent.

In the method indicated by Fig. 109 a piece of flat copper tube is passed over the wire, the free end of the latter being bent round a brass thimble, and then passed through the copper tube, in a similar manner to the connection for the solid wire in Fig. 102. The tube is then given several turns, and the complete joint well soldered. This system is reliable, and has given good results.

A distinctive terminal is indicated by Fig. 110, consisting of a brass ferrule just sufficiently wide to accommodate the two thicknesses of wire. The bolts are of the counter-sunk head variety, so that the operation of screwing a bolt home also forces the wires into the protuberances in the sides of the ferrule. Although the foregoing methods have all been extensively used, they have now given place to the thimble splice, Fig. 111, which, as a general proposition, is undoubtedly the better terminal connection. The brass thimble protects the strands from the wearing effect produced by contact with the turnbuckle or wiring lug. It is the usual practice to wrap the splice with a binding or serving of fine copper wire, or waxed twine. The efficiency of this joint with a properly made splice may be safely taken as 85 per cent. of the total strength of the wire. With this joint the point of failure, as evidenced by numerous tests, always occurs at, or near, the last tuck in the splice, at which point the extra thickness of the splice is just merging into the normal thickness of the wire. The disadvantage with all terminal connections which necessitate the use of solder is the impossibility of determining just how much the heating operation affects the strength of the wire, and also the effects of corrosion, set up by the various species of flux used in the process of soldering.

Relative Strengths.

For a given diameter the solid-wire stay possesses the greatest strength, the next best being the single-stranded cable, as the following comparison of stay strength, taken from the Report of the National Advisory Committee for Aeronautics, 1915, of America, will show:—

Streamline Wires.

Although in the quest for increased speed the number of exposed wires were reduced to a minimum, the aggregate resistance still remained considerable, this leading to the development of the swaged streamline wire, the introduction of which is generally ascribed to the Royal Aircraft Factory; and these wires are now generally used for all exposed wiring. The points in favour of them are that, properly fitted, a considerable reduction in resistance is obtained, there is a lessened liability to slacken after some use, this rendering rigging a more certain operation, and the nature of its connection obviates the use of turnbuckles.

They have been variously criticized as being expensive to produce, that the resistance may be increased if improperly aligned in the machine, and also that any fracture or flaw is less liable to be detected before complete failure during flight. In manufacture the solid rod is rolled to the section shown by Fig. 112, a certain length each end being left for the right- and left-hand thread. Two of the connections mostly used are shown by Figs. 113 and 114, the latter being preferable, as the universal joint permits of movement in two directions, which reduces the tendency of the wire to crystallize as a result of excessive vibration. To prevent wear at the points of intersection it is usual to fit acorns of fibre or aluminium, a popular form being shown by Fig. 115. Some designers still prefer to use the wire cable for interplane bracing, a fairing of wood being bound to the cables by tape at intervals, this also preventing excessive vibration.

Some years ago various attempts were made, mostly on French monoplanes, to utilize flat steel ribbon for exposed wiring, but, owing to the difficulty of successfully forming a terminal, its use never became extensive, although it may possibly be regarded as the precursor of the modern streamline wire.

It is notable that, so far, the wiring of all German aeroplanes is effected by cable, so that apparently the merits of the streamline wire are not recognized. It is also surprising that no attempt has been made to streamline the cable. A device for tying the wires and preventing friction at the point of intersection, found on nearly all enemy aeroplanes, is indicated by Fig. 116, and there are also instances of quick release devices, these being popular in this country about 1912, and now obsolete.

Although determined attempts have been made of late to entirely eliminate exposed wiring, examples of this occurring in the recent German Fokker triplane, it appears that the various alterations engendered by this procedure in the structure of the machine more than counteract the saving in head resistance.

Moreover, with modern methods of construction the ultimate strength of a wireless wing structure leaves considerable room for improvement, and the price paid for the saving is too great.

The arrival of the all-steel aeroplane would entirely alter the condition of things, as with this construction much better chances exist for the production of a reasonably strong wing structure without exposed wing bracing.