WIRES AND CONNECTIONS.
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.
[Illustration: FIGS. 103–112.--Methods of forming wire connections.]
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:--
--------------------+-----------+-------------+---------
Material. | Diameter. | Strength of | Strength
| | material. | of stay.
--------------------+-----------+-------------+---------
| inch. | lbs. | lbs.
Wire, solid | 3/16 | 5500 | 5100
Strand, single | 3/16 | 4600 | 4100
7 × 19 multi-strand | 3/16 | 4200 | 3500
--------------------+-----------+-------------+---------
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.
[Illustration: FIGS. 113–116.--Methods of forming wire connections.]
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.