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.
[Illustration: FIG. 47.--Interplane strut attachment.]
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.
[Illustration: FIG. 48.--Interplane strut attachment.]
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
[Illustration: FIG. 49.--Interplane strut attachment.]
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
[Illustration: FIG. 50.--Interplane strut attachment.]
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.
[Illustration: FIG. 51.--Interplane strut attachment.]
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.
[Illustration: FIG. 52.--Interplane strut attachment.]
[Illustration: FIG. 53.--Interplane strut attachment.]
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.
[Illustration: FIG. 54.--Plane strut attachment.]
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.
[Illustration: FIG. 55.--Interplane strut attachment.]
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.