WING-TRUSSING SYSTEMS.
Although the trussing of aeroplanes is carried out along certain
well-defined lines, there are occasional divergences from the orthodox.
The differences now existing are not nearly so great as those of
former days, this being explained by the fact that the progress of
any science or industry tends towards uniformity of method, while
practical experience eliminates the undesirable systems. This does
not necessarily mean that the present methods in vogue are incapable
of improvement, but merely denotes their suitability for present
requirements.
The Pratt Truss.
[Illustration: FIG. 56.--The Pratt truss.]
The basis of all modern trussing systems, with modifications, is
the Pratt truss (Fig. 56), familiar in bridge-building circles, the
basic principle of which is that the compression members are disposed
vertically, and while of minimum length are most favourably placed for
obtaining the maximum efficiency. There are other types of trusses
used in structural engineering, as, for instance, the Howe truss, in
which the compression members are arranged diagonally, and the Warren
lattice-type girder; but for various reasons these are not applicable
to the needs of aeronautical engineering. But a brief consideration
of the chief features of the Pratt or box-girder system of trussing
will suffice to illustrate its great advantages for aircraft work,
particularly for machines exceeding a certain span; and it is this
limiting span to which a monoplane can safely and efficiently be built
which is largely responsible for its present spell of unpopularity.
Monoplane Trussing.
From the standpoint of simplicity, the monoplane equals the biplane.
As each wing of the former may be considered as a cantilever, it
is the difficulty of adequately staying the wings above a certain
span which forms the deterrent feature, for it is obvious that, as
the span increases, in order to obtain a reasonable angle for the
wires, the king post, or cabane, must be increased in height. This
would necessitate an ungainly undercarriage, less able to withstand
rough landings, with a consequent increase in both weight and head
resistance. However, it seems that the monoplane will have a future
for sporting purposes, where the span will not exceed 30 ft., and will
probably be nearer 20 ft.
[Illustration: FIG. 57.--Monoplane wing bracing.]
Various attempts have been made to obviate this inherent defect of the
monoplane system of trussing, the first and most popular being the
king-post system (Fig. 57), in which short masts are incorporated in
the wing structure and wire-braced to the spars. From the points formed
by the crossing of the mast and spar the main bracing-wires are taken.
That this system is of real use is demonstrated by the fact that,
amongst others, the Antoinette, Flanders, and Martinsyde monoplanes
incorporated this system. It is worthy of note that this system also
characterized the huge Martinsyde trans-Atlantic ’bus, the wing-spread
being in the neighbourhood of 70 ft. Another original attempt at
improvement, the wing-bracing of the Deperdussin hydro-monoplane, is of
interest (Fig. 58). As regards the bracing, the machine was virtually a
biplane, the wings being stayed by a steel tube running parallel with
the wings, and connected to it at intervals by steel tubular struts,
with cross-bracing between, as in a biplane. The abolition of the top
wires rendered the machine of greater value for war purposes than other
tractor machines of that period. The logical conclusion of this system
is exemplified by the Nieuport scouting biplane, the lower plane of
which corresponds to the streamlined steel boom of the Dep.
[Illustration: FIG. 58.--Deperdussin monoplane bracing.]
Wireless Wing Structure.
Superficially, it would appear that the abolition of external trussing
and wiring would make for greater aerodynamical efficiency; and,
constructionally, it would be quite possible to build wings devoid of
external staying, and at the same time of sufficient strength. But when
it is considered that this would entail an excessive depth of spar at
the root of the wing, with a resultant increase of head resistance,
it is doubtful whether any appreciable advantage would accrue. In the
event of the wing becoming deformed or out of alignment, re-truing up
would be almost impossible, and would certainly require the uncovering
of the wing and partial reconstruction. Contrast this with the orthodox
wire bracing. It is simple of attachment, of relatively low cost, and
offers the utmost facility for truing up. A monoplane of note, built
without external trussing, was the special Antoinette, produced for the
French military trials of 1911. This had a span of approximately 46
ft., and the depth of spar at the root was about 2 ft. 3 ins., and at
the tip 9 ins., the consequent weight alone being abnormal.
Anchorage of Lift Wires.
The one-time practice of anchoring lift wires to various parts of the
undercarriage is bad in principle, as there is a distinct possibility
that a rough landing may damage the wire or its attachment, and
ultimately cause failure in flight. This practice undoubtedly arose
from a desire to obtain a good angle for the lift wires, a subsequent
improvement being the addition of a separate pylon or cabane.
Biplane Trussing.
[Illustration: FIG. 59.--Biplane Truss.]
The most common form of biplane truss is shown by the diagram (Fig.
59), sometimes, as in the case of various pusher types, or those for
long-distance work where a large wing area is necessary, extended to
three bays each side, which probably explains the partiality of German
designers for multiplicity of interplane struts, as, prior to the
outbreak of the war, the majority of German machines were designed and
built entirely for long distance and duration flying. By this means
light wing loading, which entails large wing area, was possible without
prohibitive weight, for by the addition of a pair of struts to the
two-bay type, a lighter wing spar for the same strength is possible.
In this type of truss the bays adjacent to the fuselage are varied
in width, in order more easily to apportion the stresses, which are
greater at the centre of the wing structure. A modification of Fig.
59 is indicated by Fig. 60, which illustrates diagrammatically the
arrangement of the Maurice Farman biplane, the improvement consisting
of the method of strengthening the interplane struts. The outer strut
is braced with a small king-post, and from this a wire is taken through
each side of the strut. On this machine the struts are of the light,
hollow-spar type, and this arrangement must therefore materially reduce
their tendency to buckling.
[Illustration: FIG. 60.--Farman wing structure.]
Another version of this system is that in which the top plane is of
greater span than the bottom, the extension thus formed being stayed
with lift and counter-lift wiring, or by means of a strut acting in
tension and compression.
Single Strut Systems.
The almost universal arrangement for the small single-seater scout
is the single bay, and from this method the progress of design has
inclined towards the elimination of as many struts and wires as
possible, which has its culminant in the type of truss embodying one
strut and one pair of wires, lift and counter-lift, each side of
the body. Quite a number of machines have incorporated the single
strut assembly, the earliest perhaps being the Brequet, and one also
remembers a small Avro scout, the strut in this case being built up
with spars and stringers, covered with fabric. The single-lift truss
is particularly suited to multiplane construction, where the chord of
the wings is narrow, and the bending moment, due to the movement of the
centre of pressure, is correspondingly reduced. A disadvantage exists
with this form of truss similar to that experienced with the wireless
monoplane truss, _i.e._ the difficulty of maintaining the correct
incidence from root to tip. However, some extraordinary machines of
recent construction embodying this feature, stand up to active service
demands, so that this defect can be of no great moment. A minor detail
consists in the circumstance of, for example, a lift wire coming adrift
or perhaps being shot away. With the single-lift truss total collapse
would ensue, but it is conceivable that the ordinary double-lift truss
offers more chances of escape.
[Illustration: FIG. 61.--Wireless wing structure.]
Another system which obviates the need for wires is illustrated by
Fig. 61, which was the particular system used on the Albatross “Arrow
biplane” of 1912. A drawback is the difficulty of readjustment, which
is the probable explanation of its failure to come into extensive use.
The direct antithesis of this arrangement, the elimination of struts,
is indicated by Fig. 62; but as this embodies all the defects of the
monoplane system of trussing, even of the attachment of wires to the
undercarriage, it must be considered of no practical utility.
[Illustration: FIG. 62.--Biplane truss without interplane struts.]
1½ Strut Machines.
The arrangement shown by Fig. 63 is responsible for the designation of
machines so built as “1½ strutters.” A later development of this system
consists of but four centre plane struts, the two struts forming an
inverted V between the fuselage longerons and centre being dispensed
with.
[Illustration: FIG. 63.--1½ strut wing structure.]
The system (Fig. 64) is illustrative of the form of staying in use on
a modern high-speed scout, and in respect of which a patent is held.
As this machine is designed with a very small gap, the lift wires
are consequently at a somewhat flat angle. The strut, about halfway
along each wing, is hinged at the point of intersection of the wires,
which, incidentally, do not run through from corner to corner, but are
attached in the centre to a fitting which also forms the anchorage
for the struts. By this method there is an apparent reduction in the
tendency of the wing spars to buckle under load between the points of
support.
[Illustration: FIG. 64.--A patented wing bracing.]
Drift Bracing.
So far the methods dealt with denote the methods of staying in a
vertical dimension, and it remains to consider the provision for
trussing in the fore-and-aft direction. There are two methods in use,
one being to brace the wings internally, which is the more general
practice, as by this arrangement the resistance of exposed wiring is
obviated, while the alternative method consists in taking wires from
various points along the wing to the nose and rear part of the fuselage.
Properties of the Various Types.
The necessity for increased size, with its inevitable sequence,
increased weight, must be realized without a very great addition to
the landing speed, the figure for the latter standing at approximately
45–50 m.p.h. This factor greatly influences the maximum wing loading
possible, without detrimentally affecting this, so that in the
design of the large machine a considerable increase in wing area is
unavoidable. This fact practically rules out the monoplane system for
the large aeroplane, as, although this arrangement possesses a superior
ratio of lift to drag to that of the biplane or multiplane, the great
span necessary to obtain the wing area is impracticable. It is quite
obvious that to brace adequately a monoplane structure of 100 ft. span
or so, a very complex system would be required, in addition to which
the spars would essentially be of larger and heavier section. The
biplane arrangement can be used successfully for spans up to 100 ft.,
and, assuming that the future commercial machine will necessitate still
greater wing area, it is a feasible supposition that the triplane, or
even quadruplane systems will be used. Certain modern triplanes have a
reputed excellent performance, the carrying capacity and engine power
being colossal. Against this we have the fact that the advantage of
the triplane system is purely structural, as aerodynamically it is not
nearly so efficient as the biplane, and it is at this stage that the
question of the limiting size of aeroplanes is encountered. Various
tests, both in model form and full size, have shown that the lift of
the middle plane of the triplane system is greatly inferior to that of
the top or bottom planes, this being due to the interference of the
free air flow by the upper and lower planes. This circumstance is an
indication that the biplane arrangement, viewed from the standpoints of
modern design, is the most economical form for future commercial use.