FUSELAGE FITTINGS.
The design and type of fitting employed for connecting the longerons,
cross and vertical struts of the fuselage, varies greatly, being
usually one of the distinctive constructional details of a machine.
This position renders uniformity of practice a comparatively
unattainable quantity, which, in view of present requirements, and
the absolute need of rapidity of output (which must commence as soon
as possible after a successful design is produced), can only be
considered as regrettable. This diversity of design is mainly the
result of the desire for originality of each individual designer,
and however commendable from this standpoint, is a position which is
almost certain to disappear with the progress of the industry. Take as
a hypothesis the case of, say, ten makes of scouting biplanes in use,
each with approximately the same arrangement of longerons and struts,
and with a similar overall size of fuselage. Each of these machines
will incorporate a different fuselage clip, which means that somewhere
highly skilled labour is being unnecessarily expended in the making
of jigs and press tools, whereas a suitably standardized clip for all
scout machines of certain dimensions would involve the making of one
set of press and bending tools only for the machines of the one type
built. Another aspect, quite as important, is the simplification of
the supply of spares. Acceleration of aircraft output, if achieved
only through the medium of small part production, is one of the most
important contributary factors towards ultimate success in the field.
Types in Use.
The sketches, explained in detail hereafter, are illustrative of some
of the many systems in use, and taken collectively fall under two
categories: (1) those in which attachment to the longeron involves
drilling, and (2) those in which the fitting clips or encircles the
longeron, friction only keeping it in position. The first method
permits of a clip of comparatively simple design, but it has the
serious disadvantage of weakening the material, and assuming the
longeron section is sufficient to account for this, then clearly a
fitting which is attached without the use of bolts would allow a
reduction in the size of a longeron (which means a saving in weight)
without depreciating the factor of safety. In the second method the
attachment is usually accomplished by the pressure of bolts, with
practically no weakening effect; but in this case the disadvantage lies
in the fact that at each point of attachment a differently dimensioned
clip will be necessary, this being due, as explained in the previous
chapter, to the longeron tapering towards the stern post. (A method
of reducing the number of different clips by suitably shaping the
longeron was also dealt with.) It is evident that most fittings must
inevitably form a compromise between the demands of production and
design, although it must be admitted that in some cases the fittings
collectively very successfully evade the requirements of both.
[Illustration: FIG. 76.--Fuselage fitting.]
The clip indicated by Fig. 76 is the particular form of construction
associated with the various versions of the Bleriot monoplane, and
favoured by the early pioneers generally. It was retained in the
Bleriot construction until some time after the outbreak of the war--as
a matter of fact, until the type was deleted for war purposes.
It is composed, as will be seen from the sketch, of simply two U-bolts,
the attachment to the longeron involving the drilling of four holes,
which constitutes the chief objection to this particular form of
clip, and has been the subject of criticism from the time of its
first appearance as an aircraft detail. The struts are slotted over
the bolt, and although this does not conform to the best principles,
it is simple, and may have been sufficient for a lightly loaded
machine. A point about this clip, which undoubtedly was the cause
of its popularity amongst the pioneers, with whom economy was an
evil necessity, is that the wires can be strained or tensioned by an
adjustment of the nuts on the longerons, thus rendering turnbuckles
[Illustration: FIG. 77.--Fuselage fitting.]
unnecessary. Fig. 77 shows the form of clip used on the Hanriot
monoplanes, and is a good example of the class of fitting bent up
from sheet metal. This is usually made to be slightly smaller than
the longeron, the pressure resulting from the tightening of the bolts
on the ends forming the cross-bracing lug, keeping it in place. The
defect of this arrangement is that any slight shrinkage of the longeron
will permit movement, and for this reason provision should be made for
the subsequent adjustment of the bolts. The struts are taken by the
lugs punched up from the body of the clip. This leaves very little
material to resist the tension of the bracing wires or tierods, but a
modification of this clip surmounts this difficulty.
[Illustration: FIG. 78.--Fuselage fitting.]
A similar clip is used by the German Aviatik firm, but it is certainly
inferior owing to the very poor connection of the struts. Instead of
the four lugs gripping the sides of the struts, they are punched up to
form a square or tenon, over which the cross-struts are mortised. In
any case it would not satisfy the standards maintained by our leading
constructors, and certainly not the technical advisers to the Air
Board. The clip indicated in Fig. 78 does not encircle the longeron,
but abuts against the two inner sides of the longeron only. The body
of this clip is a stamping, bent to a right angle, to which the square
sockets for the struts are welded. Attachment to the longeron is
effected by an eyebolt, which passes diagonally through it, this also
providing the anchorage for the cross-bracing wire. A form of this
clip has been used on a certain make of machine for a considerable
period, so that it has advantages that are not readily apparent. One
outstanding defect is existent in that the pull of the wires would tend
to lift the socket on the side opposite to the eye bolt, and this in
turn would cause distortion of the struts. A connection favoured by an
American firm is shown by Fig. 79, and possesses the merit of extreme
simplicity. The longerons are not drilled, the attachment being through
the agency of an L bolt, which also provides the anchorage for the
cross-bracing wire. To prevent movement the clip is additionally fixed
to the longeron by wood screws.
[Illustration: FIG. 79.--Fuselage fitting.]
[Illustration: FIG. 80.--Fuselage fitting.]
The method shown by Fig. 80 is that used on the Deperdussin monoplanes,
being patented by that firm as far back as 1912, and consists of two
cast aluminium sockets, bolted to the longeron. The struts, in this
case oval in section, are fastened in place by steel bushes, which
are driven through in the form of steel tube, and expanded and burred
round the socket, at the same time forming the anchorage for the
wires. This system has been used in the construction of a fast scout
of comparatively recent origin, but it embodies the same defects as
the Bleriot clip, _i.e._ four holes are needed in the longeron for
every joint; but it has the advantage over the latter in that a better
terminal is provided for the struts.
[Illustration: FIG. 81.--Fuselage fitting.]
Fig. 81 indicates the arrangement on the German L.V.G.
(Luft-Verkehrs-Gesellschaft) fighting biplane. This is an aluminium
alloy casting, fastened to the longeron by screws, and as it is not
affected by the taper of the longeron, all the fittings, or at any
rate, those in the tail portion, can be of the same dimensions. A
point which is often overlooked when using a fitting of this type is
that any strain on the wires is transmitted to the longeron by the
fastening screws only, or, in other words, the tendency of the wires
when tensioned to pull the fitting from the longeron is resisted by the
screws only. This does not impress one as being well suited to perform
the functions demanded of the average joint, and about the only detail
upon which its existence is justified is its ease of production. In the
writer’s opinion the clip, Fig. 82, is by far the finest connection
yet devised, and one that should be standardized. Its attachment is
accomplished without objectionable drilling; it provides an excellent
housing for the cross-struts; can be tightened up should shrinkage
occur in the longerons; and can be produced at an absurdly low figure.
This clip has been used on machines which have accomplished some
meteoric performances during the war, and, moreover, was designed and
in use a considerable period before the war.
[Illustration: FIG. 82.--Fuselage fitting.]
[Illustration: FIG. 83.--Fuselage fitting.]
The clip, Fig. 83, is simple and quite easily manufactured, being
stamped out of sheet metal, and bent up to shape. The lugs forming the
anchorage for the wires would have a tendency to straighten out at
the bends; but the amount of this, whether serious or otherwise, in
the absence of actual experience, is largely conjectural. However, a
fitting of this kind was used in the construction of the nacelle of a
seaplane exhibited at Olympia in 1914.
Steel Tube Fuselage Construction.
[Illustration: WELDED WIRES 18 BWC
FIG. 84.--Welded joint in steel tube fuselage.]
In certain isolated instances, the fuselage is built up of steel
tubing, and on one machine of recent design the joints throughout are
effected by welding: a detail of the attachment of the vertical and
cross struts to the longerons is shown by Fig. 84. It will be noticed
that a small quadrant-shaped piece of tube or rod is welded to the
struts, and from this are taken the bracing wires. As the welded joints
impart a certain rigidity to the structure, the fact that the wires
are exerting a side pull on the struts may be of little consequence,
although this method could hardly be used in conjunction with the
fuselage construction of average English machines. A rather unusual
feature may be noticed in the attachment of the bracing wires, which
are not finished off with the orthodox wire ferrule, but are arranged
as a loop, the turnbuckle forming the anchorage for the two ends.
The trend of design in this country seems to incline towards the clip
stamped out from sheet steel and bent up. This class of fitting can be
produced accurately and quickly, and, in the writer’s opinion, is by
far the best manufacturing proposition. Aluminium castings are quite
obsolete, and the built-up fitting, involving welding or brazing, does
not seem greatly in vogue.