FUSELAGE CONSTRUCTION.
The body, or fuselage as it is generally described, constitutes the
nucleus of the completed machine, and at the same time offers the
most interesting examples of constructional detail. It may be as well
to point out that the term “fuselage” is ordinarily applicable to a
body of a machine of the tractor type; the short body of the average
“pusher” or propeller aeroplane is termed the “nacelle.”
The material chiefly used in the construction of this component is
wood, and there are but very few instances where metal is used.
Fuselage Types.
The different types or methods of construction may be classified in the
following order:--
1. Box-girder of four longerons or rails, with cross-struts and wire
bracing (Fig. 65).
[Illustration: FIG. 65.--Arrangement of fuselage members.]
2. Tail portion of longerons, struts and wiring; in the front portion
the wire bracing is dispensed with, being replaced by diagonal wood
bracing, to which is screwed either three-ply or sheet aluminium alloy
(Fig. 66).
3. In this case wire bracing is entirely dispensed with, the four,
and occasionally six, longitudinals being connected together by cross
struts or formers cut to the required shape, the whole body being
covered with three-ply.
4. Laminated or monocoque type, formed by layers of wood and fabric,
crossed alternately and glued together.
[Illustration: FIG. 66.--Arrangement of fuselage members.]
Box-Girder Type.
Dealing with each type in greater detail, and in order of
classification, the details and methods of manufacture of type 1 may
be considered. The longerons are usually of ash or hickory, although
latterly silver spruce has come into use for this purpose, this being
due to the desire to reduce weight to the absolute minimum.
In the opinion of the writer, a spruce longeron should be of larger
section than one of hard wood, for one or two reasons. Spruce is a
soft wood, and the outside fibres are far more apt to get damaged by
a fitting which has been bolted home with too much pressure, also the
corners may get rubbed or knocked off, which all means a reduction in
strength. The use of a spruce longeron precludes any sharp bends in the
contour of the fuselage, as this wood does not lend itself to bending,
although it may be sprung to an easy curve. By disposing joints in
the longerons, it is possible to arrange the lengths so that the bend
is contained in one portion. This portion can then be of laminated
construction, _i.e._ it can be built up of a number of layers glued
together, and clamped to a block of the required shape until the glue
has set. In some cases the longerons from the engine mounting to the
rear cockpit, where additional strength is necessary, are of ash, while
aft of that, to the stern post, spruce is the material.
[Illustration: FIG. 67.--Longeron sections.]
It is usual, in this country at least, to spindle the rails to one of
the sections illustrated by Fig. 67, this spindling or channelling
running through from nose to stern post, or the front portion,
extending as far as the rear cockpit, is left solid, the tail part only
being spindled. This channelling is always stopped at the intersection
of the cross-struts with the rails, to provide the abutment for the
struts, and the extra material to compensate for any holes necessary
for the attachment of the fitting. In the shaping of the rails
longitudinally, two methods are available: they may be tapered or
gradually diminished from the front to the stern post, or the overall
section may be parallel to a point somewhere in the neighbourhood of
the pilot’s seat, and from that point diminished to the stern post. The
first method is obsolete, as all the fittings vary in size, which makes
for undue complication as well as increasing the number of jigs and
dies necessary to produce the stampings. The second method only partly
obviates this, and the only system which permits of the same size
fitting being used right through is that in which the rails are of the
same overall section throughout, but this is very rarely used.
Another arrangement consists of keeping the rail of equal thickness
for approximately 10 ft. from the engine bearers, and then diminishing
in a series of steps to the stern post. By this method only three or
four sizes of fittings are necessary. Some fittings are not affected by
the taper of the rails, and are made the same size throughout, but in
nearly every case the attachment to the rails is accomplished either by
bolts or screws. The piercing of the longeron, particularly when this
is of spruce, is hardly commendable practice, and certainly in view
of the many forms of clip fittings in use appears to be unnecessary.
A point which apparently escapes the notice of some designers, is the
necessity of some allowance being made for unfair stresses induced by
landing shocks and rough handling. There is a tendency to make the
tail portion separate from the front, the joint occurring just aft
of the rear cockpit, so that in the event of damage due to strains
transmitted by the tail skid, this portion can be detached and a new
portion substituted, which seems infinitely better than dismantling the
whole machine and returning the whole body to the works or depôt. In
the design of the body under consideration due regard should be given
to the necessity of occasional replacement of a damaged rail. Some
fittings afford the utmost facility for this, while others render this
procedure a lengthy and difficult operation.
Jointing of Longerons.
A popular method of jointing longerons consists usually of a plain butt
joint, clipped with some form of steel tube socket, or by fish-plates
flanged to clip the edges of the longerons and bolted through. A
spliced joint is sometimes used when timber is not procurable in
any great length, this consisting of an ordinary splice from 12 to
18 ins. long, glued and riveted, and afterwards, when the joint is
thoroughly set, bound with tape soaked in glue and subsequently doped
and varnished. As this is a somewhat lengthy operation the socket
method predominates. In modern aeroplanes the size of a longeron rarely
exceeds 1½ ins. square, and it will therefore be realized that this
construction is all that is possible, as, owing to the slightness of
material, no advantage would accrue from the employment of a joint of
the halved or scarfed variety.
Diagonal Wood Bracing.
A great deal of the foregoing applies to the second type, so far as the
longerons and tail portion are concerned. The diagonal wood bracing
is usually of spruce, and is, of course, heavier than wiring. The
aluminium or duralumin sheeting has latterly given place to three-ply
for the outside covering, which may be ascribed to the saving in weight
effected by its use, as a square foot of 20 B.W.G. aluminium, which is
the general thickness for this purpose, weighs 8 ozs., while a square
foot of 3/32 in. birch three-ply weighs approximately 5 ozs. This
gives a saving of 3 ozs. for every square foot of surface covered, and
moreover three-ply, properly glued and screwed or copper, nailed to
the framework, constitutes by far the better stiffening medium. The
disadvantages of this method of construction are: (1) the difficulty of
re-truing the front portion should distortion occur; (2) erection is
somewhat involved; and (3) it is heavier than the first type, although
it affords a more solid mounting for the engine, with a consequent
reduction of vibration.
Three-ply Fuselage.
The third system is typical of the method adopted for the series of
German Albatross machines. There are few, if any, examples of its use
in this country, although prior to the war a few constructors favoured
its use, and one successful monoplane of note was so built. The writer
is acquainted with one pioneer designer who very strongly believes
in this form of construction, and certain later developments in the
use of three-ply confirm this view. The advantages of this form of
construction are: (1) quickness of production; (2) great strength in
a vertical and horizontal direction; (3) the result of the longeron
being shot through would not endanger the structure to the same extent
as with a wire-braced system. Against this must be balanced the fact
that: (1) it entails a considerable increase in weight; (2) is weak
under a torsional strain, such as that produced by the combined actions
of elevator and rudder; and (3) cannot be trued up in the event of
distortion. Examples of this system in pre-war machines are afforded by
the Martinsyde and Blackburn monoplanes, although the framework in both
cases was so formed as to constitute a lattice girder. The tail portion
of the Martinsyde was lightened by cutting away diamond-shaped pieces
from each bay.
The formers of the Albatross are extremely simple. In the fore part
they are cut from three-ply, while at the rear they are just simple
frames composed of laths, reinforced where the longerons occur by
three-ply stiffeners. There are six longerons, the two middle ones
being fixed slightly more than halfway up each side, which are really
longitudinal stringers to prevent the three-ply buckling between the
points of attachment.
The Monocoque Type.
The monocoque system originated in France, several constructors having
produced machines incorporating this feature. The most successful
machine produced on these lines was the Deperdussin, and many will
recall the excellent streamline form of the machine exhibited at
the 1913 Aero Show. These bodies were built over formers of various
sections, which were removed when the glue joining the different layers
had set. The resultant shell, which was about four millimetres thick,
was then covered with fabric and varnished. Several factors militate
against its extensive adoption as a method. It is rather costly, and
does not seem to be suited to rapid production. In addition, the
attachment of such members as the chassis, wings, and interplane
struts, is more complicated. It should be noted, however, that various
modern machines are similarly built. The Borel firm produced a machine
with monocoque body, this being composed of three-ply covering on
ribs running diagonally the length of the body, and although this is
not such a lengthy operation as the Dep. system, it has not survived,
unless one considers flying-boat construction as its modern version.
A slight variation of the monocoque system is used for the bodies of
some modern aeroplanes. The framework consists of very small stringers
arranged at various points on light formers cut to the fuselage
section. To this structure is applied two thicknesses of three-ply in
the form of strips about 3¼ ins. wide, each thickness being disposed
diagonally in opposite directions, as shown by Fig. 68. This is covered
with fabric, the total thickness being no more than 1½ mm., and as this
is made up of six layers of wood and one of fabric, the fineness of the
ply-wood will be realized.
[Illustration: FIG. 68.--Arrangement of three-ply bands in monocoque
fuselage.]
It should be noted that the ply-wood strips do not completely encircle
the formers, but are jointed at the top and bottom, a light longeron
being arranged at these points.
A detail which would appear to be of great utility at the present
time is the arrangement wherein the nose of the body containing the
engine and accessories is a separate unit, and in the event of engine
breakdown can be detached and another substituted.
Fuselage Contours.
In the design of the contour of the fuselage the type of the motor
used is the determining influence. With the vertical “in line” engine,
it is possible to design a slim narrow body, while a rotary or radial
engine necessitates an increase in width, which also means increased
air resistance. With the Vee type engine, the popular practice is to
allow the tops of the cylinders to project through the cowling, which
permits of a narrower body than if the width of the body equalled the
overall width of the engine. Where a rotary engine is employed and
the mounting is of the overhung type, the width of the fuselage may
be reduced by allowing the engine to project over the sides, and the
cowling carried on an arrangement of formers and stringers, which
gradually merges into the main structure, as in Fig. 69. It is apparent
that the line of the body and that of the fairing should converge as
gradually as possible, as, should this be at all abrupt, there is a
distinct possibility that the air flow will take the course indicated
in Fig. 70, resulting in a dead air region and inefficiency.
[Illustration: FIG. 69.--Fuselage outline.]
[Illustration: FIG. 70.--Fuselage outline.]
It may be taken generally that the wider the body the greater the
weight, for the struts have not only to be made longer but also of
greater overall section. The practice in this country is to keep the
longerons parallel to the centre line on plan, as far as the rear
cockpit, tapering from that point to the stern post in a straight or
slightly curved line, as Fig. 71.
[Illustration: FIG. 71.--Fuselage outline.]
This simplifies the fittings, the sockets for the centre plane struts
are in line, and the different lengths of fuselage struts necessary
reduced to a minimum.
[Illustration: FIG. 72.--Fuselage outline.]
The plan outline of several German machines is shown diagrammatically
by Fig. 72. It will be seen that from the nose the body gradually
widens out until the maximum width, generally in the vicinity of the
front seat, is reached, from where it tapers to the tail. This shape
appears to satisfy aero-dynamic requirements more closely than either
of the foregoing examples; but in practice the difference is not
appreciable, and in any case the reduction of head resistance does not
compensate for the additional work.
[Illustration: FIG. 73.--Fuselage outline.]
[Illustration: FIG. 74.--Fuselage outline.]
In side elevation the general practice, with exceptions, is to arrange
the top longerons parallel to the line of thrust, _i.e._ the axis
of the motor, as in Fig. 73. This simplifies erection and affords a
convenient datum line for truing up.
[Illustration: FIG. 75.--Fuselage outline.]
On the German Rumpler and early Albatross biplanes, the upper longerons
are curved, as in Fig. 74, but in the most recent versions of the
Albatross they are level with the line of thrust. Fig. 75 illustrates
an arrangement where the top rails, from a point some distance along,
slope down to the nose. By this method the body weight is kept as low
as possible and the engine and accessories rendered more accessible.
Although it is usual to terminate the body in a vertical knife-edge,
formed generally by the rudder post, another arrangement, typical
of the Morane monoplane, finishes in a horizontal edge. The German
Fokker, obviously inspired by the French Morane, and the Albatross
DI, are similarly terminated. This system of tapering to a horizontal
knife-edge is not considered the best arrangement from a strength point
of view, the flat angle of the bracing wires permitting a certain
amount of movement, eventually resulting in slackness and loss of
alignment.