Classification of types. While there are a number of methods adopted in
building up the fuselage structure, the common type is the "wire truss"
in which wood compression members are used in connection with steel wire
or cable tension members. Four wooden "longerons" or "longitudinals" run
the entire length of the body and are bent to its general outline. The
longitudinals are spaced at the correct distance by wood compression
members, which in turn are held in place by wire cross bracing. This
method of trussing forms a very strong and light structure, although
rather complicated, and difficult to build. The cross-section is
rectangular, although in many cases the body is converted into a
circular or elliptical section by the use of light wood formers fastened
to the main frame.
Another well known type is the "Monocoque" body, first used on the
Gordon-Bennett Deperdussin monoplane. This fuselage is a circular shell
built up of three-ply tulip wood, thus forming a single piece body of
great strength. The three-ply shell is really a veneer, the layers
proceeding spirally around the body, each layer being securely glued to
its neighbor. Between each layer is a scrim layer of treated silk, and
another fabric layer is generally glued to the outside of the shell. The
shell is very thin, the total thickness of the three layers of wood and
fabric in modern machines being rather less than 1.5 millimeters (about
1/16 inch). In the original "Deps" this was somewhat greater, 0.15 inch.
Monocoque construction as a rule is heavy and expensive, but offers the
great advantage of strength, perfect alignment at all times, and of
offering resistance to rifle and shell fire. If the longitudinals of a
truss type are struck with a bullet, or shell fragment, the entire
fuselage is likely to fail, but a monocoque body may be well perforated
before failure is likely to take place.
The American L. W. F. Tractor Biplane has a monocoque body in which
spruce laminations are used instead of hardwood. One ply runs
longitudinally while the other two layers are spiralled to the right and
left respectively. Between each layer is a scrim layer of treated silk,
the whole construction being covered with a final layer of fabric,
several coats of waterproof compound, and four final coats of spar
varnish. When used for seaplanes the wood plies are stitched together
with strong wires to prevent separation due to dampness. Since spruce is
used in place of hardwood, the construction is lighter than in European
models, and the L. W. F. Company claim that it is lighter than the usual
truss construction. An additional advantage of the monocoque
construction is that the pilot is protected against splinters or
penetration by the limbs of trees when making a forced landing in the
brush.
Another form of monocoque construction was adopted by the French
builder, Bleriot, at the beginning of the war. The fuselage of this
machine was covered with papier-mache, the ash longitudinals being
buried in this mixture. The papier-mache is built up with glue and silk
threads. This construction is very light and strong, but is expensive
and difficult to protect against moisture. The front of the fuselage is
protected with a 3 millimeter steel armor plate to protect the pilot
against bullets and shrapnel. The papier-mache portion of the body is
not easily splintered by bullets.
A third form of monocoque, experimented upon by the author, is the steel
shell type in which the three-ply wood veneer is supplanted by a thin
steel shell. This outer shell is strengthened by suitable stiffener
angles. With a shell thickness of 0.013 inch, the strength is equal to
the strength of a wood shell and is slightly less in weight. It has the
advantage of being easily and cheaply formed into shape and is
absolutely proof against the influences of heat and moisture. It cannot
splinter, will not catch fire and offers a maximum resistance against
penetration. There is yet much experimental work to be done before the
construction is perfected.
About midway between the truss fuselage and the monocoque is the veneer
construction used on many of the modern German aeroplanes. In general,
this may be described as being a veneer shell fastened to the
conventional wood longitudinals. Stay wires are not in general use, the
veneer taking the shear due to the bending movement. Six longerons are
used instead of four, the two additional members being located midway on
the vertical sides. Transverse wood frames take the place of the
transverse stay wires used in the truss type. Examples of this type are
met with in the "Albatros de Chasse" and in the "Gotha" bomb dropper.
The single seater, "Roland," has a fuselage of circular section, with a
true monocoque veneer construction, but German-like, reinforces the
construction with a number of very small longitudinals. In this machine
there are 6 layers, or plies, of wood reinforced by fabrics. The entire
thickness of the wood and fabric is only 1.5 millimeters (1/16 inch).
Steel tube fuselage dates back to the beginning of the aeroplane
industry. In this type the wood longitudinals of the wood truss type are
replaced with thin gage steel tubes, the cross struts being also of this
material. The diagonal bracing may be either of steel wire, as in the
wood frames, or may be made up of inclined steel tube members that
perform both the duty of the stay wires and struts. For the greatest
weight-efficiency, a steel tube body should be triangular in section
rather than square. A triangular section saves one longitudinal and a
multitude of wire struts and connections since no transverse bracing is
necessary. Connections on a steel tube fuselage are difficult to make
and are heavy. They require much brazing and welding with the result
that the strength is uncertain and the joint is heavy.
A very modern type of steel construction is that developed by the
Sturtevant Company. The members of the Sturtevant fuselage are in the
form of steel angles and channels, similar in many respects to the
sections used in steel buildings and bridges. The joints are riveted and
pinned as in steel structural work. The longitudinals are angles and the
struts are channels. Crystallization of the steel members is prevented
by the use of special pin-connected joints provided with shock absorbing
washers. Owing to the simplicity of the riveted joints, there is
practically no weight due to connections, and since the weight of
connections is a large item in the total weight of a fuselage, the
Sturtevant is a very light structure. According to G. C. Loening,
engineer of the company, the fittings of a large wood fuselage weigh at
least 60 pounds. This is almost entirely saved with the riveted
connections.
[Illustration: Truss Type Fuselage of Curtiss R-4 Biplane,
Showing Motor and Front Radiator Mounted in Place. It Will Be
Noted That the Upper and Lower Longerons Are Channeled Out for
Lightness and Hence These Members Are of the "I" Beam or Channel
Form. Propeller Flange Is Shown Projecting Through the Radiator
Opening.]
A novel type of wood fuselage has been described by Poulsen in "Flight."
Eight small longitudinals are used which are held in place by three-ply
wooden formers or diaphragms. Wire bracing is used in a longitudinal
direction, but not transversely in the plane of the diaphragms. The
cross-section is octagonal, and the completed structure is covered with
fabric. For the amateur this offers many advantages since the wiring is
reduced to a minimum and all of the members are small and easily bent to
shape. It is fully as light as any type of body, for the connections are
only thin strips of steel bolted to the diaphragms with small machine
screws. No formers are needed for the deck, and the machine can be given
a close approximation to the ideal stream-line form with little trouble.
*Truss Type Fuselage*. We will now take up the construction of the truss
type of fuselage in more detail, and investigate the merits of the
different types of connections used in fastening the frame together.
Like every part of the aeroplane, the fuselage must either be right or
wrong, there is no middle course. Fig. 23 shows a side elevation of a
typical truss type fuselage built up with wood longitudinals and struts,
the tension members being high tensile strength steel wire and cable. L
and L’ are the upper and lower longitudinals, S-S-S are the vertical
struts, and T-T-T are the horizontal cross struts which run across the
frame. The engine bed is the timber marked B at the front of the body.
The upper wing is attached to the body through the "cabane" struts C,
and the chassis connections are shown at D. The stern post E closes the
rear end of the body in a knife edge and acts as a support for the
rudder and the rear end of the stabilizer. F is the seat rail which
carries the seats and supports the control yokes.
All cross bracing is of high tensile strength steel wire, or of high
strength aviation cable, these strands taking the tensile stresses while
the wood struts are in compression. In the forward portion, double
stranded cables are generally used, with solid wire applied to the after
portions. The longitudinals are of ash from the motor to the rear of the
pilot’s seat, while the rear longitudinals are generally of spruce. In
some machines, however, the entire length of the longitudinals is ash.
The latter arrangement makes a heavier, but stronger body. The struts
are usually of spruce as this material is stiffer than ash and much
lighter.
[Illustration: Diagram of Typical Truss Type Fuselage, Showing
Principal Members in Place.]
[Illustration: Fuselage of Hansa-Brandenburg Fighting Biplane.
See Page 268. Figs. 27-28-29-30. Fuselage Details. (Truss Type)]
Both the struts and longitudinals are frequently channelled out for
lightness, as shown by Fig. 27, the wooden member being left rectangular
in section only at the points where the connections are made with the
struts and cables. The channelling-out process, if correctly followed,
gives very strong stiff members with a minimum of cross-sectional area
and weight. Many captured German machines, on the contrary, have solid
longitudinals of rectangular section, wrapped with linen fabric. This
fabric strengthens the construction and at the same time reduces the
chances of splintering the wooden members in a hard landing. The fabric
is glued to the wood and the entire wrapping is then given several coats
of a moisture repelling varnish. In the older types of fuselage, the
longitudinals were often of the "laminated" class, that is, were built
up of several layers of wood glued together in a single rectangular
mass. This reduced the tendency toward splitting, but was very
unreliable because of the uncertainty of the glued joints when exposed
to the effects of heat and moisture. Laminated longitudinals are now
seldom used, particularly in the region of the motor where water and oil
are certain to wreck havoc with the glued up members.
As the stresses rapidly diminish toward the tail, it is the general
practice to taper down the section of the longitudinal toward the rear
and to reduce the section of the struts. The longitudinals are generally
kept constant in section from the motor to the rear of the pilot’s seat,
the taper starting at the latter point and continuing to the rear end.
For example, if the longitudinal section at the motor is 1 1/4" x 1
1/2", the section at the rear will be 1" x 1", the width of the struts
corresponding to this taper. While tapering is very desirable from the
weight standpoint, it makes the fitting problem very difficult since
each fitting must be of a different dimension unless the connections can
be designed so that they are adjustable to changes in the section of the
longitudinals. In one machine, the width and depth of the longitudinals
are kept constant, the variation in weight and section being
accomplished by increasing the depth of the channelling as the rear is
approached. With this design, the same fittings can be applied from one
end to the other.
[Illustration: Figs. 27-28-29-30. Fuselage Framing Members and
Details.]
Since the loading of the struts is comparatively light, they can be much
reduced in section by channelling or by chamfering, as shown by Fig. 28.
If the width and thickness is maintained, much of the interior material
can be removed without danger of reducing the strength. Sketch (A) in
Fig. 28 shows a very common method of strut reduction, the strut being
of rectangular section throughout its length, but tapered in such a way
that it is thickest at the center (d) and thinnest at the two ends (e).
To obtain the correct relation between the center end thickness requires
very careful calculation. As shown, the strut is attached to the upper
and lower longitudinals by sheet steel fittings or "sockets." Sketch (B)
shows a simple method, the rectangular strut being chamfered off at each
of the four corners, and left full size at either end where the fittings
connect it with the longitudinals. This form is not correct from a
technical standpoint, but is generally good enough for lightly loaded
struts, and has the advantage of being cheaply and easily constructed.
In sketch (C) a channelled strut is shown, the center portion being
channelled out in a manner similar to the channelling of the
longitudinals. This lightening process is most commonly adopted with the
large heavily loaded struts in the front portion of the fuselage, and at
the points where the motor bed is suspended or where the wings and
chassis are attached to the body. The black dots at the ends of the
struts indicate the bolt holes for the fittings, it being permissible to
drill holes in the ends of the struts but not in the longitudinal
members. If the strut is large enough to resist the bending stresses at
the center it will generally allow of holes being drilled near the ends
without danger of strength reduction. Again, the struts are always in
compression and hence the bolts may be depended upon to partly take the
place of the removed material in carrying the compressive stresses.
Holes should never be drilled in the longitudinals since these members
may be either in tension or compression, depending upon the angle at
which the elevator flaps are set. The hole not only destroys the
strength at the point at which it is drilled, but this reduction also
extends to a considerable distance on either side of the hole, owing to
the fibrous nature of the wood. In steel members the effect of the hole
is purely local and does not usually extend much beyond the edge of the
hole. Considering the wood beam as consisting of a series of parallel
fibers, it will be seen that severing any one of the fibers will
decrease the strength of the wood through a distance equal to the length
of the cut fiber, or at least through a distance equal to the natural
shear value of the resins that bind the fibers together.
Fuselage fittings are almost numberless in the variety of design. They
must be very light and strong, must be applied without drilling the
longerons, and should be simple and cheap to construct. They are usually
made of sheet steel of from 0.20 to 0.30 point carbon, and may be either
bent or pressed into shape. At the points where the struts are joined to
the longitudinals, the fittings connect struts and wires in three
planes, the vertical struts and fore and aft wires; the transverse wires
and horizontal struts, and the top and bottom wires that lie in a
horizontal plane. There are at least 6 connections at every strut, four
of the connections being made to the stay wires or cables. A simple
connection is therefore very hard to design.
Fig. 29 shows a typical fuselage "panel" and the interconnected members
in their usual relation. LU and LL are the top and bottom longitudinals
at the right, while LU’ and LL’ are the longitudinals at the right hand
side. The vertical struts SV and SV’ separate the top and bottom
longitudinals, while the horizontal struts SH and SH’ separate the right
and left hand sides of the fuselage body. The wires w-w-w-w brace the
body fore and aft in a vertical plane. The wires t-t lie in a horizontal
plane, produce compression in the horizontal struts SH-SH’, and stiffen
the frame against side thrust. The transverse rectangle SV-SH-SV’-SH’ is
held in shape by the transverse stay wires W-W, this rectangle, and the
stays resisting torsional stress (twisting), act against the struts
composing the sides of the rectangle. In some European machines, the
wires WW are eliminated, and are replaced by thin veneer panels, or
short wood knee braces as shown by Fig. 30. The section shows the
longitudinals L-L-L-L and the struts SV-SV’-SH-SH’ braced by the veneer
sheet or diaphragm D. This diaphragm is well perforated by lightening
holes and effectually resists any torsional stress that may be due to
motor torque, etc. Since the transverse wires W-W in Fig. 29 are rather
inaccessible and difficult to adjust, the veneer diaphragm in Fig. 27
has a great advantage. In this regard it may be stated that wire bracing
is not a desirable construction, and the substitution of solid veneer is
a step in advance.
Wire bracing has always seemed like a makeshift to the author. The
compression and tension members being of materials of widely different
characteristics are not suitable in positions where a strict alignment
must be maintained under different conditions of temperature and
moisture. The difference in expansion between wire and the wood
compression members produces alternate tightness and slackness at the
joints, and as this is not a uniform variation at the different joints,
the frame is always weaving in and out of line. Under the influence of
moisture the wood either swells or contracts, while the wire and cable
maintain their original lengths and adjustments. The result is that a
frame of this kind must be given constant attention if correct alignment
is desired.
The adjustment of a wire braced wood fuselage should be performed only
by a skilled mechanic, as it is easily possible to strain the members
beyond the elastic limit by careless or ignorant handling of the wire
straining turnbuckles. In the endeavor to bring an old warped fuselage
back into line it is certain that the initial tension in the wires can
be made greater than the maximum working stress for which the wires were
originally intended. Shrinkage of the wood also loosens the bond between
the wooden members and the steel fittings unless this is continually
being taken up. Some form of unit construction, such as the monocoque
body, is far more desirable than the common form of wire trussed wood
body.
[Illustration: Fuselage Details of De Havilland V. Single Seat
Chaser. A Rotary Le Rhone Motor Is Used in a Circular Cowl. The
Diagonal Bracing in the Front Section is Reinforced by Laminated
Wood Plates Instead of by Wires. Dimensions in Millimeters.]
*Fuselage Fittings*. In the early days of aviation the fuselage fittings
on many machines were made of aluminum alloy. This metal, while light,
was uncertain in regard to strength, hence the use of the alloy was
gradually abandoned. At present the greater part of the fittings are
stamped steel, formed out of the sheet, and are of a uniform strength
for similar designs and classes of material.
The steel best adapted for the fittings has a carbon content of from
0.20 to 0.30, with an ultimate strength of 60,000 pounds per square
inch, and a 15 per cent elongation. The steel as received from the mill
should be annealed before stamping or forming to avoid fracture. After
the forming it can be given a strengthening heat treatment. A lower
steel lying between 0.10 and 0.15 carbon is softer and can be formed
without annealing before the forming process. This material is very
weak, however, the tensile strength being about 40,000 pounds per square
inch. Fittings made of the 0.15 carbon steel will therefore be heavier
than with the 0.30 carbon steel for the same strength. The thickness of
the metal will vary from 1/32" to 1/16", depending upon the load coming
on the fitting.
A typical fuselage strut fitting is shown by Fig. 31-A in which L-L-L
are the longerons, d is the fitting strap passing over the longerons, S
and So are the vertical and horizontal struts respectively. The stay
wires are fastened to ears (b) bent out of the fitting, the wires being
attached through the adjustable turnbuckles (t). The struts are provided
with the sheet steel ferrules marked (F). There are no bolts passing
through the longitudinals L-L’, the fitting being clamped to the wooden
member. This is very simple and light fitting. Fig. 31-B is a similar
type, so simple that further discussion is unnecessary.
Fig. 32 shows a fuselage strut fitting as used on the Standard Type H-3
Biplane. We are indebted to "Aerial Age" for this illustration. This
consists of a sheet metal strap of "U" form which is bent over the
longitudinal and is bolted to the vertical strut. At either side of the
strut are through bolts to which bent straps attach the turnbuckles.
These straps are looped around the bolts and form a clevis for the male
ends of the turnbuckles.
[Illustration: Fig. 31. Typical Fuselage Strut Fittings.]
An old form of fuselage connection used on the Nieuport monoplane is
shown by Fig. 33, an example of a type in which the bolts are passed
through the longeron member. This fitting is very light but
objectionable because of the piercing of the longeron.
An Austrian aeroplane, the Hansa-Brandenberg, has a wood fuselage in
which no stay wires are used. This fuselage is shown by Fig. 23a. Both
the vertical and inclined members are wood struts. The outer covering of
wood veneer makes the use of stay wires unnecessary since the sheath
takes up all horizontal stresses, and hence forms a sort of plate girder
construction. The German Albatros also employs a wireless veneer
fuselage, the construction being shown in detail by Figs. 36 and 36a.
Three longerons are located on either side of the body, the third member
being placed at about the center of the vertical side. As will be seen,
the veneer makes the use of wire bracing and metal connections
unnecessary. The veneer also insures perfect alignment.
[Illustration: Fig. 32. Fuselage Strut Fittings of the Standard
H-3 Training Biplane.]
*Wing Connections*. The lower wings are attached to the lower
longitudinals by a special sheet steel fitting which also generally
connects to a vertical strut at this point, and to an extra heavy
horizontal strut. A sheet metal clevis, or socket, on the wing spar is
pinned to the fuselage half of the fitting so that the wing can be
easily detached when the machine is to be dissembled. At this point a
connection is also provided for the end of the inner interplane stay
wires. The horizontal strut at the point of wing attachment is really a
continuation of the wing spar and takes up the thrust due to the
inclination of the interplane stays. In the majority of cases the
horizontal thrust strut is a steel tube, with the hinged connection
brazed to its outer ends. This is one of the most important and heavily
loaded connections on the machine and should be designed accordingly.
[Illustration: Fig. 33. Fuselage Fittings of the Nieuport
Monoplane.]
Fig. 37 shows a typical wing to fuselage connection of the hinge type.
The wing spar (G) is covered with a sheet steel ferrule (A) at its inner
end. Two eye bars (B) are bolted to the wing spar, and over the ferrule,
the eyes of the bar projecting beyond the end of the spar. This forms
the wing half of the connecting hinge. The eyes are fastened to the
fuselage hinge member (H) by means of the pin (E). This pin has a
tapered end for easy entry into the joint, and is pierced with holes at
the outer end for cotter pins or a similar retaining device. The
fuselage hinge member (H) is brazed to the end of the steel tube strut
(T). This tube runs across the fuselage from wing spar end to wing spar
end.
Strut tube (T) lies on, and is fastened to, the fuselage longeron (L),
and also lies between the two halves of the vertical strut (S). The
vertical strut is cut out at its lower end for the receipt of the steel
tube (T). A steel plate is brazed to the tube, is wrapped about the
longeron (L) and is bolted to the vertical strut (S). The interplane
stay (F) is attached to the pin (E) at the point of juncture of the wing
spar eye and the fuselage member of the hinge. A collar (I) is brazed to
the tube, and forms a means of attaching the fuselage stays (D). The
drift wires (C) of the wings are attached to an eye at the end of one of
the wing spar bolts. As shown, the fitting (H) is a steel forging, very
carefully machined and reduced in weight. The inside wing ribs are
indicated by (K), from which it will be seen that there is a gap between
the end of the wings and the outside face of the fuselage.
[Illustration: Fig. 36. Veneer Fuselage Construction of the
German "Albatros" Speed Scout. Body Outline Is Obtained by
Veneer Diaphragms and no Stay Wires Are Used.]
Fig. 36-a shows the construction of the wing joint of the German
Albatros machine. The fuselage is of monocoque construction which allows
of a simple attachment to the outer shell. This is a very sturdy and
simple connection. Fig. 38-Z is the wing attachment detail of the
English London and Provincial Biplane (1916), the fuselage in this case
being of the wire trussed wood type. We are indebted to "Flight" for
this illustration.
[Illustration: Fig. 36-a. Details of Albatros Veneer Fuselage
Construction.]
In some machines the interplane stay wires are attached to a lug formed
from the attachment plate, but we do not consider that this construction
is as good as the type in which the wire is attached directly to the
wing spar pin. While the former may be easier to assemble, the
attachment of the wire to the pin eliminates any eccentricity, or
bending moment, due to the pull of the interplane stay. The attachment
in the L. W. F. insures against any eccentricity in the stay attachment,
and at the same time makes the assembly and dismounting a very simple
matter.
[Illustration: Fig. 37. Wing Connection to Fuselage.]
*Chassis Member Attachment*. The attachment of the chassis struts
generally involves some difficulty as these members usually intersect
the line of the longerons at a very awkward angle. If the wing
attachment is near the same point, as it generally is, the detail is
made doubly difficult. The chassis must be pin connected as in the case
of the wing joint so that the chassis members can be easily and quickly
removed. A detail of a chassis to body connection is shown by Fig. 39.
In this figure (L) is the lower longeron, (S) is the vertical fuselage
strut, and (C) is one of the chassis members. The upper end of the
chassis member is enveloped in a sheet steel ferrule (D) which is bolted
in place, and which is provided with a clevis at its upper end for the
attachment pin (P).
A plate (E) is bolted to the fuselage strut (S) and is passed around the
lower longeron (L), a hinge joint (H) being provided for attachment to
the chassis ferrule through the pin (P). Ears or lugs are left at (G-G)
for the attachment of the fuselage stays (B-B). On the inner side of the
plate (E) are attachment lugs for the horizontal strut (H). It will be
noted that the plate (E) is well provided with lightening holes so that
the weight can be kept down to a minimum. The pin is tapered at the end,
and is provided with cotter pin holes. The fitting in general is small,
and does not produce any great degree of head resistance, the small part
exposed being of good streamline form.
[Illustration: Fig. 38x. Wing Connection of the Albatros Reconnaissance
Biplane. Fig. 38y. Wing Attachment of Albatros Fighter with Pin Joint.
Fig. 38z. Wing Connection of London and Provincial Biplane.]
Great care should be taken in brazing or welding these fittings, since
the heat changes the structure of the metal and greatly reduces its
strength. The brazing temperature varies from 1,500 to 1,700 degrees, a
point well above the tempering heat of steel. Attempts have been made to
heat treat the metal after the brazing operation, but with very little
success, owing to the fact that the heat treating temperature is
generally at or above the melting point of the brazing spelter, hence is
likely to cause holes and openings in the brazed joints. With acetylene
welded joints the parts can, and should be, heat treated after the
welding. While this is an apparent advantage of acetylene welding, all
parts cannot be successfully handled in this manner. The welding torch
can only join edges, while the brazing spelter can be applied over
almost any area of surface. Welding is very successful in joining thin
steel tubes while in many fittings made of sheet metal, brazing is the
only feasible operation.
[Illustration: Fig. 39. Chassis Connection.]
Both methods have a common fault, in that they are unreliable. Imperfect
welds and brazing are not always apparent from the outside, actual
breakage of the part being necessary to determine the true nature of the
joint.
FUSELAGE WEIGHTS.
*Distribution of Weight*. The weight of a fuselage depends upon the span
of the wings, upon the seating capacity, and upon the weight and type of
the power plant. The weight also varies considerably with the type of
construction, that is, whether of truss, veneer, or monocoque
construction. A heavily powered machine, or one carrying more than a
single person, requires heavier structural members and hence weighs more
than a small single seater. The amount of fuel carried also has a
considerable bearing on the fuselage weight.
Probably the best method of treating this subject is to give the
fuselage weights of several types of well known machines. The reader
will then have at least a comparative basis for determining the
approximate weight. (Truss type only.)
There are so many variables that the weight cannot be determined by any
set rule or formula. Alexander Klemin in his "Course in Aerodynamics and
Airplane Design" says that the approximate weight of a bare wood truss
type fuselage is about 150 pounds for a machine having a total weight of
2,500 pounds. For small biplane and monoplane scouts weighing
approximately 1,200 pounds total, the bare fuselage frame will weigh
about 70 pounds. These figures are for the bare frame alone and without
seats, controls, tail skids or other fittings. The weights given under
the column headed "Wt. Bare" include the engine beds, tail skids,
flooring, cowling and body covering, and hence exceed the "bone bare"
estimate of Klemin by a considerable amount.
The all-steel fuselage of the large Sturtevant battle-plane (Model A)
weighs 165 pounds inclusive of the steel engine bed. A wooden, wire
braced fuselage of the same size and strength weighs well over 200
pounds, the metal fittings and wires weighing about 60 pounds alone. Ash
is used in the wood example for the longerons. The struts and diagonal
members in the Sturtevant metal fuselage are riveted directly to the
longitudinals, without fittings or connection plates. The safety factor
for air loads is 8, and for the ground loads due to taxi-ing over the
ground, a safety factor of 4 is used.
After a minute comparison of the items comprising the fuselage of the
Curtiss JN4-B and the Standard H-3, Klemin finds that the fuselage
assembly of the Standard H-3 amounts to 13.6 per cent of the total
loaded weight, and that the fuselage of the Curtiss JN4-B is 15.5 per
cent of the total. Tanks, piping and controls are omitted in both cases.
For machines weighing about 2,500 pounds, Dr. J. C. Hunsaker finds the
body weight averaging 8.2 per cent of the total, this figure being the
average taken from a number of machines.
On careful examination it will be found that the fuselage assembly
(bare) amounts to a trifle less than the wing group for biplanes having
a total weight of from 1,900 to 2,500 pounds. The relation between the
wing weight and the fuselage weight seems to bear a closer relation than
between the fuselage and total weights. We will set these different
relations forth in the following table:
PERCENTAGE OF FUSELAGE WEIGHT
──────────────────────────────────────────────────────────────────────
Fuselage Weight as
Percentage of the Total
Load
Name of ─────────────────────────────────── Wing Weight As
Plane or Body Body Body Percentage of Total
Investigator Assembly Assembly Assembly Weight
Bare and and
Equipment Power
Plant
──────────────────────────────────────────────────────────────────────
Curtiss 15.50% 17.86% 43.96% 14.15%
JN4-B
──────────────────────────────────────────────────────────────────────
Standard H-3 13.60% 17.70% 45.40% 14.52%
──────────────────────────────────────────────────────────────────────
J.C. 8.20% 11.50% 34.30% 16.50%
Hunsaker
──────────────────────────────────────────────────────────────────────
Author’s 14.96% 17.62% 46.66% 14.60%
Experience
──────────────────────────────────────────────────────────────────────
Average of 13.06% 16.17% 42.58% 14.94%
Above
──────────────────────────────────────────────────────────────────────
In the above table, the column headed "Body Assembly and Equipment"
includes the body frame, controls, tanks and piping. In the fourth
column, the radiator, motor, propeller, water, and exhaust pipe have
been added. For the average value it will be seen that the bare fuselage
is about 1.88 per cent lower than the weight of the wings. It should be
noted that the wing weight given is the weight of the surfaces alone,
and does not include the weight of the interplane struts, wires and
fittings. The weight of the wing surfaces as above will average about
0.75 pounds per square foot.
Based on the above figures, we can obtain a rough rule for obtaining the
approximate weight of the fuselage, at least accurate enough for a
preliminary estimate. If A = the total area of the wings, then the total
weight of the wings will be expressed by w = 0.75A. The weight (f) of
the fuselage can be shown as f = 13.06/1494 x w = 0.65A.
*Example*. The area of the Standard H-4 is 542 square feet total. Find
the approximate weight of the fuselage. By the formula, f = 0.65A = 0.65
x 542 = 352 pounds. The actual bare weight is 302.0 pounds. For several
other machines, the actual weight is greater than the weight calculated
by the formula, so that the rule can be taken as a fair average,
especially for a new type that is not as refined in detail as the H-3.
SIZE OF LONGERONS
The size of the longerons, that is, the section, is influenced by many
factors. As these members must resist flying loads, the leverage of
elevator flaps, stresses due to control wires, landing stresses and the
weight of the motor and personnel it is always advisable to itemize the
loading and then prepare a diagram to obtain the stresses in the
different members. This latter method is a method for a trained
engineer, but an exhaustive description of the method of procedure will
be found in books on the subject of "Strength of Materials." For the
practical man, I give the following list of longeron dimensions so that
he will have at least a guide in the selection of his material.
The length of the fuselage and power of motor are given so that the
reader can obtain sizes by comparison, although this is a crude and
inaccurate method. As the longerons taper from front to back, the sizes
of the section are given at the motor end, and also at the tail. The
size of the front members depends principally upon the weight of the
motor and the passenger load, while the rear longerons carry the
elevator loads and the tail skid shock. If the rudder is high above the
fuselage it introduces a twisting movement that may be of considerable
importance. The loads on the stabilizer, elevators and the vertical
rudder are very severe when straightening out after a steep dive or in
looping, and the pull on the control wires exerted by the aviator at
this time greatly adds to the total stress. In the front of the
fuselage, the motor exerts a steady torque (twist) in addition to the
stress due to its weight, and to this must be added the gyroscopic force
caused by the propeller when the machine is suddenly changed in the
direction of flight. The combination of these forces acting at different
times makes the calculation very difficult.
In the case of the Curtiss R-4, the front longerons taper down from the
motor 1.63" x 1.25" to a point directly behind the pilot’s seat, the
section at the latter point being 1.25" x 1.25". From this point the
rear longerons taper down to 1" x 1" at the tail. At the motor, the
section is 1.63" x 1.25". The longitudinals of the Bleriot monoplane are
laminated and are built up of alternate layers of spruce and ash. This
is an old type of machine and this practice has since been discontinued.
It will be noted that as the power is increased, the size of the front
longerons is generally increased, although this is not always the case
in speed machines. The Chicago Aero Works’ "Star" fuselage could easily
carry a 90 horsepower motor, although this size is not regularly
installed.
*Pusher Type Fuselage (Nacelle)*. Compared with the tractor biplane and
the monoplane fuselage, the body of the pusher is very short and light.
The latter body simply acts as a support for the motor and personnel
since the tail loads are carried by the outriggers or tail booms. The
motor is located at the rear end of the body and may be either of the
air or water-cooled type. The accompanying figure shows a typical pusher
type body, or "Nacelle" as it is sometimes called.
The advantages of the pusher type for military service are obvious. The
observer or gunner can be placed immediately in the front where his
vision is unobstructed, and where the angle of fire is at a maximum.
[Illustration: Typical Pusher Body Showing Wings, and Outrigger to Tail
Surfaces.]
*Twin Motored Fuselage*. Twin motored aeroplanes generally have the
power plants mounted at a point about midway between the fuselage and
tips of the wings. In almost every case, the power plants are of unit
construction, that is to say, consist of the motor, radiator and
propeller complete on one support, only the fuel and oil tanks being
mounted in the fuselage. The fuselage of the twin may be similar in
length and general construction to that of the tractor biplane, or it
may be a short "nacelle" similar to that used in the pusher type. In any
case, the observer can be located in the extreme front of the body.
An interesting and unusual construction is the body of the Caproni
Biplane (1916). A center nacelle carries the passengers, a pusher screw
being located at the rear of the central body as in the case of the
pusher biplane. On either side of the center are the motors driving the
tractor screws, each motor being encased in a long tractor type fuselage
that also supports the tail surfaces. The latter fuselage serves to
streamline the motors and takes the place of the usual outrigger
construction. There are three bodies, two tractor screws, and one pusher
screw. Somewhat similar in design is the famous German "Billy
Two-Tails," this machine being equipped with two tractor type bodies. A
motor is located in the front of each body. Each fuselage is provided
with accommodations for passengers, and is long enough to support the
tail surfaces. The Caproni and the German machine are both very large
machine and heavily powered.
*U.S.A. Sea-Plane Specifications (1916)*. These government
specifications cover a twin motored sea-plane with a central nacelle.
The body is arranged so that the forward man (observer) can operate the
forward machine gun through a horizontal arc of at least 150°, and
through a vertical arc of at least 270°, with the gun at an angle of
about 75° with the center line of the body. The muzzle must be forward
of the propeller plane. The rear man (pilot) operates a machine gun
through a vertical arc of at least 150° to the rear, and through a
vertical arc of at least 180°, with the gun at an angle of about 105°
with the fuselage center line. The muzzle must be to the rear of the
plane of propeller rotation.
The number of stays and other important connections which extend across
the plane of propeller rotation shall be reduced to a minimum. It is
considered advisable to incorporate in the design of the body such a
structure (in the plane and 8 inches forward of propeller plane) as will
prevent a broken propeller blade from severing the main body. The system
used in the construction of the cage masts used on battleships is
suggested, with a number of spruce compression members in place of
stay-wires. The clearance of the propeller tips from the sides of the
central body shall be from 5 to 12 inches. No part of the gas tanks
shall lie in the plane of propeller rotation, nor within a space 6
inches ahead of this plane.
A space extending at least 9 inches back from the rear of the observer’s
seat, and entirely across the body, must be left open and unoccupied in
order that any desired instruments can be installed therein. In the
center line of the body, a circular hole 9 inches in diameter shall be
cut in the floor of the observer’s cock-pit, the rear of the hole being
5 inches forward of the forward edge of the observer’s seat. The
flooring of the pilot’s and observer’s cockpits shall consist of spruce
strips 1/2" x 1/2 " spaced at 1/2" intervals along the longerons. No
flooring is to be placed under the seats.
The safety factor of the body and tail structure shall not be less than
2.5, the air speed being taken at 100 miles per hour with the elevator
at an angle of 20° and the fixed stabilizer surface at 6°. All wire
tension members not readily accessible for inspection and adjustment are
to be single strand high tensile steel wire. All tension stays that are
easily accessible shall be of non-flexible stranded steel cable. For
turnbuckle safetying No. 20 semi-hard copper wire shall be used. All
cable shall be well stretched before making up the connections. A load
equal to 20 or 30 per cent of the breaking load shall be applied for a
period of from two to three hours. The hard wire must undergo a bending
test by bending at a right angle turn over a radius equal to the
diameter of the wire, back and forth four times each way. No more than
four sizes of turnbuckles shall be used on the entire aeroplane
structure. The strengths and size numbers of the turnbuckles will be as
follows: No. 1 = 8,000 lbs. No. 2 = 4,600 lbs. No. 3 = 2,100 lbs. No. 4
= 1,100 lbs. Controls and fittings in the vicinity of the compasses
shall, as much as possible, be of non-magnetic material. All steel plate
and forged fittings shall be protected against the action of salt water
by baking enamel, the best standard three coat process being used. All
covered wiring and turnbuckles shall be coated by at least two coats of
Flexible Compound.
All steel tubing shall be thoroughly cleaned, slushed with mineral oil
inside, and plugged at both ends by wood plugs impregnated with mineral
oil or paraffine. All steel nuts, bolts, pins and cotter pins shall be
protected by heavy nickel plating over copper. All wood members,
especially faying surfaces, end grain butts, scarfs and joints, shall be
protected against the access of moisture before final assembly by the
best grade of varnish, or by impregnation by paraffine. All wood shall
be straight grained, well seasoned, of uniform weight, and free of
knots, pitch pockets, checks or cracks. Spruce to be of the very highest
grade of selected straight, even grained, clear spruce. It shall be air
seasoned, preferably for two years. Kiln dried wood is not acceptable.
It is highly desirable to have all bolts, pins, plate fittings and
turnbuckle ends made of chrome vanadium steel (S. A. E. Specification
6.130), heat treated to obtain the best physical characteristics. All
parts and fittings that must be bent shall be heat treated after all
bending operations are completed, and by such a sequence of treatment as
will produce the desired grain and toughness, and relieve all stresses
due to the bending. This includes sheet and forged steel fittings,
turnbuckle ends and bolts and pins. All steel parts and fittings
submitted to stress or vibration shall be heat treated in such a manner
as to produce the highest possible refinement of grain and give the
greatest possible resistance to alternating and vibratory stresses.
Where plate fittings are in contact with wooden members, sharp edges
next to the wood shall be removed. In making up and connecting steel
fittings, welding shall be used wherever possible. If impracticable to
weld, and in such cases only, brazing will be used, proper heat
treatment to be employed to restore strength and toughness of metal
after such welding or brazing. Extreme care should be taken to avoid
nicking or kinking any wire, cable or fitting. Fittings, sheet or
forged, must be free from sharp corners and supplied with generous
fillets.
In general the S.A.E. Standards will be acceptable, and these standards
for screw threads shall be used wherever possible. U.S. Standard threads
will be accepted where threaded into cast iron, cast aluminum or copper
alloys. All nuts and pins must be provided with one or more positive and
durable safety devices. In general, where it must be expected that a
structural fitting will be disassembled a number of times during the
life of the aeroplane, castellated nuts with split pins, in accordance
with S.A.E. Standards, shall be used. Wherever this is not the case,
pins or bolts shall be riveted in a workmanlike manner.
Seats shall be securely braced against both horizontal and vertical
stresses. Arrangement and dimensions of cock-pits shall be as nearly as
practicable to that indicated by the drawings (not published in this
chapter). In addition, if practicable, the pilot should be provided with
quick release arm rests. Sections of best grade of khaki on each side of
seats, in which pockets are made, should be fastened to longerons and
vertical posts in such a way as to be securely in place and yet readily
detachable for inspection of structural wiring and fittings. Safety
belts shall be provided for both seats and securely fastened. The belts
shall safely support at any point a load of 2,000 pounds applied as in
practice. Rubber shock absorbers in the safety belt system are
considered to be an advantage. The quick release device shall be as
indicated in drawings and shall reliably and quickly function. Seat pads
shall be quickly detachable in order that they may be used as life
preservers. They will be filled with Kapok or other similar material and
covered with real leather to protect it against the action of salt
water.
Suitable covers shall be provided over the top of the rear end of the
fuselage. These must be easily removed and capable of being securely
fastened in place during flight. Space shall be allowed in the body
directly in the rear of the observer’s seat for the stowage of the sea
anchor. When in use, the sea anchor shall be attached by suitable and
convenient fastening hooks to the two points along the lower longerons,
and at the junction of the two vertical struts in the rear of the front
seat. The structure must be such that it will successfully withstand the
stresses imposed by the sea anchor. Controls shall be of the standard
Deperdussin type, installed in the rear cock-pit only. The tanks for the
main supply of gasoline shall be in the fuselage and located so that the
longitudinal balance will not be disturbed by the emptying of the tank
during flight.
The above data is not in the exact form of the original specifications
and is not complete, but gives only the specifications that affect the
design of the body. These were picked out part by part from the
original.
*Army Specification 1003 (Speed Scout)*. These specifications cover the
design of land machines, the extracts given here referring only to the
safety factor. Body forward of the cockpit shall be designed for safety
factor of 10 over static conditions, with the propeller axis horizontal.
Body in rear of cockpit shall be designed to fail under loads not less
than those imposed under the following conditions:
(a) Dynamic loading of 5 as the result of quick turns in pulling out of
a dive. (b) Superposed on the above dynamic loading shall be the load
which it is possible to impose upon the elevators, computed by the
following formula: L = 0.005AV², where A is the total area of the
stabilizing surface (elevators and fixed surface), and V is the
horizontal high speed of the machine. The units are all in the metric
system. (c) Superposed on this loading shall be the force in the control
cables producing compression in the longerons.
*Fuselage Covering*. Disregarding the monocoque and veneer constructed
types of fuselage, the most common method of covering consists of a
metal shell in the forward end, and a doped linen covering for that
portion of the body that lies to the rear of the rear seat. The metal
sheathing, which may be of sheet steel or sheet aluminum, generally runs
from the extreme front end to the rear of the pilot’s cockpit. Sheet
steel is more common than aluminum because of its stiffness. Military
machines are usually protected in the forward portions of the fuselage
by a thin armor plate of about 3 millimeters in thickness. This is a
protection against rifle bullets and shrapnel fragments, but is of
little avail against the heavier projectiles. Armor is nearly always
omitted on speed scouts because of its weight. Bombers of the
Handley-Page type are very heavily plated and this shell can resist
quite large calibers.
The fabric used on the rear portion of the fuselage is of linen similar
to the wing covering, and like the wing fabric is well doped with some
cellulose compound to resist moisture and to produce shrinkage and
tautness. On the sides and bottom the fabric is supported by very thin,
light stringers attached to the fuselage struts. On the top, the face is
generally curved by supporting a number of closely spaced stringers on
curved wooden formers. The formers are generally arranged so that they
can be easily removed for the inspection of the wire stay connections
and the control leads. On some machines the top of the fuselage consists
entirely of sheet metal supported on formers, while in others the metal
top only extends from the motor to the rear of the rear cockpit.