the fuselage structure
The word fuselage is based on the French
word fuseler, which means "to streamline." The fuselage must be strong and
streamlined since it must withstand the forces that are created in flight. It
houses the flight crew, passengers, and cargo.
Fuselages are classified according
to the arrangement of their force-resisting structure. The types of fuselages we
will study are the truss and the semi-monocoque. Five types of stress act on an
aircraft in flight: tension, compression, bending, shear, and torsion. Let's
look at each one individually (figures 1-2).
Tension is the stress which tends to pull things apart.
When you try to break a length of rope, you exert a type of stress which is
called tension. (see figure 1-2a)
fig 1 - 2a tension stress
Compression is the opposite of tension. It is the
stress which tends to push materials together. When you grasp a football at both
ends and push, the ball is subject to compression. The landing gear struts of an
aircraft are also subject to compression.
This type of stress combines tension and
compression. You put a bending stress on a bar when you grasp it with both
hands and push the ends together or when you bend a paper clip. The wing spars
(interior structural members) are subjected to bending while the aircraft is in
flight. The lower side of the spar is subjected to tension, while the upper side
is subjected to compression. Obviously, some materials will break before they
bend and often are unacceptable for aircraft construction. (see figure 1-2c)
fig 1 - 2 c bending stress
Shear stress is caused by forces tending to slip or
slide one part of a material in respect to another part. This is the stress
that is placed on a piece of wood clamped in a vice and you Chip away at it with
a hammer and chisel. This type of stress is also exerted when two pieces of
metal, bolted together, are pulled apart by sliding one over the other or when
you sharpen a pencil with a knife. The rivets in an aircraft are intended to
carry only shear. Bolts, as a rule, carry only shear, but sometimes they carry
both shear and tension. (see figure 1-2d)
1 - 2d shear stress
Torsion is the stress which tends to distort by
twisting. You produce a torsional force when you tighten a nut on a bolt.
The aircraft engine exerts a torsional force on the crankshaft or turbine
All the members (or major portions) of an aircraft
are subjected to one or more of these stresses. Sometimes a member has alternate
stresses, such as compression one instant and tension the next. Some members can
carry only one type of stress. Wire and cables, for example, normally carry only
tension. (see figure 1-2e)
fig 1 - 2e torsional stress
Since any member is stronger in
compression or tension than in bending, members carry end loads better than side
loads. In order to do this, designers arrange the members in the form of a
truss, or rigid framework (see figure 1-3). In order for a truss to be
rigid, it must be composed entirely of triangles. When the load on a truss acts
in one direction, every alternate member carries tension while the other members
carry compression. When the load is reversed, the members which were carrying
compression now are subjected to tension and those which were carrying tension
are under compression. The truss itself consists of a welded tubular steel
structure with longerons (horizontal members) and diagonal braces. These
features make it rigid, strong, and light.
fig 1- 3 truss-type fuselage structure
The truss is covered with a
metal or fabric cover so that less drag will be generated. To produce a smooth
surface, the fabric cover is put on fairing strips, which are thin flat strips
of wood or metal. These fairing strips run the length of the fuselage in line
with the direction of flight.
The semi-monocoque is the most often
used construction for modern, high-performance aircraft. Semi-monocoque
literally means half a single shell. Here, internal braces as well as the skin
itself carry the stress (see figure 1-4). The internal braces include
longitudinal (lengthwise) members called stringers and vertical bulkhead.
The semi-monocoque structure is
easier to streamline than the truss structure. Since the skin of the
semi-monocoque structure must carry much of the fuselage's strength, it will be
thicker in some places than at other places. In other words, it will be thicker
at those points where the stress on it is the greatest.
fig 1- 4 semi-monocoque - type fuselage structure
Some aircraft use a
combination of construction methods. The superb Mooney range of four seat
aircraft, for instance, use a steel tube truss frame around the passenger
compartment with semi-monocoque behind.
Many modern light aircraft are
constructed using composite (glass fibre) materials. This type of construction
is effectively monocoque and is extremely strong. Extra rigidity can be achieved
by using glass foam sandwich construction.
the Glasair is a typical example of a sandwich composite monocoque design