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

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

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.

Bending

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

   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

   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 axis.
   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

traditional construction

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.

Semi-monocoque

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.