Complex frames with fixing moments

Imagine a simple frame to have its joints welded instead. If the frame was initially deficient, as shown in Fig. 11.10(b), then it would be improved by welding and would become, for many purposes, a good

approximation to a perfect frame. Furthermore, it could possibly be lighter than the pinned version, because of the omission of the diagonal member.

In the pinned case the forces in the members can be resolved as pure end-loads, with no bending. In the welded frame, compared with the pinned in Fig. 11.11, bending is transmitted to the members by means of

the frame distorting while the angles at the corners remain unchanged. Instead of pure end loads each member is subjected to a system of forces such as that shown in Fig. 11.11(d).

Fig. 11.11 Plane-frame with fixing moments at joints.

This kind of arrangement of forces and moments happens constantly in aircraft work as structural members, which have torsional and bending strength, must be arranged to take torsion and bending, and thus pay a little more fully for the carriage of their bulk. Such arrangements of members to satisfy a vast number of different stressing cases give rise to the apparent paradox of redundancy in aircraft structures. Some

examples of complex frames with fixing moments are shown in Fig. 11.12. To analyze them rather elegant strain-energy methods must be employed in place of the simpler force diagram.

Fig. 11.1.2 Three examples of redundant frames with applied loads and moments from wing and undercarriage.

Strain energy

displacement. The magnitude of the work is given by the product of the force and the distance through which it moves. In a similar way, when a force or a stress is applied to a body, strain occurs and the force is said to do work. In each of the frames shown so far each force causes a component strain in each member. Equations can be stated for each structure which, when resolved, describe the distribution of strain energy (work causing strain) between the members caused by the individual forces. Solution of the equations depends upon the principle that a load ‘chooses’ the path of least work, for structures too obey the law of conservation of energy. 11.4 Structural design

Historically the main parts of an aircraft structure are the fuselage, wings and tail. Most bodies are built in the same way as a fuselage, most aerofoil surfaces in the same way as wings. The sketch in Fig. 11.13 suggests the salient features of an aircraft structure.

Fig. 11.13 Sketch of main details of aeroplane structure.

Taking the fuselage first, the skin is usually formed of metal sheets riveted, or spot-welded, to metal frames, formers and bulkheads. Generally speaking a frame has the outline of a cross-section of a body and is built up from a number of smaller members. A former has the same outline, but is much lighter and is usually pressed from sheet metal. The centre is cut away, so that a former is really a stiffening outline for maintaining the form of the skin. In section it may be of ‘Z’ ( ) or ‘top-hat’ ( ) section: the latter being in effect two ‘Z’ sections facing each other and joined together along the upper edge. A bulkhead is a complete section cutting like a diaphragm across a body. As such it may be built up like a frame, or pressed from sheet. A bulkhead may be pierced by holes and doorways, but these are usually covered by plates and doors that form part of the load-bearing bulkhead structure.

Running lengthwise along the fuselage, supported by the bulkheads, frames and formers and, in turn, supporting the skin are the stringers. Stringers are light members that may be of ‘Z’ or ‘top-hat’ section. Related to the stringers in that they run fore and aft, but serving a major structural purpose in that they are designed to take end-loads, are the longerons. If the fuselage is viewed in elevation it is seen to be a long beam, supported by the wings somewhere between 40 and 50% of the length from the nose. There are local loads applied to the beam from tail and nosewheel, and perhaps from engine-mountings, with local

distributions of loading from payload and equipment. Tension and compression in the structure above and below the neutral axis of the fuselage must be met by end-loads in the longerons aided by the skin. In a similar way side-loads on the fuselage are met by longerons and skin. If the skin is thick enough there may be no longerons as such, the end-loads being met by an arrangement of stringers, slightly heavier in section than usual.

Across large cutouts a structure may contain internal bracing members made up of struts and ties. Depending upon the load directions in Fig. 11.10 the individual members may be either struts or ties: struts being members end-loaded in compression, ties being loaded in tension. Struts and ties are rarely intended to take torsion and bending.

shape of the skin may also be maintained by spanwise stringers that serve a major purpose in effecting a reduction in spar sections and weights, by distributing end-loads into the skin. Ribs may be built up like frames, be light as formers, or be made like bulkheads. The latter are found in wing structures used to contain fuel, without recourse to internal, separate, fuel tanks. Spars, ribs and skin form the tank surfaces.

The remaining shape of an aeroplane is largely non-structural, in that it consists of fairings, cowlings and fillets. These items are made of shaped skin, stabilized by stringers and formers.