There are two general types of fuselage construction—welded  steel  truss  and  monocoque designs. The welded steel truss was used in smaller Navy aircraft, and it is still being used in some helicopters. The monocoque design relies largely on the strength of the skin, or covering, to carry various loads. The monocoque design may be divided into three classes—monocoque, semimonocoque, and reinforced shell. !    The true monocoque construction uses formers, frame assemblies, and bulkheads to give shape to the fuselage. However, the skin carries the primary stresses. Since no bracing members are present, the skin must be strong enough to keep the fuselage rigid. The biggest problem in monocoque construction is maintaining enough strength while keeping the weight within limits. !    Semimonocoque  design  overcomes  the strength-to-weight problem of monocoque construction. See figure 4-6. In addition to having formers, frame assemblies, and bulkheads, the semimonocoque construction has the skin reinforced by longitudinal members. !    The reinforced shell has the skin reinforced by a complete framework of structural members. Different portions of the same fuselage may belong to any one of the three classes. Most are considered to be of semimonocoque-type construction. The semimonocoque fuselage is constructed primarily of aluminum alloy, although steel and titanium are found in high-temperature areas. Primary bending loads are taken by the longerons, which usually extend across several points of support. The longerons are supplemented by other longitudinal members known as stringers. Stringers are more numerous and lightweight than longerons. The vertical structural members are referred to as bulkheads,   frames, and   formers. The heavier vertical members  are  located  at  intervals  to  allow  for concentrated loads. These members are also found at points where fittings are used to attach other units, such as the wings and stabilizers. The stringers are smaller and lighter than longerons and serve as fill-ins. They have some rigidity but are chiefly used for giving shape and for attachment of skin. The strong, heavy longerons hold the bulkheads and formers. The bulkheads and formers hold the stringers. All of these join together to form a rigid fuselage framework. Stringers and longerons prevent tension and compression stresses from bending the fuselage. The skin is attached to the longerons, bulkheads, and other structural members and carries part of the load. The fuselage skin thickness varies with the load carried and the stresses sustained at particular loca- tion. 4-6 ANf0406 Figure 4-6.—Semimonocoque fuselage construction.