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Development of Aircraft Structures | |
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The very early airplanes were built from very lightweight materials such as bamboo, wood, and fabric. They were designed much like bridges were, with beam and truss construction. A truss is a series of straight bars (beams) connected by wires or thin rods on the diagonals so that the spaces in between the bars are cut into triangular spaces. The wings on the Wright flyer form a truss; the two wings used wires and bars diagonally to strengthen the wing to withstand the aerodynamic forces. The fuselage of the Bleriot XI monoplane is another good example of this construction. The insides of wings were also a type of truss construction. The bars inside were called spars, and wires were used on the diagonals to strengthen the wing. The spars, plus the spar caps at each end, were shaped to give the wing the right shape for the desired aerodynamic characteristics. This shape is often called the airfoil. The figure below shows the basic construction of the wing of the Sopwith Camel World War I fighter. As manufacturing techniques for metals improved in the early 1900's, metal rods and pieces began to replace the wooden components in airplanes. Metal skins, rolled very thin, were used because they were less affected by weather than the fabric skins. The ribs and spars of the plane were made by riveting many pieces together. When aluminum alloys became available at the end of the Twenties, these pieces were often stamped out whole from aluminum sheets. A major and important development in structural design for aircraft was the decision to use the metal skins to carry some of the aerodynamic loading. The very thin metal sheets were made thicker to help distribute the loading. For a slight increase in weight for the thicker skins, a greater savings in overall weight was realized by the designers being able to decrease the structures inside the wing! As the use of metals in the airplanes became more commonplace, the basic design of airplanes also changed. The original biplane design with struts and bracing wires, which worked so well for the first low speed flights, was no longer so efficient at the higher speeds that were coming into the field. Protuberances like the spars and wires caused more drag at the higher speeds. In addition, with essentially two wings, the induced drag for a biplane is much higher than for a monoplane. The biplane designs gradually gave way to monoplane designs. Using the metal skins to carry some of the loading gave the unbraced monoplane wings the strength to handle the aerodynamic loads without protruding struts or excess weight. Work continues in the field of aircraft structures for a better balance of weight and strength. Improving manufacturing techniques for spars and ribs, better and more consistent alloys, and composite materials are all contributing to new structural designs.
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