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Elements of Structures | |
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The basic structure of an airplane can be said to consist of three types of elements: stiffened shells, stiffened plates, and beams. On an airplane, the fuselage or the engine nacelles (the outer covering of an engine) are generally formed of stiffened shells. The top and bottom surfaces of the wings are made of stiffened plates. The wing itself can be considered an I-beam, with the top and bottom parts of the I along the skins of the upper and lower wing surfaces, and the spars inside the wing acting as the vertical bar of the I. When an element is described as stiffened, that generally means that oddly shaped pieces of metal are added to the back side of a plate or shell to strengthen it. Stiffeners allow a shell or plate to carry more load. Computing the loads on the different components of an airplane is very complex. In some geometrically simple parts, the tension loads (molecules being pulled apart), are relatively easy to compute, but compressive stresses (molecules being pushed together) are harder to calculate. Since the skins on the wings and fuselage of the plane are fairly thin (measured with respect to the width of the part), the shells or plates tend to buckle before reaching the failure point. Since buckling usually means that the component is deformed permanently, the engineers try to design stiffened shells and plates to postpone the buckling to a much higher stress level.
The best way to illustrate buckling is to use the simplest
form of buckling, which occurs in a long slender rod
or column. A piece of the rod is held vertically
and a series of weights are placed on the top of
the rod. Eventually, the weight on the top will
exceed the critical load, and the column will bend,
deforming permanently. Euler, in addition to his fluid dynamics
calculations, determined the expression to compute the critical load of
a column based on factors such as the strength of the
material (in a term called the modulus of elasticity),
the length of the rod, the shape of the rod
and its diameter or thickness.
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