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Aircraft Structure

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How can a machine be light enough to fly but strong enough to stay intact under great stress? How can a wing carry fuel and engines without breaking off? All while being made of beer can aluminum?

One key is the material aircraft are made of. Aluminum is almost always alloyed with other metals, often copper or magnesium, for vastly improved properties. Solution heat treat processes bring about striking improvements in the strength of these alloys. Steel, titanium and composites are also used.

Aircraft and bridges have a lot in common. The cantilever design is commonly found in wings. One wing supports the other in the same way adjacent bridge spans do. This cancels the need for wing braces found on many small aircraft. "W" shaped warren trusses are found throughout aircraft, especially in wings and tubular frames.

The shape of a part decides how rigid it will be. Compare a sheet of tin foil to a piece that has been formed into any shape. The difference in strength is incredible. I recently formed a structural component made of .060" aluminum alloy. The forming process changed it from being as limp as a piece of poster board into part of a critical wing spar. Aircraft use this phenomenon extensively. The same applies to solid rods of metal - a hollow tube is actually stronger. Unfortunately, a dent or kink will cause a very dramatic weakening.

Most aircraft are of the "semi-monocoque" type. The skin is stressed and provides a substantial amount of the strength. The structure is composed of a series of longerons, ribs, spars, formers, webs, stringers and bulkheads. These provide the basis for a strong structure that the skin holds together. Rivets have allowed aircraft to attain monstrous size. Countless small parts are mechanically joined to achieve fantastic strength. Riveted skin panels are strong enough to contribute greatly to the sound structure of airframe.

Riveting doublers into high stress areas cause the metal to perform like it's much thicker. But stress on the structure must not be permitted to focus sharply on a small area. Smooth transitions are critical. Some skin panels are chemically etched to gradually taper in thickness. This improves the even transmission of stress from wing tip to fuselage. Engineers go to great lengths to gradually and evenly direct stress to the major structures. The de Havilland Comet failed structurally because cracks formed at the square window openings. Aircraft mechanics can inadvertently cause a failure by increasing the strength of a repair when stress is supposed to gradually dissipate onto the skin.

While some structures must be rigid, many areas need carefully designed flexibility. The wing tips of a Boeing B-52 bomber routinely flex twelve feet. Attempts to make them solidly rigid would add massive weight and still end in catastrophe.

Thousands of small parts are built up into sub assemblies. These in turn form the main sections of the aircraft. The fuselage often contains several sections and terminates with the empenage. The vertical and horizontal stabilizers are part of this section. The rudder and elevator are then attached. Wings, engines and landing gear will complete the structure of the airplane.

Much of what we know has been paid for in lives. The structure of today's aircraft is the result of a century of development. Incredible effort goes into the structural design of aircraft. Aerodynamics, weight and balance, strength, performance and safety are meticulously perfected. The result is what we take for granted. Advances in composite materials will give us even better "flying machines" in the future.

More about this author: Dave Jackson

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