Atmospheric Flight

Structure

All the different pieces of an airplane must be held together by a sound structure, like the steel frame of a building or the chassis of a car. The structure must be strong so that the airplane doesn't break when the aerodynamic forces (weight, lift, thrust and drag) act on it. It must be stiff so that the pieces maintain their correct aerodynamic position during flight. Stiffness against twisting is particularly important during turbulent flight. The amount of force also depends on the flight speed. Design rules include a warning that a certain airplane design must not fly faster than the recommended speed. Flying faster than the recommended speed could cause the airplane's structure to break apart under the stress.

Usually airplanes can be made stiffer and stronger by using more or heavier material. It is more expensive to buy the extra material, but it can be worth it to avoid worrying about airplane's structure breaking apart. Unfortunately, for airplanes, the weight of the material is much more important than the cost. Any extra weight for the body of the airplane means less weight is available for payload (passengers or cargo). Airplane designers need to make airplanes as lightweight as possible without making it too weak. An airplane that is too heavy and strong cannot carry payload efficiently.

Airplanes are made of materials that are stiff, strong and lightweight. Early airplanes were made from fabric with lightweight, but strong wood. Later they were constructed of lightweight sheet metal. Now airplanes are made up of a mix of fabricated metal and composites that provide strength without adding a lot of weight.

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