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Structure is how the plane is built and with what materials. An airplane must be light in weight, but strong at the same time. There are many forces acting on an airplane. The structure must be able to stay together even through gusts of wind or flying at speeds above normal. Aircraft structural design is different from other structural fields (such as buildings or ships) because the plane must be both lightweight and strong. Of the four forces involved in flight (lift, drag, thrust, weight) structure affects the weight. The total weight of the plane is: the aircraft itself (empty weight) plus the passengers, crew, baggage and freight (payload), and the fuel. This is called takeoff weight. There must be enough lift to get the total weight of the airplane into the air. Engineers also consider cruising weight and landing weight. These weights are the totals of the empty weight, payload weight , and the weight of the fuel at the time. As an airplane gets ready to takeoff, several things have to be considered. The takeoff weight must be able to get off the ground before the end of the runway! Also, the distance to travel, with a heavy payload, may require more fuel than the plane carries. Tradeoffs may have to be made. Lighter payloads for shorter runways; larger airplanes with more fuel to carry heavy payloads long distances. Weight becomes very important.
The ability of the airplane to withstand the aerodynamic forces during
flight is the other important part to the science of structures. The
aircraft structural designer builds with 2 goals in mind: the first is
to minimize the load on an individual piece (or part) so that it will
last for a long time. The second is to coordinate all the pieces so
that if one unit fails, the other units will take on the load. The
first goal is called "safe life". Each unit is designed to have the
least load or force possible on it. This lengthens the life of each
part beyond the plane's expected life or until scheduled replacement.
"Fail safe" is the second goal and it means that the overall airframe
(structure) is designed so that failure in one component doesn't cause
the whole airplane to come apart.
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