Aeroelasticity is the study of the interaction of aerodynamic stresses, inertia, and elastic responses in physical structures. Such interactions can produce both static and dynamic responses. Unstable dynamic responses in components can lead to structural failure under certain conditions. Aeroelasticity is typically concerned with designing structures to be stable when subjected to a dynamic air flow. These structures are often aircraft, but they can also include bridges, wind turbines and other terrestrially-based elements.
Most materials, including metals, exhibit elastic behavior when responding to external stresses. Elastic materials will return to their original size and shape if they are not deformed beyond a critical amount. While being deformed, they will stretch or shrink according to the level of stress applied. A metal spring stretches out when pulled at the edges, but does not remain permanently deformed after it is released. In fact, even solid pieces of metal behave in this way.
In an airplane, external aerodynamic forces apply mechanical stress to the wings and main body. In terms of aeroelasticity, this stress is similar to a stress applied directly to the material—for example, from placing weights on the airplane. In response, the structure of the airplane will deform slightly due. This will slightly alter the shape of the plane, which will in turn affect the exact aerodynamic stress. In a static scenario, the structural response of the airplane will reach equilibrium with the new aerodynamic stresses.
When a structure begins to deform because of aerodynamic stresses, it will gain inertia, or momentum, as it moves to change shape. Once it reaches its new “equilibrium” position, it does not immediately stop; rather, it overshoots this position because it has gained inertia. Aerodynamic stresses may tend to restore the structure to an equilibrium shape, but sometimes an oscillation can occur. It requires friction or some kind of damping force to slow this oscillation. In other words, the structure may have an equilibrium shape, but if it picks up too much inertia each time it moves toward that shape, it will be in an unstable equilibrium.
Many people witnessed this important aspect of aeroelasticity on 7 November 1940, when the Tacoma Narrows Bridge in the U.S. state of Washington began vibrating because of high winds. The natural frequency of the bridge, which is related to how fast the bridge will vibrate, happened to be similar to the rate the wind changed directions. When this happens, the wind can cause the bridge to vibrate more and more. In the case of Tacoma Narrows Bridge, the runaway structural vibration led to the bridge’s destruction. This event led to an increase in aeroelasticity interest and research.