No aircraft structure is completely rigid, so when it is subjected to aerodynamic forces it will normally deflect by a small amount. This effect can become very important at high speeds because any change in the shape of the body can cause the applied aerodynamic forces to change, leading in turn to further deflection and further changes in load. This vicious circle can rapidly develop into aeroelastic phenomena such as flutter and wing divergence .

Flutter

Flutter occurs when a lifting surface deflects under aerodynamic load so as to reduce the applied load. Once the load reduces, the deflection also reduces, restoring the original shape, which restores the original load and starts the cycle again. In extreme cases the elasticity of the structure means that when the load is reduced the structure springs back so far that it overshoots and causes a new aerodynamic load in the opposite direction to the original. Even changing the mass distribution of an aircraft or the stiffness of one component can induce flutter in an apparently unrelated aerodynamic component.

At its mildest this can appear as a "buzz" in the aircraft structure, but at its most violent it can develop uncontrollably with great speed and cause serious damage to or the destruction of the aircraft

Wing divergence

Wing divergence occurs when a wing deflects under aerodynamic load so as to increase the applied load, or move the load so that the twisting effect on the structure is increased. The increased load deflects the structure further, which causes a further increase in load, until the structure fails.

Prediction and cure

Aeroelasticity involves not just the external aerodynamic loads and the way they change but also the structural, damping and mass characteristics of the aircraft. Prediction involves making a mathematical model of the aircraft as a series of masses connected by springs and dampers which are tuned to represent the dynamic characteristics of the aircraft structure. The model also includes details of applied aerodynamic forces and how they vary.

The model can be used to predict the flutter margin and, if necessary, test fixes to potential problems. Small carefully-chosen changes to mass distribution and local structural stiffness can be very effective in solving aeroelastic problems.

See also: Aerospace engineering