In the field of mechanics and structures, flutter refers to an aeroelastic phenomenon where a body's own aerodynamic forces couple with a its natural mode of vibration to produce rapid periodic motion. Aeroelastic flutter occurs under steady flow conditions, when a structure's aerodynamic forces are affected by and in turn affect the movement of the structure. This sets up a positive feedbackloop exciting the structure's free vibration. Flutter is self-starting and results in large amplitude vibration which often lead to rapid failure.
The aerodynamic conditions required for flutter vary with the structure's external design and flexibility, but can range from very low velocities to supersonic flows. Large or flexible structures such as pipes, suspension bridges, chimneys and tall buildings are prone to flutter. Designing to avoid flutter is a fundamental requirement for rigid airfoils (fixed wing aircraft and helicopters) as well as for aircraft propellers and gas turbine blades.
Prediction of flutter prior to modern unsteady computational fluid dynamics was based on empirical testing. As a result many pioneering designs failed due to unforeseen vibrations. The most famous of these was the opening of the original Tacoma Narrows Suspension Bridge in mid 1940, which failed spectacularly 4 months later during a sustained 67kph crosswind and became know as Galloping Gertie for its flutter movement.
During the 1950s over 100 incidences were recorded of military or civilian aircraft being lost or damaged due to unforeseen flutter events. While as recently as the 1990s jet engine flutter has grounded military aircraft.
Techniques to avoid flutter include changes to the structure's aerodynamics, stiffening the structure to change the excitation frequency and increasing the damping within the structure.