Introduction to Aircraft Aeroelasticity and Loads

11

Dynamic Aeroelasticity – Flutter

Flutter is arguably the most important of all the aeroelastic phenomena (Collar, 1978; Garrick and Reid, 1981) and is the most difficult to predict. It is an unstable self-excited vibration in which the structure extracts energy from the air stream and often results in catastrophic structural failure. Classical binary flutter (Scanlan and Rosenbaum, 1960; Fung, 1969; Hancock et al., 1985; Niblett, 1998; Bisplinghoff et al., 1996; Hodges and Pierce, 2002; Dowell et al., 2004) occurs when the aerodynamic forces associated with motion in two modes of vibration cause the modes to couple in an unfavourable manner, although there have been cases where more than two modes have combined to cause flutter and in industry the mathematical models employ many modes (see Chapters 22 and 23 where a typical industry approach is described).

At some critical speed, known as the flutter speed, the structure sustains oscillations following some initial disturbance. Below this speed the oscillations are damped, whereas above it one of the modes becomes negatively damped and (often violent) unstable oscillations occur, unless some form of nonlinearity (not considered in detail here) bounds the motion. Flutter can take various forms involving different pairs of interacting modes, e.g. wing bending/torsion, wing torsion/control surface, wing/engine, etc.

In this chapter, a simple binary flutter model is developed, making use of strip theory with simplified unsteady ...