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CHAPTER 2
FLUTTER AND LIMIT CYCLE OSCILLATIONS
Introduction
In this section, the characteristics of flutter and limit cycle oscillations (LCO) will be
discussed, as well as techniques for analysis and prediction.
Dynamic Aeroelasticity
Aeroelasticity is often defined as a science which studies the mutual interaction between
aerodynamic and elastic forces. 6 Modern aircraft structures are very flexible, and this flexibility
is fundamentally responsible for the various types of aeroelastic phenomena. Flexibility may
have advantages for structural design; however, aeroelastic phenomena arise when structural
deformations induce additional aerodynamic forces. These additional aerodynamic forces may
then give rise to additional structural deformations which will induce even greater aerodynamic
forces. Such interactions can either dampen out and lead to a condition of stable equilibrium, or
diverge and destroy the structure. The term aeroelasticity is not completely descriptive since
many important aeroelastic phenomena also involve inertial forces.
Flutter
Flutter is an instability caused by the aerodynamic forces coupling with the structural
dynamics. Flutter can occur in any object within a strong fluid flow that results in a positive
feedback occurring between the structure's natural vibration and the aerodynamic forces. If the
energy during the period of aerodynamic excitation is larger than the natural damping of the
system, the level of vibration will increase. The vibration levels are only limited when the
aerodynamic or mechanical damping of the object matches the energy input, which often results
in large amplitudes and can lead to rapid failure. Therefore, structures exposed to aerodynamic
forces are carefully designed to avoid flutter.
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