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CHAPTER 1
INTRODUCTION
Motivation
Limit Cycle Oscillation (LCO) is a sustained non-divergent periodic motion experienced
by aircraft with configurations which often include stores. The external stores consist of any
object carried on the aircraft, such as a missile, bomb, fuel tank, or pod. The primary concern
with regard to LCO relates to the pilot’s ability to carry out his mission safely and effectively.
While experiencing LCO, the pilot may have difficulty reading cockpit gauges and the heads-up
display (HUD), and an unexpected LCO condition can lead to premature termination of the
mission. There are also concerns regarding the effects of LCO on the stores and UAVs. These
issues include such things as potential degradation in reliability of components, whether or not
stores can be safely released during LCO, the possible effects of LCO on target acquisition for
smart weapons, and the effects of LCO on weapon accuracy for unguided weapons. Therefore,
the prediction of LCO behavior of fighter aircraft is critical to mission safety and effectiveness,
and the accurate determination of LCO onset speed and amplitude often requires flight test
verification of linear flutter models. The increasing stores certification demands for new stores
and configurations, the associated flight test costs, and the accompanying risk to pilots and
equipment all necessitate the development of tools to enhance computation of LCO through
modeling of the aerodynamic and structural sources of LCO nonlinearities.
Flutter, an instability caused by the aerodynamic forces coupling with the structural
dynamics, and LCO are related phenomena. This association is due in part to the accuracy with
which linear flutter models predict LCO frequencies and modal mechanisms. 1 However, since
the characteristics of LCO motion are a result of nonlinear effects, flutter models do not
accurately predict LCO onset speed and amplitude. Current engineering knowledge and theories
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