An approach developed to stabilize periodic orbits in chaotic attractors is generalized and applied to non-chaotic flap-lag instability in helicopter rotor blades. Periodic flapping and lead-lag oscillations occur in rotor blades during forward flight; the governing equations are nonlinear with periodic coefficients. Oscillations become unstable as the advance ratio of the helicopter increases. Stabilization may be achieved by control of the mean pitch angle of the blade once per period according to a discrete control law. The control law is applied to the Poincare map which governs samples of the system obtained once per period. The controller stabilizes but does not attempt to change underlying periodic orbits. This approach is particularly well-suited to systems with periodic coefficients (such as rotorcraft) since the discrete version of the system is time-invariant.
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