A method to increase the stability of periodic flap-lag dynamics in helicopter rotor blades is investigated. Instability in the flap-lag dynamics of stiff-in-plane rotors can occur as forward flight speed is increased, or if significant pitch-lag coupling is present. A method originally developed to control chaos can be applied to stabilize unstable or weakly stable periodic behavior. Stabilization is achieved using small perturbations of the mean blade pitch angle. The approach, which will be referred to as periodic active control (PAC), consists of applying discrete control to the Poincaré map associated with the nonlinear dynamical system. Control effort is applied efficiently, since it does not change, but only stabilizes underlying periodic motion. Stabilization can lead to higher safe speeds, decreased transient effects, and simplified designs in helicopters.