Vortex control for rotor blade devices

To control vortices originating at the tips of a rotor's blades rotating through the air at a revolution frequency f, separation control device(s) are actuated to periodically introduce perturbations into the airflow moving over the blades. The periodic introduction of perturbations is controlled in accordance with a periodic modulating frequency of introduction f.sub.0 while the frequency of the perturbations so-introduced is designated as f.sub.e. Vortex control is achieved when the periodic modulating frequency of introduction f.sub.0 satisfies the relationship nf.ltoreq.f.sub.0.ltoreq.f.sub.e where n is the number of blades.