The focus is on the vortex ring state (VRS) observed
at rapid descent rates. At VRS, the helical vortex filaments coming off the blades amalgamate around the
rotor disk forming a vortex ring, which periodically
detaches into the wake, causing extreme oscillations
in thrust, with periods on the order of several tens of
rotor revolutions. We discuss here the phase relation
between the thrust cycle and vorticity distribution at
the rotor disk. Maxima of the VRS thrust oscilla-
tions correlate well with the maxima of circulation,
enstrophy, and minima of enstrophy dispersion radius
observed in the vicinity of the rotor disk
