  When a rotor blade encounters reverse flow in edgewise flight at high advance
ratio, a strong vortical structure forms on the leeside of the sharp blade
edge. This has unexpected effects on blade aerodynamics. A differential onset
velocity gradient due to rotor rotation creates an envelope of reverse flow
extending from the hub. A Sharp Edge Vortex (SEV) is observed at the geometric
trailing edge, forming soon after the blade enters the retreating-blade side.
The SEV grows radially inwards in size and strength as the blade retreats. The
SEV convects and evolves along with the rotor blade until dissipation. In this
paper, a coherent vortex is observed at 240 degrees azimuth. Vortex size and
strength increase as the blade retreats, ceasing to grow before 270 degrees.
The reverse flow envelope increases at higher advance ratios with increased
vortical strength. The solid body rotation core of the vortex stretches at
lower advance ratios and shows the signs of a burst vortex in the dissipation
phase. The proximity of the SEV to the blade causes large excursions in surface
static pressures which in turn generates significant negative lift on the
retreating blade. The attached, coherent sharp edge vortex shows similar
morphological features as the leading edge vortex on a delta wing.
