VORTICITY CONCENTRATION AT THE EDGE OF THE INBOARD VORTEX SHEET

Flow visualization and velocity measurements are used to show that the outer edge of the trailing vortex sheet from an untwisted rotor blade roils up into a discrete vortical structure which is opposite in sens e and approximately one-half the strength of the tip vortex. This occu rs both in hover and low-speed forward flight of a two-bladed teeterin g rotor, and at both upstream and downstream edges of the wake in forw ard flight. The roll-up process and the trajectories of the tip vortex and the counter-rotating vortex are examined using digitized laser sh eet video images. Azimuth-resolved velocity data are used to quantify vortex strengths.