An evaluation is made of extensive three-component (3-C) particle image velocimetry (PIV) measurements within the wake across a rotor disk plane. The model is a 40 percent scale BO-105 helicopter main rotor in forward flight simulation. This study is part of the HART II test program conducted in the German-Dutch Wind Tunnel (DNW). Included are wake vortex field measurements over the advancing and retreating sides of the rotor operating at a typical descent landing condition important for impulsive blade-vortex interaction (BVI) noise. Also included are advancing side results for rotor angle variations from climb to steep descent. Using detailed PIV vector maps of the vortex fields, methods of extracting key vortex parameters are examined and a new method was developed and evaluated. An objective processing method, involving a centerof-vorticity criterion and a vorticity "disk" integration, was used to determine vortex core size, strength, core velocity distribution characteristics, and unsteadiness. These parameters are mapped over the rotor disk and offer unique physical insight for these parameters of importance for rotor noise and vibration prediction. SYMBOLS C rotor blade chord, 0.121 m C T rotor thrust coefficient, thrust/ρπR 2 (ΩR) 2 CV center of vorticity in (x,y) plane, m DNW German-Dutch Wind tunnel DLR German Aerospace Center HART HHC Aeroacoustic Rotor Test HHC Higher Harmonic Control k grid point indices j indices for instantaneous image LDV Laser Doppler Velocimetry LLS Laser Light Sheet n number associated with analytical velocity profile, Eq. 5 R rotor radius, 2 m vortex core radius, m core radius associated with Rankine vortex, m shape factor for vortex velocity profile u,v,w velocity components for x,y,z coordinates, m/s rotor hover tip speed, RΩ (218 m/s) 'spin' velocity at r C , m/s (x,y,z) TUN windtunnel coordinate system: (x TUN positive downstream, y TUN positive starboard, z TUN positive up). x,y,z PIV image frame coordinates, m, Fig. 4. α rotor shaft angle with respect to z TUN axis, deg δ vortex wander parameter, m circulation within , m/s 2 σ standard deviation of parameters derived from instantaneous images Ω rotor rotation frequency, rad/s vorticity normal to (x, y) plane, s −1 Ψ blade azimuth angle (0 0 aft), deg.