This paper provides unambiguous velocity-field proof that a strong edge vortex occurs on a rotating blade in reverse flow. The rotor blades of a helicopter encounter reverse flow during flight at high advance ratio. Reverse flow is a limiter in rotorcraft design with excursions in pitch link loads and bending moments. Stereo particle-image velocimetry is used on the flow under the retreating blade of a two-bladed teetering rotor system in a low speed wind tunnel. The results are correlated with earlier aerodynamic loads and flow visualization data using the same rotor blade planform placed in a yawed, fixed-wing position. Results obtained with the blade held fixed at several rotor azimuths and angles of attack are used to ascertain the rotation effects on the flowfield by comparison with rotating blade results. Initial results suggest that radial velocity due to rotation hinders separation and delays the formation of an attached vortex, compared to the static case. The circulation of the reverse flow attached vortex is of the same order of magnitude as the bound circulation of the airfoil section, proving that the vortex contributes significantly to the lift force in addition to the pitching moment.

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