Measurements of the vortex structure in the wake of a scale helicopter rotor in forward motion

• The vortex structure of a rotor in forward flight is characterized using stereo-PIV. • Rotor forces and moments are measured using a six-axis load cell. • Results show how collective, lateral, and longitudinal pitch settings affect vortical structures. • Findings are useful for validating numerical simulations and rotor wake models. • Lateral and longitudinal pitch variations can create corotating vortices on both rotor sides. This paper provides experimental insight into the vortex structures that form in the wake of a scale helicopter rotor in forward motion. Time-resolved stereoscopic particle image velocimetry was used to capture the vortices that form on the advancing and retreating sides of the rotor at an advance ratio of 0.20. The rotor radius was R = 425 mm and its two blades with chord length c = 35 mm spun at 1,500 RPM, resulting in a Reynolds number of U tip R / ν = 1.7 × 10 6 ( U tip c / ν = 1.4 × 10 5 ) where U tip is the blade tip speed. The collective, lateral cyclic, and longitudinal cyclic pitch settings were varied one at a time. The structure of vortices measured in the wake was compared to reference paths that we expect the vortices to take if they advect into the wake at a constant velocity without interaction. We find that the vortices on the retreating side of the rotor closely match the reference paths. Conversely, the vortices on the advancing side often exhibited large deviations from the paths. The vortices on both sides of the rotor had opposite directions of rotation when the collective pitch setting was varied. In contrast, varying the lateral and longitudinal cyclic pitch resulted in the formation of vortices with the same direction of rotation on the advancing and retreating sides. Notable vortex formation was observed on both sides of the rotor when the lateral cyclic pitch setting was varied, while vortex formation was dominant on the advancing side when varying the longitudinal cyclic pitch.