An experiment was conducted to measure the hovering performance of a rotor typical of that used on a rotating-wing micro air vehicle. The rotor was shown to have relatively low hovering efficiency that can be traced, at least in part, to its significant viscous wake and the relatively large aerodynamic losses that are associated with the wake. High-resolution flow visualization images have divulged several interesting flow features that appear unique to rotors operating at low Reynolds numbers. The vortex sheets trailing the rotor blades were found to be much thicker and also more turbulent than their higher chord Reynolds number counterparts. Similarly, the viscous core sizes of the tip vortices were relatively large as a fraction of blade chord compared to those measured at higher vortex Reynolds numbers. However, the tip vortices themselves were found to be laminar near their core axis with an outer turbulent region. Particle image velocimetry measurements have been made at various wake ages that have quantified the structure and strength of the wake flow, as well as the tip vortices. An analysis of the vortex aging process has also been conducted, including the development of a new non-dimensional equivalent time scaling parameter to normalize the core growth of tip vortices generated at substantially different vortex Reynolds numbers.
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