How water and solid boundaries affect the dynamics of a hovering rotor

Micro Aerial Vehicles (MAVs) are poised to revolutionize urban planning and diagnostics, rescue operations, transportation of goods, and home/business management. However, before MAVs become mainstream, they will need to fly much more dependably in complex environments. Our current understanding of how rotors interact with boundaries is based primarily on helicopter models designed for high Reynolds-number flows. We studied small (~20 cm diameter) isolated rotors to support better flow models of MAV-boundary interactions as they hovered above solid and water surfaces. We modeled the rotor by combining blade element theory with the potential flow-based "image method." We compared the model to experiments for multiple rotor diameters and blade pitches. Particle image velocimetry revealed deflected momentum jets between the rotor and the planar boundary. We see agreement between our experimental data and simple blade element modeling with inviscid ground effect. There are only minor differences in the effectiveness of the model between the solid and water cases. These findings suggest easy adaptation of ground effect models to differently sized rotors over either water or solid surfaces.