Fundamental Understanding of Flow Physics of a MAV-scale Cycloidal Rotor in Forward Flight

This paper provides a fundamental examination of the flow physics for a two-bladed MAV-scale cycloidal rotor (or cyclorotor) in forward flight using experimental particle image velocimetry (PIV) measurements, computational studies (2D CFD) and time-averaged performance measurements. A simple aerodynamic analysis using time-averaged flow field measurements from PIV is used to develop a basic understanding of the distribution of blade aerodynamic forces and power along the rotor azimuth. The incoming flow velocity is shown to decrease in magnitude as the flow passes through the upper half of the rotor. This is attributed to power extraction by the blades in the upper-frontal region of the rotor azimuth. Flow field measurements also show a significant increase in flow velocity across the lower half of the rotor cage. The aerodynamic analysis demonstrates that the blades accelerate the flow through the lower-aft region of the rotor, where they operate in a high dynamic pressure environment with a large positive effective angle of attack. CFD-predicted values of instantaneous aerodynamic forces reveal that the aft section of the rotor is the primary region of force production. Phase-averaged flow field measurements are also analyzed. These results reveal two blade wakes in the flow, formed by each of the two blades. Analysis of the blades at several azimuthal positions revealed two significant blade-wake interactions in the aft of the cyclorotor. The locations of these blade-wake interactions are correlated with force peaks in the CFD-predicted instantaneous blade forces, implying that unsteady aerodynamic interactions play an important role in the lift and propulsive force generation of the cyclorotor.