Effect of rotor geometry on UAV-scale cycloidal rotor hover performance
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abstract
Copyright 2018 by AHS International, Inc. All rights reserved. This paper focuses on systematic time-averaged thrust and power measurements to characterize the effect of rotor geometry on the performance of a cycloidal rotor operating at Reynolds numbers between 100,000 and 300,000. A cycloidal rotor is a revolutionary horizontal-axis propulsion device that has proven to benefit from increased maneuverability and aerodynamic efficiency at micro air vehicle (MAV) scales. The current study aims to investigate cycloidal rotor performance at significantly larger UAV-scales. Towards this, experiments were conducted for a range of rotational speeds across different blade pitch amplitudes for rotor configurations with varying airfoils, blade spans, chord-by-radius ratios, and number of blades. The study found that the optimal pitch amplitude for symmetric pitch kinematics was highly dependent on the configuration due to changes in rotor inflow and flow curvature effects. An airfoil thickness as high as 25% of chord was capable of efficiently generating thrust and thicker airfoils provide efficient operation over a wider range of pitch amplitudes. Changing the blade span showed negligible change in thrust and power per unit area and power loading. Changing the chord-by-radius ratio resulted in increases in thrust at a fixed speed and power loading up to a current optimal ratio of 0.66. Increasing number of blades resulted in a steep decrease in thrust per unit blade area. Examining all of the tested configurations allowed for an optimal solidity range to be found of between 0.30 and 0.40. Based on the 31 unique configurations tested at 6 pitch amplitudes each in the present study, at an operating Reynolds number of 200,000, the optimal cycloidal rotor configuration had a chord-by-radius ratio of 0.66, 3 blades featuring a blade aspect ratio (span/chord) of 4 and a NACA 0020 airfoil, rotor aspect ratio (span/diameter) of 1.33 and pitch amplitude of 40 deg and produced a FM of 0.6.
