Flow Control Strategies for Improved Aerodynamic Efficiency of Micro-Rotorcraft

Two novel flow control strategies for improving the aerodynamic performance of micro-scale rotors are presented in this paper. First, the effect of surface temperature and heat transfer is investigated using numerical simulations and experimental work. The basic idea is to take a direct advantage of heat transfer that dominates micro-scale systems to enhance lift, reduce drag, and increase the envelope of operation of airfoils. It is shown that varying wall temperature has significant impact on the airfoil pressure distribution at the micro-scale, because the thickness of the thermal boundary layer is very substantial in comparison to the airfoil chord and the viscous boundary layer. The experimental measurements show good agreement with numerical predictions. Next, the improvement in aerodynamic performance of micro-scale rotors using unsteady blade motion is investigated. The objective is to utilize dynamic blade pitching motion to delay the onset of stall, enhance the lift and improve the micro-rotor efficiency. A micro-rotor system featuring piezoelectricaly actuated controllable twist rotor blades is developed and tested in hover. Dynamic excitation of the micro-blade in torsion results in significant improvement in the micro-rotor thrust in the post-stall regime. The experimental measurements also showed good agreement with the numerical predictions.