Flow control strategies for improved aerodynamic efficiency of micro-rotorcraft

This thesis is aimed at demonstrating substantial improvements in aerodynamic efficiency of micro-rotorcraft. The work investigates the effect of airfoil surface temperature and heat transfer, and unsteady blade pitching motion on the performance of micro-scale rotors. Prior to testing of new strategies to enhance performance, the baseline aerodynamic performance of the micro-rotor system was quantified. This study indicated that the micro-rotors displayed adequate lifting capacity, however the overall hovering efficiency was very poor compared to full-scale rotors. These results highlighted the need for new strategies to enhance aerodynamic performance of micro-scale airfoils. The improvement of aerodynamic efficiency of small-scale airfoils using surface temperature and heat transfer was investigated using numerical simulations, asymptotic analysis and experimental work. The basic idea was 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. The numerical simulations show that although varying surface temperature does not produce significant impact at the full-scale, its effect is very pronounced at the micro-scale. The asymptotic theory demonstrates that most of the effect actually comes from the heat transfer in the much smaller nose region of the small-scale airfoil. The experimental measurements show good agreement with numerical predictions. The improvement in aerodynamic performance of micro-scale rotors using unsteady blade motion was also investigated using numerical simulations and experiments. The objective was to use dynamic blade pitching motion to delay the onset of stall, enhance the lift and improve the micro-rotor efficiency. A micro-rotor system featuring piezoelectrically actuated controllable twist rotor blades was developed and tested in hover. The piezoelectric actuation system had sufficient control authority and was able to generate significant blade unsteady pitching deformations. Excitation of the blade in torsion resulted in significant improvement in the micro-rotor thrust in the post-stall regime. The experimental measurements also showed good agreement with numerical predictions.