Optimization of Coaxial Rotor System for a Gun-launched Micro Air Vehicle

Abstract

The paper discusses an experimental parametric study to maximize the hover efficiency of a coaxial rotor system for a micro air vehicle (MAV) that could potentially be launched from a 40 mm grenade launcher to achieve improved mission range and endurance. Prior to testing the rotors in the coaxial configuration, isolated rotor tests were performed on acustom-built hover test stand. The rotor diameter was kept constant at 6 inches (0.015 m), and used two untwisted carbon fiber blades with rectangular planform shape. The blades utilized thin circular cambered plate airfoil sections for improved performance at low Reynolds numbers (Re ≈ 30,000). The parameters that were varied include blade thickness-to-chord ratio (t/c), chord length, pitch angle, and Reynolds number. Thickness-to-chord ratio did not have a significant impact on figure of merit below 4%. Increasing the blade chord at a constant Reynolds number and t/c significantly improved hover efficiency until a chord length of 16.6 mm (solidity, σ=0.14). Across all the cases tested, the optimal pitch angle was around 16- 17 degrees. The optimal rotor from the isolated rotor experiments was used as the baseline rotor for coaxial rotor testing where the effect of vertical rotor separation was investigated with the same blade pitch angle for upper and lower rotors. Overall, the rotor separation had negligible effect on the performance of the upper and lower rotors for a separation distance range from 0.5R to 2R. The highest figure of merit obtained for the coaxial rotor was around 0.55 at Re = 30,000. Across all the vertical separations and disk loading the torque-balanced coaxial rotor system produced almost 1.5 times the thrust of an isolated rotor, which was set at the same pitch angle.