Investigation of Coaxial Rotor Performance for a Gun-launched Micro Air Vehicle

Abstract

This paper presents an experimental parametric study to maximize the hover efficiency of a coaxial rotor system for a micro air vehicle (MAV) that could be launched from a 40 mm grenade launcher. Towards this, isolated rotor experiments were first conducted to optimize the performance of a single rotor at low Reynolds numbers (Re) through varying parameters including blade pitch angle, thickness-to-chord ratio (t/c), chord length, and Reynolds number. Results showed that t/c had minimal impact on the figure of merit FM) below 4%, while increasing blade chord length significantly improved hover efficiency until a chord length of 16.6 mm (solidity = 0.10). The optimal pitch angle for the isolated rotor was around 16 degrees, and the maximum FM was 0.59 at Re = 70,000. Coaxial rotor experiments were performed using the optimal isolated rotor as the baseline. The vertical separation between the upper and lower rotors had negligible impact on performance for a separation distance range from 0.5R to 3R. However, below a separation distance of 0.5R, the thrust and FM of the lower rotor increased and that of the upper rotor decreased. The highest FM obtained for the coaxial rotor was around 0.60 at Re = 30,000, which remained relatively constant across all vertical separations tested. The coaxial rotor system produced almost 1.66 times the thrust of an isolated single rotor. When compared at the same disk loading, the power loading (thrust/power) for the coaxial rotor was similar to that of an isolated single rotor. A comprehensive analysis (RCAS) was used to model the MAV-scale isolated rotor in hover and the analytical predictions agreed well with the experiments. In order to generate the airfoil lookup table for the RCAS model, water tunnel experiments were conducted at Re = 44,000 using a scaled-up wing with the exact same circular-cambered plate airfoil used in the coaxial rotor blades.