Rotor Hover Performance and System Design of an Efficient Coaxial Rotary Wing Micro Air Vehicle

Abstract

Size restrictions force MAVs to operate in a low Reynolds number aerodynamic regime that results in poor aerodynamic performance of conventional airfoils and rotor configurations.

A computerized hover test stand was used for the systematic testing of single and coaxial small-scale rotors. Thin circular arcs were chosen for blade manufacturing because of their good aerodynamic characteristics and simplified parameterization. Influence of airfoil geometry on single rotor hover performance was studied on untwisted rectangular blades. Non rectangular blades were used to study coupled airfoil and blade parameters. Performance gains were obtained by introducing large negative twist angles over short radial distances at the blade tips. A parametric study of the blade geometries resulted in maximum figures of merit of 0.65.

Coaxial rotor performance at torque equilibrium was explored for different trims and operating conditions. It was found that the upper rotor was marginally affected by the lower one at spacings larger than 35% of the rotor radius, and that it produced about 60% of the total thrust. Experiments showed that power loading was maximized when higher collectives were used at the lower rotor, resulting in sizable differences in rotational speed between rotors.

The CFD solver INS2d was used for a two-dimensional parametric aerodynamic study of circular arc airfoils. Lift, drag, and moment coefficients were explored over a range of Reynolds numbers. Lift predictions were satisfactory; however, drag was under-predicted at low angles of attack. The CFD database was integrated to a BEMT rotor model through a parameterization that coupled blade planform with twist distribution and airfoil shape. Thrust and maximum FM predictions were satisfactory for rectangular and non-rectangular blades with maximum cambers of 6% and below. The BEMT model was extended to the coaxial rotor case, producing good thrust and power predictions with errors within 5% of the experimental measurements. The approach validated the use of analytical and numerical tools commonly used in full-scale analysis, and proved to be a powerful system design tool.

A fully functional coaxial MAV was developed based on the aerodynamic studies performed. It has been used as a testing platform for control system and algorithms.