Generalized Aerodynamic Optimization of Hovering Coaxial Rotor Blades

This paper presents the development and application of a generalized approach to the aerodynamic optimization of hovering coaxial rotors. The generalization lifts the constraints of previous theories, and enables dis-similar rotor diameters, dis-similar rotational speeds and thrust-sharing percentage of the upper and lower rotor of a coaxial pair to be incorporated as design variables. This increased versatility was necessitated by the emergence of all-electric VTOL UAVs which feature independently driven rotors, and have unique operational requirements. The optimization methodology begins with the derivation of the generalized momentum theory for coaxial rotors, and then extending the formulation to become an optimization technique which determines optimal combinations of rotor diameters and rotational speeds for a specified thrust-sharing percentage. These results are next used as inputs to determine the optimal dis-similar geometries for the upper and lower rotor blades. The optimization is demonstrated on a multi-rotor VTOL UAV with three coaxial pairs of rotors. The results show that high Figure of Merit is achieved, and this is a contributing factor in achieving high hover endurance.