Aerodynamic Design Optimization of Proprotors for Convertible Rotor Concepts

Trades in the aerodynamic design of proprotors to power convertible-rotor aircraft have been examined. The most important design challenges are to maximize overall aerodynamic efficiency in both hover and forward flight, as well as preserving adequate stall margins for maneuvering flight. To better assess proprotor performance, a new formulation of the blade element momentum theory for highspeed propellers and proprotors was developed. The approach uses an efficient and robust numerical method to solve simultaneously for the axial and swirl induced velocity components. The efficacy of the approach was validated against measurements of the performance of two NACA high-speed propellers at advance ratios up to 2.5 and tip Mach numbers up to supersonic conditions. The importance of calculating accurately the swirl component of the induced velocity is emphasized. Parametric studies and design optimization studies were performed for different convertible rotor aircraft platforms with the end goal of developing a better understanding of the tradeoff that would be needed for the development of advanced proprotors to power such aircraft. The effects that solidity, diameter, rotational speed, blade twist and taper, number of blades, tip sweep, and airfoil characteristics have on proprotor performance were all explored. Particular importance was given to proprotors with variable tip speed, and the relative advantages of variable diameter versus variable rotational shaft speed concepts.