Aerodynamic Design Optimization of Proprotors for Convertible-Rotor Concepts

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Trades in the aerodynamic design of proprotors that could be used to power convertible-rotor aircraft have been examined. The key design challenge is to maximize overall aerodynamic efficiency of the proprotor in both hover and forward flight, while preserving adequate stall margins for maneuvering flight and compressibility margins for high speed flight. To better assess proprotor performance, a new formulation of the blade element momentum theory for high-speed propellers and proprotors was developed. This approach uses an efficient and robust numerical method to solve simultaneously for the axial and swirl induced velocity components in the wake of the proprotor. 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 goal of developing a better understanding of the tradeoffs that would be needed for the development of advanced proprotors to power such convertible-rotor 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 that may have variable tip speed, and assessing the relative advantages of variable diameter versus variable rotational shaft speed concepts. Proprotors with variable blade twist were also considered. It was found that significant improvements in proprotor performance may only be practically realized by varying one or more of diameter, shaft speed, or blade twist during flight.