Rotor Optimization Framework Introducing Discrete Airfoil Selection with Computational Fluid Dynamics Validation

Helicopter rotor blade optimization is fundamentally challenged by the complexity of airfoil design. Along the blade span in hover, and across both span and azimuth in forward flight, local conditions vary from low-speed stalled flow to transonic compressible flow and even include reverse flow regions. Parameterizing the airfoil shape to accommodate these extremes introduces a high-dimensional and computationally expensive design space. Consequently, most prior studies either use a single fixed airfoil or optimize only geometric parameters such as twist and taper, treating the airfoil as a predefined element. This work addresses this gap by proposing a discrete airfoil selection methodology integrated into a multi-objective rotor blade optimization framework based on the midfidelity Maryland Free Wake analysis. A pool of well-known rotorcraft airfoils is used, and four optimization scenarios are explored to quantify the importance of airfoil choice. The first two scenarios retain the baseline airfoil with and without tapered thickness. The third scenario enables automatic thickness tapering linked to airfoil selection. The fourth scenario introduces two airfoil selection parameters for the inboard and outboard regions and one parameter to control their transition. The multi-objective optimized blades from all four scenarios are validated using high-fidelity computational fluid dynamics (CFD). The results show that including discrete airfoil design parameters in the design space provides meaningful performance gains, achieving a higher figure of merit in hover and an improved lift-to-drag ratio in forward flight. The results confirm the efficacy of this pragmatic and previously underexplored design strategy during the preliminary rotor design stage, although further high-fidelity CFD-based refinement remains necessary for robust final rotor design.