Airfoil Optimization for Helicopter Rotor Blades: A Trade-off between Drag Reduction, Lift Capability and Aerodynamic Stability

Abstract

Defining the blade geometry of a helicopter rotor is a complex process that requires balancing performance in hover and forward flight, while maintaining stability and low vibrations. Performing direct rotor optimizations provides accurate results but with high computational costs; for this reason, a lighter approach based on numerical optimization of airfoils in critical sections of the HART II rotor was adopted in this thesis. The geometry was described using the IGP algorithm, and unsteady CFD simulation with DDES was integrated into a single-objective optimization with constraints, combining surrogate models and evolutionary algorithms. A fundamental constraint was to maintain the maximum thickness of each section, necessary to ensure structural requirements, allowing only variation in its position along the radius. In addition, a study was conducted on the tab angle, which proved to be a key parameter for improving aerodynamic moment stability. The second step of the work consisted in analysing the impact of the new optimized airfoils on the rotor in forward flight and hover, keeping the same planform and twist distribution. The results show that 2D optimization, as an alternative to selecting existing airfoils, is a valid approach for developing more efficient and robust configurations, even if defining a trade-off concept is not always straightforward, since an improvement in one flight condi�tion may result in penalties in another.