The influence of the blade pitch angle on the Cₜ, Cₚ and FM characteristics of the Mi-8 and UH-60 rotors in S₀–S₃ configurations

The article presents a comparative aerodynamic and energy analysis of the influence of the blade pitch angle on the performance characteristics of helicopter rotor systems representing different scale classes. The study focuses on two representative main rotor configurations and examines four blade geometries (S₀–S₃), ranging from a baseline profile to optimized variants with modified twist and tip geometry. The research is based on a combined theoretical and analytical approach using dimensionless thrust and power coefficients (Cₜ and Cₚ), as well as the integral efficiency indicator known as the figure of merit (FM), which enables a unified comparison of rotors with different diameters and loading conditions. Analytical approximations of the functional dependencies Cₜ(θ), Cₚ(θ), and FM(θ) are constructed to describe the relationship between blade pitch angle and energy balance under steady operating conditions. The results demonstrate that an increase in the pitch angle leads to an approximately linear growth of the thrust coefficient, while the power coefficient exhibits a nonlinear increase due to the combined effects of profile and induced losses. As a consequence, the FM efficiency curve shows a pronounced maximum at an optimal pitch angle range, beyond which further increases in pitch result in a rapid decline in energy efficiency. It is shown that rotors with smaller swept disk areas and higher specific power loading exhibit greater sensitivity of FM to variations in pitch angle, whereas larger rotors demonstrate a wider efficiency plateau and lower sensitivity to tuning errors. The transition from the baseline blade configuration to optimized geometries (S₂–S₃) leads to a noticeable reduction in power losses, an expansion of the high-efficiency operating range, and a decrease in the sensitivity of FM to pitch variations. The scientific novelty of the study lies in the unified analytical framework used to compare pitch-angle-dependent efficiency characteristics across multiple blade configurations and rotor scales. The practical significance of the results is associated with their applicability to the optimization of collective pitch settings and the development of adaptive rotor control strategies aimed at improving energy efficiency and operational stability under variable flight conditions.