Analytic Model of Proprotor Forces and Moments at High Incidence

The paper presents an analytical model for estimation of proprotor aerodynamic loads at elevated incidence angles. Previous theories have concentrated on either small incidence angle for aircraft stability analysis, or edge-wise flow for helicopter forward flight. This development attempted an engineering method that covers full incidence angle range from $0$ to $\pi / 2$. Blade element theory was applied to known proprotor geometry, and off-axis loads including normal force and in-plane moment were obtained in closed form based on thrust and torque in axial condition. The model was found to be sufficiently accurate over a broader flight conditions compared to classical models, and computationally more efficient than numerical methods. Hence it could be easily used as a preliminary design and analysis tool for future convertible aircraft proprotors. The paper further discusses a dedicated wind tunnel campaign on proprotor off-axis load measurement. Experimental data from the test campaign was considered in model validation. The results suggested that the model was capable to accurately estimate proprotor performance in nominal flight regimes