Influence of Blade Bending and Torsion on Propeller Whirl Flutter

The whirl flutter aeroelastic instability is a major concern for propeller-driven aircraft. Classical models generally involve a propeller with rigid blades mounted on a flexible support. However, with modern engines trending toward slender and lighter blades, concerns arise about the influence of blade flexibility on this phenomenon. This study addresses this question by conducting an in-depth analysis of the impact of blade elasticity on whirl flutter stability. A structural model of a propeller is developed using beam theory, complemented by two other models to predict motion-induced aerodynamic loads. The first approach employs a quasi-steady assumption, while the second incorporates unsteady flow effects through a state-space formulation. These models are combined to evaluate the individual impact of blade deformation in out-of-plane bending, in-plane bending, and torsion on whirl flutter. The combined effect of all these deformations is then analyzed, revealing that blade flexibility has a significant stabilizing influence for conventional blade stiffness values. It also gives rise to unusual phenomena, such as forward whirl instabilities, which do not occur with rigid blade propellers. The main insight from this research is that blade elasticity can no longer be overlooked, as it plays a significant role in whirl flutter stability.