Impact of Spanwise Propeller Placement and Aerodynamic Interaction Effects on Whirl Flutter of Wing-Twin-Propeller Configurations

This paper investigates the impact of aerodynamic interactions on the whirl flutter boundary of wing-twin-propeller configurations. A coupled wing-pylon-propeller model is developed in the Rotorcraft Comprehensive Analysis System (RCAS), where the wing is modeled using uniform inflow and the propeller wake is modeled using the viscous vortex particle method (VVPM). The study examines the effects of spanwise propeller placement and rotation direction by first analyzing a single-propeller configuration and subsequently extending the analysis to twin-propeller configurations. The analyses are performed for both rigid and flexible wings, with the latter designed such that whirl flutter governs the instability boundary. Results show that spanwise propeller placement strongly influences whirl flutter stability, with outboard locations exhibiting higher flutter speeds. Aerodynamic interactions between the wing and the propeller are found to be generally destabilizing, reducing the whirl flutter speed across all spanwise locations, with the largest reduction observed at outboard positions. In twin-propeller configurations, amplitude modulation is observed due to coupling between closely spaced modes enabled by wing flexibility and aerodynamic interactions. The inboard propeller is found to govern the instability boundary across all spanwise arrangements. Inter-propeller aerodynamic interactions are generally destabilizing, particularly for closely spaced configurations, while contra-rotating propeller configurations exhibit slightly higher flutter speeds due to the reduced impact of interaction effects. These results provide insight into the directional nature of aerodynamic interaction effects on whirl flutter in wing-twin-propeller configurations, highlighting their configuration-dependent behavior and generally modest influence on the whirl flutter boundary.