Rotational-Stiffness Sensitivity of a Low-Frequency Aeroelastic Branch in a Wingtip-Connected Modular UAV

This study examines how the rotational stiffness of two wingtip interfaces changes the low-frequency aeroelastic response of a free–free, three-module unmanned aerial vehicle. A finite-element model coupled with doublet-lattice aerodynamics is analyzed using the p–k method. The response associated with the lowest retained oscillatory crossing of the locked reference is followed along single-channel and prescribed multi-channel stiffness paths using complex-eigenvector modal assurance criteria. Competing crossings and discrete unequal-interface cases are reported separately. Rotation about the spanwise joint axis (DOF 5) produces the largest target migration, from approximately 11.7 to 31.7 m/s; the other two channels affect this target less, although DOF 4 can introduce a lower-speed competing response. Unequal-interface cases change candidate ordering without establishing a universal asymmetry law. A five-case Nastran–ZAERO comparison reproduces the selected stiffness trends, with an approximately 10.2% difference in low-frequency crossing speeds. The contribution is a comparison of channel-dependent target migration and candidate competition under common modeling assumptions. The results constitute a branch-specific numerical sensitivity analysis under linear, zero-structural-damping assumptions; they do not establish a global or experimentally validated flutter boundary.