Coupled Aerodynamics–Structure Analysis and Wind Tunnel Experiments on Passive Hinge Oscillation of Wing-Tip-Chained Airplanes

This study examines the wing hinge oscillations in an aircraft concept that employs multiple wings, or small aircraft, chained at the wing tips through freely rotatable hinges with minimal structural damping and no mechanical position-locking system. This creates a single pseudo long-span aircraft that resembles a flying chain oriented perpendicular to the flight direction. Numerical calculations were conducted using the vortex lattice method and modified equations for a multi-link rigid body pendulum. The calculations demonstrated good agreement with small-scale wind tunnel experiments, where the motion of the chained wings was tracked through color tracking, and the forces were measured using six-axis force sensors. The total CL/CD increased for the chained wings, even in the presence of hinge joint oscillations. Furthermore, numerical simulations assuming an unmanned airplane size corroborated the theoretical attainment of passive stability with high chained numbers (≥9 wings), without any structural damping and relying solely on aerodynamic forces. Guidelines for appropriate hinge axis angle δ and angle-of-attack regions for different chained wing numbers to maximize passive oscillation stability were obtained. The results showed that wing-tip-chained airplanes could successfully provide substantially large wing spans while retaining flexibility, light weight and CL/CD, without requiring active hinge rotation control.