Passive Deformation-Driven Lift Enhancement via a Modular Compliant-Joint Flapping Wing Mechanism

This study presents a novel flapping wing mechanism featuring a modular, passive compliant joint installed between the inner and outer wing sections. Unlike traditional rigid or actively actuated designs, this mechanism enables the wing to passively bend during the flapping cycle through aerodynamic and inertial forces, without requiring additional control inputs. The compliant joint is implemented using 3D-printed PETG material and is designed with interchangeable geometries that allow quick adjustment of torsional stiffness. This modularity makes it possible to systematically investigate how structural flexibility influences aerodynamic performance in flapping flight.

Three joint configurations—rigid (CJ NON), moderately compliant (CJ 12), and highly compliant (CJ 08)—were tested in a low-speed open-circuit wind tunnel at incoming flow speeds of 0, 3, and 6.6 m/s, with a constant flapping frequency of 2 Hz. Vertical forces were measured using a six-axis load cell, and wing kinematics were tracked with high-speed cameras.

The key findings show that only the compliant configurations (CJ 12 and CJ 08) were able to produce net positive lift at a forward speed of 3 m/s, while the rigid CJ NON configuration failed to generate significant lift. Notably, CJ 12 demonstrated a distinct first-stage deformation near the downstroke reversal that contributed an additional lift peak, and a second-stage deformation that mitigated negative lift during the upstroke. These passive deformations occurred at consistent phases of the flapping cycle, indicating a repeatable aerodynamic benefit linked to structural flexibility.

This research highlights the aerodynamic advantages of incorporating compliant mechanisms into flapping-wing systems. The ability to generate lift-enhancing deformation passively—without added weight or complexity—offers new insights for the development of lightweight, energy-efficient bioinspired aerial vehicles.