Dipteran-insect-inspired thoracic mechanism with nonlinear stiffness to save inertial power of flapping-wing flight

This paper presents the design, analysis, and characterization of a compliant thoracic mechanism that saves inertial power for flapping-wing micro air vehicles. Lightweight polyimide film hinges were previously integrated into a compliant flapping-wing mechanism to reduce friction. However, these were not stiff enough to fully recover wing's inertial energy into elastic energy. To store adequate elastic energy using film hinges, we develop a compliant thoracic mechanism with nonlinear stiffness characteristics by mimicking a Dipteran insect's flight thorax. This thoracic mechanism consists of rigid plates and polyimide film hinges connected to form a closed shell structure. It has a nonlinearly increasing stiffness so that it can slow the wings down rapidly toward the end stroke and subsequently help reverse the wings. It demonstrates almost full recovery of inertial power for 10-cm span flapping wings up to 25 Hz. As a result, it only expends 2% of the total mechanical power on inertial power at 25 Hz. In contrast, the rigid-body mechanism with no elastic storage expends 23% of the total mechanical power on inertial power when the same wings beat at the same frequency. With the capability of elastic energy storage, this compliant thoracic mechanism saves power expenditure ranging from 20 up to 30% to produce the same thrust, in comparison with the rigid-body flapping mechanism. This study shows that power saving is effective only if elastic energy storage is well tuned to recover the wing inertial power.