# Optimization of Ornithopter Energy Efficiency Through Spring-Induced Harmonic Motion

## TL;DR

This paper shows that adding a lightweight spring to flapping-wing drones can significantly improve their energy efficiency and flight performance.

## Contribution

The novel contribution is demonstrating that a passive torsional spring tuned near resonance can enhance ornithopter efficiency without complex control systems.

## Key findings

- An optimally tuned spring-assisted system achieved up to threefold improvement in thrust efficiency.
- Indoor flight tests showed a 12.8% increase in average endurance with the spring-assisted configuration.
- The spring-assisted system produced smoother stroke reversals, indicating reduced energy losses.

## Abstract

Ornithopters generate lift and thrust through periodic flapping-wing motion. While control-based optimization has been widely studied to improve the flight efficiency of ornithopters, passive mechanical tuning remains underexplored. This study investigates whether integrating a lightweight torsional spring can passively tune a flapping-wing system toward resonance to reduce input power and enhance aerodynamic performance. We evaluated springs of different stiffness on a 3D-printed, motor-driven flapping rig, recording input voltage and current as well as flapping frequency and thrust. Wing kinematics were analyzed using high-speed video, and free-oscillation tests identified a resonant period of ~0.14 s (~7.1 Hz). Experimental results show that an optimally tuned spring-assisted system achieves up to a threefold improvement in thrust efficiency and up to a twofold improvement in kinematic efficiency, compared to the no-spring baseline. Indoor flight tests using a commercial ornithopter (MetaFly) confirmed the improvement, showing a 12.8% increase in average endurance. The spring-assisted configuration also produced smoother stroke reversals, consistent with reduced energy losses. These results demonstrate that a low-complexity, lightweight torsional spring tuned near resonance can provide an effective passive means to enhance both energy efficiency and aerodynamic output in flapping-wing UAVs, serving as a practical, low-cost complement to control-based optimization methods.