Insect-inspired passive mechanisms in hovering flapping wing micro air vehicles: a review

Micro air vehicles (MAVs) operating at ultracompact scales under low Reynolds number regimes confront inherent aerodynamic constraints. While fixed and rotary-wing systems suffer efficiency losses from dominant viscous forces, flapping-wing MAVs (FWMAVs) circumvent these constraints through unsteady aerodynamic mechanisms. However, the challenge of integrating propulsion, actuation, and control within restricted volumes of FWMAVs necessitates biohybrid solutions leveraging insect-derived passive mechanisms. These mechanisms exploit inherent dynamic properties and natural physical interactions rather than programmed controllers or auxiliary power sources, effectively addressing fundamental engineering challenges through mechanical simplification and energy demand reduction. This review systematically examines passive mechanisms in hovering FWMAVs across biological foundations and engineered implementations. First, strategies for replicating insect wing motion patterns are introduced. Then, the intrinsic properties of flapping wings as well as effects on aerodynamic performance and flight stability are discussed. Further, comparative evaluations are presented between conventional FWMAVs and emerging beyond-natural designs combining biological principles with engineered innovations. Finally, research frontiers in passive mechanisms applications are discussed, whose implementation will help to expand FWMAVs' operational envelopes and enhance mission versatility.