Passive Pitch Mechanics of Elastic Flapping Wings

Abstract

Many flapping wing micro air vehicles (FWMAVs) utilize a flexible joint that allows the wing to passively rotate about the pitching axis. Generally, simple rigid body models are used to estimate the passive pitching dynamics. However, evidence suggests elastic wings increase aerodynamic force generation and expend less energy relative to rigid wings. As a result, elastic wings are becoming an integral part of FWMAV design. But, the effect of wing elasticity on passive pitching mechanics is unclear. To explore this, we develop a coupled model of an elastic wing attached to a flexible pitching joint. Aerodynamic moments are included through a simple blade element approach. The model is applied to an idealized insect forewing subject to prescribed roll rotation. The simulation results suggest (1) aerodynamic moments, not rigid body inertia or elastic forces, are primarily responsible for lift-generating passive pitch, (2) joint stiffness influences pitching mechanics more than wing elasticity does, and (3) flexible wings can increase net lift by as much as 20% if the pitching joint is mistuned. The framework developed in this paper can be used to design and optimize FWMAV systems in terms of both elastic wings and flexible passive pitch joints.