Role of Wing Flexibility on Force Production and Power Economy During Climbing Flight in a Fruit-Fly

A numerical investigation of the unsteady aerodynamics of a fruit-fly model with actively flexing wings in climbing flight is presented. This mode of insect flight involves a high power demand, making it an ideal regime to investigate the role of wing flexibility in aerodynamic performance and energy efficiency. Inspired by the wing flexion patterns observed in real insects, time-varying wing deformation profiles were imposed directly on the wing surface to capture the key aspects of flexible wing flight. Simulations were carried out using a three-dimensional multi-grid entropic multi-relaxation time lattice Boltzmann solver, ensuring that intricate wing-wake interactions are captured. The effect of flexion amplitude, its timing with respect to the stroke, and change in stroke-plane angle as the fruit fly ascends on wake structures and energy efficiency was systematically investigated. The results show that wing flexibility reduces the peak input power for the same mean vertical force, as it helps to keep the vortices attached to the wings and enhances the wake capture mechanisms. In addition, tuned flexibility increases the maximum achievable advance ratio (ratio of the climbing velocity to the mean wing velocity) at a given climbing angle. Excessive wing flexibility degrades the lift owing to a reduced angle of attack, resulting in a higher power loss. This study offers insights into how lift and energy efficiency can be maximized by bio-inspired wing deformation during climbing flight. Such insights are highly beneficial for the design of next-generation flapping wing micro-air vehicles for vertical or climbing maneuvers.