The gust-mitigating potential of flapping wings

Abstract

Nature's flapping-wing flyers are adept at negotiating highly turbulent flows across a wide range of scales. This is in part due to their ability to quickly detect and counterract disturbances to their flight path, but may also be assisted by an inherent aerodynamic property of flapping wings. In this study, we subject a mechanical flapping wing to replicated atmospheric turbulence across a range of flapping frequencies and turbulence intensities. By means of flow visualization and surface pressure measurements, we determine the salient effects of large-scale freestream turbulence on the flow field, and on the phase-average and fluctuating components of pressure and lift. It is shown that at lower flapping frequencies, turbulence dominates the instantaneous flow field, and the random fluctuating component of lift contributes significantly to the total lift. At higher flapping frequencies, kinematic forcing begins to dominate and the flow field becomes more consistent from cycle to cycle. Turbulence still modulates the flapping-induced flow field, as evidenced in particular by a variation in the timing and extent of leading edge vortex formation during the early downstroke. The random fluctuating component of lift contributes less to the total lift at these frequencies, providing evidence that flapping wings do indeed provide some inherent gust mitigation.

Living in a trash can: turbulent convective flows impairDrosophilaflight performance

“…We suggest that this increased frequency could serve to improve flight control, especially in the region at the middle of the chamber where an uprising plume dominates and presents the fastest flow speeds (figure 3a). Elevated flapping frequency has previously been proposed to enhance control authority, by reducing the time between wing strokes and thus the delay in updating control input to the wings [29], and to reduce the impact of random, turbulent flow perturbations on force production by flapping wings [30]. In agreement with our results, hummingbirds and moths flying in varied vortex shedding conditions increase their stroke frequency approximately 10% as compared to when flying in laminar flows at the same speed [7,8], and bumblebees flying in fully mixed turbulence at higher speeds increase their frequency by approximately 3% [24].…”

Avoiding topsy-turvy: how Anna's hummingbirds (Calypte anna) fly through upward gusts

“…Hoblit, 1988;Moulin and Karpel, 2007). In contrast, flapping wings may themselves mitigate the aerodynamic effects of unexpected gusts because lift fluctuations are known to decrease as flapping frequency is increased (Fisher et al, 2016). These effects, along with the ability of hummingbirds to alter wing kinematics on a stroke-by-stroke basis (Cheng et al, 2016a), may explain why the wings-dominated technique is associated with a less pronounced pitch response.…”

Dynamics and flight control of a flapping-wing robotic insect in the presence of wind gusts

“…The effects of wind disturbances on flapping flight are different from fixed-wing and rotary-wing flight in nature owing to unsteady aerodynamics. Literature related to wind gusts and flapping flight could be categorized into either the study of flapping-wing aerodynamics in the presence of gusts or turbulence [30][31][32][33][34] or the study of stability in forward and lateral flight [35][36][37][38][39][40]. Research focusing on aerodynamics typically employs computational fluid dynamics (CFD) or flow visualization to quantify instantaneous (as opposed to stroke averaged) flow around the wings and deduce corresponding aerodynamic properties such as lift and drag coefficients.…”