Flapping wing aerodynamics: from insects to vertebrates

Abstract

More than a million insects and approximately 11,000 vertebrates utilize flapping wings to fly. However, flapping flight has only been studied in a few of these species, so many challenges remain in understanding this form of locomotion. Five key aerodynamic mechanisms have been identified for insect flight. Among these is the leading edge vortex, which is a convergent solution to avoid stall for insects, bats and birds. The roles of the other mechanismsadded mass, clap and fling, rotational circulation and wing-wake interactions -have not yet been thoroughly studied in the context of vertebrate flight. Further challenges to understanding bat and bird flight are posed by the complex, dynamic wing morphologies of these species and the more turbulent airflow generated by their wings compared with that observed during insect flight. Nevertheless, three dimensionless numbers that combine key flow, morphological and kinematic parameters -the Reynolds number, Rossby number and advance ratio -govern flapping wing aerodynamics for both insects and vertebrates. These numbers can thus be used to organize an integrative framework for studying and comparing animal flapping flight. Here, we provide a roadmap for developing such a framework, highlighting the aerodynamic mechanisms that remain to be quantified and compared across species. Ultimately, incorporating complex flight maneuvers, environmental effects and developmental stages into this framework will also be essential to advancing our understanding of the biomechanics, movement ecology and evolution of animal flight.

Mechanics of the thorax in flies

“…Sane, 2003;Chin and Lentink, 2016). These forces primarily result from aerodynamic mechanisms owing to leading edge vortices produced during wing translation (Ellington et al, 1996), rotational mechanisms during pronation and supination of the wing , and other mechanisms such as 'clap-and-fling' (see Glossary) which are observed more commonly in miniature insects (Weis-Fogh, 1973).…”

Unsteady high-lift mechanisms from heaving flat plate simulations

“…The kinematics and mechanics of flapping flight for insects such as clap and fling, wake capture, and delayed stall have been thoroughly investigated and reported, as summarized by Shyy et al [3] and more recently by Chin and Lentink [4]. Of particular interest is the delayed stall phenomena, in which a leading edge vortex (LEV) on the upper edge of the wing enhances lift throughout the downstroke, which has been shown experimentally [5,6] and computationally [7,8].…”

Inspiration for wing design: how forelimb specialization enables active flight in modern vertebrates

“…The kinematics of individual wingbeats also differ among birds and bats [124], primarily due to differences in how they flex their wings during upstrokes [59]. Whereas downstroke kinematics remain relatively consistent, upstroke kinematics vary among birds with flight speed and wing shape.…”

“…While significant progress has been made in recent years, many gaps still exist in our understanding of vertebrate flight, especially with regard to manoeuvring flight and flight in different natural environments [124]. Additionally, many of the active control mechanisms identified for birds and bats have only been studied through the use of theoretical [153,154], computational [156] or robotic models [155].…”

“…With a robot, this constraint is removed. Robotic models, for example, have been used to study the effects of different aerodynamic mechanisms in flapping animal flight [124,174] and to isolate effects of individual kinematic variables in a flapping wing [25]. Robotic platforms will enable testing how different materials perform as aerofoils, how different wing shapes perform in different flight conditions and how integrated filoplume-like sensors might enhance flight.…”