The flight of insects has fascinated physicists and biologists for more than a century. Yet, until recently, researchers were unable to rigorously quantify the complex wing motions of flapping insects or measure the forces and flows around their wings. However, recent developments in high-speed videography and tools for computational and mechanical modeling have allowed researchers to make rapid progress in advancing our understanding of insect flight. These mechanical and computational fluid dynamic models, combined with modern flow visualization techniques, have revealed that the fluid dynamic phenomena underlying flapping flight are different from those of non-flapping, 2-D wings on which most previous models were based. In particular, even at high angles of attack, a prominent leading edge vortex remains stably attached on the insect wing and does not shed into an unsteady wake, as would be expected from non-flapping 2-D wings. Its presence greatly enhances the forces generated by the wing, thus enabling insects to hover or maneuver. In addition, flight forces are further enhanced by other mechanisms acting during changes in angle of attack, especially at stroke reversal, the mutual interaction of the two wings at dorsal stroke reversal or wing-wake interactions following stroke reversal. This progress has enabled the development of simple analytical and empirical models that allow us to calculate the instantaneous forces on flapping insect wings more accurately than was previously possible. It also promises to foster new and exciting multi-disciplinary collaborations between physicists who seek to explain the phenomenology, biologists who seek to understand its relevance to insect physiology and evolution, and engineers who are inspired to build micro-robotic insects using these principles. This review covers the basic physical principles underlying flapping flight in insects, results of recent experiments concerning the aerodynamics of insect flight, as well as the different approaches used to model these phenomena.

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The aerodynamics of insect flight

▪ Abstract “What force does an insect wing generate?” Finding answers to this enduring question is an essential step toward our understanding of interactions of moving objects with fluids that enable most living species such as insects, birds, and fish to travel efficiently and us to follow similar suit with sails, oars, and airfoils. We give a brief history of research in insect flight and discuss recent findings in unsteady aerodynamics of flapping flight at intermediate range Reynolds numbers (10–104). In particular, we examine the unsteady mechanisms in uniform and accelerated motions, forward and hovering flight, as well as passive flight of free-falling objects. The results obtained by “taking the insects apart” helped us to resolve previous puzzles about the force estimates in hovering insects, to ellucidate basic mechanisms essential to flapping flight, and to gain insights about the efficieny of flight.

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Dissecting insect flight

Unsteady aerodynamics and flow control for flapping wing flyers

SUMMARY Here we show, by qualitative free- and tethered-flight flow visualization,
that dragonflies fly by using unsteady aerodynamic mechanisms to generate
high-lift, leading-edge vortices. In normal free flight, dragonflies use
counterstroking kinematics, with a leading-edge vortex (LEV) on the forewing
downstroke, attached flow on the forewing upstroke, and attached flow on the
hindwing throughout. Accelerating dragonflies switch to in-phase wing-beats
with highly separated downstroke flows, with a single LEV attached across both
the fore- and hindwings. We use smoke visualizations to distinguish between
the three simplest local analytical solutions of the Navier–Stokes
equations yielding flow separation resulting in a LEV. The LEV is an open
U-shaped separation, continuous across the thorax, running parallel to the
wing leading edge and inflecting at the tips to form wingtip vortices. Air
spirals in to a free-slip critical point over the centreline as the LEV grows.
Spanwise flow is not a dominant feature of the flow field – spanwise
flows sometimes run from wingtip to centreline, or vice versa –
depending on the degree of sideslip. LEV formation always coincides with rapid
increases in angle of attack, and the smoke visualizations clearly show the
formation of LEVs whenever a rapid increase in angle of attack occurs. There
is no discrete starting vortex. Instead, a shear layer forms behind the
trailing edge whenever the wing is at a non-zero angle of attack, and rolls
up, under Kelvin–Helmholtz instability, into a series of transverse
vortices with circulation of opposite sign to the circulation around the wing
and LEV. The flow fields produced by dragonflies differ qualitatively from
those published for mechanical models of dragonflies, fruitflies and
hawkmoths, which preclude natural wing interactions. However, controlled
parametric experiments show that, provided the Strouhal number is appropriate
and the natural interaction between left and right wings can occur, even a
simple plunging plate can reproduce the detailed features of the flow seen in
dragonflies. In our models, and in dragonflies, it appears that stability of
the LEV is achieved by a general mechanism whereby flapping kinematics are
configured so that a LEV would be expected to form naturally over the wing and
remain attached for the duration of the stroke. However, the actual formation
and shedding of the LEV is controlled by wing angle of attack, which
dragonflies can vary through both extremes, from zero up to a range that leads
to immediate flow separation at any time during a wing stroke.

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Dragonfly flight: free-flight and tethered flow visualizations reveal a diverse array of unsteady lift-generating mechanisms, controlled primarily via angle of attack.

The free flapping flight of the dragonfly Sympetrum sanguineum and the damselfly Calopteryx splendens was filmed in a large flight enclosure at 3000 frames s-1. The wingtip kinematics are described for these flights. Despite the two species being similar in size, the damselfly flew with wingbeat frequencies half those of the dragonfly. The damselfly could perform a clap and fling, and the proximity to which the wings approached each other during this manoeuvre correlated with the total force produced during the wingstroke. The dragonfly beat its wings with a set inclination of the stroke planes with respect to the londitudinal body axis; the damselfly, in contrast, showed a greater variation in this angle. Both species aligned their stroke planes to be nearly normal to the direction of the resultant force, the thrust. In order to achieve this, the dragonfly body alignment correlated with the direction of thrust. However, the damselfly body alignment was independent of the thrust direction. Velocities and accelerations were greater for the dragonfly than for the damselfly. However, non-dimensional velocities and accelerations normalised by the wingbeat periods were greater for the damselfly.

Dragonfly flight. II. Velocities, accelerations and kinematics of flapping flight

SUMMARY The steady slow climbing flight of a dragonfly, Sympetrum frequens, was filmed and analysed. By using the observed data, the mechanical characteristics of the beating wings were carefully analysed by a simple method based on the momentum theory and the blade element theory, and with a numerical method modified from the local circulation method (LCM), which has been developed for analysing the aerodynamic characteristics of rotary wings. The results of calculations based on the observed data show that the dragonfly performs low speed flight with ordinary airfoil characteristics, instead of adopting an abnormally large lift coefficient. The observed phase advance of the hindwing, Adi — 80° can be fully explained by the present theoretical calculation. Similarly, the spanwise variation of the airloading and the time variations of the horizontal force, vertical force, pitching moment and torque or power can be definitely estimated within a reasonable range of accuracy in comparison with the flight data. The distribution of loading between the fore and hind pairs of wings is also clarified by the calculations.

Flight Mechanics of a Dragonfly

The "ro" and "kai" are kinds of oriental sculls used to propel and to steer small boats and have been utilised for a long time in Japan. Analytic and experimental studies are performed to understand the mechanisms of locomotion of the ro and kai. Simple momentum theory and the local circulation method, together with the blade element theory, are applied to obtain the hydrodynamic characteristics of the ro and these are compared with experimental test data. The results show that the matching of the ro with the hull is excellent in the hydrodynamic sense and that the operation of the ro is also conducted with optimal efficiency.

Toyohiko Furuta

Papers

Flight Mechanics of a Dragonfly

Hydrodynamic Analysis of the Sweeping of a "Ro"—an Oriental Scull