Aerodynamics of insect flight

Insects are the earliest, most numerous and smallest fliers in the world. They can hover, fly forward, climb and descend with ease while demonstrating amazing stabilities, and they can also maneuver in impressive ways like no other organisms could. Although the wing of an insect beats at high frequency, the wing's relative velocity is small owing to the small wing length. As a result, the mean lift coefficient of wing required to balance the insect weight is relatively high, about 1.5–2, much higher than that of an airplane at cruising flight. The Reynolds number of insects' wings is small, ranging from about 10 to 3 500. How the required high-lift coefficient is produced at such low Reynolds number? Researchers are very interested in this question and in recent years, significant progress has been made in the area. Works before 2005 have been discussed in detail in several review papers, and in this article, we review the advances made since 2005. We begin with an overview of the flapping kinematics and basic equations of fluid dynamics. It is followed by a summary of the works before 2005. Then we review the advances made since 2005, dealing in turn with measurement of wing motion in freely-flying insects, leading-edge vortex, effect of wing deformation and corrugation, vortex wake of flapping wings, ground effect and aerodynamic interaction between wings and body, flight of tiny insects, flight of butterfly and dragonfly, and maneuvering flight. Finally, we make remarks on the state-of-the-art of this research field and speculate its outlooks in the near future.