The aerodynamics of miniature insect flight

The smallest example of powered flight currently known to humans is that of miniature insects, with wing lengths typically no greater than 1 mm. Flight in this domain is characterised by Reynolds numbers of the order of 10, meaning that viscous flow effects are more pronounced and, consequently, representative lift-to-drag ratios are significantly low. Most notably, at miniature scales, there is a transition from insects with wings made of continuous membranes to wings predominantly made up of bristled appendages. Yet, there remains very little understanding of how the structural arrangement of bristled wings interacts with the aerodynamics. In addition to their unique wing morphologies, the wing kinematics employed by miniature insects are also distinct. While flight is classically characterised via a lift force as the primary component for counteracting weight, miniature insects use swimming-like flapping profiles in which drag plays a distinctly more pronounced role in opposing gravity. Relative to the broader field of insect aerodynamics, the miniature domain has only recently begun to receive widespread attention from aerodynamicists, yet developing our understanding in the miniature field provides an opportunity to advance our capacity to inform the future design of miniature flying robots. To that end, the purpose of this review is to collate together the progress made thus far, in order to generate a perspective with regards to our current understanding of flight in the miniature domain.