Perspectives on low-Reynolds-number aerodynamics: shape, motion and structure

Some of the most interesting areas in aerospace science and technologies are on either higher, faster, and larger systems or lower, slower, and smaller flying capabilities. In this paper, we present our perspectives on the aerodynamics related to small, fixed-wing as well as flapping-wing flight vehicles. From an evolutionary viewpoint, flyers have gone through many iterations, adaptations, and optimizations to balance their biological functions, including flight. In the low-Reynolds-number regime, the aerodynamic characteristics around a solid object differ from those observed at the scale of passenger-airplanes. Consequently, the optimal airfoil and wing shapes vary with vehicle size. As vehicle dimensions vary, non-proportional scaling between surface areas and weight shifts the dominance of physical mechanisms, leading to distinct operational parameters and technical requirements. With smaller flight vehicles, structural flexibility as well as anisotropic material properties become more pronounced, which causes qualitative changes in aerodynamics. The flapping motion of the wings, the interactions between wings, the synergistic characteristics of wing and tail, and the development of soft structures for better agility and flight performance are discussed. Low-Reynolds-number aerodynamics require collaborative innovation to optimize shape, motion, and structure of vehicles in accordance with the scaling laws. Together, progress on these fronts is reshaping the design paradigm of air vehicles and other types of robots with shrinking physical dimensions and more versatile capabilities to meet wider ranges of missions.