As already mentioned, there are several prominent features of MAV flight: (i) low Reynolds numbers (104–105), resulting in degraded aerodynamic performance, (ii) small physical dimensions, resulting in much reduced payload capabilities, as well as some favorable scaling characteristics including structural strength, reduced stall speed, and impact tolerance, (iii) low flight speed, resulting in an order one effect of the flight environment such as wind gust, and intrinsically unsteady flight characteristics. The preferred low Reynolds number airfoil shapes are different from those typically used for manned aircraft in thickness, camber, and AR. In this chapter, we discuss low Reynolds number aerodynamics and the implications of airfoil shapes, laminar–turbulent transition, and an unsteady free stream on the performance outcome.
Schmitz (1942) was among the first to investigate the aerodynamics for model airplanes in Germany, and he published his research in 1942. His work is often considered to be the first reported low-speed wind-tunnel research. However, before him, experimental investigations of low Reynolds number aerodynamics were conducted by Brown (1939) and by Weiss (1939), in the first two (and only) issues of The Journal of International Aeromodeling. Brown's experiments focused on curved-plate airfoils, made by using two circle arcs meet at maximum camber point of 8%, at varying locations. The wing test sections were all of 12.7 cm × 76.2 cm, giving an aspect ratio of 6. In all cases, the tests were conducted at a free-stream velocity of 94 cm/s.