text: Reynolds numbers are used in airfoil design to (among other things) manage "scale effect" when computing/comparing characteristics (a tiny wing, scaled to be huge, will perform differently). Fluid dynamicists define the chord Reynolds number , where  is the flight speed,  is the chord length, and  is the kinematic viscosity of the fluid in which the airfoil operates, which is  for the atmosphere at sea level. In some special studies a characteristic length other than chord may be used; rare is the "span Reynolds number", which is not to be confused with span-wise stations on a wing, where chord is still used.