Low Reynolds Number Airfoil Design
Low Reynolds numbers make the problem of airfoil design difficult because
the boundary layer is much less capable of handling an adverse pressure
gradient without separation. Thus, very low Reynolds number designs do not
have severe pressure gradients and the maximum lift capability is restricted.
Low Reynolds number airfoil designs are cursed with the problem of too much
laminar flow. It is sometimes difficult to assure that the boundary layer
is turbulent over the steepest pressure recovery regions. Laminar separation
bubbles are common and unless properly stabilized can lead to excessive
drag and low maximum lift.
At very low Reynolds numbers, most or all of the boundary layer is laminar.
Under such conditions the boundary layer can handle only gradual pressure
recovery. Based on the expressions for laminar separation, one finds that
an all-laminar section can generate a CL of about 0.4 or achieve a thickness
of about 7.5%. (Try this with a simple airfoil design code such as PANDA.)