Aerodynamics and Design for Ultra-Low Reynolds Number Flight

Chapter 8
performance ramifications seen in more traditional applications. Properly configured
airfoils will continue to operate in a steady-state manner while significant portions of the
section are separated, albeit with reduced performance.
At more conventional Reynolds numbers, separation is usually a rapid precursor to stall,
but at ultra-low Reynolds numbers it is a prevalent feature of the normal operating range
of an airfoil. This leads to a fundamental difference between airfoil design for more
conventional Reynolds numbers and design for ultra-low Reynolds numbers. Typical
high Reynolds number airfoil design considers separation only as a factor at the extremes
of airfoil performance, such as near maximum lift or in designs emphasizing long runs of
laminar flow; much more effort is spent on transition and other sources of drag that come
into play long before separation is an issue. At ultra-low Reynolds numbers separation is
the issue.
Small changes in the Reynolds number cause large changes in drag, and as the Reynolds
number is reduced, section L/D quickly falls to single digits. Maximizing performance
requires operation at or near maximum lift, causing separation and its impact on the
effective camber to be dominant performance factors. Within the scope of this study, as
the Reynolds number is reduced, the maximum steady-state lift coefficients are seen to
generally increase. Since laminar separation is essentially independent of Reynolds
number, the reduction in the steepness of the recovery gradient due to boundary layer
growth dominates, delaying the onset of separation and permitting higher maximum lift
coefficients.
These results are part of an even broader conclusion which provides a prime motivation
for continued research. As the Reynolds number drops, the dramatic increase in viscous
effects, such as the rapid growth of boundary layers and the prevalence of separation, do
not simply washout the sensitivity to the detailed shape; the study of NACA 4-digit
sections demonstrates that at ultra-low Reynolds numbers, geometry variations still have
a tremendous effect on the aerodynamic performance of an airfoil. The impetus for two-
dimensional research and design at these Reynolds numbers is further strengthened by
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