Aerodynamics and Design for Ultra-Low Reynolds Number Flight

Chapter 3
42
The primary effect of substituting a constant thickness profile is a uniform increase
in drag across the operating range of the section. Drag polars for the NACA 0002 and
both plate sections are shown in Figure 3.17. The drag increase ranges from 9% at
Re=6000, down to 5% at Re=1000. The increase is comparable to changing from a 2%
thick airfoil to a 5% thick airfoil, but the plates suffer penalties in L/D that are minimal
compared to an actual increase in thickness. An increase in section thickness results in a
significant reduction in the lift curve slope and lower maximum steady-state lift
coefficients, but the constant 2% thickness plates exhibit no reduction in lift curve slope
relative to the 2% thick airfoil. An actual increase in thickness would result in a 20% to
25% reduction in maximum L/D. The penalty for the constant thickness plates is only
5% at Re=1000 and Re=2000, increasing to 18% at Re=6000.
The leading edge shape effects the formation of the leading edge separation bubble. At
Re=1000, both plates are fully attached past α=4.0. Once leading edge separation
occurs, the radiused plate gains less than one-half degree of angle of attack for
equivalent bubble lengths. Leading edge separation bubbles appear on the plates earlier
than on the airfoil, but the leading edge stall on the airfoil occurs very quickly. The net
effect is a minimal penalty in lift for the plates. At Re=6000, the leading edge separation
bubble forms almost immediately on the blunt section, but does not form on the radiused
plate until 1.5 degrees later. Avoiding a blunt profile is advisable, but as the Reynolds
number and maximum section thickness are reduced, the details of the thickness
distribution become less relevant and the camber-line becomes the dominant factor in
performance.