DESIGN OF LOW REYNOLDS-NUMBER AIRFOILS .1.

The 9.5%-thick low Reynolds number (Re) airfoils ASM-LRN-003 and ASM-LRN-007 were designed for high section lift-to-drag ratios (CL/CD∞) at 2.5 × 105 < Rec < 5.0 × 105, assuming laminar separation and transition control for minimum CD∞ with turbulators. At the upper low-drag CL limit (CL ≈ 1.3 to 1.4), 67-69%- chord laminar flow with substantial concave pressure recovery is expected on the upper surface. At CLdesign the lower surface is fully laminar. To maximize (CL/CD∞), the rear lower surface is undercut and aft-loaded, inducing a relatively strong local flow deceleration at lower CL and correspondingly higher flight dynamic pressures. The resulting higher airloads on the flexible rear wing structure (similar to soaring bird wings with their chordwise feathers and quills) deflect and decamber the airfoil, thereby decreasing the pressure rise and enabling all laminar flow at lower CL on the lower surface. An undercut front lower surface with a correspondingly sharper leading edge raises (CL/CD∞)max further, narrowing, though, the low-drag CL range. With optimum transition control, (CL/CD∞)max = 120 at Rec = 2.5 × 105 (CL = 1.40) and 166 at Rec = 5.0 × 105 (CL = 1.35) result, using Drela’s code for airfoil ASM-LRN-003. The low-drag CL range increases substantially by deflecting a 16%-chord cruise flap. © 1989 American Institute of Aeronautics and Astronautics, Inc. All rights reserved.