6.0
Aerodynamics 6.1 Airfoil Design The method for selection of airfoil profiles was dictated by two main criteria. First, in order
to maximize the extent of natural laminar flow on the upper surface (thus reducing friction drag), the selected airfoils needed a favorable “rooftop” shape pressure coefficient distribution 24. Second, the airfoil geometry must have moderate pitching moment coefficients to avoid larger horizontal tail planform areas required to balance the aircraft. In order to obtain a reasonable baseline airfoil, a study of 15 low Reynolds number, natural laminar flow airfoil geometries (available on University of Illinois at Urbana-Champaign’s 25 web portal) was conducted. The airfoils were analyzed using the DesignFoil software on the merit of the maximum extent of laminar flow at CL 0.65 (selected in Sec. 2.1). Figure 19 presents a summary of the airfoil selection.
(a) (b) Fig. 19 Airfoil selection pool. a) Section L/D v. CM0 b) Section L/D v. CL0. It is noteworthy that the airfoil NLF415/E393 while having outstanding L/D and CL0 performance, attains too large of a moment coefficient, making it unsuitable as a wing airfoil.
Using these airfoils, multiple combinations of upper and lower surface curves were analyzed in order to select the best performing combination. NASA Langley’s NLF-415 was selected as the upper airfoil profile for both root and tip. The NL722343 was selected for the lower root airfoil profile and NLF 0215F was chosen for the lower tip airfoil profile. Camber adjustment was performed on the root and tip airfoils to increase their section cruise L/D. As the extent of laminar flow plays a significant role in airfoil performance, CFD analyses (using ANSYS CFX) were then performed to verify the locations of transition to turbulence as well as general airfoil characteristics. 32