Design and Performance of Low Reynolds Number Airfoils for Solar-Powered Flight

Abstract

Airfoils with flat panels on the suction side and a flat pressure side are investigated for use with flat solar arrays for low speed solar-powered flight. Over 200 designs were evaluated using a coupled panel method/boundary layer analysis code (Drela, M., “XFOIL: An Analysis and Design System for Low Reynolds Number Airfoils,” in Conference on Low Reynolds Number Aerodynamics, June 1989). The best four designs were fabricated and their aerodynamic performance measured experimentally for design validation. Experimental aerodynamic analysis was conducted in the 2'x2' wind tunnel at the University of Michigan, Ann Arbor. The lift was measured using a force balance over a range of angles of attack from -5 to 20 degrees and at four Reynolds numbers: 60,000, 100,000, 200,000 and 250,000. The airfoils drag was determined from wake velocity profiles measured using twodimensional Particle Image Velocimetry at Reynolds numbers of 60,000 and 250,000. The flow over the suction side of the airfoils was also examined for laminar separation bubbles using Particle Image Velocimetry. The aerodynamic performance of the airfoils is compared to that of a SD 7080 airfoil, which is an airfoil section frequently used in low Reynolds number applications. The airfoil section that showed best performance is the BC 3X92, which has three flat panels on the suction side and a maximum thickness of 9.2 percent. Its drag is comparable to that of the SD 7080, and has comparable or higher maximum lift coefficient. The BC airfoils with flat panels typically generate more lift than the SD 7080 at almost all angles of attack because of the larger mean camber. From the flow field data, regions of separated flow and laminar separation bubbles are identified at a Reynolds number of 60,000, but none is observed at a Reynolds number of 200,000. The measured lift curves and the location of laminar separation bubbles of the BC airfoils are compared to XFOIL estimates. Such comparisons show that XFOIL predicts the trends of the lift curve well, especially at the higher Reynolds numbers. However, XFOIL predicts many features in the lift curve that are not observed on the measured lift curves perhaps due to the unconventional airfoil geometry used in this study. In particular, XFOIL consistently predicts the formation of laminar separation bubbles at most angles of attack, which were not found in the experiments.