Low Speed Wind Tunnel Testing of Aerofoil Family for Solar Powered Aircrafts

Abstract

The scope of this report covers the design and testing of a low Reynolds number aerofoil family used for aircrafts propelled by solar energy. The main constrain is the top surface which is defined by the mechanical properties of the solar cells and has a significant effect on the aerodynamics of the whole aerofoil. The aim was to provide reliable data which are in general sparingly available due to the fact that the technology is recent and not widely shared. Another objective was to upgrade the aerodynamic parameters of the aerofoil by using modifications that included gurney flaps, in-plane gurney flaps, trailing edge wedges, and lower surface modifications. Finally, using an iterative process the best configuration could be found and the data were used to improve the low-speed analytical model used to design aerofoils for solar powered models. The reports treats broadly about the experimental technique used to test the aerofoil and problems encountered during low-speed wind tunnel testing. The baseline aerofoil was designed to operate at 400000 to 600000 Reynolds number and generate the coefficient of lift (cL) between 0.7 and 0.9. The plain aerofoil without any modifications reached an aerodynamic efficiency (cL/cD) of 81 at 0.83 cL. It had a negative pitching moment (-0.12 for low Re) which further decreased for high Re (-0.57 at 2000000Re). Surface flow visualisation test revealed that there is a separation bubble at about 2% x/c and that separation occurs at the bottom surface at about 40%x/c. Parts designed to remove the latter phenomenon proved to be aerodynamically inefficient (maximum cL/cD of 71). The best improvement was the in-plane serrated 2% gurney flap, which achieved 140 cL/cD at 0.71 cL. The 0.8% gurney flap also showed an increase in the aerodynamic efficiency (109 cL/cD at 0.81 cL). Due to the time constrains of none of the leading edge devices could be checked. This, apart from further testing, should be the main aim for future research.