Influence of Turbulence on Cambered and Symmetrical Airfoils at Low Reynolds Numbers

Small unmanned aerial vehicles (UAVs) are becoming more common as electronic systems decrease in size and weight. These aircraft fly below a Reynolds number of 250,000, where, in clean flow, the wing boundary layer undergoes laminar-to-turbulent transition for a significant portion of the wing chord. However, many UAVs fly at low altitude, where significant levels of turbulence are encountered due to terrain roughness. The present study was carried out to understand how this turbulent flow changes the performance of the UAV wing-in particular, how this influence may vary with the camber of the airfoil form. Two airfoils, a NACA0012 and a NACA4412, were tested in relatively clean and highly turbulent flows in a wind tunnel. Testing was conducted at Reynolds numbers of 50,000-200,000, with turbulence intensities from 1.3 to 15%. Under increasing turbulence intensities, in contrast to prior flat plate research, the maximum lift coefficient was seen to decrease by up to 30% for the cambered airfoil, compared with a 5% rise for the symmetrical form. The influence of the Reynolds number on both airfoils decreased as the turbulence intensity increased, whereas camber maintained a point of difference.