Low-Reynolds Number Behavior of the Leading-Edge Suction Parameter at Low Pitch Rates

Abstract

Small unmanned aerial vehicles (UAVs) are becoming increasingly popular as low-cost observation and research platforms. However, these aircraft operate low in the atmosphere, exposed to the turbulent wakes of man-made and natural roughness features. As such, it is important to estimate their unsteady aerodynamic performance. The leading-edge suction parameter (LESP) is one criterion used in vortex methods for predicting flow separation, which can arise in both steady and unsteady flows. The critical LESP represents the maximum suction supported in the leading-edge region, equivalent to the flat-plate behavior of thin airfoil theory. This parameter was investigated at low pitch rates for a NACA0012 airfoil at Reynolds numbers applicable to small UAVs, to examine the hypothesis of a constant critical value existing for an arbitrary motion. It is seen that the critical LESP shows a strong dependence on pitch rate, with a piecewise linear fit being obtained. A higher pitch rate induces a greater pressure-field lag, promoting a higher peak LESP for flow separation, and a lower value for reattachment. These peaks in the LESP agree well with the instances of separation and reattachment. Raising the Reynolds number also acts to raise the critical LESP for a given reduced pitch rate.