Time-Averaged Aerodynamics of Sharp and Blunt Trailing-Edge Static Airfoils in Reverse Flow

Two-dimensional wind-tunnel experiments have been conducted on three airfoils held at static angles of attack through 360deg at a Reynolds number of Re=1.1x105 to evaluate the influence of trailing-edge shape on time-averaged force and flowfield measurements. The present study focuses on airfoil performance in reverse flow to advance the understanding of this flow regime for high-speed helicopter applications. It is shown that the drag of a NACA 0012 airfoil in reverse flow is more than twice as large compared to forward flow due to early flow separation, similar to a flat plate. Two blunt trailing-edge airfoils are considered in this work: an elliptical airfoil and the DBLN-526. Both airfoils exhibit a rapid increase in lift at low angles of attack in both forward and reverse flows. The drag of the elliptical airfoil in reverse flow is significantly lower than the NACA 0012 for 5<rev<17deg. Lift was calculated via a circulation box method applied to time-averaged flowfield measurements and compared to force measurements. The findings presented here give fundamental insight into the selection of airfoils for the inboard section of a rotor blade for optimal performance in both forward and reverse flows.