Reynolds Number Effects on Airfoils in Reverse Flow

Abstract

This work is aimed at providing an improved understanding of the impact of the radial Reynolds number distribution that exists in the reverse flow region of a helicopter operating at high advance ratios. Time-averaged sectional airloads and flow fields were measured experimentally for four airfoils in forward and reverse flow at Reynolds numbers between 3.3× 10 and 1.0× 10. Two airfoils with a sharp geometric trailing edge (NACA 0012 and NACA 0024) and two airfoils with a blunt geometric trailing edge (a 24 % thick elliptical airfoil, and a 26 % thick cambered ellipse airfoil) were tested. This work shows that the airloads for a NACA 0012 in reverse flow (a “thin” airfoil with a sharp aerodynamic leading edge) are insensitive to Reynolds number due to early flow separation. The airloads of thicker airfoils are found to be more sensitive to Reynolds number. In reverse flow, a NACA 0024 airfoil exhibits a decrease in the magnitude of the airloads with increasing Reynolds number for −3 ≤ −αrev ≤ 15deg. The lift curve of an elliptical airfoil becomes more linear with increasing Reynolds number. The character of the lift curve for the cambered ellipse airfoil in both forward and reverse flow changes drastically for Re ≥ 3.3 × 10. This includes a large shift in the zero-lift angle of attack. These results give insight to the design of high-speed helicopter rotor blades by examining the sensitivity of airloads to the range of Reynolds numbers encountered in the reverse flow region.