Reynolds Number Effects on Rotor Blade Sections in Reverse Flow

The retreating blade of a high-advance-ratio rotor encounters a wide range of Reynolds numbers when passing through the reverse flow region. The present work was aimed at providing an improved understanding of Reynolds number effects in both forward and reverse flow. Time-averaged sectional airloads and surface oil flow visualizations were obtained experimentally for four airfoil cross sections at Reynolds numbers between 3.3x10(5) and 1.0x10(6). Two airfoils with a sharp geometric trailing edge (a NACA 0012 and a 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 are insensitive to Reynolds number due to early flow separation, because it acts as a thin airfoil due to the sharp aerodynamic leading edge. The airloads of thicker airfoils were found to be more sensitive to Reynolds number. In reverse flow, the NACA 0024 exhibits a decrease in the magnitude of the airloads with increasing Reynolds number for -3 <= -alpha(rev) <= 15 deg. 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 changes drastically for Re >= 3.3 x 10(5) in both forward and reverse flow. These results provide insight for 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.