Reverse- and Cross-Flow Aerodynamics for High-Advance-Ratio Flight

Abstract

High-advance-ratio, high-speed rotorcraft flight enters complex aerodynamic regimes that, in many cases, have either been neglected or lie on the edge of models applied in comprehensive rotor code analysis. Specifically, rotor blades will experience large areas over the rotor disk where reverse-flow and cross-flow effects cannot be neglected during the design and analysis of an efficient rotor at high advance ratios. As experimental evaluations are expensive, a cost-effective alternative is the use of computational fluid dynamics (CFD) codes to develop sectional airfoil tables, as well as to understand the physics of these two flow effects. Numerical experiments have been performed, with correlation to experimental databases, that illustrate that CFD methods are an excellent alternative if advanced turbulence models are employed to correctly model the physics of separated and yawed flows. Correlation with an existing yaw equivalence model shows that corrections in the supercritical Mach range and high angles of attack tend to over predict the coefficients. At high radial flow conditions, the drag on the NACA0012 airfoil is equivalent to flat plate friction drag.