Vortex aerodynamics of rotors at high advance ratios

Abstract

Rotor operation at high advance ratio is important for high-speed and compound rotorcraft concepts. The operation of helicopter rotors in reverse flow has taken on new significance in the context of co-axial rotors. A rotor moving edgewise at a high advance ratio, encounters reverse flow on parts of the retreating portion of the rotor disc. Predicting rotor stability and pitch link loads is complicated by the presence of unsteady pitch, yaw and rotation effects. Predictions using comprehensive codes have shown large differences from full-scale experimental data. Prior approaches have modeled these using flow separation with airfoil data modified for yaw, vortex shedding, dynamic pitch oscillations and reverse dynamic stall of an airfoil. However, the highly 3-dimensional flow phenomena do not conform to approaches based on 2-dimensional airfoil aerodynamics. The present work delineates the nature of flow around a rotating blade in reverse flow by integrating the results from fixed wing experiments with rotating wing experiments. The work focuses on a strong 3D vortex similar to those seen on delta wings that would develop over the sharp edge at high yaw, providing an avenue for vortex lift aerodynamic analyses. The fixed wing and rotating wing experiments were performed on a tethered rotor blade with NACA0013 profile. Fixed-wing results from load measurements and flow visualization showed that the sharp-edge vortex (SEV) is a primary feature in reverse flow when the blade is yawed either forward or backward. The aerodynamic loads conform with analytical model using Polhamus Suction Analogy, thus showing significant contributions from vortex-induced lift and pitching moments. In summary, it is apparent that the reverse flow regime should be modeled and analyzed as a case of SEV formation under the very sharply swept blade immediately after 180 degrees azimuth. The SEV evolves as the sweep decreases with increasing azimuth. In the regime before 240 degrees, an attached, strengthening SEV may be expected. At some moderate sweep (azimuth beyond 240 degrees in our case) the vortex bursts and detaches from the surface. Thereafter it convects with the blade, but induces strong pressure effects on the blade surface even as far as 300 degrees azimuth. The blunt edge flow is highly 3-dimensional, and has much less flow separation and unsteadiness than might be predicted from 2-dimensional airfoil aerodynamics.