BLADE TWIST EFFECTS ON HOVERING TILT ROTOR FLOW FIELDS

The flow field of highly-twisted rotors in hover was analyzed using a thin-layer Navier-Stokes analysis. The objective of the investigation was to understand how various fluid dynamic phenomena affect tilt rotor hover performance and how these phenomena differ for a conventional helicopter blade, especially near stall conditions. Differences in rotor performance, blade loads, skin friction, flow separation location, and flow field vorticity were investigated at near-stall conditions as the blade twist was systematically changed from that of a tilt rotor (full-scale V-22) to a rotor with a linear twist. Starting with an existing Navier-Stokes analysis for hovering helicopter rotors, boundary conditions at the blade root and far-field were modified. A no-slip boundary condition at the blade root and a far-field inflow boundary condition based on mass conservation were implemented. These modifications produced noticeable effects on calculated section loads and performance. As the twist was changed from a tilt rotor blade twist to a conventional helicopter blade twist, while maintaining thrust, the separated region near the tip grew. This caused the section in-plane force near the tip to increase significantly, which caused the rotor torque to increase, finally resulting in degraded performance. The flow in the blade inboard region (r/R< 0.50) remained attached and well-behaved for all twist cases and thrust conditions analyzed. The calculations indicated that separation on the linearly twisted blade was closely coupled with the tip vortex, while separation on the tilt rotor blade was shock-induced. In addition, the study showed that twist distributions with more inboard twist than the V-22 can provide better hover performance.