Flow Field Analysis of Highly-Twisted Rotors in Hover

The flow field of highly-twisted rotors in hover is studied using a thin-layer Navier-Stokes analysis. The objective of the investigation is 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. One of the key physical differences between a tilt rotor and helicopter blade is the built-in twist. The tilt rotor blade has a nonlinear twist distribution with extremely high twist at the blade root; the helicopter blade has a low, linear twist over the majority of the blade span. Blade twist is used as the focal point of this study. Differences in rotor performance, blade loads, surface pressures, skin friction, flow separation location, and flow field vorticity are investigated for near-stall conditions as the blade twist is systematically changed from that of a tilt rotor to a linear twist. The study of twist effects on stall behavior of a full-scale tilt rotor blade suggests that separation on the linearly twisted blade is closely coupled with the tip vortex, while separation on the tilt rotor blade is shock induced. As the twist is changed from a tilt rotor blade twist to a conventional helicopter blade twist, while maintaining thrust, the separated region near the tip grows. This causes the section in-plane force near the tip to increase significantly, which causes the rotor torque to increase, finally resulting in degraded performance. The flow in the blade inboard region (r/R < 0.50) remains attached and well-behaved for all twist cases and thrust conditions analyzed. The twist variation study suggests that twist distributions with more inboard twist than the V22 provide better hover performance.