Aerodynamics of a Yawed Blade in Reverse Flow

The retreating blades of rotorcraft operated at high advance ratios will experience reverse flow through a sector encompassing a wide range of blade azimuth angles. There is a great deal of uncertainty in the blade aerodynamic loads under these conditions. This is a limiting factor when trying to improve the flight speed envelope of helicopters. Previous studies and work have used two-dimensional aerodynamic approaches for the reverse flow area, making the assumption that aerodynamic forces behave similar in magnitude but opposite in direction. There have been no 3-dimensional considerations being taken into account nor was vortex induced lift considered. We hypothesize that the reverse blade flow field includes phenomena similar to the formation of a leading edge vortex on highly-swept, sharp-edges delta wings. An approach is being developed to understand aerodynamic contributions to blade loading beyond linear theory, where vortex-induced lift might be significant. Rotor blades at highly yawed angles relative to the wind can be thought of as very low aspect ratio wings. Since reverse airfoils are thought of as sharp edges, theoretically it should stand that a reverse finite wing at high yawed angles could be considered as a slender delta wing. The main aim of this work is to progress towards testing this above hypothesis. Experimental data is collected from a scaled version of a rotor blade exposed to the reverse flow at various azimuth positions representing the retreating side of the disc, in a 1.07m×1.07m low turbulence wind tunnel.