Lifting Surface Blade Model for Comprehensive Rotorcraft Analysis

Current comprehensive rotorcraft analyses typically use lifting line theory coupled with 2D look-up tables to determine section lift, drag and pitching moment along the rotor blade. These methods cannot directly capture important 3D flow effects that influence chordwise loading and unsteady aerodynamics. Computational fluid dynamics (CFD) codes have been coupled to comprehensive rotorcraft analyses to provide this capability, but these coupled solutions are too computationally intensive for daily design work. This paper describes work performed to investigate the effectiveness of using a vortex lattice lifting surface blade model in comprehensive rotorcraft analyses to bridge the gap between current lifting line models and full CFD blade airload solutions. A vortex lattice blade model was used to directly compute section lift and pitching moment, including both circulatory and noncirculatory unsteady airloads, within the 3D rotating environment. The method was coupled with a free-vortex wake model and predictions were compared with experimental data, model problems and other methods. It was observed that with a modest increase in computation time, a comprehensive rotorcraft analysis using a vortex lattice blade model can obtain significantly improved predictions of blade pitching moment over a lifting line model, particularly in flight conditions that involve blade vortex interactions.