Analysis of Download and Rotor/Airframe Interactions in Hover

The losses in rotorcraft hover performance from airframe download have long been an important issue in rotorcraft design. However, given the increased focus on advanced tiltrotor, compound, and coaxial configurations, there is a more pressing need for physics-based prediction methods suitable for use early in the design cycle. In addition, airframes that feature substantial blockage of the main rotor downwash will typically involve significant interaction with the rotor, having an important effect on installed rotor performance. This paper centers on an assessment of the ability of two contemporary analysis tools to address the modeling of fuselage/rotor interaction in hover, with a focus on cases involving a single main rotor. Correlation studies using a comprehensive rotorcraft analysis and a cut-cell octree Cartesian CFD method are presented, to assess their ability to predict installed hover performance and download at a level sufficient to support conceptual and preliminary design. While significant limitations and development needs remain, these models produce encouraging results when applied to analysis challenges relevant to the study of new configurations.