Goals and Strategies for Open Fan Design

This paper highlights recent challenges associated with the design of an open fan propulsor for next generation civil aircraft in the high subsonic flight regime. The concept comprises a transonic propeller-rotor and a subsequent guide vane, both subject to pitch-variability in order to account for the strong variations in flight conditions over the entire mission profile. The engine-scale design was aiming for high technological maturity and was driven to comply with a high number of industrially relevant requirements to ensure a competitive design, meeting performance requirements in terms of high efficiency levels at cruise and maximum climb conditions, operability in terms of stability margins, good acoustic characteristics and structural integrity. During the design iterations, very fast 3D-RANS-based optimizations were used merely as a conceptual design tool to derive sensitivities which were used to support and justify major design choices in addition to established relations from propeller theory and common design practice. Those design driven optimization efforts were complemented with more sophisticated CFD analysis focusing on rotor tip vortex trajectories and resulting unsteady blade row interaction to optimize the guide vane clipping. The resulting open fan design will be the very basis for wind tunnel experiments of a downscaled version at low and high speed.