The role of nacelle drag in open fan design

Abstract This paper investigates the performance of an open fan operating in the high subsonic flight regime, with emphasis on the interaction between the blades and the nacelle and its impact on thrust, drag and overall efficiency. Strong spatial gradients in static pressure induced by the curved geometry of the nacelle can locally accelerate or diffuse the flow over rotor and stator blades. These effects may enhance blade performance and yield efficiency gains of up to 5% compared to configurations with less-curved, drag-reduced nacelles. However, such gains are accompanied by increased nacelle form drag, requiring a careful balance between blade performance and installation losses to maximise overall efficiency. The study compares two optimised propulsors. Each design meets the same aircraft-level requirements and stems from RANS-based multi-point optimisations. One configuration is optimised for maximum uninstalled performance only, while the other targets maximum overall installed efficiency by accounting for nacelle drag together with the blade performance. These differing objectives lead to an uninstalled efficiency difference of approximately 5%. The computational fluid dynamics (CFD) results are complemented by simplified actuator-disc-based considerations to quantify installation efficiency and assess its sensitivity to nacelle contour variations. The paper discusses key design trade-offs and highlights the importance of including geometric details such as the core inlet and nozzle in CFD analyses. It furthermore presents CFD-based propeller maps across the full flight envelope. These maps decompose thrust and drag contributions accordingly and enable a detailed evaluation of the open fan performance at cycle and system level.