Reverse thrust aerodynamics of ultra-high bypass variable-pitch turbofans.

This thesis constitutes the first research project into reverse thrust aerodynamics of variable-pitch turbofans within the Cranfield Rolls-Royce UTC in Aero System Design, Integration & Performance. The study focussed on development and validation of a steady-state (RANS) CFD research model, including the presentation of initial results. The engine model was developed primarily around NASA’s publicly available 22” Advanced Ducted Propulsor (ADP) variable-pitch fan. The 3D fan & OGV models were modified and scaled to suit the correct engine size for the study. An annular engine model was then developed, which consisted of an optimised 0D thermodynamic model, a 2D preliminary gas-path & component sizing model, and a 2D preliminary aero-line design for the nacelle & engine exhaust systems. These 2D models were extruded to provide 3D axisymmetric definitions. The resulting engine design was considered to represent a near-future ultra-high bypass ratio turbofan, of conventional geared architecture. Furthermore, in accordance with the project requirements, the engine satisfied the ‘middle-of-the-market’ thrust class (40,000 lbf/178 kN). A 3D pylon model was created, to mount the engine under the wing of NASA’s DLR F11 airframe, in accordance with the outcomes of previous turbofan installation aerodynamics studies conducted at Cranfield. The airframe was scaled to the approximate size & weight characteristics of a Boeing 757 aircraft. The research model was tested with the VPF at a single speed/pitch setting for reverse thrust, where it was found that flow within and around the engine was highly influenced by the fan’s rotational speed. The outer radius of the fan produced a reverse stream exiting the nacelle inlet at all tested landing speeds, which remained attached to the nacelle outer surface until re-ingestion through the bypass nozzle. Engine core distortion was measured during reverse thrust operation, which was most prominent at higher landing speeds (with peak DC60 = 0.21). The study concluded with new understandings of the challenges associated with a real-world (installed on airframe) VPF-reverse-thrust capable engine. More research is recommended to quantify the overall aerodynamic drag on the aircraft, and to test alternative fan pitch /RPM configurations.