Development of a Research Model to Study the Operability of a Variable Pitch Fan Aero Engine In Reverse Thrust

A rationale for the level of model fidelity required to provide the most representative flow field information to ascertain the feasibility of using a Variable Pitch Fan (VPF) in a modern high bypass ratio aero engine to generate reverse thrust is described in this paper. This is done by comparing the 3D RANS flow field solution for a newly developed reverse flow VPF design from two research models: i) isolated engine model in which the bypass duct, guide vanes, splitter and VPF are wrapped in an axisymmetric nacelle and placed in a generic far-field domain and b) integrated model in which the engine is installed to an airframe in landing configuration through a pylon and placed in a far-field domain bound by a rolling runway. The flow field solution obtained at an aircraft landing speed of 80 knots indicates that even though both models can predict the general flow patterns, there are substantial differences in parameters such as the amount of reverse stream, circumferential distribution of flow properties and flow development downstream of the engine. These differences impact the levels of reverse thrust generated, flow distortion entering the core engine and resultant airframe forces. This study makes the case that it is necessary to use an integrated model that includes a full engine nacelle installed on an airframe, to answer design questions for engineering the VPF system to generate reverse thrust. INTRODUCTION Modern ultra-high bypass ratio civil gas turbine engines have bypass ratios in the range of 10 – 14 to increase engine propulsive efficiency. The fan stage in such high bypass ratio engines is susceptible to aerodynamic instabilities at offdesign conditions due to low design fan pressure ratios dictated by engine thermodynamic cycle requirements. The VPF with variable stagger angle fan blades can be used to avoid such unstable operating regimes and optimise fan operation without resorting to a variable area bypass nozzle. The variable area nozzle for high bypass ratio engines tend to be complex and heavy enough to offset the advantages obtained from high bypass ratios. Therefore, the VPF with lighter actuation systems in the fan spinner is a more viable design alternative. A number of comparative studies between the VPF and the variable area nozzle note that the VPF can provide up to 7 % improvement in the aircraft mission fuel burn (Denning, 1972; Halliwell and Justice, 2012; Mazzawy, 2012; Krishnan et al, 2013; Yang and Tang, 2018). The direction of airflow through the VPF can be reversed if the blade aerofoils are rotated by 90 ̊ in the clockwise or anti-clockwise direction (Moore and Osborn, 1979). Therefore, the VPF can be used to generate reverse thrust by causing flow to be sucked in from the bypass nozzle exit and ejected through the fan inlet. The bulky nacelle based cascade thrust reverse unit can be eliminated when the VPF is used for reverse thrust. Shorter ‘slim-line’ nacelles that reduce installation drag and aircraft weight can then be designed without the constraint of housing the thrust reverser cascades (Hamilton and Welge, 1992). The concept of using a VPF system to generate reverse thrust has been demonstrated by the NASA Advanced Ducted Propulsor (ADP) and Quiet Clean Short-haul Experimental Engine (QCSEE) programs. A number of VPF designs with optimized performance in different portions of the aircraft mission profile and the capability to induce reverse flow were developed. Detailed exploration of the behaviour of one such VPF design was carried out in both