Performance modeling and optimization assessment of variable pitch fan for ultrafan engine

In the pursuit of ever more fuel-efficient aircraft, commercial turbofan engines are trending towards ever higher bypass ratio. However, the progression encounters great challenges in fan stability margin and nacelle weight and drag. As an elegant solution, the variable pitch fan helps maintain the stability margin while enabling the application of a short slimline nacelle. These advantages make the variable pitch fan technology be an attractive option for future ultrahigh bypass ratio turbofan engines. To get a comprehensive understanding of the potential benefits of the variable pitch fan, this paper presents a one-dimensional model of variable pitch fan for quick steady-state performance estimation. Based on the performance model, the preliminary characteristics analysis of variable pitch fan at typical working conditions is performed. Results show notable variation in airflow and engine thrust can be achieved by changing fan pitch meanwhile maintaining a constant fan rotational speed. The thrust adjustment capacity of variable pitch fan can be used to realize thrust reduction during throttling. This paper puts forward a combined throttling method. In comparison with conventional fan rotational speed-down method, the new throttling approach has a 1.5%-2% improvement in cruise SFC.