Fuel Burn Benefits of a Variable-Pitch Geared Fan Engine

Abstract

The compact form and unique variable-pitch mechanism of a variable pitch composite fan blade offer significant advantages over traditional propulsion systems. The new fan makes geared, extremely high bypass ratio engines more viable and hopefully will lead to significant improvements in propulsive efficiency and specific fuel consumption. The objective of the current work was to demonstrate analytically that the improved performance characteristics will also result in a reduced fuel burn, by carrying out a mission analysis firstly on a baseline engine/airframe combination and secondly on the same engine core combined with the new fan & LP turbine, installed in the aircraft. The work is intended to consolidate previous findings and the results of the study are intended to enable development of the new re-fanned engine to proceed with greater confidence and make it more attractive to potential investors & customers. The paper summarizes the generation of a GasTurb11 model of a CFM56-7B24 high bypass ratio turbofan the baseline engine for the program – and a derivative, an ultra-high high bypass ratio variable-pitch, geared turbofan, which used the same core. The baseline engine has a fan diameter of 60.5-inches, a bypass ratio of 5.3, a mass of 5,226 lbm and has a specific fuel consumption of 0.3679 lbm/lbf-hr at its design point (sea-level static) conditions. The new derivative engine, an ultra-high high bypass ratio variable-pitch, geared turbofan designated the VPF56 has a fan diameter of 80-inches, a bypass ratio of 11.98, a mass of 6,829 lbm and has a specific fuel consumption of 0.2787 lbm/lbf-hr at the same design point. The CFM56-7B24 baseline engine and the VPF56 variable-pitch fan derivative engine were each installed in the KC-135R airplane and flown over a simplified mission representative of a tanker used for air re-fuelling. It was found that the VPF56 variable-pitch geared fan engine consumed 3.8% more fuel than the CFM56-7B24 baseline engine. This was largely due to the significantly higher ram drag at the cruise speed and altitude selected. The VPF56 performed better at all other segments of the flight. The result does not diminish VPF potential since the previous findings on the benefits of improved propulsive efficiency were emphasized quite stronglyly and it is recommended that studies continue over a wider range of design variables. A great deal was learned about the operability of variable-pitch fans end this knowledge will be applied to ensuring the role of VPF in the future intelligent engine arena.