Engine Design Studies for a Silent Aircraft

The Silent Aircraft Initiative is a research project funded by the Cambridge-MIT Institute aimed at reducing aircraft noise to the point where it is imperceptible in the urban environments around airports. The propulsion system being developed for this project has a thermodynamic cycle based on an ultra-high bypass ratio turbofan combined with a variable area exhaust nozzle and an embedded installation. This cycle has been matched to the flight mission and thrust requirements of an all-lifting body airframe, and through precise scheduling of the variable exhaust nozzle, the engine operating conditions have been optimized for maximum thrust at top-of-climb, minimum fuel consumption during cruise and minimum jet noise at low altitude. This paper proposes engine mechanical arrangements that can meet the cycle requirements and, when installed in an appropriate airframe, will be quiet relative to current turbofans. To reduce the engine weight a system with a gearbox, or some other form of shaft speed reduction device, is proposed. This is combined with a low-speed fan and a turbine with high gap-chord spacing to further reduce turbomachinery source noise. An engine configuration with three fans driven by a single core is also presented and this is expected to have further weight, fuel burn and noise benefits.