The Silent Aircraft initiative

The Silent Aircraft Initiative is focused around a core of research projects in five strands: Airframe, Engines, Integration, Operations and UK Economy. The work carried out at both institutions has included measuring and recording the noise levels produced by different parts of conventional aircraft, and comparing with models of potential aircraft designs, including the blended wing-body aircraft pictured left. Together they are looking at ways to reduce noise at take-off, and landing, while keeping the engine and plane designs technically feasible and economically viable.

Reducing noise at take-off

The engines are the largest sources of noise from aircraft and therefore to meet the Silent Aircraft noise target a very new and novel engine design is required. A team of researchers from Cambridge University are working with engineers at Rolls-Royce in Derby to produce a completely new engine design. This needs to balance the reduced jet speed required for noise reduction against the level of thrust required for take-off. The team has found that the total exhaust area must be about three times as large as that of today’s conventional jet engines in order to achieve this balance.

Ensuring good cruise performance

Larger engines increase the drag experienced during cruise, which counteracts the benefits on fuel consumption of a quiet engine. One team is looking at reducing the amount of drag contributed by the engines by changing the way they are mounted on the airframe, making for an even more efficient plane. Instead of hanging them below the wing, they are looking to embed them in the aircraft to shield people on the ground from the noise.

One of the designs currently being considered by the integration team is illustrated top left, while the engine team are tackling the issues of having to bring the inlet air to the engines through ducts. The distorted airflow leads to very high loads on the fan blades that go beyond the limit of current designs. They have also figured out how to reduce the impact of a large exhaust area on weight and fuel efficiency by using a variable geometry exhaust system.

Reducing noise at landing

It is hard to believe, but the airframe of a landing aircraft is now about as noisy as the engines. The airframe team is looking at ways to generate drag - to slow the aircraft down for landing - more quietly. One of the biggest sources of airframe noise comes from the undercarriage, which up to now has been used as a useful source of drag on approach. The airframe and operations teams are looking at different descent approaches designed to reduce noise on landing.

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