The approach of the 'Silent' Aircraft

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With air travel predicted almost to double over the next 20 years, aircraft noise has become an important issue. Could we ever invent an aircraft quiet enough to improve the quality of life for residents living under flight paths and close to airports? This question, and many others, were answered on Friday 9 September when Dr Tom Reynolds from Cambridge University’s Department of Engineering gave the BA Isambard Kingdom Brunel Award Lecture at Trinity College Dublin, as part of the BA Festival of Science.

Dr Reynolds is a representative of a unique research project – the Cambridge-MIT Institute’s Silent Aircraft Initiative – which is developing the design for a future civil aircraft so quiet that its noise would be imperceptible to the public beyond the perimeter of an urban airport.

In an interactive lecture, The Future of Civil Aviation: The Approach of the Silent Aircraft, he showed a model and played a short animated film showing the futuristic-looking Silent Aircraft in flight. Using volunteers from the audience and such everyday noise sources as hairdryers, he demonstrated how we perceive sound, and the sheer scale of the challenge facing engineers trying to reduce aircraft noise.

The Silent Aircraft Initiative is funded by the Cambridge-MIT Institute (CMI), a UK government-sponsored joint venture between the University of Cambridge and the Massachusetts Institute of Technology (MIT). The Initiative brings together 35 researchers from the two universities with representatives from many parts of the civil aerospace and aviation industry to develop the design for a plane that is radically quieter than current passenger aircraft.

During his talk, Dr Reynolds discussed work the combined team is doing on three major noise sources: the engines, the undercarriage, and the airframe – the physical structure of the plane. The engines are an obvious source of noise, he said, with their huge fans at the front, fast moving machinery at their core in the shape of compressors and turbines, and hot, fast-moving exhaust air flowing out of the rear, which creates turbulence and noise as it mixes with the still air around it.

But he also pointed out that once the engines are throttled back for landing, about half the noise comes from the flow of air over the airframe. And he discussed why so much noise is made by the undercarriage.

He used demonstrations to illustrate these issues, and the audience at the lecture then heard about some of the novel solutions to these problems that the team is exploring. These include:

1. putting the engines above the aircraft, so that the body of the plane itself shields the ground from noise;
2. embedding them in long ducts, muffled with acoustic liners, to reduce the noise; and
3. designing an advanced engine. Relocating the engines inside the airframe raises many engineering challenges.

Dr Reynolds finished the lecture with a comparison of a conventional aircraft, and what the Silent Aircraft could sound like in flight, and then answered questions from the audience. He said:

"The 'Silent' Aircraft Initiative is a Cambridge-MIT Institute programme to develop concept designs and procedures for ultra-low noise commercial aircraft. The project is a true team effort, not only involving researchers from the University of Cambridge and MIT, but also numerous partners from private industry, government agencies and community groups on both sides of the Atlantic. As such it is a great honour for me to be able to represent the project with this year’s BA Isambard Kingdom Brunel Lecture."

Dr Tom Reynolds gave his talk from 12.45 to 13.45 on Friday 9 September in the MacNeill Lecture Theatre, Hamilton Building, Trinity College Dublin. The lecture is also one of the items from the festival being webcast live, and the archive of the webcast is now available at http://www.cusp.org.uk/festival

Last updated:09/09/05