March 28 2005 The Cambridge-MIT Institute has begun a fascinating initiative that brings together leading academics from Cambridge University and the Massachusetts Institute of Technology (MIT) with leaders in the civil aerospace and aviation industry to develop the design for a plane that is radically quieter than current passenger aircraft. CMI’s ‘Silent Aircraft’ project has a bold aim: to discover ways to reduce aircraft noise dramatically, to the point where it would be virtually unnoticeable to people outside the airport perimeter. This initiative is bringing together leading academics from Cambridge University and the Massachusetts Institute of Technology (MIT) with representatives from all parts of the civil aerospace/aviation industry. This unique community will be working together, sharing knowledge and developing the design for an aircraft whose noise emissions would barely be heard above the background noise level in a typical built-up area.

Partners already include BAA, British Airways, the Civil Aviation Authority, Lochard, Marshall Aerospace, National Air Traffic Services, the Royal Aeronautical Society, and Rolls-Royce.

Partners in the project include Boeing, British Airways, the Civil Aviation Authority, Lochard, Marshall Aerospace, the Royal Aeronautical Society, and National Air Traffic Services. They also include Rolls-Royce, which has made available its multi-million pound suite of design and analysis tools to help the research. Additionally, the project team plans to include representatives of community groups opposed to aircraft noise.

Professor Ann Dowling, from Cambridge University’s Department of Engineering, is one of the project’s leaders.

“The civil aviation industry is already introducing small, incremental decreases in aircraft noise”, she says. “But we are aiming for a radical change in noise levels - so that beyond the perimeter of the airport, the noise of aircraft flying would be imperceptible to the public.”

Both in the United States and Europe, aircraft noise has been one of the major factors holding back the growth of air transport. Airports that have tried to expand to increase capacity and ease delays have met resistance from local groups, worried about the impact of increased aircraft noise.

The leaders of CMI’s ‘Silent Aircraft’ initiative claim that designing silent aircraft will help overcome this problem, as well as providing a major boost to the UK aerospace industry, and helping UK airlines operate more productively.

Professor Ed Greitzer is the project’s leader at MIT. He says: “Although there have been a number of improvements in aircraft design, the overall shape of passenger aircraft has in general not altered a great deal over the last 40 years. A new departure in this project is to start with the idea of having a major decrease in noise as a primary design variable. With this, we envisage consequent major changes in aircraft configuration and operations. ‘Silent’ aircraft would help meet passenger demand for more flights, by creating opportunities for new airports, and allowing increased operations at existing airports.”

CMI’s ‘Silent Aircraft’ initiative is one of four new Knowledge Integration Communities (KICs) that CMI has set up. These KICs aim to find new ways in which academia and industry can work together and exchange knowledge to push forward research in areas where UK industry has a demonstrable competitive position - like aerospace. The Silent Aircraft KIC also aims to enhance this position by engaging with youngsters of all ages to enthuse them about aviation, and thus help ensure a continuing supply of talented individuals into the industry in years to come.

In one knowledge exchange initiative, the students on the project team studying aircraft engine design will spend a month seconded to Rolls-Royce at Derby. Staff from Rolls-Royce will then come back to Cambridge to continue the work. They will be using some Rolls-Royce design codes as they undertake a fundamental redesign of the elements of the engine that contribute most to noise.

Dr Nigel Birch, Chief of Noise Engineering at Rolls-Royce plc, says: “This is a bold project to address what is an increasingly acute environmental problem for the civil aviation industry in the UK. Rolls-Royce is undertaking its own research in aircraft engine noise reduction, but to achieve a very large step change in the performance of aircraft requires a radical integrated approach to the design and operation of future aircraft and flight systems. This was identified by the recent UK Cabinet Office sponsored Aerospace Innovation and Growth team as key to continuing the success of the UK aerospace industry. We believe that the breadth and depth of skills and knowledge available at Cambridge and MIT will be a valuable contribution to research in this area. Crucially, CMI has succeeded in engaging in the project all parties who can actually implement the results of the research in real projects where the gains can be exploited. We are highly committed to this project and look forward to its success.”

Andy Kershaw, Manager of Environmental Affairs for British Airways, says: “We welcome continued research into quieter aircraft technology, and especially the work that aims to optimise operational procedures within capacity constraints, in the context of improving UK competitiveness. Minimising noise from flight operations is a priority for British Airways, and we look forward to the contribution the project can make towards this goal.”

Iain Young, Chief Test Pilot of Marshall of Cambridge, says: “Environmental factors play an increasingly important part in modern airport operations and noise is a key issue. A reduction in aircraft noise would make a significant contribution to the development of regional airports through reducing the noise impact on local communities. We are delighted to support this initiative and look forward to participating in an important and interesting programme.”

Immediate success

One of the project’s first triumphs was an experimental procedure that substantially reduces the noise of descending aircraft, and tangible real world technology availability for current commercial air carriers is already within sight to the efforts of a research team led by Professor John-Paul Clarke of MIT's Department of Aeronautics and Astronautics.

In addition to improving the lives of people living and working along airport approach routes, the new procedure reduces aircraft engine emissions and fuel consumption.

In November 2004, Professor Clarke, a key member of the Silent Aircraft Initiative, visited the UK where he gave a presentation about his work to members of the Associate Parliamentary Engineering Group (APEG). Now he has unveiled the results of tests conducted at a regional US airport, which proved that so-called 'Continuous Descent Approaches' can be used successfully, and confirmed their environmental and economic benefits.

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.

A steeper descent, for example, would keep the aeroplane higher (and therefore out of earshot) for longer, and requires the plane to fly more slowly, the airframe team says. Making a steeper descent provides more power to fly the aircraft – just as you can take your foot off the accelerator in a car going down a steep hill – so the engines of the airplane are throttled back. The power has to be reduced even more, as the plane also has to slow down so that it is at the right speed for landing, and it is this slower approach that helps make it quieter. The noise generated by the airframe reduces very quickly as you slow down, so even a slightly slower approach can make a big difference in terms of noise.

The operations team are undertaking trials at airports in the UK and the US to test out different descent approaches and to measure the impact on ground noise levels and fuel efficiency.