AIRLINES and aircraft makers know all too well that those who live near airports or below busy flight paths view them as the neighbours from hell. And the problem could get far worse. The industry is desperate to maintain today鈥檚 5 per cent annual growth in passenger numbers. Unfortunately this growth roughly equates to a tripling of air traffic by 2030. Governments won鈥檛 allow the industry to triple its noise pollution, so something has to give. 鈥淨uiet design is now a fundamental requirement for new aircraft,鈥� admits Billy Glover, Boeing鈥檚 director of environmental performance.
The biggest guns in the aerospace industry, including Boeing and Airbus Industrie have already established research programmes to tackle aircraft noise. But a major new effort to address this issue began last month, when the Cambridge-MIT Institute (CMI), a joint venture between MIT in Boston and the University of Cambridge in the UK, launched a project to develop designs for quieter aircraft.
CMI鈥檚 Silent Aircraft initiative is kicking off by recognising that meeting future noise regulations might require a revolution in design. To this end, it is exploring the potential of flying wings or 鈥渂lended wing-body鈥� aircraft. The engines are mounted on top in a noise-absorbing enclosure lined with layers of steel and foam that are tuned to damp the worst engine frequencies (see Diagram). And mounting the engines on top should shield people on the ground from the worst of the noise.
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This will help at take-off, when most of the noise from an aircraft comes from its engines. But during landing it is the airframe ripping through the air that makes the din. Worst of all are the flaps that extend from the trailing edges of the wings. These increase the surface area of the wing to make sure it has enough lift to stay in the air as it slows. But the turbulent airflow across and between them creates strong vibrations: it is like blowing across the reed in a wind instrument, says aeronautical engineer Tom Hynes at Cambridge.
So why not get rid of flaps altogether? That is the aim of a technique called 鈥渂oundary layer ingestion鈥�, says Hynes. During normal flight, lift is enhanced by a thin 鈥渂oundary layer鈥� of slow-moving air that hugs the wing like a smooth skin, allowing the fast airflow to move cleanly past it without becoming turbulent. But during landing, as the aircraft slows and the wing slices the air at a greater angle, the boundary layer at the rear of the wing becomes turbulent and can separate from the wing entirely. This reduces lift and increases noise.
However, if you suck in that turbulent layer through holes in the rear two-thirds of the upper surface of the wing, you can pull the smooth airflow back towards the wing surface, boosting lift (see Diagram). The same kind of airflow control could eventually be used to reduce drag, and even replace control surfaces such as ailerons or rudders too.
Blended wing-body aircraft with this design are unlikely to fly for at least 15 years. In the meantime, the consortium is pressing for changes to the way aircraft make their approaches to airports, extending the approach path into a smoother, continuous descent that has been shown to cut noise pollution (快猫短视频, 22 November, p 26).
