By Jo Twist
BBC News Online science staff
Our skies in 20 years' time could be a lot quieter, if research by engineers on "silent aircraft" really takes off.
One of a number of designs being considered by SAI
Cambridge-MIT Institute (CMI) engineers have been working on new styles of planes that would be barely audible.
No increase in noise around airports is a key requirement for expansion plans, particuarly at Heathrow and Stansted.
Prof Ann Dowling, CMI's Silent Aircraft Initiative (SAI) leader, said it was vital to take a fresh look at aircraft design to achieve quiet skies.
"Our hope is to ensure noise is central in future designs, and to show in conceptual design what can be achieved if you give it really high priority," she told BBC News Online.
The research includes collaboration with key industry partners, like Rolls-Royce, to use some of its design codes to fundamentally alter a number of the elements of engine technology that contribute most to noise.
Noise at take-off is generated when engine exhaust air mixes with surrounding, calmer air. High-speed fans and compressors sucking air into the engines to produce thrust also make for unpleasant high-pitched whining.
And during cruising and at landing, noise is created by airflow over the body and other parts of the craft.
The ultimate aim of SAI is to make an aircraft with noise that is "barely perceptible", or equal to normal background urban noise, Prof Dowling says.
Anti-noise campaigners have welcomed the steps SAI has made since it started in late 2003.
John Stewart, head of Hacan ClearSkies, a lobby group fighting expansion at Heathrow, thinks SAI is the most technological initiative he has seen in decades.
"It is a genuinely interesting initiative and, in that sense, a little bit different from some of the vaguer promises we get from the aviation industry and the government about how technology will solve everything over the next 20 to 30 years," he told BBC News Online.
Partners like Rolls-Royce expect to see technological gains
"The current technology just will not do that."
But the government has now accepted that the noise climate is getting worse, says Mr Stewart, and it is planning for it.
The UK government has estimated that there will be a 4% rise in air passenger traffic every year for the next 30 years.
That makes for very busy skies, and technological innovation developed now is crucial to coping with that demand.
But making noiseless planes is hard.
SAI starts with the proposition of what an aircraft would look like if low-noise was a key design requirement.
"You can't just do it by incremental changes to current designs," says Prof Dowling. "So we have started again and said what will it look like? It needs much more integration of the air frame and the engine."
Under the UK government's 30-year aviation strategy, both Heathrow and Stansted would get new runways.
At Stansted, it is estimated that 14,000 people would be affected by noisier skies.
A recent report by the Civil Aviation Authority suggested that a third runway at Heathrow could mean that, by 2015, over 800,000 people would experience noise levels above those recommended by the World Health Organization.
The Heathrow runway will not get the green light unless there is no rise in noise.
One of the biggest challenges is reducing the noise of aircraft on take-off, as well as reducing emissions at cruising level. For that, the aircraft geometry has to be reconfigured.
One of the most popular concepts for future aircraft has been the "blended-wing body" design, originally devised by aerospace firm McDonnell Douglas. Prof Dowling believes a blended-wing concept is a strong contender for future craft.
"What looked like a better starting point for us was the flying wing, where you have a triangular shape of aircraft," she explains.
"If you put engines on top of the craft rather than underneath, the air frame shields some of the sound."
It is a starting point, and within the three years SAI has to run, the final result will be different. But already, SAI's designs mean there will be a requirement for radically different aircraft.
US manufacturer Boeing has been considering blended-wing bodies that can also be altered to balance good fuel efficiency with lower noise levels.
"It takes a long time to develop a new aircraft in aerospace, so we can't expect instant fixes," says Prof Dowling, who predicts we will not see really innovative, silent aircraft over our heads for another 15 to 20 years. "But if you don't start, you don't get there," she says.
One short-term option which is being examined by the Massachusetts Institute of Technology (MIT) in the US is to change the way aircraft approach a runway.
This means existing craft would fly differently on descent, at a fixed throttle setting. Tests on a method of approach developed by MIT have reduced noise by five decibels.
Silence is golden
The development of silent planes could have profound economic impacts, which SAI is also looking at in detail, especially for the viability of an airport's location.
If there are quieter planes, people may no longer object to having an airport in their region or neighbourhood, argues Prof Dowling.
Mr Stewart agrees, although he admits it is an ambitious goal and would like to see the planes working in the flesh - or metal - before being convinced.
His concern is that silent aircraft design will give airlines an excuse to bring in many more planes.
SAI's developments look far beyond even the new planes about to come into service
If the new technology produces planes as quiet as the engineers are now proposing, more planes may not be a problem in noise terms.
"If [SAI] achieves what it is setting out to do, it will mean that aircraft noise will only be a problem for people within spitting distance of the airport. That's a fundamental change."
While it is a vision groups like Hacan might well welcome, in the short and medium term, the problem of air noise remains, explains Mr Stewart.
Building on previous Engineering and Physical Sciences Research Council (EPSRC) funded research, which developed computer models for noise reduction, SAI is a £4m academic and industry-supported project at CMI.