The future is quiet

Here's a vision of the brave world of tomorrow. An airplane that is so quiet that you wouldn't hear it outside an airport. Published in the Guardian today:

Engineers have unveiled what they hope is the future for commercial airliners - a radical "flying wing" designed to be so quiet that no one outside an airport will be able to hear it.

The SAX-40 would be 25% more fuel-efficient than modern planes and carry 215 passengers up to 5,000 nautical miles (5,750 miles) at a maximum speed of 600mph. The blended wing design concept, which could come into commercial service by 2030, is a result of the £2.3m Silent Aircraft Initiative (SAI), a three-year collaboration between Cambridge University and the Massachusetts Institute of Technology.

"It's got a bold aim - by starting from scratch to design an airplane that has noise reduction as a major design consideration - so quiet that its noise would be imperceptible outside an urban airport in the daytime," said Ann Dowling of Cambridge University, who led the British side of the project.

Though airliners have been in commercial service for more than half a century their basic design has not changed. A tube-like fuselage with engines hanging under the wings has been the default design because it can be easily scaled up and down in size and is easy to maintain.

Blending the fuselage and wing has been confined mostly to military planes such as long-range stealth bombers. Engineers chose this shape for the SAX-40 because it is more aerodynamic and produces less air turbulence over the body, hence less noise. Using hundreds of microphones, engineers tested the sound produced by many of the new components. Their simulations predicted the aircraft noise would be 63 decibels at the airport perimeter, the equivalent of standing on a busy street.

"It's the integral system design that enables the low noise and not one particular technology," said Zoltan Spakovsky of MIT.

The primary noise reduction idea has been to put the engines above the body of the aircraft. This allows the fuselage to screen the noise from the ground by reflecting sound waves upwards.

The wings have been simplified, removing the need for flaps and slats - a major source of noise when a plane lands.

"On approach to land the flow over the wings and the landing gear produces much of the noise," said Dr Spakovsky. Half of the noise an aircraft makes on its approach to a runway is produced this way, and the faster it approaches the more noise it makes. The blended wing of the SAX-40 means the whole body provides lift for the aircraft, allowing it to make a slower approach.

Other ideas include lining the engines with sound-absorbing materials and making them longer so that acoustic mufflers can be added on to the ends. They also have adjustable exhaust nozzles that keep noise down at takeoff but open up at cruising altitude to maximise fuel efficiency.

"We set a major target for low noise but at the beginning of the project we didn't know what the impact would be for fuel burn," said Prof Dowling. "This design has reduced fuel burn and noise but probably if we scrapped the noise we could go still further in terms of reduced fuel burn."

The engineers calculated that the SAX-40 would achieve 149 passenger miles per gallon compared with 121 for a Boeing 777. By comparison a Toyota Prius hybrid car gets 144 passenger miles per gallon.

John Green of Greener by Design, which promotes sustainable aviation, said he was initially sceptical of the silent aircraft initiative. "Three years on I have to concede that the SAI has surpassed my expectations by quite a margin," he said.

"The team has produced a high-risk but credible design."

The conceptual design will now be carried forward by the industrial partners in the silent aircraft initiative. Several dozen companies were involved, including Boeing, Rolls-Royce and British Airways.

Further development of the SAX-40 into a commercial airliner could take several decades - the new Airbus A380, for example, took 17 years to design and build.

"This is a conceptual design and there are many technological barriers that need to be overcome to introduce these technologies into commercial use," said Cesare Hall, an engineer at Cambridge University. He outlined challenges such as developing the strong composite materials needed to produce the oval-shaped hull and improving modern jet engines to work with the SAX-40 design.

Prof Dowling said: "What we've shown here is the kinds of technologies and trade-offs and advantages they might bring. There are significant technical challenges to be overcome if we're to see an aircraft based on that concept.

"Some of the individual technologies one might see on more conventional looking aircraft in the nearer future."

What's interesting about this is the way Cambridge University and MIT worked together with industry on a specified goal for a set timescale. Their budget was quite small but they showed that, if you want to solve a problem, their model is a good one. In the process, they involved lots of undergraduate scientists and funded several dozen PhDs and Masters degrees. A good example of how to make young scientists enthused by what they are doing.

Aviation is the bete noir of climate change so what's the point of this exercise?

At the launch of the SAX-40 design, I asked one of the scientists involved if they would be able to do the same sort of focused project to reduce fuel consumption in airplanes because, pace anyone living near an airport, climate change is a bigger deal than aircraft noise. He said it would easily be possible. They already got 25% more efficiency with this design - imagine what you could do with a focused project to cut down carbon emissions from airplanes.