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Last Updated: Monday, 6 November 2006, 17:09 GMT
'Silent aircraft': How it works
Engineers from the University of Cambridge and the Massachusetts Institute of Technology have unveiled a radical design for a "silent aircraft".

The team says any noise from the concept aircraft, known as the SAX-40, would be "imperceptible" beyond the boundaries of an airport. It would also burn far less fuel than conventional planes.

The design is the result of three years' work and includes both new and existing technologies.

Here, the BBC News website details some of the design's key features.



Boeing 747 takes off
Turbulent air around conventional aircraft creates a lot of noise

The shape of the plane is what is known as a "blended wing" design. This hybrid design uses the wings of a conventional plane smoothly blended into a wide tailless body.

As turbulent airflow, generated by irregular surfaces, causes noise, the designers tried to make the airframe as smooth as possible.

The aerofoil shape of the body means that it also contributes to the aircraft's lift, meaning it can make a slower approach on landing, again reducing noise.

The improved lift also means that the plane can do away with flaps on the wings, which are a major source of airframe noise on conventional aircraft.

Because the design does not need a tail, used to provide additional lift and stability on conventional craft, it also cuts down on turbulent airflow and noise from the back of the plane.

The design, made of lightweight composites, also improves the fuel efficiency of the craft whilst cruising.


Artist's impression of the SAX-40 plane in flight (Image: SAI)
The engines are embedded into the body of the aircraft

The engines of the SAX-40 are embedded within the blended wing design with the air intakes on top. This means that the upper surface of the aircraft shields people on the ground from engine noise.

The engines are also mounted deep within the intake ducts, lined with mufflers, to maximise the noise absorption.

By embedding the three engines in the aircraft frame, it also reduces drag and therefore noise.

The "ultra-high bypass ratio turbofans", as they are known, are also arranged in a novel way to minimise noise output.

Instead of having one large fan, they have three arranged side-by-side. The smaller fans means the noise from each one is easier to absorb with surrounding "acoustic liners", or muffling materials.


Artist's impression of the SAX-40 plane in flight (Image: SAI)
The exhaust system optimises the engines performance

The output of the engines is channelled through what is known as a "variable area exhaust nozzle".

This means that the cross sectional area of the exhaust can be changed to generate different amounts of thrust and to maximise the engine's performance.

At take-off the exhausts would be open-wide to generate the maximum amount of thrust. Whilst cruising they would reduce in size to burn fuel more efficiently.

They can also be rotated, or "vectored", to generate thrust in the optimal direction for take-off and landing.

The exhaust are also lined with "mufflers" to reduce the noise of the engines.


Artist's impression of the SAX-40 plane on the runway (Image: SAI)
The landing gear of the SAX-40 is designed to minimise noise

Turbulent air swirling around the undercarriage at take-off and landing are major sources of noise.

To reduce this, the SAX-40 would have fairings to cover the wheels and braking systems, creating as smooth a flow of air as possible. This could reduce the noise from the landing gear by up to 7dB.

However, by doing this it makes the landing gear more difficult to stow and service, and also makes cooling the brakes more difficult.


Artist's impression of the SAX-40 plane in flight (Image: SAI)
The trailing edge of the wings minimises turbulent mixing of air

When turbulent air moving over the top surface of the wing shoots off the trailing edge it abruptly meets non-turbulent air. The result generates a huge amount of noise.

To minimise this, the SAX-40 would have "trailing edge brushes", a series of long, thin protrusions off the back of the wing.

These allow a smoother transition between turbulent and non-turbulent air and could reduce trailing-edge wing noise by up to 4dB.


Airbus A380
The Airbus A380 uses the drooped design on the front of its wings

The leading-edge of the wings are slightly drooped. These further help improve the lift of the aircraft, particularly at lower speeds.

To cut-down on the amount of noise generated by air whistling through a slat between the main wing body and the leading edge, the gap is covered in a flexible material.

The edges would be stowed whilst cruising for optimum performance

This drooped design is already being used on the Airbus A380.

A look at the new 'silent aircraft'

'Silent aircraft' design launched
17 Aug 05 |  Beds/Bucks/Herts
Airport joining 'silent' jet team
18 Jan 05 |  Beds/Bucks/Herts
Flying into a silent sky future
27 May 04 |  Science/Nature
Joint quest for silent air travel
20 May 04 |  Cambridgeshire

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