Design for low noise
At take-off the engines are the largest sources of noise from an aircraft. The Silent Aircraft Initiative low noise target is achieved by:
- installing the engines embedded within the fuselage with intakes above the wings to shield much of the engine noise from listeners on the ground
- novel ultra-high bypass engines with a variable-area exit nozzle. This means the engines can operate for low noise with low speed exhaust jets at take off and during climb, and then be optimised for minimum fuel burn in cruise
- throughout the climb, the thrust, nozzle settings and climb rate are optimised for low noise, subject to meeting legal requirements
- long engine exhaust ducts provide space for extensive acoustic liners to absorb the sound
On approach, the airframe on a conventional aircraft is as noisy as the engines. On the ‘Silent’ Aircraft the approach noise is reduced by:
- an airframe design that enables the aircraft to approach at lower speed and so land further down the runway. Further reduction in airframe noise on approach noise is due to
- low-noise fairings on the undercarriage
- advanced airfoil trailing edge treatment
- a deployable drooped leading edge on the wings and vectored thrust, which are used to enable low-speed flight without more noise. There are no flaps or slats
- the engines are designed for low noise, have a low idle thrust and they are able to spool up quickly if a go-around is necessary
Low noise is therefore not achieved by a single design feature but results from many disciplines integrated into the design and operation of a noise-minimising aircraft system.
- Advanced airfoil trailing edge treatment
- Airframe shielding of forward propagating engine noise
- Exit nozzles rotate to provide thrust vectoring in combination with the elevons this gives quiet drag via increased induced drag
- Optimised extensive liners for low engine noise
- Variable area exhaust nozzle and ultra-high bypass ratio engines at take-off for low jet noise
- Deployable drooped leading edge for quiet approach
- Faired, low noise undercarriage for quiet approach
- Advanced centrebody design enables a low approach speed, thereby reducing the airframe noise sources on approach
- Engines have a low idle thrust enabling low approach speed
Design for low fuel burn
The airframe is highly efficient. It is an ‘all-lifting’ design. Although based initially on the Blended-Wing-Body concept, it makes use of a novel centre-body shape with leading edge carving. This balances the aerodynamic forces without the need for a tail, and enables an optimal wing design with an elliptical lift distribution and low cruise drag. The resulting lift to drag ratio of 25 to 1 is some 10% better than other all-lifting designs such as the Blended-Wing-Body and about 33% better than current aircraft. The weight and drag are reduced by embedding the engines in the airframe. The aircraft wake is further reduced by ingesting the air near the aircraft into the engines. Careful attention has been given to the inlet duct design to minimise the flow distortion at the fan face. Finally, the area of the exit nozzle is set to operate the engines at optimum efficiency throughout cruise.
- Variable area exhaust nozzles tuning engine for optimum cruise efficiency
- Embedded, boundary layer ingesting, distributed propulsion system for reduced fuel burn
- Advanced centrebody design for excellent lift to drag ratio
- Elliptical lift distribution at cruise for excellent lift to drag ratio
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Last updated:06/11/06
