Over the past decades, our investment in technology has helped us to make our engines significantly quieter, by understanding and reducing the various components of engine noise.
The pure turbojets and early turbofans of the 1960s were dominated by high jet exhaust noise. Contrast this with the modern very-high-bypass-ratio turbofans, such as recent members of our Trent family of engines. These have significantly reduced jet velocities for the same thrust and consequently make much less noise. Advances in materials and manufacturing technology have allowed these engines to deliver impressive reductions in aircraft noise, whilst simultaneously reducing fuel burn.
However, increasing the bypass ratio even further would not deliver such significant reductions in aircraft noise. We have therefore explored methods for reducing jet noise without increasing bypass ratio, through promoting faster mixing of the jet exhaust with the atmosphere whilst minimising the turbulence created in the mixing process. For example new nozzle lip treatments (such as the serrations featured in the flight testing of the Rolls-Royce/Boeing Quiet Technology Demonstrator and the Rolls-Royce/Airbus 'Low Interior Noise Fan Nozzle' research programme) are now entering service on various aircraft, including the Boeing 787 powered by our Trent 1000.
Fan noise reduction
We continue to improve design features of the fan system to deliver minimum noise. Factors we take into account include the number of rotating blades and static vanes, their detailed geometries, the distance between these two rows of blades/vanes, and the fan's rotational speed. Key issues are the fan's aerodynamic and mechanical performance, and the manufacturing complexity and cost. Similar techniques are employed to reduce turbine and compressor noise sources.
The overall system is optimised by harnessing the power of modern computers to model the detailed aerodynamic flow over the blades and their mechanical vibration.
Acoustic liners in the nacelle (engine housing) play an important role in reducing turbomachinery noise before it escapes from the engine, converting acoustic energy into very small amounts of heat.
We have used key manufacturing, materials and design technologies to increase the effective acoustic areas in the nacelle without increasing the overall nacelle length, and to enable acoustic liners to be employed reliably in areas where the engine conditions are more extreme. Attention to detail is important, and the zero-splice intake liner (which first entered service on the Airbus A380 with our Trent 900 engine) has been very effective in reducing fan noise at aircraft departure, far greater than might be expected for a relatively small increase in acoustic liner area.