Case Studies - Aviation

QinetiQ is aiding Rolls-Royce plc in the design and development of jet noise silencers for its future range of civil aero-engines.

More and more people want to fly, but the growth of civil air traffic is constrained by the level of aircraft noise impacted on the community. The result is ever increasing pressures to reduce aircraft noise through both source noise reductions and operational procedures. With new legislation imposing further restrictions on noise levels, manufacturers and operators are faced with new challenges if the industry is to continue to grow.

Whilst the use of high bypass ratios in modern aero-engines has brought several significant benefits, particularly in fuel-efficiency and noise, the jet itself still remains a significant noise source, particularly at takeoff. A major new focus, therefore, is to reduce that jet noise.

Through the application of technologies, skills and capabilities developed over many years within the Department of Trade and Industry's CARAD (Civil Aircraft Research and Technology Demonstration) programme, QinetiQ is helping Rolls-Royce plc to reduce jet engine noise.

The most promising approach is the use of castellated or serrated nozzles in a 'separate jets' configuration, with short or three-quarter length fan cowls in high bypass-ratio engines whose design is driven by considerations of size and weight.

QinetiQ is now actively pursuing the development of these and similar devices on behalf of Rolls Royce for application to its range of Trent engines.

Key to the work is the large anechoic chamber at QinetiQ, one of the largest in the world, which is specially designed for this type of research. Models fitted with various silencer treatments are operated at realistic exhaust conditions, in terms of pressure and temperature. The noise measurements can then be made in the true 'far-field' enabling the full-scale impact of the treatment to be assessed by Rolls-Royce plc and QinetiQ.

The first anechoic chamber there was built in the early 1960's, and the increasing demand for quieter aircraft led to construction of the current anechoic chamber known as the Noise Test Facility. It is 27m (88ft) long and 26m(85ft) wide and 14m(46ft) high, large enough for far-field noise measurements to be made on usefully large jet models.

Exhaust noise test capabilities include:

• Jet temperatures up to 1100K available in the innermost streams, with an outer stream limited to 500K.
• Flight simulation by a 0.53m (21in) diameter nozzle at up to 0.35 Mach number and 0.65 for smaller nozzles.
• Rig internal noise levels that allow satisfactory measurements down to jet velocities around 150 m/s (500 ft/s).
• Noise measurements up to 120Deg to the jet axis.