While aviation's carbon emissions have grabbed the headlines, European industry's efforts to cut noise pollution have been largely overlooked
The protests surrounding the UK's Heathrow airport in August may have been focused on carbon emissions, but local inhabitants would argue that noise is a more immediate issue. This is despite four decades of success in tackling aircraft noise that has seen a 20 decibel (dB) decrease in take-off noise, a 99% reduction in the power of the sound relative to the turbojet aircraft of the 1950s.
Noise is a key feature of the European Union Advisory Council for Aeronautical Research in Europe's long-term targets. By 2020, this pan-European industry/government body wants technologies to reduce perceived aircraft noise to half the average levels achieved in 2000. All such reductions are relative to previous dB measurements of aircrafts' recorded noise levels. The dB was originally defined as a measure of relative acoustic power as perceived by the human ear, and the ear has a logarithmic response.
Perceived noise, measured in EPNdB, differs from sound pressure level in dB because it takes into account the frequency sensitivity of human ear and the duration of exposure to the noise. It is used to measure airport noise because it takes into consideration the time it takes for an aircraft to pass overhead.
Aircraft types are certified with noise limits in regard to their mass and the phase of flight. For an Airbus A380 using Rolls-Royce Trent 900 engines the certified noise limit is 108dB for arrival. The aircraft actually achieves 98dB. This is measured from beneath the aircraft when its on the 3e_SDgr glidescope.
Guided by ACARE's targets, aircraft noise reduction is the subject of several projects planned under the EU's latest environment-focused, multi-year Seventh Framework research programme (FP7). These will follow on from an earlier Fifth Framework project called SILENCE(R) - for Significantly Lower Community Exposure to Aircraft Noise - which ended in June. SILENCE(R)'s studies of engine noise-reduction technologies have been spread across 17 different areas. Co-ordinated by French propulsion company Snecma, these were divided into eight work packages with different companies taking the lead on each. Together, the results of these studies provide a picture of what a SILENCE(R)-derived engine for a next-generation single-aisle airliner could look like 10 years from now.
Nacelle shape
The first difference is the nacelle shape. No longer is it a straightforward flat inlet - the intake lip is negatively scarfed, looking like a jutting lower jaw and shielding fan noise from the ground. In flight tests of a CFM56 engine on an Airbus A320, the scarfed inlet was found to reduce perceived noise by around 2.5EPNdB when the observer was at a 60˚ angle to the engine. But at more than 120˚ the intake made little difference.
Looking inside the SILENCE(R) engine's nacelle, the intake's "zero splice" passive acoustic liner is a small, easy-to-miss change that is already delivering benefits. An Airbus study using acoustic finite-element-model simulations showed that splices have a major effect on acoustic liner performance, and the zero-splice design has a continuous surface without the joins in conventional inlet liners constructed from two or three pieces.
Today Rolls-Royce uses the liner on the Airbus A380's Trent 900 engine and, according to SILENCE(R) researchers, it provides a reduction in fan-tone noise of 4-7dB at take-off and 2dB on approach. A 2dB reduction in sound pressure level means a less than half reduction of the fan-tone noise generated, while a 4-7dB reduction means R-R has cut the sound's power by over 50%.
SILENCE(R)'s zero-splice liner work is contributing to Airbus's own studies of a noise-dampening nacelle intake - the next-generation reduced acoustic mode scattering engine duct system - that could become a feature of the new A350 XWB wide-body twinjet.
Looking towards the back of the SILENCE(R) engine, low-noise fan and core nozzles are serrated to improve the mixing of core and bypass exhaust flows, while internal and external exhaust plugs block and attentuate core noise.
Multiple variations of exhaust plugs were analysed under SILENCE(R), using CFD simulations and scale model tests. Those designs showing the best performance were selected and full-scale plugs manufactured for static and flight tests, some of which involved a CFM56-5B engine on an A320.
Qinetiq participated in a related low-noise exhaust nozzle work package, conducting noise tests in its 27m (88.5ft)-wide anechoic chamber. "It was as far as we wanted to go, producing fundamental models. It is now for the airframe makers if they want it," says Qinetiq air division noise and acoustics team capability leader Craig Mead.
Its partners in the work package were Rolls-Royce Deutschland and French aerospace agency ONERA. The Qinetiq chamber, designed for jet engines in the 1970s, was used to test 1/10th-scale model nozzles, with noise measurements taken at a distance of 12m (39ft). Qinetiq tested nozzle designs for engines with bypass ratios of up to 11 - equivalent to the A380's Trent 900 powerplant - and for a regional jet engine with a bypass ratio of five. ONERA's anechoic chamber was used to test a nozzle for an engine with a CFM56-equivalent bypass ratio of six, and this was flight tested on an A320.
The nozzle designs were all serrated, to promote mixing, with the shapes changing for each bypass ratio because of the variations in velocity between core and bypass airflows.
Returning to the front of our notional SILENCE(R) engine, its ultra-high bypass ratio (UHBR) fan has been designed specifically for low noise. It differs from a conventional swept fan in having a more exaggerated contour and greater twist relative to the root.
Advances in CFD have made it possible to optimise sophisticated geometries to realise the quieter fan. But computing power is not sufficient to eliminate actual testing and the fan was put through its paces at the AneCom Aerotest's fan rig test chamber in Germany.
Related EU projects also played their part in design for SILENCE(R)'s low noise UHBR fan. R-R chief noise specialist Andrew Kempton says: "The VITAL [Environmenally Friendly Aero Engines] project looked at low weight fan technology for UHBR. It is an enabling technology because a larger fan's greater mass would cause problems."
noise-control technology
Immediately behind the UBHR fan, the SILENCE(R) engine's fan duct incorporates active noise-control technology. Fan noise is measured by microphones inside the duct, then analysed, and signals sent to actuators mounted on the stator vanes to generate anti-noise that cancels out much of the fan noise. Putting microphones and actuators inside the fan duct brings its own set of challenges and active noise control is deemed to have a low technology readiness level of 3, requiring further substantial development work.
Deeper inside the engine, SILENCE(R) looked at the potential for noise reduction in a high-speed low-pressure compressor through improved design of inlet guide vanes and stators. It also demonstrated that highly loaded low-pressure turbines can be acoustically designed to maintain noise levels.
Gear turbulence
The SILENCE(R) programme also looked beyond the engine, and included work on low-noise landing gear designs that involved more windtunnel scale-model and flight tests. Testing of 1/10th-scale nose and main landing gears in German aerospace agency DLR's Braunschweig acoustic windtunnel found that the interaction between multiple gears caused turbulence that increased low-frequency noise.
Shielding of the gears is viewed as the answer and SILENCE(R) found it reduced the noise generated by the downstream gears. It also discovered that aligned gears were quieter than independent gears - two nose landing gears aligned in the flow were up to 3dB quieter. To validate this work following wind tunnel tests, landing gear fairings were flight tested on an Airbus A340 in 2003.
SILENCE(R) was originally to be a four-year project, but was extended to six due to various delays. Per Kruppa, the former European Commission official overseeing the study, says: "It was extended - all integrated platform projects were extended due to delays in development - but there were more reasons than that for SILENCE(R). [Any] delays and you can have to wait for six months at times for windtunnel availability if you miss your window." R-R's Kempton agrees that high demand for windtunnel and flight test resources brought additional schedule pressures to an already ambitious project.
Overall, the project partners concluded that the SILENCE(R) technologies could deliver ACARE's medium-term goal of a 5dB reduction in aircraft noise. But to achieve the long-term noise reduction targets, industry now knows that active and adaptive control has to be developed for use in engines and deployed in combination with future unconventional airframe designs.
Source: Flight International
