Aeronautics

Wings of silence

Air traffic at London Heathrow airport in the UK (an economic powerhouse for the country and Europe’s biggest airport in terms of passenger numbers) grew by more than 65% between 1980 and 2003. Fortunately aircraft have also been getting quieter. Over the same period, the number of noise pollution victims (those living in an area exposed to more than 57 decibels) has been reduced by a factor of almost four. And the battle against the decibels looks set to continue. We take a close look at the successful Silence(R) technology validation project in La Baule (France), which came to an end on 27 June 2007.

The fan of the Trent 900 engine manufactured by Roll Royce, which equips the Airbus A380, considerably reduces engine noise. © Rolls-Royce plc 2006 The fan of the Trent 900 engine manufactured by Roll Royce, which equips the Airbus A380, considerably reduces engine noise. © Rolls-Royce plc 2006
During the approach phase, the airframe is just as noisy as the engine. The sound sources presented on the image of a Fokker 70 are the landing gear, flaps, and jet engine. ©NLR During the approach phase, the airframe is just as noisy as the engine. The sound sources presented on the image of a Fokker 70 are the landing gear, flaps, and jet engine. ©NLR

There is a ceaseless flurry of activity behind the window. The steel condors await their turn on the taxiway.Before they take off for Beijing, passengers watch a long-haul aircraft belonging to a Latin American airline land on the runway. In 2006, Heathrow Airport, lying 25 km west of London, saw more than 1 300 take-offs and landings per day.

Paying the price in noise

In recent decades, air transport has brought the regions of the world closer together. Every year, two billion passengers take to the skies via a web of air routes linking peoples, countries and cultures. Economic exchanges are also undertaken, and market globalisation has followed the air corridors. Indeed, aviation creates opportunities by facilitating meetings between people and transporting products and services quickly over long distances. So much so that greater air traffic is one of the prerequisites for the development of Europe’s regions, as well as for maintaining the competitiveness of its aerospace industry.

However, the flip side of the coin is first and foremost noise pollution, with repetitive noise peaks around airports. The serious impact of noise pollution on residents’ health ranges from hearing impairment to psychological, pathological and physiological effects from sleep disturbance, such as high blood pressure.

Battle against the decibels

The problem is not new. High-bypass-ratio turbofan engines emerged in the 1970s and reduced noise levels by 20 decibels (dB), effectively diminishing perceived noise to a quarter of its original level. Indeed, the decibel scale has followed an exponential downward curve, as has the impact on human ears. In bypass engines, hot gases from the primary airflow are surrounded by a secondary (or bypass) airflow, thereby reducing the speed of exhaust gases and the resulting noise when these gases mix with the ambient air.

However, such advances are not sufficient in this day and age. Economically speaking, noise pollution imposes an estimated financial toll on Europe of €24 billion per year (1). Furthermore, ICAO (the International Civil Aviation Organization) has enacted the “Chapter 4” standard for a new category of quieter aircraft, which came into application in 2006. This forces public and private stakeholders to continue the fight against decibels.

This fight has been fought for more than six years, notably by means of Silence(R), the largest ever European project to focus on aviation noise pollution, with a total budget of more than €110 million, 50 % of which is financed by the European Union. Eugène Kors, Silence(R) coordinator at Snecma (a major French engine manufacturer for commercial and military aircraft and space vehicles) tells us all about the project: “Silence(R) has validated technologies for reducing noise at source. The project has been a resounding success: after conducting research into operational procedures, it has achieved a 5 dB noise reduction, meeting the medium-term goals of Vision 2020.”

Of aerodynamics…

Aircraft noise arises from turbulent airflow. This causes pressure variations, in turn generating sound waves of varying frequencies. “The main sources of aircraft noise are components in both the airframe and the engine.” (2) Noise levels increase during the landing and take-off phases when the leadingedge wing slats, high-lift flaps and landing gear are out.”

“In the aerodynamics field, Silence(R) has brought improvements to landing gear, chiefly by means of fairings to reduce drag.” Although researchers have focused on fairings since 1998, the problem remains to be resolved entirely: while they do provide a more aerodynamically streamlined shape, fairings making landing systems heavier and more complex.

This has caused engineers to shift their focus onto placing ducts along landing-gear legs and on wheel and axle shapes, in order to limit the whistling sound they produce. This research is essential because landing gear is responsible for a massive 50 % of aerodynamic noise. “In the approach phase, the noise from landing gear is equivalent to the noise from the engines.”

… and engines

Although it is true that, during the approach phase, the engine emits only half the total noise because it is running at 55 % efficiency, the engine is still the main noise pollution factor during the flight phase. Most of the research validated by Silence(R) focuses on propulsion-system acoustics, in particular the noise output from the engine pod or nacelle (fairings on the engine), a highly critical component in terms of safety because it absorbs most of the propulsion force.

To trap some of the sound frequencies generated in the combustor and turbines, Silence(R) researchers have developed an acoustic treatment for air inlets called Negatively Scarfed Intake (NSI). This geometric shape has a 10° angle of curvature that modifies the directional model of noise radiation, angling more noise upwards. This air intake device, mounted on an Airbus A320 equipped with CFM56 engines, underwent flight tests as part of the Silence(R) project where it achieved significant noise reductions on both approach and take-off.

“Zero Splice”, an all-in-one success

However, the most immediate success story has been the development of “Zero Splice” technology. When conducting tests, researchers observed a dispersion of sound waves via the manufacturing joints in the acoustic panels lining the internal walls of the engine air inlets. The panels channel the complex aerodynamic phenomena produced in this area, especially around the blades.

These acoustic barriers are usually formed from two or three parts and the assembly joints (or splices) reduce acoustic performance. Apart from failing to completely shroud the internal wall, the splices themselves transmit sound waves. These sources of noise resonance lead to sound dispersion, which shifts from circumferential mode to other modes, significantly increasing perceived noise levels on the ground.

The “Zero Splice” principle is simple: it is an internal lining made in a single piece without splices or joins. To produce it, engineers had to overcome a number of design and manufacturing challenges, including creating a retractable mould. During real-scale tests on a Rolls-Royce fan, the “Zero Splice” panels achieved genuinely conclusive results, with significant noise reduction. Developed by Airbus, the “Zero Splice” panels will incorporate the ducts for the new-generation engines to be used in the Airbus A380.

Already in industrial application

One of the advantages of the new lining is that most of the noise reduction is achieved at frequencies where the noise levels are greatest. Furthermore, as weight and drag remain unchanged, fuel burn does not increase. The Airbus A380 could carry an extra payload of 10 tonnes without producing more noise. This technology could extend beyond the Airbus fleet to become a world standard.

The Airbus zero-splice technology carried off the prestigious 13th Decibel d’Or aeronautics award in December 2006 in recognition of the 0.4 dB reduction it has achieved on take-off by reducing sound pressure from the fan by 7 dB. “Further applications should follow close on the heels of this first Silence(R) industrial application success. They should provide a host of business opportunities and potential benefits for producers and customers.”

Silence revolution

In spite these short- and medium-term efforts and their rewards, noise abatement techniques are reaching their limits in modernday civil aircraft. However, building quieter engines almost always involves a compromise that reduces engine performance. To raise our sights to more ambitious goals, it will therefore be necessary to completely rethink aircraft design.

Delphine d’Hoop

  1. According to the 2007 update of the CALM strategic paper Research for a Quieter Europe in 2020, October 2004: www.calm-network.com/SP_2020_update07.pdf
  2. All quotes are from Eugène Kors.

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Silent Aircraft

The design of the silent and more fuel-efficient bird of the future. © 2006 Cambridge-MIT Institute The design of the silent and more fuel-efficient bird of the future. © 2006 Cambridge-MIT Institute

The silent aircraft of the future are perhaps not so far off. Aware of the need for large-scale new developments for air carriers, a number of major stakeholders in the sector have set themselves the task of producing a completely new design. During the three-year period from 2003 to 2006, the University of Cambridge in the UK  and the Massachusetts Institute of Technology (MIT) in the USA headed up the Silent Aircraft  Initiative (SAI) to seek new advances and integrate them into a futuristic aircraft design.

With its delta wing shape, the aircraft of the future will, according to its designers, be able to carry 250 passengers, and be “almost imperceptible” outside the airport perimeter. Furthermore, it is expected to consume significantly less fuel than modern-day aircraft because its aerodynamic properties will give it much greater lift.

The innovations mainly take the form of a new shape, a curved wing with a wide body and no tail (to reduce induced drag and air turbulence behind the aircraft). Smooth curves delineate the regular surfaces of a lightweight composite structure designed to increase energy efficiency during cruise flight. Presented in December 2006, the engines of the silent aircraft are incorporated into the curved sweep of the wings, with the air inlets situated on top of the airframe. The major manufacturing firms that participated in the project – including Rolls Royce and Boeing – expect this revolutionary new aircraft to come into operation around 2030.



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To find out more

  • Silence(R)
    51 partners – 16 countries ( AT - BE - CH - DE - DK - EL - ES - FI - FR - IT - IR - NL - PT - RO - SE - UK )
    www.snecma.com.
  • Silent Aircraft Initiative
    www.silentaircraft.org