In mid-December 2023, a demonstration organized in Ireland revealed the great concern of citizens about the noise of commercial aircraft. Protesters gathered outside the offices. where Dublin Airport Authority (DAA) was debating a planned expansion of its facilities. The chant chanted by the protesters was “Wrong way DAA”, a slogan that could be translated as “Not like that, DAA”.
Such reactions are common across Europe, as airports seek to expand their operations while residents in areas close to flight routes demand solutions to the noise caused by planes taking off and landing. In September, Schiphol Airport in the Netherlands limited flights due to noise levels, dismaying airlines, which suffered a significant financial hit as a result. This tension between commercial interests and those of citizens has led the research community to look for solutions to ensure that airplanes can fly emitting less noise. In view of the nearly 30,000 daily flights in Europe, preventing associated health problems has been one of the main drivers of research.
“Airplanes make a lot of noise, especially on takeoff, but also on landing,” explains Werner Haase, a German aeronautical consultant and aerodynamics expert, with a doctorate from the Technical University of Berlin, who adds: “Being asleep and a plane passing over your house can be very disruptive. “The social impact of this type of noise is very high.” Although the planes are now 75% quieter that three decades ago and road transport produces much greater noise pollution than aviation, the increase in air traffic has caused many people to be affected by the noise level.
In Europe, 22 million people suffer from “chronic irritation” due to environmental noise, including that of airplanes, according to a report by the European Comission which cites associated conditions, such as cardiovascular disease, stress and tinnitus. That report also indicates that 12,000 premature deaths occur each year due to prolonged exposure to environmental noise and that 12,500 school-age children experience learning difficulties associated exclusively with aircraft noise.
Noise pollution is the second leading environmental cause of health problems after air pollution, according to the United Nations World Health Organization. Through the Zero pollution action plan2021, the European Commission aims to reduce the number of people suffering from chronic discomfort from transport noise by 30% during this decade.
Redesign of 'flaps' and 'slats'
INVENTOR, an EU-funded project, aims to make landings less disruptive for citizens living near airports. The project focuses on the landing gear and in the flaps and the slats —which are located at the front and rear of the wings, respectively. When an airplane approaches an airport, the landing gear extends and the landing position changes. flaps and slats to increase lift at low speed, explains Dr. Eric Manoha, director of the computational aeroacoustics research unit at the French aerospace research center ONERA.
“The planes follow a gradual descent trajectory along a predetermined route. This means that planes constantly fly over the homes of the same group of people at low altitude,” says Manoha, who directs the INVENTOR project, which began in May 2020 and will run until the end of October 2024. This initiative, which brings together the main aeronautical manufacturers in Europe, Airbus and Dassault, as well as the landing gear manufacturer Safran Landing Systems, tries to find a solution to the noise generated by both flaps and the slats on the wings as well as the landing gear.
Although aircraft engines are one of the main sources of noise during takeoff, ensuring that flaps, slats and landing gear generate less noise could produce a large overall improvement during approaches and landings, when the engines are less noisy. “Between 30% and 50% of the noise caused by airplanes in these phases of flight comes from the fuselage, not the engine,” says Manoha.
The INVENTOR team is testing a series of changes to the design of aircraft to reduce the decibel level, with the aim that the aviation sector can apply the proposals that arise from the project. One such change would be to place a passive porous structure in front of the landing gear to alter the flow of air that interacts with the structure, which would dampen noise. Another of them has to do with the installation of an active device in the landing gear that expels air. This would create an air curtain which would again reduce noise by diverting the airflow.
It is also important to consider the landing gear when trying to reduce engine noise. flaps of the wings, which extend during takeoff and landing to increase lift and drag. “The landing gear wake affects the flaps. And that causes noise,” explains Manoha.
The project team has incorporated a porous material into the flap to reduce the noise generated by the air flow around it. It is also evaluating different options to reduce noise from the slats through porous materials, as well as the noise generated by the mechanical devices that serve to extend the slats.
Predict to reduce decibels
Quieter airplanes will emerge more from the cumulative effect of all these small design improvements than from any one particular change. For example, the latest generation narrow-body aircraft emit between 75 and 80 decibels during landing. “Our goal is to achieve a reduction of between one to three decibels per element of the fuselage. Every small step is progress,” says Mahona.
In any case, those EU citizens residing near airports should continue to be patient. While some technological solutions could be incorporated into existing aircraft, the most significant improvements in air traffic noise will only come with the next generation of airliners.
Another independent EU-funded project, led by Werner Haase and which has just concluded, took a more general approach to the problem. Called DJINN, the project focused on improving the simulation methods used by aircraft manufacturers to predict the noise of future aircraft. The project, which began in June 2020 and ended in November 2023, used advanced numerical analysis methods and high-performance computing to predict fluid flows and improve the aerodynamic and acoustic performance of aircraft. .
Their findings will help aircraft manufacturers experiment with design options to reduce noise. “These methods have become more precise than those used by the industry today to calculate noise. This is something very important for the sector, si
nce what they are looking for is to improve their designs,” says Haase. Unlike the INVENTOR project, DJINN examined the noise generated by both the engines and the fuselage, and included aircraft manufacturers Rolls-Royce and Safran Aircraft Engines as participants in the project.
Engine covers
One of the possible design changes studied by the DJINN team is to give a toothed shape to the motor outlet. Serrated engine covers, already featured on some Boeing models, have been shown in some tests to reduce aircraft engine noise.
Until now, Airbus has chosen not to use serrated engine covers, citing poor effectiveness against noise and that there are other options to reduce noise, such as the use of flaps porous. “All options have their pros and cons. The question of which approach is most appropriate remains open,” says Haase. In any case, while these changes may not be immediately incorporated into airplanes, both Haase and Manoha are optimistic about the prospects for reducing noise in airplanes given the advances that are being made in research.
In this context, the aviation sector has an incentive to accelerate these changes, as European airports increasingly regulate landing and takeoff fees based on aircraft noise levels. This practice responds to growing concern for the environment and public protests such as the one last December in Ireland. “Airplane noise is affecting the lives of several people every day. Thanks to new technologies we will be able to reduce these inconveniences,” concludes Manoha.
The research described in this article has been funded by the EU Horizon programme. The opinions of those interviewed do not necessarily reflect those of the European Commission.
Article originally published in horizonthe European Union's research and innovation magazine.
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