Small is beautiful: How a tiny device cuts aircraft noise

A tiny device, shown here mounted upstream of a circular opening used to equalise the pressure in wing fuel tanks, reduces the whistling noise produced by the Airbus 320. [Photo courtesy of Lufthansa]

A tiny device, shown here mounted upstream of a circular opening used to equalise the pressure in wing fuel tanks, reduces the whistling noise produced by the Airbus 320. [Photo courtesy of Lufthansa]

This article is part of our special report Aviation.

SPECIAL REPORT / The name – vortex generator – sounds more complex than the device itself. Yet the small component developed by German researchers provides a big solution to noise produced by one of the most widely flown passenger aircraft.

Under pressure from airlines and new regulations, aircraft manufacturers are locked in a fierce competition to produce planes that are easier on the ear and the environment. During rollouts of new or revamped models of engines and aircraft at the Farnborough International Airshow this week, manufacturers are touting new or updated commercial products billed as the cleanest and quietest ever.

The vortex generator, however, required no big shift in technology or major manufacturing investment. Developed by the German Aerospace Centre, or DLR, the device diverts wind from vents on the underside of wings on the Airbus 320. It reduces the sound radiating from the aircraft in its clean configuration, just before the landing gear and flaps are deployed for landing.

For Jan Werner Delfs, who heads DLR’s Department of Technical Acoustics, the A320’s noise is like the sound produced by blowing across the opening of a beer bottle, though at many magnitudes of difference. In the airplane’s case, the whistle occurs when air passes over circular openings used to equalise the pressure in fuel tanks mounted inside each wing.

“If you listen to landing aircraft you can always say this is an A320,” Delfs explained in a telephone interview from the northern German town of Braunschweig. “You believe those tones are coming from the engines, but they are really coming from those holes.”

‘Very annoying’ tone

The DLR attached a 5-cm triangular piece of aluminium sheet metal upstream of the two vents on each wing, in order to divert the air flow and stop the whistle. A decade of research went into the vortex generator, which stems from efforts to mask a similar whistle produced when air blows over the gun ports of warplanes.

“You will certainly very much notice the difference,” Delfs said, noting that it knocks about six decibels off the sound contour of the A320. “It’s not only the question of decibels, by the way. It’s also a question of the kind of sounds and tones, (which) are usually perceived as much more annoying than some featureless noise without tones. If you have a rushing by of something, it’s not very annoying, but if you have a distinct tone, it’s very annoying. So it’s not just the decibels, its the way [the noise] is perceived.”

This year, Germany’s Lufthansa began installing the device on more than 150 A320s and its sister models, the A319 and A321, and announced that it was deploying new aircraft with the device pre-installed. The airline – Europe’s second carrier in passenger numbers – said it was part of its overall scheme to make airplanes quieter.

A Lufthansa spokesman told EURACTIV that the noise appears to affect only the Airbus line of single-aisle jets. He said the airline was spending “a single-digit million amount” to retrofit its fleet.

An Airbus spokesman at Farnborough said the device is available as a retrofit on its aircraft and is already being installed on new-model A320s. Other Airlines, including Air France, have announced that they are installing the sound-reducing component on the affected Airbus aircraft.

Investing in the future

Both airline and components manufacturers are rolling out other technologies that reduce both noise and air pollution. The world’s dominant aircraft manufacturers, Airbus Group and the Boeing Company, are competing with each other at this week’s biennial Farnborough trade show to announce new or revamped aircraft that are billed as quieter and offer better fuel efficiency.

Boeing, for instance, announced a stretch version of its signature 737 line – the 737 Max 8 – that it says is 20 percent more fuel efficient than its Next Generation 737, which it previously sold as one of the most efficient mid-size passenger jets. Airbus, meanwhile, announced a revamp its  long-range 330 jet, promising a 14 percent reduction in fuel consumption over its predecessor.

Both airlines say these aircraft combine new engines and aerodynamic features to make their airlines quieter as well as cleaner on the environment.

Aviation engineers say today’s newest engines are on average 75% quieter than those produced just a few years ago, and one engine expert told EURACTIV the race is on to make the aircraft body itself quieter.

“Before, the designers of the airplane hadn’t had to worry to worry about noise very much because the airplane noise was buried by the engine noise – was overwhelmed by the engine noise,” said Alan H. Epstein, vice president for technology and environment at Pratt & Whitney, the Connecticut-based aircraft engine manufacturer. “And now the engine people have cut it down to about equal and the next generation of engines will cut it down still more. That leaves the airplane people exposed – they have to get to work, and they are starting to, to reduce the airframe noise.”

New EU regulation

Yet even as planes become quieter, the steady growth of air traffic means noise remains a political bombshell in Europe, which leads the world in noise-based flight restrictions. EU states are obliged to limit noise around airports under a 2002 regulation, and the European Parliament and Council approved on April 16, 2014 new aviation noise rules (Regulation 598). The new regulation, which is due to take effect in 2016, puts the EU in line with the International Civil Aviation Organization’s “balanced approach” to noise reduction through measures such as requiring the use of modern aircraft, quieter ground-control operations and – as a last resort – restrictions on nighttime flying.

Still, it could take years for new technology and policies to pay off. More immediate solutions might be found with relatively simple developments, like the vortex generators.

“This tone problem and the vortex generator is rather exceptional,” said Delfs of DLR’s Department of Technical Acoustics. “It’s an exceptionally simple means to get rid of this noise.”

Other noise-reduction efforts may be far more complex and take longer to develop, he said. “At the moment, we are at a stage where any further decibel reduction takes a lot of effort because today’s aircraft are [already] relatively silent.”

“By fitting these vortex generators to our Airbus short and medium-haul fleet, we are continuing to invest in active noise protection”, Kay Kratky, member of the Lufthansa German Airlines Board, said in a statement announcing the retrofit. “In addition to the extensive modernisation of our fleet over the next few years, this is one of several steps that we are taking to reduce noise. It shows our commitment to working towards a balance between the interests of aviation and local residents, especially at our hubs.”

In announcing a new version of its signature 737 passenger jet at the Farnborough International Airshow in England, Ray Conner, Boeing Commercial Airplanes president and chief executive, said: "This new 200-seat 737 MAX 8 ensures we'll retain our leadership in comfort, capacity and lower operating costs in the heart of the single-aisle market. With this increase in capacity and confidence in our engine and airplane testing, we're on track to deliver a 20 percent more fuel efficient product than today's Next-Generation 737."

Fabrice Brégier, president and chief executive of the Airbus Group, told journalists at the Farnborough International Airshow that the company’s A330neo - a revamped version of an existing transcontinental passenger jet - offers “the latest in fuel-saving technologies combined with the best-in-class comfort, reliability and operational efficiency of the A330.” Airbus says the plane will use 14% less fuel per flight and use quieter engines.

The EU’s Clean Sky Joint Technology Initiative, launched in 2008 to encourage public-private cooperation with a goal of cutting fuel consumption and reducing noise.

It began with 1.6 billion EUR in funding over seven years and the Commission proposed a second phase, Clean Sky 2, backed by 1.8 billion EUR under the Horizon 2020 research and innovation programme. Industrial partners are to contribute another 2.2 billion EUR through 2020.

Clean Sky seeks to reduce carbon dioxide and nitrogen oxide emissions and reduce aviation noise by up to 75% from today’s levels.

>>Read our LinksDossier: Single European Sky: Towards greener air travel?

Backed by EU funding, researchers are looking at ways of reducing the drag an associated noise when air passes through landing gear bays and wing flips. Schemes include covers that would pop into place to divert the flow of air away from the landing gear cavity, and producing wings with “gapless profiles” that would be quieter, more fuel efficient and help reduce the jolts passengers feel on approach to the airport.

  • June 14-20: Farnborough International Airshow
  • June 13, 2016: EU regulation on airport noise takes effect
  • 2020: Clean Sky 2 research programme to expire

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