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The acoustic profile of a jet engine’s fan intake and exhaust is shown.

Designing quieter aircraft via acoustic simulation

Greater fuel efficiency and low emission requirements have grown into such an urgent imperative in aircraft design that it often overshadows an equally significant factor in air travel—noise.

Noise is as much a limiting factor on air travel as fuel efficiency and reduction of emissions. Airports around the world restrict takeoff and landing hours to protect surrounding areas from excessive jet aircraft noise. This is at the same time that air travel’s popularity and the world economy’s reliance on overnight air freight are increasing and driving demand for more flights, not fewer.

The aerospace industry has responded to the noise challenge by designing quieter aircraft as governments all over the world have demonstrated they are willing to manage airport noise by restricting flight times in many different ways, such as curfews, surcharges, noise level limits, and quotas.

Before aerospace manufacturers can produce quieter aircraft, however, they must provide engineers with the necessary design processes and technology tools. The current development model in effect at most manufacturers does not fully consider acoustics until too late in the process to achieve significant noise reduction. To balance noise and fuel economy with all of the other considerations that go into designing aircraft, aerospace engineers must be able to weigh the effects of their design innovations from a new aircraft’s inception.

The noise backlash

Concerns about excessive aircraft noise surfaced in the 1970s and have spurred increasingly stringent regulations ever since.

The supersonic Concorde jet liner is probably the most famous victim of the noise backlash. The British-French aircraft was restricted to relatively few landing sites by authorities in Europe and North America concerned about excessive noise. Though many factors influenced the aircraft’s demise after almost 30 years in operation, those restrictions contributed to decisions not to develop a new version.

Homeowner complaints led the U.S. Congress to give the U.S. FAA authority in 1968 to set noise standards for new airplane designs. The FAA designated three generations (stages) of aircraft by noise level and laid out a schedule for phasing out the loudest.

In Europe, where the air travel safety agency Eurocontrol predicts a 16% increase in air travel by 2018, European Union nations have strict controls on Stage II aircrafts and are considering a proposal to phase the loudest Stage III aircrafts out of fleets that service EU airports.

All this comes in spite of the fact that aircraft have become 75% quieter since the 1970s, according to the European Union.

The goal: quiet efficiency

Even as noise concerns were swirling around air travel, the seemingly endless increases in aviation fuel costs made more efficient aircraft an immediate necessity. This is significant to noise control because fuel-efficiency measures can conflict with efforts to make planes quieter.

Counter-rotating jet engines, for example, consume less fuel than conventional engines but they’re also louder. Making parts and fuselages quieter has traditionally meant adding weight to reduce vibration. Today, however, aerospace companies are experimenting with composites and high-strength metals to reduce weight. Lightness can increase vibrations.

Nevertheless, it isn’t impossible to balance noise and fuel efficiency. Engineers have surmounted higher obstacles. Even with new and lighter materials, there are opportunities to reduce noise through the shape of the fuselage, engine nacelle design, and insulation distribution, for example.

There’s a proviso, however. Engineers must be able to determine how their noise-reduction adaptations will perform long before the design reaches the prototype stage. Late-stage changes are prohibitively expensive, so engineers have limited options for noise control if a design is even 50% finished.

By the same token, incorporating a noise-saving idea into a design without testing it runs the risk of finding out during prototyping that it affected the aircraft’s performance, or didn’t yield the results it was supposed to. To innovate early in the process while minimizing the risk of costly mistakes, engineers need tools that can simulate the noise profile of a single component, such as the nacelle, so they can see the immediate effect of their innovation, and the overall noise profile of the aircraft.

Acoustic simulation software tools have been in the aircraft industry for 20 years, though at all but a very few companies they have been poorly integrated into early stage design processes.

Airbus and Rolls-Royce are among the exceptions. They teamed up in 1999 with Free Field Technologies (now an MSC Software company) to develop Actran acoustic simulation software that can model entire systems and handle both engine noise and airframe noise (as induced by turbulent flows around the aircraft).

The earliest acoustic simulation software was too complex for every engineer in the process to learn, but Airbus has worked steadily over the past 15 years to “democratize” it. Today, acoustic simulation is fully integrated into its design processes.

Any engineer in Airbus’ design process can initiate an acoustic simulation calculation. They change values for parameters such as speed, temperature, and altitude in a simulation model and submit it for calculation. Actran runs the simulation and reports the results to engineers. Airbus engineers can use Actran at the beginning of the process to get a broad idea of which design is the most promising.

As they get closer to a final design, they can adjust the parameters for optimal performance. This system eliminates guesswork and needless iteration. It avoids costly late-stage errors through constant simulation that reveals when an idea is going awry.

Whatever a company’s sound management challenges, Airbus shows that it’s possible to infuse acoustic simulation into design processes from beginning to end. This approach gives engineers the acoustic simulation intelligence they need to create quieter aircraft. The industry needs quieter aircraft so it can grow, which it won’t be allowed to do if it reduces the quality of life around airports.

Dr. Jean-Louis Migeot and Dr. Jean-Pierre Coyette, co-founders of Free Field Technologies (FFT), an MSC Software company, wrote this article for Aerospace Engineering.

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