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Clean Sky IDEAS. A breath of fresh air

Noise reduction in the aircraft cabin is beneficial to passenger wellbeing and comfort during flight. But equally important is reducing aircraft noise for the engineering, maintenance and service personnel who work in the airport ramp area – the working environment where aircraft are parked, boarded, unloaded or loaded and refueled. One of the contributing factors to aircraft noise is cabin air conditioning systems, but Clean Sky’s IDEAS project is progressing towards innovative solutions to mitigate the issue.

"Aircraft air conditioning systems use fan-based technology to mix fresh and recirculated air, and these systems emit quite high frequency noise which has to be mitigated because it creates noise in the cabin which is annoying for passengers. They also generate 'ramp noise' outside of the aircraft which is an issue for people working at the airport, plus there are noise regulations that aviation must be compliant with" says Dr. Eng. Estelle Piot, Head of research unit for Instability, Transition and Acoustic Control at ONERA, the French Aerospace Lab.  

Manufacturers of air conditioning systems limit fan-based noise by using porous materials inside the ducts that convey air throughout the cabin. However, as aircraft design shifts towards the wider use of electrical systems it becomes desirable, from an energy efficiency standpoint, to switch towards more electrical environmental control systems (E-ECS) in the cabins. This in turn presents new and unique challenges which are being addressed through Clean Sky's IDEAS (Innovative DEsign of acoustic treatments for Air conditioning Systems) project.

"The plan, defined by topic leader Liebherr Aerospace, is to use an air pump to mix the hot and cold air instead of the usual rotating fan. But the issue we have is that an air pump generates low frequency and mid frequency noise at around 1 kilo Hertz – exactly the range of frequency which the human ear is very sensitive to. This brings us to the objective of our IDEAS project which is to develop innovative and efficient acoustic liners to replace the porous material for this kind of noise source, because porous materials are not effective at all in the mid or low frequency range, unless we use a really large amount of foam, and there is not enough space available for this" says Dr. Piot. "So the objective is to develop an acoustic liner which will be compact, very efficient acoustically, with low weight, low price and other industry prerequisites which will be needed in the future for more electric air conditioning systems".

To accomplish this, the IDEAS consortium is pooling the numerical and experimental capabilities of project coordinator ONERA with the complementary advanced materials and manufacturing expertise of two SMEs, ATECA and Poly-Shape. 

"The relevant experience of these partners will enable the maturation of innovative acoustic liners technologies, and to bring these new concepts from the research laboratory to an industrially relevant environment, i.e. from TRL3 to TRL6, through close cooperation with the Topic Manager" says Christina-Maria Margariti, Project Officer at Clean Sky. "A modal detection tool adapted to the industrial setup is in development, to assess the noise source created by the jet pumps of the air system, and a compact innovative acoustic liner will be designed in order to mitigate this noise source all over the frequency range, while meeting the strict weight, costs and tight space restrictions". 
There's a 'bigger picture' consideration too. In addition to the noise reduction objectives of the IDEAS project, E-ECS systems provide potential for fuel savings in future aircraft through more efficient use of aircraft energies and by removal of the need to draw bleed air from engines, as explained by Clean Sky Project Officer Michel Goulain:

"With today's air conditioning systems you have take-off from the engine which decreases the power of the engine, so we're trying to find other solutions which don't rely on engine take-off. In the future we'll have more hybrid or electrical propulsion and then it will be easier in an aircraft to produce electricity, and with on-board electricity we'll be able to use electricity in different areas – one of them is air conditioning. Because it's a new system inside the aircraft, the air, instead of coming from the engine, will come from outside the aircraft through a dedicated air inlet to feed the E-ECS, and as you can imagine it can be noisy, so that is another important reason for this project" says Goulain.

Now into the final year of the 36 month project, ONERA's Dr. Piot reports on the current status of IDEAS as it transitions through its testing phase, with the news that "right now, ONERA is designing the innovative liners and we are working with ATECA and Poly-Shape which are devising the necessary manufacturing techniques for building a full scale prototype of the innovative duct liners. We're at the stage of the project where we have designed the innovative concept and have carried out simulations and lab scale prototypes to test its efficiency, so now we're confident in being able to reach a high noise damping in the low and mid frequently range". 

Part of the prototype will be 3D printed using innovative lightweight polymers that are able to sustain very high temperatures. Another part of the prototype will be made of perforated composite panels to minimise the weight of the final prototype. 

"Weight is really the most stringent condition we have had to fulfill. But it's also the most innovative part of the project – to build something using some really hard materials that are almost as light as porous foam would have been, though these were not an option. And that is the most important and most difficult part of the project" says Piot. 

"We'll be using a real air pump which emits noise and we are putting the prototype downstream, and then we measure with a dedicated microphone which has been designed and manufactured during the project by ONERA. By using advanced microphone post processing measurements and treatments we will be able to assess the true acoustic efficiency of the liner inside the duct downstream of the air pump" adds Piot, concluding that "ATECA and Poly-Shape are currently manufacturing the full scale prototype, and within the first quarter of this year we will test it in the Liebherr Aerospace facility with a representative air pump in realistic conditions in order to check if indeed we have achieved the acoustic attenuation we are hoping for".