A holistic approach to greener, cleaner and quieter aircraft
By sucking air through tiny holes in the leading edge of the wings and tail fins of aircraft, manipulating airflow and modifying wing flaps, a large-scale EU-funded project is developing and testing a range of innovative aeronautics technologies that will improve fuel efficiency, reduce noise and lower the environmental footprint of aircraft.
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The AFLoNext project, coordinated by Airbus and involving 40 partners in 15 countries, is helping to lay the foundations for a greener air transport industry, focusing on technologies that should have a significant impact on the performance of existing and future generations of passenger aircraft. In combination, the technologies could cut fuel consumption by more than 10 %, lowering greenhouse gas emissions and operating costs for airlines, while improving aircraft efficiency and reducing noise pollution around airports.
“In AFLoNext we are not reinventing the wheel, but building on earlier research that has shown promise and bringing it closer to maturity and commercial viability, primarily through extensive large-scale testing,” explains project coordinator Martin Wahlich of Airbus Operations’ Flight Physics Research and Technology centre in Germany.
Among the more mature technologies being studied in the project is hybrid laminar flow control (HLFC) on aircraft fins and wings. Laminar flow occurs when air passes smoothly over a surface, much like water flowing around a smooth rock in a stream. But if the rock is rough or the water is flowing fast enough, turbulent eddies form. The same thing happens at a micro-scale, as aircraft move through the air in cruise conditions – turbulent air builds up around the wings and tail, increasing drag and fuel consumption.
HLFC is achieved by incorporating a precision-engineered mesh of tiny holes – each around 60 microns in diameter – that suck in turbulent air, thereby increasing the laminar zone on the profile to reduce drag. By applying HLFC to the tail fin and outer wing, the AFLoNext researchers estimate that reduced drag will enable fuel savings of around 9 %.
To counter the risk of the tiny holes becoming blocked by insects at low altitudes, an innovative shielding system has been developed, using the high-lift system on the leading edge to protect the micro-perforated areas.
AFLoNext will test the HLFC system applied to a vertical tail plane on the Airbus A320 ATRA test aircraft at the German aerospace centre DLR, while also conducting tests on models on the ground to check system integration issues.
From hybrid to active air flow control
Additional fuel savings are also possible from a related technology called active flow control, in which compressed air is forced through tiny nozzles to manipulate airflow, effectively moving turbulent air away from the plane surface to increase lift in key areas. AFLoNext will test active flow control on the outer wing, the wing trailing edge and on the junction between the wing and the pylons that carry the engines.
“Active flow control can not only produce fuel savings of several percent, but it is likely to be an essential enabler if more efficient engines, such as ultra-high bypass ratio engines, are fitted to aircraft using the standard under-wing design,” Wahlich explains.
Ultra-high bypass ratio engines use less fuel to produce the same or more thrust than standard jet engines, but they are also larger in diameter, which would require them to be mounted higher on the wing to ensure ground clearance. Raising the engines would require the high-lift system in that area to become smaller, thus reducing lift. But active flow control can mitigate this loss of lift by locally manipulating the airflow to maintain or improve high-lift performance, especially during take-off and landing.
Vibration and noise reduction, too
AFLoNext researchers are also working on technologies to mitigate vibration and noise, focused on improvements to wing flaps, the undercarriage and landing gear. An advanced numerical method to simulate airflow impacting the main landing gear is being developed in the project, and will be validated during test flights of DLR’s A320 ATRA. This method could enable airframe designs that would reduce airflow-induced vibration, allowing lighter materials to be used and lowering aircraft weight. Meanwhile, modifications to the flaps through the use of porous metal foam, which will also undergo in-flight testing, could significantly reduce noise.
“Making aircraft quieter is a major priority of airlines, especially amid the expansion of airports close to residential areas and increasing air traffic,” Wahlich says. “In AFLoNext we are focusing on technologies that are demanded by the aviation industry, and we are adopting a holistic technical approach to proving their effectiveness and viability.”
Some of the technologies are closer to maturity than others, but Wahlich predicts that most will be incorporated incrementally into newer versions of existing aircraft models, and play a role in future aircraft designs in which reducing noise pollution, increasing fuel efficiency and minimising environmental impact will be important considerations.
In that regard, the AFLoNext results will feed into the Clean Sky 2 initiative, a public-private partnership between the European Commission and the European aeronautics industry under the Horizon 2020 Research and Innovation Programme that aims to achieve a series of ambitious environmental goals for the aviation sector by 2050, including a 75 % reduction in CO2 emissions and 65 % reduction in aircraft noise.