With sustainability fast becoming its No. 1 design priority, Airbus has unveiled details of aerodynamic and propulsion research efforts that, if successful, could underpin fuel-saving technology for its next-generation airliner designs.

The work on drag-reducing boundary layer ingestion (BLI) and bio-inspired wing design concepts is part of the company’s accelerating drive to study aerodynamic advances for both incremental and step-change gains in fuel efficiency. The effort is also aligned with a range of initiatives aimed at developing sustainable production methods.

The renewed focus on reducing emissions and improving efficiency is being particularly felt in Europe in the wake of the “flying shame” aviation carbon emissions backlash that has reduced air travel, particularly in Scandinavia, since late 2018. Sparked largely by calls made by teenage climate activist Greta Thunberg, Swedish travelers have taken increasingly to trains instead of aircraft, and airports in the country have seen year-over-year passenger numbers drop for seven consecutive months, while 2018 marked the country’s weakest overall growth in passenger numbers in a decade.

Airbus, which announced a research pact with Scandinavian carrier SAS on increased use of hybrid and electrical power at its Innovation Days technology briefing on May 22, says the focus on reducing emissions is now fundamental to its plan to support sustainable, carbon-neutral growth in aviation. Although the manufacturer says today’s airliners are approximately 80% more fuel efficient per passenger-kilometer than 50 years ago, the forecast growth in air travel over the next 20 years threatens to significantly increase the environmental impact of aviation.

“Sustainability . . . are we feeling pressure? The answer is ‘yes,’” says Jean-Brice Dumont, Airbus executive vice president of engineering. “We are working in parallel with technology teams so that we can mature the technologies and prepare options for what a greener, zero-emissions aircraft might be, with whatever solutions could be available at the earliest possible time. It’s not a fashionable trend, it’s a mega, irreversible global trend. We have to be working on it, and we are very serious about it,” Dumont vows.

The challenge will be made even tougher by the continuing growth of the global commercial airliner fleet, which Airbus estimates will number 50,000 by 2037. “We have committed to the goals of capping carbon growth by 2020 and, beyond that, to slice down NOx [oxides of nitrogen] emissions by 90%, CO2 by 75% and perceived noise by 65% by 2050,” says Airbus Chief Technology Officer Grazia Vittadini. “There is no single technology that will enable us to achieve these ambitious targets. It will be more through the coherent development of several technologies like advanced materials, hybrid propulsion, electrification, autonomy, connectivity, artificial intelligence and improved industrial systems,” she adds.

The focus on novel aerodynamic and propulsion concepts comes as Airbus looks to introduce laminar-flow technology into its product line. The company is close to completing tests of the A340-based BLADE (for Breakthrough Laminar Aircraft Demonstrator in Europe) testbed, and recently began flight-testing a hybrid laminar-flow-control experiment on the vertical tail of its A350-900 prototype.

Developed and flight-tested under the European Clean Sky 2 research initiative, BLADE has evaluated two transonic laminar-flow wing sections mounted outboard on the A340. “This started in 2017, and the flight-test campaign will finish this summer. We are pretty confident from what we have seen that this solution has industrialization potential,” says Vittadini.

Studies of boundary layer ingestion under the Airbus Nautilius project are also attracting interest, says Dumont. In joint work with French aerospace research organization Onera, Airbus is evaluating the potential benefits of BLI for an A320-size aircraft. With twin tail-mounted ultra-high-bypass ratio turbofans, the Nautilius is a follow-on to an Onera study that examined the potential benefit of BLI on the Nova configuration, which has its engine inlets integrated into the sides of the aft fuselage.

Compared with a conventional configuration in which the engines are mounted on the rear fuselage, Nova’s partially embedded engines require 5% less power for the same performance. The benefit comes from the propulsors ingesting the slow-moving fuselage boundary layer and reenergizing the wake, reducing drag. Onera’s results, similar to those seen in BLI evaluations by NASA, encouraged Airbus to go further with the Nautilius, which positions the engines to ingest 100% of the fuselage boundary layer.

“We have been working on BLI concepts for over a decade, and the idea now is to make it real and fly demonstrators to show it is a viable configuration and saves energy onboard,” says Dumont, adding that a potential benefit of the Nautilius is that it “can be combined with a hybrid propulsion system.” He also cautions that, “at the end of the day, the changes to the configuration are quite significant, so we are checking to see if it is worth it. I think with BLI just by itself we won’t save as much as we need to, but if it is more of a combination with alternate propulsion, then you have a solution.” 

Flight tests of an Airbus UK-developed scaled demonstrator, AlbatrossOne, are also beginning as part of investigations into the potential weight and drag reduction benefits of a higher aspect-ratio wing with moving tips. The main innovation in the concept is a semiaeroelastic hinge that enables the wingtips to be stowed for gate access, like on the Boeing 777X, but also to move freely in flight for load alleviation.

Similar in scale to NASA’s PTERA testbed, which was used to evaluate inflight wing folding for drag reduction, the AlbatrossOne closely resembles an A321, with a swept wing and underslung engines. Described as “industrially relevant,” the model has been built to demonstrate the main aeroelastic characteristics of the special hinge, including the motion of the freely folding wingtips on landing.

The outer 25% of the wingspan is movable on the model, which Airbus says made its first flight earlier this year. The wing is built with proportional structural damping and stiffened to avoid the tip flapping and coupling with wing bending. The designers say that, although an initial straight-wing test model with free-moving tip sections did exhibit limit-cycle oscillation, this does not occur on the swept-wing model.

To reduce wing-bending moments from the hinge to the wing root compared to a conventional wing, the hinge is designed not to transmit bending moment. Unlike the hinges on the 777X or the North American XB-70, which pioneered the use of inflight wingtip folding (primarily for lateral stability and control rather than load alleviation), the AlbatrossOne hinges are not aligned with the direction of flight, but angled outward, almost perpendicular to the leading-edge sweep angle.

AlbatrossOne designers say wind-tunnel tests at the University of Bristol indicate that this flare angle generates a nose-down change in angle of attack when the tip folds up. The tip motion is also associated with an incremental aerodynamic download, which reduces load on the structure and, at the same time, enhances static stability.

“The beauty of this principle is the hinge automatically activates when it detects a gust, and is fast enough to alleviate the load by opening up the tip, then moves back to the extended position for best cruise performance. This significantly lightens the wing itself and all the interactions between the wing and body, reducing the need to strengthen the wing center box,” says Dumont.

Initial flight tests have verified basic handling and stability with the wingtips locked and unlocked, and will be followed by flights in which the tips will be unlocked in flight. In the longer term, the research may incorporate an actuator to enable the tips to be relocked in flight and folded on the ground.