An eventual successor to current regional turboprops could feature a strut-braced, high-aspect-ratio composite wing with morphing leading and trailing edges, depending on the outcome of an ongoing research and technology project.
Led by Airbus Defence & Space and part funded through the EU’s Clean Aviation programme, the HERWINGT project is attempting to develop a wing for a future regional aircraft that is 15% more fuel efficient and 20% lighter at component level than current state-of-the-art designs.
Launched in January 2023 and running until the end of 2025 as part of Clean Aviation’s first phase, the project is now approaching a series of critical milestones that will allow it next year to begin constructing more than a dozen separate hardware demonstrators.
Using a twin-engined aircraft as its baseline, trade studies carried out so far have allowed the HERWINGT team to narrow down its preferred architecture to a strut-braced configuration rather than a traditional cantilevered wing.
Sebastian Pellicer Sotomayor, project co-ordinator and member of the chief engineering R&T team at Airbus Defence & Space, says the strut-braced wing proved to be the most capable of supporting the high-aspect-ratio required for better aerodynamic efficiency.
With the wing’s aspect ratio to rise to 17 from 12 on the ATR 72-600 being used as the project’s reference aircraft, Pellicer says the strut-braced wing was lighter than the competing cantilevered design – which would have required significant reinforcement at the root to combat the “high bending moment” of the long, thin wing.
Pellicer does not specify the span of the new wing but points out that it will be longer than the ATR’s 27m (89ft).
According to the project’s calculations, any aerodynamic penalties from the strut are “not that bad” when measured against the weight benefits of the structure, he adds.
Specifications for the wing have been driven by HERA, another Clean Aviation project, which is looking at the aircraft-level requirements for a future regional turboprop. That foresees an aircraft with a maximum take-off weight of about 30t, against 23t for the ATR 72-600, which will be capable of transporting up to 100 passengers on routes of 270-540nm (500-1,000km).
Work to create a detailed digital mock-up of the strut-braced wing is now under way, says Pellicer. But in parallel, analysis is ongoing of a design suitable for a distributed propulsion configuration – based around two 1.1MW electric motors and a single parallel hybrid engine on each wing. Peak power output appears to be around 8MW, based on preliminary data.
Pellicer says it is too early to comment on the likely design, save to say that based on initial studies, a wing combining both the strut-braced architecture and the distributed propulsion is “not the preferred option”.
While he “can’t discard it” at the moment, “the preliminary data we have is that for the distributed propulsion we will have to use a smaller aspect ratio” due to the weight of the individual motors “and when you do that the strut is not that important”.
He expects to have selected the preferred configuration by the end of the year.
In the meantime, preliminary design activities for multiple wing components are scheduled to be completed by September, followed by a critical design review phase towards year-end. Once passed, that will enable production of the project’s 16 physical demonstrators to proceed in early 2025, ahead of mid-year testing.
Designed and built by the HERWINGT consortium’s 26 members – drawn from both academia and industry – the individual demonstrators cover all aspects of the wing, including the torsion box, centre box, outer wing box, and leading and trailing edges.
In fact, the project is evaluating four different designs for the leading edge alone, each featuring different manufacturing technologies or embedded systems, such as anti-ice protection or structural health monitoring; morphing leading and trailing edges – the flap and aileron – will also be tested.
Those morphing surfaces, designed by Italy’s Politecnico Milano and TU Delft in the Netherlands, are intended to improve the aerodynamic performance of the wing through enabling automatic changes to the geometry without the need for a hinge line on the wing surface – “it gives you a smooth transition”, says Pellicer, and enhances laminar flow and reducing drag.
They will be incorporated into different demonstrators to test both the function of the morphing mechanism itself – integrated within the control surface – and, in the windtunnel, to validate the aerodynamic benefits.
Crucially, what the project provides is a basket of what Pellicer calls “ingredients” – technologies that have been tested and matured in a particular demonstrator but that, later on during the integration in the wing, may be used in multiple components. So, although the anti-ice system may be embedded in the thermoset leading edge for the tests, if it proves successful, it could eventually be incorporated into a final thermoplastic design.
“Maybe we will conclude that a particular technology is not optimal or suitable for the dedicated use that we originally intended; maybe we will decide that it is better used in the flap or somewhere else,” he says.
“Hopefully we will converge into a final configuration of the wing with plenty of mature and demonstrated technologies by the end of next year. But to be totally honest with you, I don’t expect all the technologies to succeed,” he says.
Ahead of the start of production activities in 2025, the project has begun preparing tooling and carried out “low level tests” or derisking studies.
For example, along with composite material supplier Hexcel, it has proved that liquid resin infused composite can achieve the thickness necessary for the wing. “It was a great success because we have expanded the capabilities of the material,” says Pellicer.
HERWINGT sits within Clean Aviation’s Hybrid-Electric Regional Aircraft topic area and, as such, is working as closely with the other projects within that stream: HERFUSE for the fuselage, HE-ART for the propulsion system, HECATE for the electrical distribution system, and HERA for the whole aircraft architecture.
“I am really happy with the flow of information between the different projects,” says Pellicer.
With Clean Aviation intending to move to a second phase from 2026, including flight tests, the HERWINGT project is already considering how it will advance its technologies to the next stage as a single demonstrator wing.
Pellicer sees a modified ATR 72-600 as a flying testbed as “the most likely outcome” but cautions that it is too early to confirm that strategy. It is also unclear at this stage “what level of modification we will be allowed to do in the wing”. Any major changes would require agreement from both ATR and the European regulator, he notes.
