Taking a giant leap forward

Airbus semi-aeroelastic hinge project leader, Tom Wilson and AlbatrossONE chief engineer, James Kirk discuss the potential of freely flapping wing-tip technology for future aircraft.

The AlbatrossONE demonstrator has successfully achieved a new milestone: a ‘gate-to-gate’ demonstration with wing-tips that are 75% longer than those tested in the first phase. This latest flight test campaign proves freely flapping wing-tips can alleviate wing loads and avoid tip stall for improved aircraft performance.

Thanks to its uncanny ability to travel over long distances with little fatigue, the albatross seabird has a lot to teach aeronautical engineers about improving aircraft performance. And the Airbus AlbatrossONE project team is taking keen interest in this majestic seabird, putting the principles of freely flapping wing-tips - capable of reacting and flexing to wind gusts - to the test.

This small-scale, remote-controlled aircraft demonstrator, which features ‘semi-aeroelastic’ hinged wing-tips, recently completed a successful second flight test campaign.

Q) The AlbatrossONE project is inspired by the albatross seabird. How did this unique seabird inspire Airbus engineers?

Wilson: The albatross’ wing-tips are actually somewhat analogous to semi-aeroelastic hinged wing-tips. The albatross can ‘lock’ its wings at the shoulder to travel long distances, but when faced with wind gusts, it can ‘unlock’ its shoulder to better navigate wind speeds. Semi-aeroelastic hinged wing-tips behave in the exact same way.

Kirk: Also, as a neat coincidence, the semi-aeroelastic hinged wing-tips’ long span could have an aspect ratio (i.e. the wing span to its width or ‘chord’) of around 18 (versus 9 or 10 for today’s aircraft). This is exactly the same ratio as that of the largest albatrosses. Wing span and aspect ratio are important for reducing aerodynamic drag.

Q) What makes ‘semi-aeroelastic hinged wing-tips’ so innovative?

Kirk: Semi-aeroelastic hinged wing-tips enable an aircraft to ‘surf’ through wind gusts without transferring the bending loads (i.e. external load that produces bending stresses within a body) to the main wing. This means we require less material, such as carbon fibre-reinforced polymers, to make the wing strong enough to withstand the gust loads, thus reducing the weight of the aircraft. Also, the length of the wing-tip can be extended without adding weight to the wing because the extra loads from the longer wing-tip are not passed to the main wing.

Q) How does this impact aircraft performance?

Wilson: Semi-aeroelastic hinged wing-tips are remarkable because they would enable a step change in aircraft performance: a major increase in wing span with minimal impact on wing weight would reduce drag, leading to significant reductions in fuel burn and CO₂ emissions. Lift-induced drag accounts for about 40% of a large aircraft’s drag. But this figure falls as the wing span increases. The semi-aeroelastic hinged wing-tips’ span could potentially be increased beyond 50m without increasing wing weight.

Q) The AlbatrossONE team completed a second flight test campaign in July 2020. What was the aim and what did you achieve?

Kirk: Throughout 2019, we completed a series of innovative ground-based tests, which confirmed aspects such as mass properties, wing stall behaviour, and wing-tip release and recovery mechanisms. All of these tests were highly successful. Following a first flight test campaign, we conducted a second to successfully perform a ‘gate-to-gate’ demonstration. This involves moving the wing-tips from vertical to horizontal position before flight, and back again after flight. We also enabled the wing-tips to flap just before lift-off to improve roll control and navigate a high load during flight. They were then locked into planar for efficient cruise.

Wilson: The ‘gate-to-gate’ demonstration also enabled us to prove freely flapping wing-tips can alleviate wing loads, while increasing roll rate compared to fixed-wing tips and avoiding tip stall during landing. During a separate flight, we also demonstrated how to safely land an aircraft using freely flapping wing-tips without hitting the ground or stops.

Q) Will the semi-aeroelastic hinged wing-tip concept be applied to future aircraft? And if so, when?

Wilson: Now that proof-of-concept has been achieved at small scale, we’ll increase our efforts to mature the technology at a larger scale.

Kirk: There’s still a lot of engineering work required before we can prove it’s a viable product. But the project team is motivated to achieve this goal and to inspire other engineers to think ambitiously about future aircraft!

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