As manufacturers strive to reduce the fuel burn and emissions of their next generation of commercial airliners, longer wings are near the top of the list.
Extending the span would reduce cruise drag but could prevent aircraft using existing airport gates.
Enter the folding wingtip, already a feature of the in-development Boeing 777X. Airbus also is looking at folding wingtips, but a group within the manufacturer is planning to go a step further. If a folding wingtip is to be installed for ground use, why not use it in flight?
Increasing wing aspect ratio reduces lift-induced drag, which accounts for more than 30% of aircraft drag. Extending the span also increases wing weight but using the folding wingtip for load alleviation in gusts and maneuvers promises to minimize the weight penalty for higher aspect ratio.
A freely flapping hinge does not pass bending moment, so if the wingtip is free to flap in gusts the additional span does not increase the bending moment on the wing. Also, if the hinge is angled relative to the flow over the wing, it is statically stable because aerodynamic stiffness limits the flapping.
Airbus calls the technology the semi-aeroelastic hinge—semi because it can be locked and unlocked to freely flap in flight. The promise of the technology was shown in 2019-20 flights of a subscale unmanned model, the AlbatrossONE, developed by Airbus UK.
Hints that Airbus planned to demonstrate the technology by modifying a Cessna Citation business jet surfaced early in 2021. Now, The Air Current has revealed the demonstrator project is called X-Wing and involves fitting a Citation VII with a new composite wing and fly-by-wire controls for unmanned flights.
An Aviation Week source familiar with the project says it is aimed at testing a 30%-scale version of 52 m-span (171 ft.-span) wing with a moveable wingtip section for potential application to a single-aisle transport. The moveable tip section on the Citation VII will be 2 m long, compared to 2.4 m for the current A320 “sharklet” winglet.
The wingtip will be attached to the end of the high-aspect-ratio composite test wing via an electrically powered actuating hinge mechanism incorporating a drive gearbox and clutch. The system will operate in two main modes. In the first, the electric motor will drive the gearbox to position the wingtip to specific angles for various flight modes as well as takeoff and landing. The second mode, which involves declutching the drive mechanism, will enable the wingtip to move freely on its semi-aeroelastic hinge.
The hinge system will receive position commands from the flight control computer. Commands will be to move the tip in various degree increments up and down, or to disengage the clutch. Target is to begin flight tests with the flapping wing section in late 2023, the source said.
Wing Of Tomorrow
Tom Wilson, semi-aeroelastic hinge project leader, and James Kirk, AlbatrossONE chief engineer, at Airbus UK in Filton, England, gave a briefing on the technology and its progress at the American Institute of Aeronautics and Astronautics’ virtual SciTech 2021 conference in January.
A320-family aircraft have a wingspan of 36 m, for an aspect ratio of nine. Airbus’ Wing of Tomorrow research program is developing a 45-m-span, aspect-ratio-14 composite wing for an A320-class aircraft, enabled by ground-folding wingtips that allow the aircraft to still fit a standard 36-m Code C gate.
“With the semi-aeroelastic hinge we hope to add approximately another 7 m, for an aspect ratio of 18,” said Wilson. Going from 45 m to 52 m will reduce induced drag, which is inversely proportional to the square of aspect ratio, “but because of the huge load alleviation potential we hope to get this aerodynamic gain without having to incur the weight penalty.”
A flight starts with the wingtips folded at the gate. The tips are folded down while taxiing out and unlocked during the takeoff roll. “We do this for handling qualities so we can reduce the roll damping and roll the aircraft faster, because with the high mass when the wing is full of fuel it’s difficult to roll the aircraft with the increased span,” Kirk said.
Once the aircraft takes off, the wingtips are left free for first-segment climb. Then in second-segment climb the tips are locked and recovered to the plane of the wing. “The folding mechanism will find the wingtips, lock them and then pull them back down to a planar position. With the wingtips down in this planar configuration they are fully loaded, and we have a fully effective lifting span, so we get the maximum efficiency for cruise, minimizing induced drag,” he said.
“Then, when we don’t want these long thin wings generating a high bending moment during a gust or maneuver, we unlock the wingtips and allow them to be free to offload the wing,” Kirk said. “The wingtips have been unlocked and are completely free about the hinges, passing no bending moment,” he added.
“We have to detect the gust on the nose of the aircraft, and then there’s a race. We have to send the signal from the detection sensor to the hinge to release the wingtip before the gust arrives on the wing,” Wilson said. “It’s a question of a few hundred milliseconds. The release system has to be very quick.” After the gust or maneuver, the wingtips are locked and recovered to continue efficient flight.
Because it can be difficult to fit high-lift devices such as leading-edge slats to folding wingtips, during the landing flare the semi-aeroelastic hinge could be used to angle the tips up so there is a geometric reduction in the angle-of-attack of the tip, helping prevent wingtip stall. “So you can improve your low-speed performance,” he said.
“At the moment, the concept is that the wingtips are locked during the flare,” Wilson said. For safety, the semi-aeroelastic hinge system may be treated similarly to a thrust reverser, requiring a 10-9 probability of catastrophic failure, he said. After landing, the tips go into their ground folded position.
The AlbatrossONE project was named after the albatross because of a unusual feature of the long-soaring bird. A sheet of tendons allows an albatross to lock its shoulder and keep its wing outstretched for an extended time without using its muscles. When it needs to flap its wing, the albatross can unlock its shoulder.
AlbatrossONE was a 1/14th-scale (7%) radio-control model of an A321 with a 52-m-span wing, giving the electric-powered model about a 4-m span. The objective of the project was to demonstrate that freely flapping wingtips are an idea worth pursuing for load alleviation, to investigate the handling qualities and to show that a wingtip could be recovered after it was released.
“We wanted to convince Airbus and the aviation community that this could work, and the best way to do that was to build it and fly it,” said Kirk. “In 20 months, we went from a clean-sheet design to a flying aircraft on a very, very low budget. We achieved this with an army of intern, graduates and apprentices. Some would call it slave labor,” he joked.
To save time and money, the model was scaled physically and not dynamically, so some aeroelastic effects of flapping wingtips could not be explored. “We basically ensured the 1G spanwise lift distribution was scaled so we had a representative loading on the wingtip,” he said.
In February 2019, the model made two flights from Aston Down airfield in England, AlbatrossONE becoming the first Filton-manufactured aircraft to fly since the Concorde. The folding mechanism had yet to be manufactured, so the wingtips were fixed for the first flight and freely flapping for the second.
Bending moments measured on both flights clearly showed the load alleviation benefit of allowing the wingtips to freely flap, Kirk said. The second flight also showed the folding wingtips were statically and dynamically stable throughout the flight. The first flight showed the fixed wing suffered tip stall. On both flights, the aircraft exhibited Dutch roll instability, a yaw-roll coupling.
The model was improved for the second phase of flights, with upgraded instrumentation, flight controls and weight reductions to allow a bigger battery to increase flight endurance to 5 min. Tip stall was fixed by adding leading-edge droop to the wing and a yaw damper was added to solve the Dutch roll issue.
The folding mechanism and control system was installed, enabling inflight release and recovery of the wingtips as well as ground folding. One half of the aircraft was then mounted on the side of van and driven down the runway as quick and cheap way to verify the stall fix as well as check the lock, release and recovery operation of the semi-aeroelastic hinge.
Next came a tether test. Attached to a cable, the model was flown round and round in a circle inside a building. “The original idea of the tether test was yaw damper tuning, but it became much more,” Wilson said. The test also looked at handling qualities, wingtip folding angle as aircraft angle-of-attack and sideslip changed, as well as some failure cases with the hinges. “And in addition to that it was a good shakedown of the aircraft and test processes, and also the team ahead of flying,” he added.
“We even tested our safety parachute,” Wilson said. “We discovered that if you have a small airplane which is fully instrumented and you have a large building and a cable, then you can do a low-risk test and get a lot of useful information out of it.”
Phase 2 flight testing began in July 2020 at Shenington, another former military airbase in England. The goal of these flights was to demonstrate full gate-to-gate functionality of the semi-aeroelastic hinge technology. Bending-moment data from the flights showed a reduction with the wingtips unlocked. Roll-rate data showed “a free wingtip is equivalent to having no wingtip in terms of roll damping,” Kirk said.
AlbatrossONE also demonstrated what happens if there is a failure in flight and the wingtips cannot be locked and the aircraft has to land with them freely flapping. Modeling suggested aerodynamic stiffness and damping is sufficient to retard the motion of the wingtips so they do not hit the ground. A bounce landing of the AlbatrossONE confirmed this, the tips staying at least 10 deg. above the plane of the wing. “This is telling us that landing with fee wingtips is very much a feasible proposition,” said Wilson.
There was a question about how the flapping wingtips would behave in sideslip. Another tether test showed that, at high angle of attack, as the sideslip angle approaches the hinge flare angle, the tip collapses against the wing. “When we design a future aircraft with this technology, we will have to make sure we have stoppers so that, if the aircraft is in extremely high slideslip at low speed, collapsing of the wingtip can’t happen,” he said.