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'Morphing' winglets make for greener aircraft
:: 24 January, 2009
AIRLINE passengers gazing out of their cabin windows could soon be met with an unfamiliar sight - their aircraft's wing tips changing shape mid-flight.
That is because aircraft makers Boeing and Airbus are developing moveable winglets - the fins at the end of a wing - in a bid to build greener, quieter planes.
The US military has been exploring morphing wings for its uncrewed aircraft for five years. In such planes, a long, high-lift wing useful for loitering can quickly be transformed into a sharp delta wing to dive towards a target, thanks to the use of composite skins that slide out to alter its shape. Such radical changes would be far too risky on a civil airliner, where any failure of the technology could be fatal, so research has focused on morphing winglets that alter the shape of the wing more subtly.
Radical shape-changes are too risky for civil aircraft, so morphing winglets subtly alter the wing shape
Perfected at NASA in the 1970s, conventional, fixed winglets are small, upward extensions to a plane's wing which disrupt the production of the swirling wake vortices that normally stream from a flat wing tip. This minimises the drag experienced by the wing, making it easier for the plane to cut through the air, which in turn boosts its fuel efficiency. Winglets fixed at an angle of around 25 degrees from the vertical - known as the cant angle - can cut an aircraft's fuel consumption by 3 to 5 per cent.
By designing these winglets to move during the flight, changing their angle for take-off, climb, cruise and landing approach, for example, Boeing and Airbus hope to improve fuel efficiency even further, while reducing the noise the aircraft makes during landing.
Boeing has filed a patent application on a winglet that moves using shape-memory alloys (SMAs). Engineers working on an Airbus project are planning to file three patents on their own morphing winglet technology, says Askin Isikveren, chief engineer on the Airbus-funded "Morphlet" project at the University of Bristol in the UK.
"The 3 to 5 per cent variation in fuel efficiency today results from using fixed winglets optimised for cruising flight. If we vary the cant angle during flight we can maintain a 5 per cent fuel reduction all through the flight."
Allowing the winglet to flatten completely will also give the wing extra lift at low speeds. "That means less thrust from the engines is needed and so we can have a quieter landing approach," Isikveren says.
The firms have very different ideas about how to build their morphing winglets, however. Boeing wants to move its winglets using SMA panels that change shape in response to an electric current or heat. The firm also proposes placing a small flap at the trailing edge of the winglet - effectively a mini-rudder - to fine-tune the winglet's performance.
Isikveren thinks SMAs are the wrong choice. "The power needed to support an SMA is excessive - it's a huge drain on the electrical system of the aircraft," he says.
Instead, his team have stuck with standard, lightweight electric motors to move hydraulic rams. To ensure the winglet remains streamlined, the team has developed a concertina-like flexible skin made of corrugated Kevlar fabric which surrounds the wing/winglet joint.
Both teams face considerable safety challenges before their technologies can be certified, however. Phil Irving, a damage-tolerance expert at Cranfield University near Milton Keynes, UK, cautions that the repeated flexing of the SMA structure in Boeing's design could gradually break down the bonding between the winglet and the rest of the composite wing. "There are difficulties with the long-term performance of these smart structures," he says.
Simon Waite, a structures specialist with the European Aviation Safety Agency in Cologne, Germany, says another "interesting" challenge will be to ensure morphing winglets cannot interfere with other control surfaces, like the nearby ailerons which produce roll. For instance, if a change to a winglet caused the aircraft to roll to the right, the ailerons would have to compensate. Isikveren is confident that aircraft control systems will cope with this.
With planned airport expansions such as the third runway at London's Heathrow in the pipeline, the need for greener, quieter aircraft has never been greater.
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