The art of folding

AMMay18Feature - wings1
AMMay18Feature - wings1

Wings that can fold or change shape in flight – yet without needing bulky control systems – could revolutionise flight efficiency. Lou Reade reports.

Aeroplane wings are designed for rigidity. After all, they must withstand the rigours of aviation. However, their fixed position is not optimal for every aspect of flight, and all the various conditions between take-off, cruising, banking and landing. Operations such as lifting the flaps for landing help to change wing dynamics, but this involves shifting one rigid design in relation to another.

For some time, researchers have been looking into the idea of more ‘flexible’ wings that can change their shape with more fluidity. An example is a wing whose tips can be moved up and down during flight.

While the idea of folding wings is not new – and was studied on the XB-70 Valkyrie aircraft back in the 1960s – it is usually achieved using a bulky arrangement of motors and hydraulic systems that can weigh down the aircraft.

Now, Nasa has tested a lightweight shape memory alloy that allows aircraft to fold their wings during flight. The material is controlled by temperature and fashioned into a tube-shaped actuator. When heated, the tube twists, which moves the wing’s outer portion up or down.

The development forms part of Nasa’s Spanwise Adaptive Wing (SAW) project. The shape memory alloy is built into to an actuator on the aircraft, which folds the wings in the same way as a heavy hydraulic system – but with just 20% of the weight.

“Folding wings has been done in the past, but we wanted to prove the feasibility of doing it using shape memory alloy technology – which is compact, lightweight, and can be positioned in convenient places on the aircraft,” says Othmane Benafan, co-principal investigator for the SAW project.

Nasa has tested a shape memory alloy that can fold aeroplane wings in flight, and is 80% lighter than traditional hydraulic systems

In a recent series of flight tests, the researchers used the material to fold the wings either upwards or downwards by 70° during flight. More flights are planned for later this year, which will bend the wings in both directions during flight.

Folding wings could reap huge aerodynamic benefits for both sub- and super-sonic aircraft, says Nasa. On subsonic aircraft, such as commercial planes, folding wings could improve controllability and reduce the need for heavy components, such as the tail rudder. This would reduce fuel consumption and allow pilots to adapt more easily to flying conditions by adjusting the wings automatically.

However, Nasa believes the most significant benefits of folding wings in flight would be for with supersonic flight.

Matt Moholt, principal investigator on the SAW project, says: “Folding the wing tips downward – to ‘ride the wave’ in supersonic flight – will reduce drag, which may result in more efficient supersonic flight. It could also increase performance, as you transition from subsonic to supersonic speeds. This is made possible using shape memory alloy.”

Flexible flaps

Nasa has also carried out research into wing flaps – and has found that making them from flexible materials could boost flying efficiency and cut aircraft noise.

The flap, manufactured by US-based Flexsys, was developed under the Adaptive Compliant Trailing Edge (ACTE) project and uses a ‘twisted’ configuration: here, the inboard and outboard sections of the flap are deflected in opposite directions – which changes the centre of lift on a wing during flight. This could be used to alleviate gust loading more effectively – which could lead to the creation of lighter wings.

In tests, the inboard section was deflected down by 2.5°, whilst the outboard sections were deflected up by the same amount.

“When incorporated with new wing designs, ACTE will make aeroplanes more efficient, quieter, and lighter – which helps reduce overall fuel burn,” said Kevin Weinert, ACTE project manager.

Nasa has also examined the acoustic benefits of the technology. In a series of test flights, it fitted the flexible, twistable wing flaps onto aircraft and flew them over an array of microphones. (The project also measured the noise from landing gear and nose cones.)

The ACTE flap was recently flown to validate its ability to reduce vortices off wing flaps at high speeds, and in May 2017 demonstrated the first ever flight of a twisted flap configuration. In addition to its aerodynamic benefits, the ACTE flap also produces lower airframe noise levels, according to project manager Kevin Weinert.

“ACTE technology reduces noise, though that wasn’t part of the original intent,” said Weinert.

The original testing did not measure noise directly, but the researchers believed that, because the ACTE flap reduced vortices off the flaps, it would also be quieter.

“I see this being something that may be integrated into aviation within the next 10 years, as reducing noise for communities around airports is a goal of aircraft manufacturers,” he said.

Super designs

Meanwhile, Joaquim Martins – an aerospace engineer who leads the Multidisciplinary Design Optimisation Laboratory at the University of Michigan – has used a supercomputer to develop shape-changing wing designs that burn less fuel.

“We're bridging the gap between an academic exercise and a practical method for industry,” he states.

Flexible, twistable wing flaps, developed in the ACTE project, could boost fuel efficiency and cut aircraft noise

Martins and colleagues have developed a new wing design that could burn 2% less fuel than current designs. He says that improved wing designs could improve fuel efficiency by up to 10%.

“Research on new materials and morphing mechanisms will make morphing systems lighter, more energy efficient, and more economical,” he says. “It is just a question of time before we see aircraft wings with morphing capabilities that seem impossible today.”

He says that using a technique called multidisciplinary design optimisation can create more detailed computer models. However, it requires the use of parallel supercomputers to run complex calculations on thousands of design variables.

“We try to get each simulation to be of the order of a few minutes – and the optimisation to converge within eight to 48 hours,” he says.

Boeing and Nasa are both testing prototypes based on wing designs suggested by his optimisations. However, applying these algorithms in industrial process will require extra work – such as integrating them with manufacturing costs, to test whether the more efficient aircraft designs can be built economically.

Foldable wings have existed since the earliest days of aviation – when the Wright brothers used pulleys and levers to twist the trailing edges of wings. However, as aeroplanes got more sophisticated the systems to control them became very heavy.

Now, the availability of new materials and more raw computing power have made it possible to design and build new types of wing – which could one day have a huge effect on flight efficiency.

www.nasa.gov

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