The way birds change the shape of their wings during flight is inspiring smaller aircraft that are more agile, efficient and stealthy. Stuart Nathan reports
Man has been a master of the air for a century now. Flying is commonplace and many different varieties of aircraft fulfil various roles in both military and civilian applications. But as the defence sector’s use of unmanned aerial vehicles grows, its aerospace engineers are looking to more complex methods of flight.
Aircraft fit into two types: fixed-wing or rotor-craft, which generate lift by forcing air to flow over a rigid aerofoil. An aircraft that can change the shape of its aerofoil would have a different range of mid-air tricks; it could be more efficient in flight, using less fuel; it could be more agile; and it could even be more stealthy.
The interest in morphing wings stems from natural flight. Engineering is, to a large degree, derived from the natural world: engineers seek to emulate natural materials and their properties and look at how evolution has solved various problems, as these tend to be the most elegant solutions. Every living thing that flies uses morphing wings, which means they can overcome one barrier to man-made flying structures: flight at low Reynolds numbers.
The Reynolds number is derived from the speed of movement in a fluid, the density and viscosity of the fluid and the size of what is moving. It describes how the fluid moves over the moving object (it also applies to a fluid moving within a pipe or past a fixed object). Large aircraft, travelling fast in thin air, have a very high Reynolds number and can generate lift purely by the shape of their wings, creating higher pressure below than above, which pushes the aircraft upwards.
But small aircraft, flying low and slow, have lower Reynolds numbers, which means the resistance of the air becomes more important. Birds, insects and bats get past this problem by deforming the shape of their wings so they can create patterns of turbulence in the air, which, in ways that are not fully understood, generate lift and thrust. For very small aircraft — particularly small UAVs — they may represent the only way to fly.
They also help birds make a transition into different types of flight. Watch a bird of prey, for example, as it flaps to take off, stretches its wings out to glide, brings them in to jink and turn rapidly, folds them to dive and flares them to brake and lift off again. While aeronautical engineers have harnessed aerodynamics and designed control surfaces that can duplicate some of these tricks, only a morphing wing will allow them the full range of versatility.
The swift is one of the most accomplished fliers of all birds, making it an important model for morphing-wing vehicles. Related to hummingbirds, swifts spend most of their lives in the air, landing only to lay eggs and raise chicks; they can spend as much as four years on the wing, even sleeping while flying. They are also aerobatics experts, making amazing swoops and high-speed turns.
A team of Dutch and Swedish scientists based at Delft Technical University has used studies of the swift to develop a morphing-wing flier called RoboSwift. Based on studies by David Lentink of Wageningen University, RoboSwift has crescent-shaped wings that it can sweep in, going from a wide shape, almost at right angles to its fuselage, to a narrow, swept-back configuration. Each wing consists of four curved members that mimic the feathers of the swift’s wing, sliding over each other to narrow and curve back the wing shape.
Lentink’s studies showed that the swift uses its different wing shapes for different sorts of flight. Held out stiffly, the wings generate the maximum amount of lift; the birds use this to fly slow and straight and the wing configuration lets them glide with minimum effort at night. For speed, they sweep their wings back and also sweep back to make tight turns, even though the extended configuration would generate more lift: the forces experienced in a turn would break the wingbones if the wings were held straight, so they compromise by losing altitude in a turn.
RoboSwift first flew last year. With a 50cm wingspan and weighing around 100g, the aircraft is powered by a nose-mounted electric propellor that stops and folds away during gliding flight to conserve batteries. The Dutch police force is funding the project as it sees applications for the RoboSwift in surveillance work.
In the US, meanwhile, DARPA, the Defence Advanced Projects Agency, has its sights on morphing-wing technology for the nano air vehicle (NAV) programme. NAV is aiming to bring UAVs indoors by developing aircraft the size of a hummingbird and capable of similar tricks.
‘The NAV programme will push the limits of aerodynamic flight and power conversion for very small flapping-wing air-vehicle systems,’ said programme manager Todd Hylton. The end-point of the programme is an aircraft weighing around 10g that can fly forwards at 10m/sec and hover for extended periods; that can withstand a wind gust of 2.5m/sec and fly indoors; and can be controlled from up to a kilometre away. ‘These goals will stretch our understanding of flight at these small sizes and require novel technology development,’ he added. Californian company AeroVironment, which manufactures hand-launched reconnaissance UAVs called Ravens used by US forces in Afghanistan, has risen to the challenge by developing a tiny aircraft that it has dubbed Nano Scout. While the Ravens seem small — about 1.4m across and weighing 2.75kg — they dwarf Nano Scout, which is just 7.5cm across.
Nano Scout is modelled on a hummingbird, with wings that perform the characteristic backwards and forwards sweep of these tiny aeronauts. The fruits of the first phase of NAV, the Nano Scout prototype is the first flapping-wing aircraft to be able to hover, move backwards, forwards, left and right. Powered by an on-board battery, it can fly for 20 seconds — something that AeroVironment’s project manager and principal investigator on the project, Matt Keennon, is hoping to improve.
Unlike the propellor-driven RoboSwift, Nano Scout generates all its lift and control through only two moving, aerodynamic parts — its wings. It doesn’t even have a tail. Throughout its development, Keennon’s team used extra lift-generators, such as propellors and a tail, to work out how the moving, morphing wings could perform both tasks, but gradually removed these.
‘From the first day of the Phase I effort, we knew our biggest challenge would be to develop a viable propulsion system and after that the control system would be the next extreme challenge,’ Keennon said. The development project involved testing more than 90 different wing configurations.
But the combined lift-propulsion-control system it developed is inherently unstable; once it is flying, it immediately wants to tumble out of control, making it impossible for a human pilot to fly without assistance from an automated control system. It’s also extremely power-hungry: the 20 second flight drained the battery.
Despite the difficulties, DARPA has extended its contract with AeroVironment to develop Phase II of Nano Scout. This, Keennon explained, involves optimising the aircraft for longer flight times — which will mean either making it less power-hungry or finding a way to get more power on-board, with suggestions including incorporating solar power or inductive charging. The team is also looking at how the craft can make the transition from hovering to forward or backward flight, and reducing the size, weight and acoustic signature. ‘All of these are distinct technical challenges in their own right that actually conflict with each other.’
The project remains ambitious: AeroVironment hopes to complete Phase II within a year. By this time, Keennon hopes to have demonstrated hover and forward flight using a ground-control video display. And even then, the project has some way to go: Nano Scout, despite its name, can’t scout. It can’t carry a payload, the motivation for the whole programme. Sensors and miniaturised cameras are becoming smaller and lightweight, but any extra weight will cause Keennon further problems.
In the UK, research at Bristol University is taking a different approach, focusing on the effects of different kinds of wing-morphing on the flight characteristics of a flying-wing type aircraft, aimed at developing a slow-flying UAV. The team, led by Prof Michael Friswell, has developed an experimental aircraft with wingtips that pivot up and down, from 90° above the main line of the wing to 90° below, which can make the aircraft turn, climb and dive.
Other studies include the effect of twisting the wing — a technique used in the first powered aircraft, the wood, paper and wire wings of which were very flexible. Layers of shape-memory alloy could make wing shapes that snap into two different stable shapes when heated and cooled. Such mechanisms, Friswell believes, are key to a morphing aircraft that is stealthy and that maximises fuel efficiency.
To see flight tests of the early phase of the AeroVironment NanoScout, click here: http://www.youtube.com/watch?v=Cov7-XWUa18