Swift by design, drone by nature

Hovering, darting, gliding, diving and braking, the fleet-winged contraption in the video below is at the cutting edge of biologically-inspired flight.

Weighing the equivalent of two tablespoons of flour, this wispy ornithopter (flapping-wing aircraft) mimics the aerobatic manoeuvres of one of the world’s fastest birds, the swift. It was developed over years of research by an international team from Australia, Singapore, China and Taiwan – but it’s just the beginning of a new era of drone technology.

One of the creators, aerospace engineer Javaan Chahl from the University of South Australia, says ornithopters are safer, quieter and more versatile than typical quadcopter drones.

“Conventional hovering drones using multiple rotors have a range of disadvantages: they can’t glide, and the speed of the blade tips is quite fast, leading to noise and hazard,” he says.

“Ornithopters have larger wings that flap comparatively slowly when hovering. These wings can also act like the wings on a normal aeroplane, allowing much more efficient forward flight than a multirotor.”

Using a combination of flapping wings and tail control, ornithopters not only can more efficiently hover and cruise, they also can stop quickly to avoid collisions, and so have the potential to safely navigate crowded areas.

These half-bird, half-insect contraptions aren’t exactly a new idea. They were considered by Leonardo da Vinci, who sketched a flapping-wing flying machine in the 15th century, though the first ornithopters capable of flight weren’t developed until the 19th century, when model versions were powered by rubber bands, gunpowder charges or steam engines.

In recent years, research into robotic ornithopters has been on the rise. Prototypes have thus far been too inefficient and slow to be agile – a problem Chahl and colleagues overcame by drawing inspiration from nature.

Birds, bats and insects all independently evolved flight in an incredible example of convergent evolution. Each flying system has its own pros and cons, adapted to perform specific functions over millions of years, and scientists have long studied these systems in order to learn how to replicate them.

This field – biomimetics – informed the design of this new ornithopter. Instead of modelling the prototype after a single species, the researchers mixed and matched, drawing strategies from both birds and insects.

“The flight manoeuvre was modelled after the swift,” Chahl explains. “The actual wing design and actuation is much more comparable to an insect.

“We have observed the basic principles and implemented to the best of our engineering ability. We are actually working towards exactly copying wings and mechanisms from real insects, but that is harder and will take longer.”

Developing this prototype was already challenging; Chahl’s colleagues at the National University of Singapore went through countless wing and mechanism designs.

“The mechanisms and wings require extreme patience to make, and there are many small parts that can be damaged,” Chahl says. “A heavy landing often ended in some hours of repairs.”

The wings are made from an elastic mechanism, which minimises wobble and increases energy efficiency.

“Our ornithopters make use of the ‘clap and fling’ effect,” Chahl adds. “The two pairs of wings flap such that they meet, like hands clapping. This makes enough extra thrust to lift their body weight when hovering.”

A large tail also creates turning force, allowing the ornithopter to perform aggressive aerobatic manoeuvres and switch quickly from horizontal to vertical flight.

The biggest challenge was creating “an efficient gearbox that wouldn’t bind in the presence of the large reciprocating wing beat forces,” Chahl explains; they’re currently working on a better mechanism for this that resembles the system in insects.

Their tiny prototype is also fairly fragile, so durability is a major issue – and making it more robust will also make it heavier.

But even in its current state, this ornithopter can pull off some amazing moves that will likely mean its applications will be centred around people – partly because it’s so agile, and partly because its flapping wings are much safer than rapidly-spinning propellers.

The researchers envision that smaller versions may be able to substitute for pollinating insects in indoor environments without damaging vegetation, or they could be used to inspect delicate machinery. By carrying a small camera and electronics, ornithopters could also be used for crowd and traffic monitoring or surveying forests and wildlife.

This kind of research has a long way to go – but it seems like an exciting time to be in the field.

“There are a number of groups working on this type of platform throughout the world, although we have probably created one of the most efficient by doing a lot of analysis and refinement,” Chahl says.

Other groups use different types of technology. While Chahl and colleagues use brushless motors to drive the wings, others use piezo electric elements or servo motors.

“In terms of the state of the field – well, you don’t see any other viable flapping wing drones yet,” he says.

While several other prototypes are notable, they are very light and slow with limited thrust.

“We have had a lot of emphasis on manoeuvre and power-to-weight ratio,” Chahl explains. “We can do aggressive aerobatics while remaining efficient, which is new.”

Still, there’s a lot to learn; Chahl’s co-researcher, Yao-Wei Chin from the National University of Singapore, says they are a long way from truly replicating biological flight.

“Although ornithopters are the closest to biological flight with their flapping wing propulsion, birds and insects have multiple sets of muscles which enable them to fly incredibly fast, fold their wings, twist, open feather slots and save energy,” he says.

“Their wing agility allows them to turn their body in mid-air while still flapping at different speeds and angles. At most, I would say we are replicating 10% of biological flight.”

What we really need, according to Chahl, are synthetic muscles for improved control – and as of yet there is no good substitute.

“The key limitation is that insects and birds have much more sophisticated control of the wing than our crank mechanism,” he says. “All of the opportunities to improve performance and precision of flight come from having a better mechanism driving the wings. Swifts are far more agile, more efficient and more aerodynamic – they are in a different league.”

Of course, swifts are not the only efficient flier. Birds and insects come in many shapes and sizes, and it may also be worth exploring if there is an optimal size and weight for ornithopters.

In principle, an even larger ornithopter could be built – but perhaps not as big as the one sketched by da Vinci, who envisioned a flying machine large enough to carry a human. Unfavourable scaling laws likely exist, as we see when we look to nature.

“Very large birds like condors and eagles do not hover, yet small birds like hummingbirds can,” Chahl explains. “We expect that when we design a large ornithopter we will discover that the wings will be too heavy, or the flapping mechanism will use too much power or be too heavy.

“It is wise to look at nature for evidence, and the evidence indicates that a condor’s weight and wingspan might be the outer limit.”