Bio-inspired drones: the natural evolution of UAVs
Image credit: Alireza Ramezani, Caltech, Greg St. Martin
We are familiar with drones that look high-tech but rather robotic, sprouting multiple sets of rotors or with long, imposing, fixed wings. Yet the UAVs of the future might look less like vehicles and more like insects, birds or even bats. Here we look at three nature-inspired examples.
Animal Dynamics is a company at the forefront of the bio-inspired approach. Formed as a spin-out from Oxford University in 2015 by serial entrepreneur Alex Caccia and Oxford professor of biomechanics Adrian Thomas, the project began as a shared hobby to create a human-powered machine based on fish propulsion. The hobby turned into a company when the duo took on a challenge from the UK’s Defence Science and Technology Laboratory (Dstl) to build a tiny, ultra-efficient reconnaissance drone able to cope with challenging wind conditions.
Skeeter was born, an insect-inspired UAV weighing less than 200g and the size of a biro. The tiny drone has flapping wings that enable it both to hover and to fly forwards. The wing design makes it resistant to wind gusts as well as being both safer and quieter than normal drones, and it operates off standard rechargeable batteries.
“We have a vehicle that is able to do everything,” says Caccia. “It has a greater performance efficiency than a propeller-driven craft of the same size, weight and power.”
Skeeter is currently being developed for defence contractors and Caccia expects it to be operationally ready by 2021. “It will be really useful for helping people in dangerous situations, to show them what’s hiding behind a wall or building,” he says. “The reason it’s such a good approach is that you can carry it with you.”
Animal Dynamics has developed another bio-inspired UAV called Stork, which is designed to address the last-mile logistics problem in dangerous situations. The goal is to supply frontline troops without the casualties and expensive damage that can occur by current methods like helicopter, truck or even quadbike.
Animal Dynamics’ solution is an autonomous-powered paraglider that is efficient, lightweight and super-robust. “The wing design is bio-inspired,” adds Caccia, “and the frame and chassis design is more skeletal than mechanical so it flexes a bit, which absorbs impact on landing.”
Stork weighs in at 20kg and can carry up to 10kg of payload more than 50km with a short take-off and landing. It can be packed into a small space and, once unpacked, can be ready to fly in 10 minutes, all of which make it perfect for challenging frontline resupply conditions. It is also low-cost compared to current ground or air supply solutions and doesn’t risk the lives of human operators.
Stork is being trialled by contractors, and Animal Dynamics is already testing a scaled-up version that can carry 120kg payloads.
It’s not only insects that can hover and fly but some birds as well, which is one reason a team of US researchers chose the hummingbird as the inspiration for their drone. “Most birds can only fly forward,” says Professor Xinyan Deng from Purdue University, one of the researchers on the project. “But the high wingbeat frequency of hummingbirds means they can sustain hovering. On the other hand, they can also do very manoeuvrable flight because their wings are deformable so they have the advantages of both insects and birds.”
This manoeuvrability means Deng’s hummingbird drone can fly figures of eight, turn 180 degrees in a few wingbeats and even do a 360-degree body flip. The team achieved this agility by mimicking the deformable wings of hummingbirds and by making each wing individually controlled so that subtle differences in wing angle and shape can make large changes in flight direction.
The hummingbird drone is able to absorb impacts and is safe enough to be plucked from the air by hand. It is also quiet compared to quadcopters, has more efficient forward flight and is resistant to wind gusts. Its efficiency draws on the hummingbird’s amazing migratory ability, which enables it to cross the Gulf of Mexico sometimes in one flight.
“They have to fight all the time against gusts and disturbances,” says Deng. “But it’s the flapping motion itself, the unsteadiness of the aerodynamics, which is naturally rejecting those disturbances, which gives them more energy.”
The hummingbird drones are still in the development stage – they aren’t yet carrying their own power source – but Deng sees them coming to market within five years. She envisages applications in search and rescue where hummingbird drones, with their small size and manoeuvrability, could navigate very confined spaces such as earthquake-hit buildings to search for survivors.
When it comes to ease of manoeuvring, it might not be birds but mammals that take the crown. This is one reason another US team has adopted a mammalian star flier as the inspiration for their drone.
“Bats can make a lot more agile, aggressive manoeuvres than birds,” says Soon-Ji Chung, a researcher on the team and professor of aerospace at Caltech. “They can make 100-degree turns dramatically and can also do upside-down perching.”
Bats achieve this agility with thin, flexible wing membranes that can morph during flight to create asymmetric wing shapes. Chung and his team reverse-engineered this design to create their own wing membranes made from pressed silicon as thin as a human hair. The membrane is stretched across two independently controlled and highly structured wing frames. “It has five independently actuated joints,” says Chung, “and these joints are moved by five independent micro motors. There are additional passive joints also so altogether you can fold the left and right wings independently.”
Chung envisages applications in the construction industry where Bat Bot’s small size, manoeuvrability, silence and safety aspects, plus its ability to perch at odd angles (not yet developed), would make it an ideal means of monitoring and updating construction progress. “With multiple drones you can create a 3D model of your building in almost real time,” says Chung. “That can be used in comparison with your CAD design to make sure you are building the structure properly.”
Although the Bat Bot (pictured above) is not yet commercially available, Chung is already working on scaling up the design to human-sized vehicles like flying cars and ambulances that could beat congestion on the ground.
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