Loons, gulls, puffins and petrels are just some of the 100 species of birds that can both fly and swim. These diving birds can dive into the water to swim to their prey, then rise into the air and fly away.
Inspired by these naturally aquatic fliers, engineers at MIT and EPFL in Lausanne, Switzerland, designed a robot that can swim underwater, then emerge from the water and continue flying in the air, much like diving birds.
The “Flapping Wing Air-Aquatic Vehicle” (FAAV) weighs just under 300 grams (about half a pound) and is intended to lend a hand scientists study the mechanics that enable diving birds to fly through air and water.
The robot has a central body, or fuselage; two malleable, flapping wings; and steerable tail. The wings and tail can be exchanged for different sizes. In experiments conducted in a reservoir and at a local lake, engineers identified combinations of wing size, flapping frequency and tail angle that enable the robot to seamlessly transition from swimming through water to piercing the surface to flying through the air.
Their results which will appear in the magazine today could lend a hand scientists understand how diving birds adapt their flight mechanics to navigate through air and water – media with very different physical properties. The project could also bring to market a recent class of drones and air-water vehicles. Scientists predict that such winged robots could be used in oceanography to fly to and collect samples from water regions that conventional ocean ships would otherwise not be able to access.
“Our vision is that oceanographers, marine biologists, and members of coastal communities will launch this robot from a boat or from shore to fly close to an area of interest, such as an iceberg or port facility, or over a pod of whales,” says Raphael Zufferey, an assistant professor of mechanical engineering at MIT. “It would dive into the water to take a measurement or collect a sample, then fly back to deliver the data at a fraction of the cost of traditional methods. It could then return to dive for more.”
Zufferey is the lead author of the recent study, which includes co-authors from EPFL and Northwest Indian College in Bellingham, Washington.
Flight mechanics
At MIT, Zufferey is in charge AURA Laboratorywhere he and his students construct air and water vehicles inspired by nature’s biomechanics. The robots they build are petite and designed to discreetly explore and monitor the condition of oceans and waterways.
As part of their recent work, the team aimed to design a vehicle that could fly in the air and underwater. Any such vehicle would have to adapt to and transition between two very different substances. Water is 1,000 times denser than air, and moving through one or the other requires completely different mechanics. Or at least that’s what people might assume.
“Some adaptations need to be made to make this transition work. But there is a solution that exists in nature,” Zufferey says. “Birds like puffins can fly very fast in the air, they can dive and swim in water at 3 meters per second. They can do really amazing things. So we knew it was possible. It’s just that no one had tried it in a mobile robotic system.”
To find out how diving birds fly, the team reviewed the scientific literature and collected available data on puffins, petrels, kingfishers and other diving birds. They observed that the smaller birds flap their wings about 10 times per second while flying in the air and about four times per second while swimming in the water. Larger birds have a slightly lower flapping frequency in both air and water due to their larger wingspan.
With bird biomechanics in mind, the team developed a winged robot designed to flap at a frequency similar to that of real diving birds.
Making a jump
The recent robot roughly resembles a bird, with a body, two wings and a tail. The body contains a battery and a waterproof electric motor that drives the crankshaft, which in turn pumps the wings up and down at a set frequency. The wings are made of slim membranes coated with hydrophobic nanoparticles that lend a hand shed water. The tail is motorized so it can change angle, making it easier for the robot to climb or dive.
The wings can be exchanged for different sizes. The researchers produced and tested three sets of wings: petite (60 centimeters wide), medium (80 centimeters) and vast (100 centimeters). They conducted experiments first in a petite water tank and then in Lake Geneva in Switzerland.
In their tests, they placed the robot underwater, about half a meter below the surface. They programmed the wings to flap at a specific frequency and the tail to tilt at specific angles as the robot flew. They then observed the conditions under which the robot successfully surfaced, emerged from the water, and rose into the air.
The robot performed multiple flights with different wing sizes, flapping frequencies, and tail angles. Overall, the team found that the robot was able to reliably fly, swim, and switch between water and air when flying with medium-sized wings. Wing flexibility is key; the wings must be malleable enough to minimize the amplitude of flapping in the water, and also forceful enough to keep the robot in the air.
The researchers also found that the robot could swim through the water at a speed of almost 1 meter per second, flapping at a frequency of about 5 hertz, or five flaps per second. The robot could fly through the air at about 20 feet per second, flapping at a similar frequency. The robot’s flapping speeds and frequency were similar to those of real diving birds.
They determined that to make a jump from water into the air, the robot should be tilted at 70 degrees – a relatively vast angle that prevents the robot’s wingtips from touching the surface of the water as it rises into the air. Any steeper and the robot will tip back into the water.
Interestingly, this combination of wing size, flap frequency, and tail tilt allowed the robot to swim underwater, rise to the surface, and fly without something many diving birds need: feet. When birds such as puffins and ducks lift off the surface of the water, they paddle by flapping their wings and flicking their tails. Surprisingly, Zufferey and his colleagues found that, at least in robotics, flying out of the water does not necessarily require a paddling maneuver.
“If you look at birds, most of them have to paddle on the surface to fly away. The question was, do we need the same thing with robots? Turns out we don’t,” Zufferey says.
In the future, the team will refine the design of the wings to allow them to not only rotate, but also flap up and down. They will also test the robot’s performance in turbulent conditions, such as swimming through coarse water and flying against the wind. They then hope to deploy the vehicle to lend a hand answer ocean science questions.
“One of the main challenges in ocean science is frequently collecting data from multiple locations, which is what this robot could do in the future,” Zufferey says. “It can be sent not only weekly, but hourly. It can fly at high speed, dive on the return flight, deliver the data and fly out multiple times.”
This work was partially supported by a scholarship grant under the Marie Skłodowska-Curie Actions.