Originally published in MIT News here.

Over 100 bird species both fly and swim. Such diving birds plunge into water to swim after prey, then leap back into the air to fly away. Inspired by these naturally aquatic aviators, engineers at MIT and EPFL in Lausanne, Switzerland, have designed a robot that can swim underwater, then flap out of the water to continue flying through air, much like diving birds. 

The FAAV (Flapping-wing Aerial-Aquatic Vehicle) weighs less than 300 grams (about half a pound) and is designed to help scientists study the mechanics that enable diving birds to fly through air and water. Overall, the robot is made up of a central body (fuselage), two flexible wings that flap, and a steerable tail. The wings and tail can be swapped out for different sizes. 

In experiments carried out in a water tank and at a local lake, the engineers identified combinations of wing size, flapping frequency, and tail angle that enable the robot to smoothly transition from swimming through water to breaking through the surface to flying through the air. The robot invention is meant to help scientists understand how diving birds adapt their flight mechanisms to move through air and water—mediums with very different physical properties. 

The FAAV design could easily be adapted for a new class of aerial-aquatic drones and vehicles. Such winged robots could be deployed in oceanography to fly to, and sample from, aquatic regions that would otherwise be too dangerous for traditional ocean vessels to access. “Our dream vision is for oceanographers, marine biologists, and members of coastal communities to launch this robot from a boat, or from shore, and it would fly close to the area of interest, such as an iceberg or a port facility, or over a pod of whales,” says Raphael Zufferey, assistant professor of mechanical engineering at MIT. “It would dive into the water to take a measurement or collect a sample and fly back to deliver the data at a fraction of the cost of traditional methods.” 

Zufferey is the lead author of the new study, which includes co-authors from EPFL and Northwest Indian College in Bellingham, Washington. For MIT, Zufferey is head of the AURA Lab, where he and his students engineer aerial and aquatic vehicles inspired by biomechanics in nature. The robots built are small, designed to unobtrusively explore and monitor the health of oceans and waterways. 

The FAAV had to adapt to and transition between air and water—which is 1,000 times denser than air. Moving through these two different elements required different mechanics. Through extensive research the team noticed that smaller birds flap their wings around ten times per second when flying through air, and only four times per second when swimming through water. As birds get larger, their flapping frequency slows due to their wider wingspans. With the biomechanics of birds in mind, the team developed a winged robot designed to flap at similar frequencies to that of actual diving birds. 

How It Works 

The robot roughly resembles a bird, with a body, two wings, and a tail. The body contains a battery and waterproof electric motor that drives a crankshaft, which in turn pumps the wings up and down at preset frequencies. The wings are made of thin membranes that are coated with hydrophobic nanoparticles to help wick away water. And the tail is motorized, enabling it to change its angle to help the robot fly up or dive down. 

Researchers fabricated and tested three sets of wings: small (60 centimeters wide), medium (80 centimeters), and large (100 centimeters). They carried out experiments first in a small water tank, then in Lake Geneva in Switzerland. During testing, robots were placed underwater at about half a meter below the surface. The wings were programmed to flap at certain frequencies and the tail to pitch at certain angles throughout flight. The researchers observed under which conditions the robot successfully swam toward the surface, out of the water, and into the air. 

The team found the robot was able to reliably fly, swim, and transition between water and air when it flew with medium-sized wings. Flexibility in the wings was key; the wings needed to be flexible enough to minimize flapping amplitude in water and also firm enough to keep the robot aloft in the air. It was found that the robot could swim through water at about 1 meter per second when its wings flapped at 5 Hertz. At this same frequency, the robot cold fly through the air at a distance of six meters per second. These speeds and flapping frequencies are similar to that of actual diving birds. 

To make the transition from water to air, it was found that the robot required a pitch of 70 degrees. Although relatively steep, this angle kept the robot’s wingtips from touching the water’s surface as it flapped up and into the air. Any steeper angle would tip the robot back into the water. Interestingly, this combination of wing size, flap frequency, and tail pitch enabled the robot to swim underwater, launch off the surface, and fly, without something that many diving birds require—feet. 

When birds such as puffins and ducks take off from the water’s surface, they paddle their feet, along with flapping their wings and pitching their tails. Surprisingly, Zufferey and his colleagues found that, at least in robotics, the act of flying out of water doesn’t necessarily require a paddling maneuver. “If you look at birds, most birds need to paddle at the surface to take off. And the question was, do we need the same for robots? And it turns out we don’t,” Zufferey says.

Going forward, the team is improving the design of the wings to enable them to turn in addition to flapping up and down. They will also test the robot’s performance under turbulent conditions, such as swimming out of choppy waters and flying through wind. 

This work was supported, in part, by a Marie Skłodowska-Curie Actions fellowship grant.

Reprinted with permission of MIT News. 

For information: 

MIT

EPFL Switzerland

Northwest Indian College