Watch a ‘flying fish’ drone hover in the air and then swim underwater

It's at home buzzing over dry ground, but then plunges into a pool. Here's how the flying machine works.
a drone flies over the water
Aaron Burden / Unsplash

At a conference last week, a group of engineers presented a fascinating new drone capable of flying through the air—and operating underwater. While it’s only a prototype, the researchers had to solve interesting problems in order to create a working aerial-aquatic quadcopter. 

A large group of researchers from seven universities and laboratories throughout China and Hong Kong contributed to what they’ve dubbed the TJ-FlyingFish. In the paper presented at the Institute of Electrical and Electronics Engineers (IEEE) 2023 International Conference on Robotics and Automation (ICRA) in London, they described how they developed the 3.6-pound quadcopter. 

According to the research paper, there have been previous multi-rotor aerial-aquatic hybrid prototypes, however, they have mostly relied on “standard aerial hardware constructions with water resistance.” In other words, instead of creating a drone truly capable of flying through the skies and also operating underwater, most researchers have designed aerial drones with some waterproofing so the machines don’t stop working when they splash down. 

The issue is that while both water and air are technically fluid mediums, they have vastly different properties. The different viscosities and densities affect how the propulsion system should operate, as well as the overall design of the drone. To operate in the air, the drone needs to be able to overcome gravity. To operate effectively underwater, the drone needs to be neutrally buoyant and able to generate enough thrust to overcome water resistance. 

As a result, achieving effective thrust is different in both mediums. The propulsion force is determined by the amount of mass that the propellers move. To fly, the drone needs high-speed propellers to throw a lot of lightweight air around as fast as possible. To move through the much denser water, though, the drone needs slower speed high-torque propellers. Instead of using two sets of propellers, the engineers designed an innovative system that uses one motor and two separate gearboxes: one rigged for aerial flight and the other for underwater movement. This has the advantage of keeping weight low, though it makes for an inherently complex system.

[Related: The Army’s Black Hawk helicopter replacement is a speedy tiltrotor aircraft]

When the drone takes off, it works like a regular quadcopter. The propellers force air down and give it enough lift to fly, and they are able to tilt and rotate independently so it can maneuver and hover. It has enough battery power to hover for six minutes. But it’s when it lands in water that things get interesting. 

The drone is slightly negatively buoyant, so when it hits the water it slowly starts to sink. Then, one way for it to travel underwater is for the whole drone to rotate, so the propellers pull it sideways through the water. Alternatively, the drone’s body can stay upright, and it can maneuver by tilting the props in different ways. These two systems enabled by the tilting propellers and dual gearbox allow it to operate effectively underwater and maneuver in three dimensions (just as it can in air) in a way that a waterproof drone equipped solely with traditional aerial propellers can’t. 

A huge amount of the researchers’ effort went into making the drone efficient underwater, rather than just designing a waterproof quadcopter. As a result, the prototype’s underwater performance is surprisingly good. It can spend around 40 minutes submerged and has a maximum dive depth of just under 10 feet. It was designed to be lightweight, so there was a tradeoff with waterproofing. Future prototypes will likely be able to go deeper. 

Most impressively, the drone can also take off from the water. By rising to the surface and rotating its propellers, it generates enough lift to get back into the sky. 

Although only a prototype, it’s hard not to imagine uses for a drone like this. The researchers suggest remote sensing operations and disaster rescue, but it could also be used to conduct civil and military surveys, capturing incredible video footage, and inspire sci-fi authors. It could also lead to waterproof drones, which could save some people a lot of money

Take a look at this cool aerial-aquatic drone in action, below.