Tiny Drone Flies Like A… Jellyfish?

The ungainly flapping motion of this latest tiny flying robot closely mimics the swimming motion of a jellyfish.

It's a bird … it's a plane … it's a jellyfish? The latest tiny drone may not be sleek or stylish, but it's certainly a departure from the usual flying robot design.

In trying to think outside the box, researchers at New York University have come up with a drone design that mimics the movements of swimming jellyfish.

"Lots of designs kind of mimic flapping insect wings," said Leif Ristroph, an applied mathematician at New York University in New York City. But he knew from his previous research that those designs don't stay stable in flight without some help.

Insects have built-in sensors and feedback that help them stay upright, he explained.

Drones based on insect wings need the same support. But motors, sensors, and batteries add weight, which becomes problematic for people looking to design smaller and smaller drones.

"I wanted to design something that had stability without the stability-sensor needs," Ristroph explained in an interview.

He came up with a drone propelled by four wings, which the mathematician presented earlier this week at a fluid dynamics conference in Pittsburgh, Pennsylvania. Instead of sticking the wings out to the side like those on an airplane, Ristroph pointed them downward and had them flap back and forth to push his robot up into the air.

From the beginning, the design had good lift capabilities, Ristroph noted. The drone also showed signs of being inherently stable: It would automatically right itself so that it was always flying up, he said, without the need for extra sensors.

But it wasn't until the applied mathematician built his drone that he realized it moved remarkably like a jellyfish. (See "5 Surprising Drone Uses [Besides Pizza Delivery].")

A Diaphanous Design

"One of the important things to keep in mind is that whether it's an insect, a bird, a fish, or a jellyfish, they're all playing by essentially the same rules in physics," said Brad Gemmell, a University of Texas, Austin, ecologist who studies how animals move through the water.

Moving through air is similar to moving through water. And regardless of an organism's shape, if it wants to propel itself along, it has to generate thrust, explained Gemmell, who was not involved in the new research.

Jellyfish move in essentially two ways. For smaller species that are a few centimeters in size, they use jet propulsion, he said. The animal contracts its umbrella-shaped bell, squeezing out water behind it and creating a doughnut-shaped ring of water called a vortex.

That vortex pushes the jellyfish forward, said Gemmell.

Larger jellyfish add a movement called rowing to their jet-propulsion capabilities. Rowing is when the loose edge of their bell flaps around, creating additional vortices that help to generate thrust.

Those loose edges almost push against the fluid like a rower pushes an oar against the water, Gemmell explained. "That's what the flying object in the video showed."

Improving the Design

Ristroph is working on improving his drone. Now it needs to be connected to a power source with a tiny wire, but he'd like to see it fly free one day.

The mathematician is also working on other unusual designs for drones. He's working on one that works like an umbrella opening and closing in order to generate lift.

"I'm personally interested in schemes that no one in nature or the engineering laboratory has thought of," Ristroph said.

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