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01/17/2014

A jellyfish flying machine? Why not

Aerojelly

Leif Ristroph decided to make a flapping-winged aircraft that didn't imitate anything in nature. (Photo credit: Leif Ristroph)

 

Ornithopters are one of those things that engender a bit of obsession among scientists, engineers and tinkerers, but that most people have never heard of.

An ornithopter is an aircraft that flaps its wings like a bird or an insect. Leonardo Da Vinci designed one in the late 15th century, and in 2010 engineers from University of Ontario became the first to fly one powered by a human. You can make an ornithopter that runs on a rubber band with instructions found online

Leif Ristroph, a math professor at New York University, built one with $15 worth of parts. Except Ristroph's ornithopter is a little different. It doesn't fly like a bird or insect at all. It flies like a jellyfish. 

Most efforts to construct an ornithopter up until this point have been "biomimetic": they mimick the movements of things that fly in nature. But Ristroph's flying machine works with a pulsing motion more akin to a jellyfish swimming (see the video at the bottom of this post). 

"The main thing I wanted to do is point out that we could explore options that are not necessarily in nature. We can dream up other ways of flying, and of course we have in the past – there's nothing really quite like an airplane or helicopter in nature," Ristroph told the Star.  

The problem with moths, bees, and other horizontal wing-flappers is that their flight mechanism is unstable. When insects and birds are hit by a gust of wind and tilted to one side, they can make tiny adjustments that allow them to stay aloft. But ornithopter designers have had to come up with all kinds of super-smart feedback control systems and extra tails to compensate.

Ristroph did his PhD on flapping-wing aerodynamics at Cornell, and "after staring at a lot of high-speed video of insects flying, I certainly had an urge to build one."

But when he set about designing an ornithopter he wanted to make something super minimalistic, so he went down a totally different path from insect-like flight altogether. 

One of the images he had in his head was of a flapping umbrella opening and closing. Out of eight to 10 different designs, this one showed the most promise. But looking at video of the prototype in the air, Ristroph and his colleagues realized the four wings opening and closing looked much more like a jellyfish propelling itself underwater than an umbrella. 

The design is self-righting, and doesn't require any feedback control from the user. The 10 centimetre prototype caused quite a stir at a fluid dynamics conference in November, and a paper on the project with the title "Stable hovering of a jellyfish-like flying machine" was published in this week's issue of Journal of the Royal Society Interface.

While ornithopters might seem like just a quirky project, they have important applications in the real world. 

The aerodynamism of fixed-wing airplanes breaks down as designs get smaller. Hovering aircraft -- helicopters -- work better. But while aerodynamic theory of flapping-winged flyers is still a frontier field with lots of unknowns, there are some indications that flapping could work even better than hovering in very tiny aircraft.

"That kind of jives with the fact that all these insects which are small flap their wings," says Ristroph.

It's not hard to imagine the different ways that tiny, super stable flying machines could be useful to humans, from search-and-rescue to surveillance. 

"Obviously there's a lot of military interest in these things. But you can also imagine using a small maneuverable flyer that can actually hover and move around in crowded environments. That might be a good way to explore a collapsed building in a search-and-rescue mission, for example." 

James DeLaurier, a professor emeritus at the University of Toronto Institute for Aerospace studies and the inventor who oversaw the human-powered ornithopter project that took flight in 2010, called the jellyfish design "very clever." For the types of tasks its intended for, DeLaurier noted that the prototype would have to be developed to steer itself.

He also pointed out that in the 1950s, a Canadian Aviation Hall of Fame inductee named Charles Luttman explored the use of pulsing surfaces and created an "umbrellathopter" that was powered by a rubber band.

  

Kate Allen is the Star's science and technology reporter. Find her on Twitter at @katecallen.

 

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