Researchers at Cornell University in New York are looking to design wing shapes and arrangements, as well as flapping patterns for flying micro-robots.

Starting with videos of dragonflies flying, the team has developed a computational fluid dynamics model of dragonfly flight. “We have compared our computed results with forces on a transducer on a tethered dragonfly and they agree,” Professor Jane Wang told Electronics Weekly.

The equations of fluid dynamics are known but hard to apply, particularly to flexible surfaces. “To compute well is actually very hard,” said Wang, “It is still a challenge to develop models for objects containing a moving boundary.”

One surprising finding from the model is that dragonflies fly using drag rather than lift.

Drag is generally thought to be an inefficient process, but dragonflies must be doing something right because they have been around since before the dinosaurs.

This computational fluid dynamics shot show dragonfly flight. clockwise votices are blue, anti-clockwise are red, and the wings are black

“Dragonflies have a very odd stroke. It’s an up-and-down stroke instead of a back-and-forth stroke,” she said.

“Dragonflies are one of the most manoeuvrable insects, so if they’re doing that they’re probably doing it for a reason. But what’s strange about this is the fact that they’re actually pushing down first in the lift.”

Aeroplanes and bird wings use aerodynamic lift to carry weight.

A Welsh dragonfly

“The dragonfly uses a lot of aerodynamic drag to carry its weight. That is weird, because with airplanes you always think about minimising drag. You never think about using drag,” said Wang. “The question bothered me: is it possible to use drag and still be efficient?”

It turns out the wing changes direction just as its boundary layer separates and its wing stalls - when it is in a state Wang calls ‘dynamic stall’. “It reverses the wing, makes a new boundary layer and makes use of it,” she told EW.

The next question for Wang is whether engineers can use these ideas to build a flapping machine as efficient as a fixed-wing aircraft.

“I want to build insects on a computer as a way of learning why almost all things that move in fluid use a flapping motion,” said Wang. “Whether it’s a fish which flips its fins or a bird, they’re actually using the same principle.

Now Wang and her team have a reliable model, they are going to search for flapping arrangements suitable for robot flight. “Most ways to move a wing up and down are inefficient,” she said. “We need to find the sub-set that are efficient. The next piece of work is to use genetic algorithms and produce optimisation.”

www.cornell.edu