Like dandelion fluff, insect legs can take advantage of updrafts to fly high.
DENVER — For these insects, flight is all in their legs.
Ghost cranes float lazily on the breezes, their six long legs spread and their wings completely still. Wind tunnel experiments are beginning to reveal how they stay aloft. This information helps researchers build miniature flying machines that mimic the relaxed flight strategy of cranes, physicist Sarahi Arriaga-Ramirez reported March 17 at the American Physical Society’s World Physics Summit.
The Ghost Crane of the East (Bittacomorpha clavipes) lives in the eastern United States. Flies only live about a week when fully mature, during which time the insects mate but do not eat. “They have very limited energy and need to conserve it,” says Arriaga-Ramirez of the University of California, Berkeley.
When viewed with high-speed cameras in the still air of a laboratory, ghost cranes flap their wings to ascend, holding their legs vertically, presumably to reduce drag so they can fly more easily. But that changed in the updraft of a wind tunnel: The insects held their wings still and spread their legs upward to form an inverted cone shape, reminiscent of an upside-down umbrella or a fluffy dandelion seed – both of which are capable of catching the wind. The flies floated in the breeze because of the drag caused by their legs.
As conditions changed, the shape of the cone adjusted. In a more powerful updraft, the arrangement of the flies’ legs became narrower.
To understand the impact of changing the shape of the cone, the researchers made larger-than-life 3D printed models of a ghost crane and dragged them into a tank of mineral oil. The exchange of air for oil simulated the conditions that a real, smaller fly would experience, because on a small scale, air acts like it’s more slimy. The results revealed that a narrow cone produced less drag than a flatter cone, by approximately 20%. This suggests that the crane legs are adjusting to provide a smoother lift.
The insect’s original mode of transportation has inspired researchers to tinker with various designs of small, energy-efficient aerial vehicles. One option used a material called shape memory alloy to allow the legs of the robot crane to bend on command. Such materials return to their previous shape when an electric current passes through them. Coils made from these shape memory alloys changed legs from straight to bent, and vice versa.
It’s unclear how much ghost cranes control their legs and how much they simply blow in the wind. So the team also tried legs with flexible joints that allow passive movement. As wind speed increased, the legs automatically folded, giving the vehicle a narrower shape.
This results in very stable flight, Arriaga-Ramirez said. Even if the airflow around the aerial vehicle is disturbed, the vehicle, like the ghost crane, is not disturbed.
