Experiment with parrotlet reveals flight research faults

Wearing protective goggles and chin strap and flying through a field of lasers and microparticles, a parrotlet named Obi has helped researchers explain the way animals generate enough lift to fly and reveal that models currently used for the research are faulted.

The experiment was designed to compare commonly used models in the literature to figure out how much lift a bird, or other flying animal, generates based off its wake, explained Diana Chin, a graduate student in the lab of Stanford University mechanical engineer David Lentink. "What we found was that all three models we tried out were very inaccurate because they make assumptions that aren't necessarily true."

Researchers rely on these models to understand how animals support their weight during flight. The results are commonly referenced for work on flying robots and drones inspired by the biology of these animals.

The question was whether models of lift based on an inaccurate idea of an animal's wake were valid.

For the experiment, Eric Gutierrez, former graduate student in the Lentink lab and lead author of a study published in the December issue of Bioinspiration and Biomimetics, made parrotlet-sized goggles using lenses from human laser safety goggles, 3D-printed sockets and veterinary tape. The goggles had reflective markers on the side so the researchers could track the bird's velocity. Then he trained Obi to wear the goggles and to fly from perch to perch.

Once trained, the bird flew through a laser sheet that illuminated nontoxic, micron-sized aerosol particles, its wing motion disturbing the particles to generate a detailed record of the vortices created by the flight. The particles swirling off Obi's wingtips created the clearest picture to date of the wake left by a flying animal.

Past measurements had been taken a few wingbeats behind the animal, and predicted that the animal-generated vortices remain relatively frozen over time, like airplane contrails before they dissipate. But the measurements in the new experiment revealed that the bird's tip vortices break up in a sudden dramatic fashion. "Now, whereas vortex breakup happens far away behind the aircraft, like more than a thousand meters, in birds, it can happen very close to the bird, within two or three wingbeats, and it is much more violent," said Lentink, who is the senior author on the paper.

The team applied each of the three prevailing models to the actual measurements they recorded and from that generated three different estimates of the amount of lift Obi generated with each wingbeat. They then compared those calculated estimates of lift to the actual lift measured in a previous study carried out using a sensitive device developed by the Lentink lab.

What they found is that to varying degrees, all three models failed to predict the actual lift generated by a flapping parrotlet.

The research highlights challenges in developing flying robots based on what's known about animal flight. "Many people look at the results in the animal flight literature for understanding how robotic wings could be designed better," Lentink was quoted as saying in a news release. "Now, we've shown that the equations that people have used are not as reliable as the community hoped they were. We need new studies, new methods to really inform this design process much more reliably." Endit