Birds provided the initial inspiration for the Wright brothers’ flyers, and yet their first aeroplanes did not try to imitate flapping flight. Technology has advanced considerably over the last century, and yet still jet and airplane wings are mostly immobile. Can we learn more from the flapping flight of birds? And do the same processes work at the small scale of small insects? How do bats fly so well with completely different wings? Here are some of the things engineers are learning from biologists who study animal flight.
Everyone’s favorite garden bird has revealed another trick among its unique flying capabilities: the aerial roll. A paper by Ravi et al. in Current Biology describes how “Modulation of Flight Muscle Recruitment and Wing Rotation Enables Hummingbirds to Mitigate Aerial Roll Perturbations.” To perform the roll maneuver — which involves a challenging continuous perturbation — the hummer must alter its wing tip trajectory (from a figure 8 to an oval), alter its tail fan position and use different muscles on its left and right wings. This must be an instinctive ability, because young birds were seen to master it on the first attempt.
The birds also augmented flight stabilization by adjusting body and tail posture to expose greater surface area to upwash than to the undesirable downwash. Our results provide insight into the remarkable capacity of hummingbirds to maintain flight control, as well as bio-inspiration for simple yet effective control strategies that could allow robotic fliers to contend with unfamiliar and challenging real-world aerial conditions. [Emphasis added.]
Barn owls get extra lift from their tails, reports Nature. A two-minute video shows how it works.
Scientists in London filled a flight tunnel with thousands of tiny soap bubbles to reveal the motions of air as the owl flies through them. As expected, vortices formed under the wing tips, providing lift. Unexpectedly, additional vortices formed under the tail tips, providing additional lift. This trick also reduced drag, the video shows. Small fixed-wing aircraft use their tails for stability, and would become unstable if this occurred, but the owl can adjust its body and tail quickly to compensate, so it gets maximum lift with minimum drag. “Engineers might be able to copy this trick for use in aircraft that actively stabilize themselves,” the narrator says.
Birds and Insects
Drones are all the rage for everyone from hobbyists to governments. Small drones face aerodynamic challenges that heavy craft do not. Brown University researchers came up with a rather surprising wing design: one that lacks the curved airfoil on the front of the wing. They got the idea from birds and insects.
The new wing replaces the smooth contour found on the leading edges of most airplane wings with a thick flat plate and a sharp leading edge. Counterintuitive as it may seem, it turns out that the design has distinct aerodynamic advantages at the scale of small drones. In a paper published in Science Robotics, the researchers show that the new wing is far more stable than standard wings in the face of sudden wind gusts and other types of turbulence, which often wreak havoc on small aircraft. The wing also provides an aerodynamically efficient flight that translates into better battery life and longer flight times.
Probing deeper, the scientists found that the boundary layer (a “thin layer of air that’s directly in contact with the wing”), which is full of vortices and turbulence on large planes, is laminar on flat wings. Turbulence helps the boundary layer stay attached to the wing in large planes. Laminar flow, however, easily detaches from a flat wing, creating instability. The biomimetic separated-wing design keeps turbulence at a consistent point, preventing detachment even when gusts of wind hit the craft. The result is “more consistent lift and overall better performance.” Somehow birds and insects had that all figured out.
In Scientific Reports, the open-access journal from Nature, Wainwright et al. explore the phenomenon of mass migration of small insects high in the atmosphere. In “Linking Small-Scale Flight Manoeuvers and Density Profiles to the Vertical Movement of Insects in the Nocturnal Stable Boundary,” they discuss the aerodynamic challenges these insects face during their night-time migrations. The tiny flyers are well-equipped for the challenge.
Movement in the nocturnal boundary layer (NBL) presents very different challenges for migrants compared to those prevailing in the daytime convective boundary layer, but we found that Lagrangian stochastic modelling is effective at predicting flight manoeuvers in both cases. A key feature for insect transport in the NBL is the frequent formation of a thin layer of fast-moving air — the low-level jet. Modelling suggests that insects can react rapidly to counteract vertical air movements and this mechanism explains how migrants are retained in the jet for long periods (e.g. overnight, and perhaps for several hours early in the morning).
Using lidar (a type of high-frequency “radar” using infrared beams instead of radio waves), the scientists were able to detect mass upward migrations of insects about an hour after sunset in Oklahoma. With lidar, they could see insect clouds in the air and follow them.
This Great Plains location is situated in the ‘Mississippi flyway’ where nocturnally-migrating insects ride warm southerly nocturnal low-level jet winds, easily covering distances of several hundred kilometres in a night’s flight.
The migrating insects include aphids, leafhoppers and moths. They fly upward en masse into an “insect layer” about a kilometer high in the night-time atmosphere where they are carried along by jet streams of air currents. But to get there, these small biological aircraft have to fly vertically without the aid of convection currents or updrafts. Light as they are, they are heavier than air and would fall without powered flight.
The average vertical air motion during this time is close to zero (upwards at ~0.03 m s−1 in the hour after sunset) which is about a tenth of the unaided ascent rates (~0.2 m s−1) of which small migrant insects are capable and our data shows a median insect ascent rate of 0.07 m s−1 during the main period of dusk ascent (Fig. 3) across the full two-month period investigated. This validates previous assumptions that small insects emigrating at dusk actively climb to altitude with minimal atmospheric assistance, in stark contrast to small insect migration in the well-mixed daytime convective boundary layer which relies on assistance from thermals.
Night-time migration is preferred because the boundary layer is not as high as it is in the daytime. As a result, masses of tiny insects swarm about an hour after sunset up into the sky into a freeway of air currents that carries them long distances all night long.
The phenomenon of an “insect layer” moving living creatures over long distances is analogous to the daily ascent and descent of plankton in the ocean, which contribute to mixing of ocean waters. Those tiny creatures, too, must swim with their tiny flippers on their own power for considerable distances. These migrations are not passive processes driven entirely by currents. They require life forms that actively participate in their travel. How do they know where and when to go?
Flying is not necessarily easy for the insects in the atmospheric freeways, either. They have to rapidly respond to turbulence, wind gusts and other complications that dynamically change the boundary layers. These surfers in the sky ride are ready. They ride the pipeline!
Modelling suggests that insects can react rapidly to counteract vertical air movements and this mechanism explains how migrants are retained in the jet for long periods (e.g. overnight, and perhaps for several hours early in the morning). This results in movements over much longer distances than are likely in convective conditions….
Is it not remarkable that such tiny flyers are capable of making use of natural air currents and boundary layers that form between day and night? None of this would be possible without sophisticated flight instrumentation and programming designed into each tiny creature.
An Inspired Heart
It must be difficult for a Darwinian to think about these natural masters of heavier-than-air flight, but evolutionary biologist Pamela Yeh loves her life’s work as a birdwatcher. Nature featured the young biologist’s research, showing her with binoculars watching a population of urban juncos (a type of sparrow) from her UCLA campus.
Most of my work is done here on the campus of UCLA, where I am in this picture. I also go with my students into the Californian mountains to see how juncos’ appearance and behaviour vary between their natural and their city habitats. There is something so joyful, so wondrous, about going into the on-campus ‘field’ to study birds — sometimes I feel I know a little secret about the natural world, right here. It makes my heart sing.
The only evolution in her research appears to involve slight differences between related species — i.e., microevolution. “It’s amazing that I can walk from one building to another and see our birds,” she adds. “It’s a reminder that nature isn’t something we go to — it’s where we are. For me, that’s very inspiring.”