A sweltering day in India brought out the local giant fruit bats (also called Indian flying foxes) to keep cool in the river. Normally nocturnal, they made a rare daytime appearance to beat the heat. Wildlife photographer Hardik Shelat was lucky enough to catch these awesome images of the bats in flight. True to their name, the animals have wingspans ranging from 1.2 to 1.5 meters, which should give them some impressive lift, even when gliding down near the water. (Image credit: H. Shelat; via Colossal)
Tag: bats
Portraits of Flight
During lockdown, photographer Doris Mitsch turned her eyes to the sky and began capturing these mesmerizing composite images of animals in flight. Vultures, crows, starlings, gulls, and bats all feature in her series. Some images, like “Lockdown Vulture (Signature)”, feature a single bird’s movement over a minute. Others show entire flocks over extended periods.
I love how the images capture a sense of speed. Given equal timing between images, the lines with more space between each snapshot of a bird indicate a faster speed. It’s a bit like having particle image velociometry frames stacked atop one another! (Image credit: D. Mitsch; via Colossal)
Bats in Ground Effect
As pilots can tell you, flying near the ground (or an open expanse of water) gives one an aerodynamic boost. Essentially, the surface acts like a mirror, reflecting and dissipating the wingtip vortices that create downwash. That reduces the power necessary to fly, as long as you’re flying within about a wingspan of the surface.
Theoretically, flapping fliers like bats and birds should also benefit from this ground effect, but measurements have been hard to come by. A new study using bats trained to fly in a wind tunnel provides some of the first detailed measurements of ground effect for flapping animals. The researchers found a 29% reduction in the power necessary for flight when in ground effect compared to being out of it! That’s twice the savings predicted by modeling, meaning we still have a ways to go to accurately capture the physics of flapping flight under these circumstances.
Such a substantial savings also strengthens arguments for flight developing from the ground up. Using ground effect, surface-dwelling animals could have evolved flight gradually, taking advantage of the energy savings offered by sticking close to the surface. (Image and research credit: L. Johansson et al.; submitted by Marc A.)
Hovering
Nectar-drinking species of hummingbirds and bats are both excellent at hovering – one of the toughest aerodynamic feats – but they each have their own ways of doing it. Hummingbirds (bottom) use a nearly horizontal stroke pattern that’s quite symmetric on both the up- and downstroke. To keep generating lift in the upstroke, they twist their wings strongly midway through the stroke. So although hummingbirds get most of their lift from the downstroke, they get quite a bit from the upstroke as well.
Bats, on the other hand, use an asymmetric wingbeat pattern when hovering. Bats flap in a diagonal stroke pattern, using a high angle of attack in the downstroke and an even higher one during the upstroke. They also retract their wings partially during the upstroke. This flapping pattern gives them weak lift during the upstroke, which they compensate for with a stronger downstroke. Compared to non-hovering bat species, nectar-drinking bats do get more lift during the upstroke, but they’re nowhere near as good as the hummingbirds. The bats compensate by having much larger wings compared to their body size. Bigger wings mean more lift.
In the end, the two types of hovering cost roughly the same amount of power per gram of body weight. That’s great news for engineers designing the next generation of flapping robots because it suggests two very different, but equally power-efficient methods for hovering. (Image credit: Lentink Lab/Science News, source; research credit: R. Ingersoll et al.; via Science News; submitted by Kam Yung-Soh)
Flying on Flexible Wings
Bats are incredible and rather unique among today’s fliers. Like birds, they flap to produce their lift and thrust, but where birds have relatively stiff wings, a bat’s wings are flexible. The thin webbing of skin stretched between the bat’s finger joints has muscles inside it that fire as the mammal flaps. This means that the bat may actively control just how stiff its wing is as it flies.
Compared to other natural and manmade fliers, the bat is incredibly agile and stable, able to recover from wind gusts in less than a full wingbeat cycle. They also have some incredible acrobatic capabilities. When preparing to perch, a bat loses almost all of its aerodynamic lift but still manages to maneuver itself so it flips over and grabs hold. Check out the full video above to learn more about these fascinating animals. (Video and image credit: Science Friday; research credit: S. Swartz and K. Breuer)
Editor’s Note: I’ll be travelling through the end of the month with limited email access. The blog should continue posting uninterrupted, but if you contact me, just know it may be awhile before I can get back to you. Thanks! – Nicole
Hairy Tongues Help Bats Drink
Nectar-drinking bats, honey possums, and honeybees all use hair-like protrusions on their tongues to help them drink. In bats, these papillae have blood vessels that swell when drinking, stiffening the hairs. To investigate this drinking mechanism, researchers built their own version of a bat tongue by fabricating hairy surfaces and testing how well they trapped viscous oil when dipped and withdrawn. Through a combination of experiment and mathematical modeling, the researchers found that the optimal fluid uptake depended on the density of hairs, fluid viscosity, and the withdrawal speed. When they compared their results to actual bats, honey possums, and honeybees, they found that those animals’ tongues have hair densities very close to the predicted optimal value, suggesting that their model is capturing the important physical mechanisms that have driven evolutionary advantages for these species. (Image and research credit: A. Nasto et al.; submitted by Kam-Yung Soh)
Flying with Large Ears
Evolution often requires compromise between competing effects. Large-eared bats, for example, rely on the size of their ears to aid their echolocation, but such large ears can hurt them aerodynamically, thus limiting their flight. Results from a recent experiment, however, suggest that large ears are not a total loss aerodynamically speaking. Researchers used particle image velocimetry to study the wakes behind free-flying, large-eared bats and found significant downward flow behind the bats’ bodies. This indicates that the bats generate some lift with their ears, body, and/or tail. The position and tilt of the ears in flight is similar to forward swept wings, which the authors suggest could help contract the wake behind the ears and reduce its negative influence on flow over the wings. Although the evidence is not yet conclusive, the study does suggest that large ears may be more aerodynamically beneficial than they appear. (Image credit: L. Johansson et al./Lund University, source; via Jalopnik)
The next FYFD webcast will be this Saturday, May 21st at 1pm EDT. My guests will be Professor Jean Hertzberg of the University of Colorado at Boulder and Professor Kate Goodman of the University of Colorado at Denver. Dr. Hertzberg is the creator of the course Flow Visualization, an interdisciplinary course combining engineering, art, and fluid dynamics. It’s a class (and website) that’s been an inspiration for me and FYFD since the early days! Dr. Goodman, an expert in engineering education, earned her PhD studying the Flow Viz course and its impact. This will be wide-ranging discussion – with everything from experimental fluid dynamics and engineering education to art, photography, and hopefully even cardiac fluid dynamics!
(Original images: P. Davis et al.; B. Moore; L. Swift et al.)
Nectar-Eating Bats
Nectar-eating bats have evolved to use several methods to drink. Some bats, like the Pallas’ long-tongued bat (top), use a lapping method. Hair-like papillae on the bat’s tongue increase the contact area with the nectar, helping to draw the fluid up in viscous globs as the bat repeatedly dips its tongue into the nectar. The orange nectar bat (middle and bottom), in contrast, has a tongue with a long central groove. This bat’s tongue stays submerged as it drinks. Researchers hypothesize that muscle action along the tongue, combined with capillary action in the narrow groove, allow the bat to actively pump nectar up to its mouth. It’s worth noting that the edges of the bat’s tongue do not curl around to touch, so the bat is definitely not using suction as one would with a straw. (Image credit: M. Tschapka et al., source)
Fine-Tuning Flight
We humans generally use fixed wings for flight, but in nature, flapping flight dominates. As an animal flaps, it extends or draws in its wings during key points of the cycle in order to change its aerodynamics. But this control can be more than just a matter of stretching their wings. Recent work on bats shows that they can fine-tune the stiffness of their wings’ membrane using tiny, hair-thin muscles. Each muscle is too slight to change a wing’s shape on its own, but by firing synchronously–tensing on the downstroke and relaxing on the upstroke–the bat can manipulate its membrane stiffness and thereby affect its wing shape. Moreover, the timing of the muscles’ action changes with flight speed, suggesting that the bats are actively controlling their aerodynamics during flight. (Video credit: Swartz-Breuer lab/Brown University; via Futurity; submitted by Boris M)
Flapping to Fly Efficiently
High-speed video shows that bats achieve some of their efficiency in flight by pulling their wings inward on the upstroke, as seen above. While this does affect drag forces on the wing slightly, the primary energy savings comes from the inertial ease of lifting the folded wing. Much the way it is easier to lift your arm when it is folded than when you stretch it outright, it takes less energy for the bat to lift a folded wing than one that is fully extended. (via Wired Science)
