Sky slitherers

November 12, 2011 at 7:10 pm

Say the word “glide” out loud, and you’re likely to think first and foremost of wings. Most gliding creatures have some kind of wing-like structure to aid in their battle against gravity, from the patagium of the flying squirrel to the wide, papery wings of the Javan cucumber seed.

Most, but not all.

Meet genus Chrysopelea, a group of five species of snakes that have developed the ability to glide, despite a lack of any kind of limb, appendage, or wing-like structure so slow their descent. Its secrets lie in the way it morphs and moves its body midair, producing aerodynamic forces that exceed the weight of the snake, giving it the spectacular ability to slither through the air.

Launch sequence from varying angles

Launch sequence from varying angles

The snake begins by hanging from a branch by its tail, making a J-like shape, from which it springs up and out. This springing gives the snake a bit more height from which to glide, as well as providing a little extra velocity boost, to help create lift. As it becomes airborne, the snake flattens its body from just behind its head to the vent by flaring out its ribs and contracting its stomach, creating a concave cross-sectional shape similar in many ways to an air foil. The snake then positions itself to have a high angle of attack, and begins to undulate laterally in an S-shape.

A- Normal body shape B- Flattened body shape

A- Normal body shape B- Flattened body shape. From J. Socha (2011). See References below.

Lift it caused by these combined factors: the increased surface area and air foil-like shape of the snake’s body when flattened, the high angle of attack the snake is able to take on, and the undulating of the body, which increases the velocity of the glide as the snake undulates with higher amplitudes. The best gliding species of these snakes, the paradise tree snake, is able to reach glide ratios as high as 4.2.

Pretty amazing  for a creature shaped more like a piece of rope than an airplane.

References:

By Solange | Posted in student post | Tagged , , | Comments (2)

Spinning and gliding seeds

November 10, 2011 at 3:34 am

A previous post by a student in last quarter's class briefly discusses the autorotation abilities of the maple samara, then talked about a monocopter based on the samara's design. The maple samara is certainly fascinating - but it is not the only seed with interesting aerodynamic properties. In fact, it is described as "crude" in relation to some other gliding and rotating seeds due to its inferior symmetrical airfoil (Alexander 51). I'd like to talk about the aerodynamics of some other types of gliding and rotating seeds.

A samara is merely a seed that develops a winglike shape attached to it. It is widely accepted that samara-producing plants evolved as an tradeoff: the resources required to produce a wing several times larger than the seed could be put to making more seeds, but seeds that get spread by the wind have a much higher chance of producing a viable plant. (Benkman). Many samaras are not capable of gliding or even efficient autorotation; however, simply being draggy is a huge adavantage. Distance traveled is equal to time times velocity, so for a given wind speed a slower-falling seed will spread further and thus be less likely to compete with its siblings for nutrients and water. (Benkman).

(All files are free-use images from Wikimedia Commons, available under attribution-sharealike licenses. Click on them for larger sizes, author information, and the full licenses.)

The non-gliding samara of the Mixed Bushwillow, Combretum zeyheri

Ash seeds are capable of an unusual type of autorotation. Instead of generating lift with a miniature wing, it is shaped as an elongated cylinder. When it falls, small imperfections on its surface cause it to start spinning, and due to the Magnus Effect that spinning generates lift. The Magnus Effect is a not-wholly-understood effect caused by a body spinning in a viscous fluid, where drag effects cause a force perpendicular to the spin axis. It's why curveballs curve, and why backspin causes a golf ball to carry further, and why ping-pong players can pull off ridiculous shots. In this case, it greatly impedes the fall of the ash seed, causing it to drift further (Alexander 53).

The Magnus Effect illustrated

Ash seeds (Fraxinus americana)

The champion of aerial samaras, though, is the Javan Cucumber, Alsomitra macrocarpa, a vine in the pumpkin family. It forms a wing up to 13cm (five inches) across. The glide ratio is as high as 4:1 and its drop speed just 0.3 to 0.7 meters per second (Azuma); thus, from a 30-meter tree, a seed can glide 120 meters (400 feet) and stay aloft for over a minute - and those numbers can easily increase by orders of magnitude with a strong wind, as like a glider the samara can ride the wind to gain altitude. It formed one of the earliest examples of biomimetics when in 1904 Austrian inventor Igo Etrich used it as the basis of a stable wing design, the ancestor of today's flying wing bomber designs. (Vincent).

A. macrocarpa samara


This is just a sampling of the great variety of aerial seeds. Many flowers like dandelions have their seeds drift on the wind. Thai plants related to poison ivy deploy multiple blades to spin to the ground. And tumbleweeds use the wind to uproot the whole plant to spread seeds! (Armstrong)

Sources

By David Sindel | Posted in student post | Tagged , , , | Comments (2)

The canopy ants: Prof Steve Yanoviak speaks in EK131

November 5, 2011 at 9:35 pm

Ant graphic 2 NYC 2006

Graphic adapted from the New York Times article on the gliding ants. See references below.

Canopy ants use "directed aerial descent" to return to their home tree trunk when they fall from a branch. They were the first known case of gliding in an insect with no wings— a completely unexpected discovery. They also glide intentionally backwards toward the tree, with their abdomen (gaster) leading and their head tailing, which is a behavior never recorded before.

The whole maneuver forms a J-shaped trajectory, consisting of:

  • a vertical drop (uncontrolled parachuting)
  • quick body alignment, with the abdomen pointing to the tree
  • a glide in a steep angle toward the tree trunk

Prof. Steve Yanoviak, at the University of Arkansas, who discovered the gliding behavior of ants, spoke to the class via Skype on Sept. 14, 2011.

A video of his guest appearance can be viewed, downloaded and sync'ed to your iPod visiting the course's iTunes U page.

This is an account of Prof. Yanoviak's Q&A session in "Bio-aerial Locomotion".

Answering the first question from a student, he told us that he was working in a project in Panama, spending a lot of time up on the trees. Up there looking down, he said, it's natural to think about how things fall. Between collecting data, he dropped some ants from the tree, and something didn't look quite right about the way they fell. For several years, this stayed in his mind, but he did not pursue the question. But about 5 years after that first observation, while in Peru in an unrelated project, he brushed off a bunch of ants from himself, and they all fell gliding back to the tree. That was the "wow" moment.

Canopy ant in gliding posture. Photo by Yonatan Munk.

Canopy ant in gliding posture. Photo by Yonatan Munk.

Another student wonders why this behavior could not be related to the evolution of flight in other insects. Prof Yanoviak notes that ants are secondarily wingless, and they are very derived as a species. To understand how wings evolve, we have to look at insects that pre-date the evolution of wings, you have to go to a very primitive group. This is fortunately now the case with the follow-on work with bristletails.

What about the shape of the legs, which are shaped like a paddle? It turns out this is not relevant, since many other ants can glide, but they have cylindrical legs. However, the paddle-shaped hind legs make these ants better gliders. But they are also useful for other reasons in their environment: it enables them to avoid predation, or being trampled on by monkeys, and so on.

Prof Steve Yanoviak demonstrates the gliding maneuver using his stuffed toy ant

Prof Steve Yanoviak demonstrates the gliding maneuver using his stuffed toy ant

How do the legs help the glide? Using a toy ant, Prof. Yanoviak demonstrates how the ants fall from the canopy: they come in to the tree trunk rear-end first, and hold their hind legs and middle legs to the sky, similar to a skydiver. This puts the moment arms up above the center of gravity, allowing them to control better.

Is he still working on this topic? Although the big, burning, question has been answered, there are about a dozen other smaller questions that are waiting to be answered, but he currently does not have the time or the funding to pursue them. But he's working on some new projects, like the ecology of how ants interact with vines in the forest—ants preferentially use vines to climb in the forest, they use vines like their preferred highways, and he'd like to understand why they do that. The forest canopy limits the way ants can move to very limited path routes to access food. Vines are very efficient routes, and they can be defended from other insects.

How does he stay safe on top of trees? He uses regular climbing gear, throwing the rope around a big branch with a slingshot and a lead weight, and then strapping himself with a climbing harness; he is always tied to the tree as he climbs. If the tree is healthy, he is safe.

How much control do the ants have while they glide? "It's amazing," he says, and continues,

"You know, it's one of these things that, when I sit in my office and I think about it and I write about it, it's almost like 'hm, I really don't want to exaggerate this, is this really happening, as much as I think it is?'. And then I get out into the forest and I drop and ant, and it's just phenomenal. They make split-second decisions about their trajectory. They are falling very fast, between 4 and 5 m/s; that's fast fast, and I've seen them fall and they'll see, somewhere along that descent, they'll see a shiny spot, and you can see the ant change direction and start heading to that shiny spot, and at the very last second they realize, I'm assuming, they realize this is not the correct target and they change direction, and they'll find a new target, and it's only happening in a matter of a couple of seconds. It's really phenomenal."

And how do they figure out where they fell from the tree?

"It turns out, it doesn't really matter which tree trunk they land on. The density of trees in the tropical forest is so much that [...] if they go to the wrong target, they go to a different tree, as long as they climb up, they are very likely to find the pheromone trail of their sisters in the same colony. So all they have to do is get themselves back up on the canopy, and they are basically home free. Everything is so well connected up there, and these particular ants, they have very large colony sizes and very large foraging region, so if you fall out of the tree, especially if it's your nest tree, and you climb back up, you're almost certain to find either one of your nest mates, or a foraging trail of one of your nest mates."

And a final student question: Out of all the things that you've studied and researched, what have you found most interesting out of your results?

"You know, the gliding ant stuff, it really trumps everything else, I think. Most scientists, especially most field biologists, go through their entire career without making any kind of interesting discovery—by interesting I mean 'interesting to the public'; everything I do is interesting to me, I love my job, but the things that excite me maybe wouldn't excite you guys on a daily basis—so, there's nothing more rewarding than being able to make a discovery that people read in The New York Times and are going to get excited about, or that it's going to appear in Highlights magazine or Ranger Rick. To me, as a scientist, when the gliding ant story was in Ranger Rick, I felt like I'd come full circle, as a human being, you know, as a profesional; because, I remember sitting in the dentist's office as a kid, reading Ranger Rick, and getting excited about the little snippets about biology in there and reading them over and over again sometimes, and when the ants got in there, I thought 'oh yeah, that's it, this is more exciting than any other publication I've ever had.' "

References

By Lorena Barba | Posted in lectures | Tagged , , , | Comments (2)

Welcome EK132 students

October 23, 2011 at 5:33 pm

Tomorrow starts a new EK131/132 cycle, with a renewed cohort of freshmen starting their exploration of "Bio-Aerial Locomotion". Welcome new students!

To start, you probably want to find out more about what this course is about. You'll find that in my Welcome to “Bio-aerial Locomotion” blog post. You'll notice that it says that you will be an author in this blog. In fact, your name is already listed in the Authors page.

During this course, you'll get the chance to author a minimum of two articles in this blog. They should be brief posts, with embedded graphics or media, on a topic of your choosing related to the course material. You are lucky in that the previous group of students has left a rich set of examples and inspiration! They have also left some suggestions for this course—including the idea that we should have a couple of graded quizzes in class, to encourage learning the material. You can thank them for this change!

Other guidelines for your blog assignments:

  1. The post should not veer off on an unrelated topic; it should make some link with the course content in terms of the biological manifestations of falling, parachuting, gliding and flying.
  2. The BU academic conduct code should be followed. Observation of plagiarism rules particularly apply. This means that direct quotes from any source must be properly attributed and marked with quotes (and be pertinent to a point).
  3. The blog is of course public. So don't plagiarize, as your honor is on the line!

Note that all lectures from the previous running of this course are available to watch on iTunes U. We will add to this media collection when we cover some new material.

Again, welcome! and I hope you like this course.

By Lorena Barba | Posted in lectures | Tagged | Comments Off on Welcome EK132 students

Solving the Mystery of the V

October 22, 2011 at 12:41 am

When you look up in the sky you just may happen to see a group of birds flying  in a V formation. But have you ever wondered why they do this? Its not because the "Mighty Ducks" told them to stick together or because it is just a random coincidence, it actually has a purpose.

flyinh v

The actual reason why birds fly in a V-shape is for two reasons, the formation helps reduce drag so that the birds experience less resistance than they would if they were to fly alone and also the formation helps to let the birds communicate more easily.

The reason why the birds experience less drag is because of the wingtip vortices that are created. These vortices are generally undesirable because they create a downwash that increase the induced drag on a wing in flight. However, this downwash is also accompanied by an upwash that can be beneficial to a second wing flying behind and slightly above the first. The birds that fly behind other birds experience a decrease in drag and that makes the long flight that they go through much easier.

trailing-vortex

The birds that fly behind experience a upwash and gain free lift and they can fly at a lower angle of attack. When the angle of attack is decreased the drag is also lowered and the bird does not need to flap its wings as hard and as often in order to maintain flight. Researchers discovered that as pelicans fly the heart rate is much lower when flying in  a V formation compared to flying alone. Another study has shown that a flock of 25 birds can fly 70% further than a solo bird using that same amount of energy.

drag

Not all the birds benefit however. The lead bird does not experience reduced drag because it is flying into undisturbed air. Also,The presence of the two birds flanking the leader helps to dissipate the downwash off the lead bird's wingtips and reduces the induced drag this bird experiences. These two flanking birds also benefit from a similar reduction in drag if outboard birds flank them as well. In other words, the birds in the middle of each of the lines forming the V are in the best position. The lead bird does not always have to deal with the burden of harder flight because when the bird gets tired it will drop back to a middle position which is the easiest and another bird will take the lead position. The birds that fly in the farthest back positions will also switch to a middle position in order to have a less difficult time flying.

So that is the reason why birds fly in a V position. So next time you see a flock of geese or duck flying above you in the sky, know that they are not only trying to look cool, but they are also using the forces of aerodynamics in order to reduce the effort that they have to use. Nature is much more complex than we even know.

Source:

http://www.aerospaceweb.org/question/nature/q0237.shtml

By David DeCaprio | Posted in student post | Tagged , , | Comments (2)