Chrysopelea- The “Flying” Snake

September 22, 2011 at 4:10 pm

Certain species of the “flying” snake are able to glide as far as 330 feet in the air. What makes them so aerodynamic and gives them such controlled flight has been interesting to watch and inspiring to the area of biomimetics.

One of the most important factors of  the snake’s ability to glide is its ability to flatten its body by sucking in its stomach and ribs. This causes the snake’s body to flatten and mimic a concave wing that traps air.

Tara_Dalton_figure-sm

An analysis of the snake’s gliding behavior reveals how it utilizes this flattened body. After launching itself off a tree, the snake thrashes so that it can reduce its speed from 6 m/s to a terminal velocity of 4 m/s . It also points its head upwards and its tail downwards to create an angle of attack from 18 degrees to 32 degrees. The size of the snake matters too. Researchers from the University of Chicago discovered that body length and wave amplitude of the undulations were important in determining flight behavior.  Small snakes were better glide longer distances than larger ones. These behaviors represent factors that explain how the snake creates a lift force greater than its weight as proven by:

L = (1/2) CL ρ V2 S

where CL=lift coefficient, ρ= air density, V=velocity, and S=surface area

Now that we understand how the snake can generate lift, we can see how the flying snake is able to control its gliding. In order to steer, the snake uses lateral undulation (the S-shaped movement of snakes). This motion also generates more lift as it changes the air pressure above the snake, thereby creating an upward force.

At the end of the video we see that the snake is able to change direction completely while it is in midair. With the information from above, it seems pretty reasonable that the snake can do that now. It just so happens that the snake thrashes to manipulate its angle of attack and surface area to slow down. Then it undulates to create its own upward force and whips its tail for propulsion.

References:

Chrysopelea. Wikipedia. <http://en.wikipedia.org/wiki/Chrysopelea>

Flying ophidians! Physicists uncover how snakes soar between trees. Scientific American. <http://blogs.scientificamerican.com/observations/2010/11/24/flying-ophidians-physicists-uncover-how-snakes-soar-between-trees-video/>

How Snakes Can “Fly”. National Geographic. <http://news.nationalgeographic.com/news/2010/11/101124-flying-snakes-fly-science-darpa-dod-socha/>

University of Chicago researchers reveal secrets of snake flight. University of Chicago. <http://www.uchospitals.edu/news/2005/20050513-snake.html>

By Kevin Ma | Posted in student post | Tagged , | Comments (4)

Wallace’s Flying Frog

September 22, 2011 at 1:18 pm

It's a bird, it's a plane, no! It's Wallace's Flying Frog! Located in the tropical jungles of Malaysia and Borneo, one of the FEW aerial amphibians on this planet is the Wallace's Flying Frog. Sizing in at about 4 inches (about the size of a tea-cup), these thrifty and quick frogs annoy and pester their predators. Named after the British naturalist Alfred Russel Wallace (the man who first studied and described these species), this type of frog is known for parachuting almost 50 feet away from its location. Many reasons contribute to why they "fly". They may be in danger of a predator, looking for prey, trying to mate, or even attempting to lay eggs. Whatever the reason, this amphibian is amongst the largest to do so.

After understanding why they "fly", I was curious to learn how. I had to know the science and physics of its ever-so-unique defense and mating mechanism. When they leap, they spread out their  four-webbed feet and catch the air around them. This, coupled with their incredible loose skin flaps, helps them glide, or parachute, through the air effortlessly and smoothly. Their over-sized toe pads help them land smoothly, whether it be on a close tree branch or even the ground. Their flight is quite remarkable, and makes it easy for them to attack their pray (which is mainly insects).

If your ever in the tropical jungles of Malaysia or Borneo, look for the frog's distinct bright green color tagged with a dark black foot webbing. This webbing discerns them from their aerial neighbors.

Wallaces Flying Frog

References

Laman, Tim. "Wallace's Flying Frog." Nationalgeographic.com. National Geographic. Web. 22 Sept. 2011. <http://animals.nationalgeographic.com/animals/amphibians/wallaces-flying-frog/>.

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By Gabriel Beru | Posted in student post | Tagged , | Comments (4)

Falling: “The tale of the gecko”

September 12, 2011 at 8:42 pm

The gecko has the fastest air-righting response ever measured.

The gecko has the fastest air-righting response ever measured.

When falling, geckos are able to right themselves turning their body in mid-air, and always land safely on their feet. It is fascinating to watch the slow-motion videos of the lizard dropping from a belly-up position, then using a swing of the tail to turn around into a skydiving posture. Even more fascinating is to understand the simple physical principle that explains their maneuver: conservation of angular momentum.

The research group of Prof Robert Full at UC Berkeley has conducted exciting work explaining how the lizards use their tail for their mid-air righting prowess—the fastest righting reflex ever measured (about 1/10 of a second). Below is a short video which summarizes the findings.

The results of a first set of experiments, performed by PhD student Ardian Jusufi, were published in the Proceedings of the National Academy of Sciences¹. Among other observations, they describe the air-righting reaction in detail, as follows:

  1. as soon as it falls (belly-up), the gecko spreads its legs wide
  2. it then pitches its tail so that it points downward, perpendicular to its body
  3. the tail now swings around the longitudinal axis of the gecko, which produces a counter-rotation of the body due to conservation of angular momentum
  4. the body executes a full turn, to end right-side up, and the tail stops turning
  5. the gecko keeps falling, arms extended like a skydiver

The scientists then built an analytical model of the righting mechanics. The angular momentum of a body is the product of its moment of inertia and its angular velocity:

angularmomentum

At the start of the fall, the gecko has zero angular momentum (there are no external torques acting on it). If one considers the body segment separate from the tail, then the sum of the angular momentum of each piece must continue to be zero:

angmomentumconservation

A simple geometrical model can be used: the body as a rigid elliptical slab, and the tail as a thin cone rotating perpendicular to the body. With biometric data, a realistic estimate of the moments of inertiaof body and tail are entered into the equations, and then a change in the angle of the body is obtained from a change in the angle of the tail:

angular-ratio

This simple model correctly predicts the reorientation of the body of the gecko! Now, there is enough information to build a device that can emulate the behavior of the gecko: the RightingBot².

References

¹ "Active tails enhance arboreal acrobatics in geckos", A Jusufi, D Goldman, S Revzen, R Full, PNAS Vol. 105, pp. 421-–4219 (2008) [pdf]

² "Righting and turning in mid-air using appendage inertia: reptile tails, analytical models and bio-inspired robots", A Jusufi, D T Kawano, T Libby, R J Full, Bioinspiration & Biomimetics, Vol. 5, pp. 1–12 (2010) [doi]

Links

By Lorena Barba | Posted in lectures | Tagged , , , , | Comments (13)

Welcome to “Bio-aerial Locomotion”

September 6, 2011 at 1:37 pm

This is the blog of the engineering freshman seminar course titled "Bio-Aerial Locomotion", taught by Prof. Lorena Barba at Boston University. This course is one of several options of the Introduction to Engineering series (EK 131/132) at the College of Engineering.

The course aims to motivate the subject of bio-inspired engineering, characterized by seeking examples in the biological world of the desired function in the engineered creation. In particular, we seek examples of aerial locomotion in the increasingly sophisticated forms of: falling, parachuting, gliding and flying.

Why "bio-aerial locomotion"?

"Aerial locomotion" is the term most often used to refer to the self-generated movements of any animal through the air. I've added the prefix "bio-" simply to be more explicit that we are focusing on the solutions that Nature has found to maneuver in the air medium.

flying creatures

In Nature, flight has evolved four times: insects, pterosaurs, birds and bats. (Images from various online sources.)

Human-designed flying devices are barely over 100 years old, which is short in historical terms, but minuscule in evolutionary terms. In Nature, flight has evolved very efficiently (much more efficiently than in engineered flight) at least four times: in insects, pterosaurs, birds and bats.

Fossil records allow a detailed study of the evolution of flight. We know that many and diverse adaptations have allowed organisms to overcome weaknesses or take advantage of good physiological traits. For example: size matters¹. A very small flying animal will need to flap its wings a lot faster than a larger one. You'll think immediately of the hummingbird. Also, the resistance of moving through the air becomes ever more difficult for tiny organisms. So there is in fact a minimum effective size for flight. On the other end, large animals will have difficulty creating enough power to fly, and the largest bird ever to have flown (Argentavis magnificens) was about 70 kg in mass. Recent research has determined that it could not have taken off without either jumping from a high point or running downhill.²

But in addition to powered flight, many biological organisms have evolved forms of passive flight and ways to maneuver in the air. When falling, geckos are able to right themselves turning their body in mid-air, and always land safely on their feet. Some species of snakes can glide to the ground while slithering their body to adjust their shape; and samaras (winged seeds like the maple seed) slow their descent as they spin, so the wind will take them farther aiding dispersal of the tree species.

In this course, we will discuss a selection of interesting cases in Nature of: falling, parachuting, gliding, flying & soaring. Parachuting and gliding can be thought of as flying without any power; the only difference between the two is the angle of the flight path. By "parachuting", we mean falling with the aid of an aerodynamic braking effect, reducing the rate of descent. To be precise in the distinction with "gliding", parachuting is defined as having a trajectory of more than 45° from the horizontal. Flying squirrels, for example, really glide: they can achieve trajectories at an angle of about 27º from the horizontal (a 2:1 glide ratio).

The various examples from Nature on staying aloft and maneuvering through the air are inspiring engineers today to design new devices, such as micro-air vehicles and robots. In this course, we'll glimpse at the modern activity of bio-inspired engineering in fields like aeronautics and robotics.

References

¹ "The nature of flight. The molecules and mechanics of flight in animals", Philip Hunter, EMBO reports (2007), Vol. 8: 811–813.

² "The aerodynamics of Argentavis, the world's largest flying bird from the Miocene of Argentina", S Chatterjee, RJ templin, KE Campbell Jr., PNAS (2005), Vol. 104: 12398–12403.

By Lorena Barba | Posted in lectures | Tagged , , , , , , , , , | Comments Off on Welcome to “Bio-aerial Locomotion”