Flying in Circles

October 21, 2011 at 7:36 pm

Thousands of years ago, early hunters discovered that a single bend in a piece of wood gave it some interesting aerodynamics properties. Tossed vertically, a boomerang will fly in a loop, returning to its thrower. How is such a seemingly simple object able to move through the air so remarkably? When we examine the boomerang closer, we find that its design is unintentionally rather complex, producing a number of forces working simultaneously.

Firstly, The two arms of the boomerang essentially act like wings, both having a thin side and a rounded side, forming airfoils. Since it will be spinning around its central point counterclockwise though, and the airfoils are facing different directions, the leading edges are on opposite sides of the boomerang.

Unlike how wings traditionally cut through the air, a boomerang is thrown vertically. To make it easier to visualize its movement, one may want to think of it more like a propeller. This “propeller” creates the same horizontal lift that an ordinary one does.

But a propeller is attached to a plane. Its blades are moving perpendicular to the direction of the aircraft. The boomerang, on the other hand, is moving in the direction that it is spinning.

Thrown correctly, it is spinning counterclockwise. What this means is that, at any moment, the arm that is on top is spinning in the direction that the entire boomerang is traveling, while the arm on the bottom is spinning in the opposite direction. Thus, the top arm is moving relatively faster, creating more lift than the bottom one.

Since there is always greater lift being produced at the top of the boomerang, it seems that this should push it over (like if one were to apply force to a stationary wheel), so that it would be parallel to the ground. Why is this not the case?

Simply, it is an example of the gyroscope effect. Put clearly by Tom Harris, “When you push a spinning wheel, for example, the wheel reacts to the force as if you pushed it at a point 90 degrees off from where you actually pushed it”. The force was applied to a single point on the boomerang, so as the arms spin, so does the point that was pushed. Consequently, the boomerang turns to the left instead of falling over. It begins to fly in a circular motion, eventually returning to its thrower.

To fully understand the physics of a boomerang, I would highly recommend watching the animations on the HowStuffWorks website that I posted, since I unfortunately could not embed them.

Sources:

HowStuffWorks: Boomerangs

GSU Hyperphysics: Boomerangs

By Luke Loreti | Posted in student post | Tagged | Comments (3)

Hummingbird’s Wake

October 21, 2011 at 7:33 pm

One of my favorite things about spring and summer is lying in an open field watching the clouds go by. Every once and a while a plane will go by leaving an enormous wake of what looks like white clouds behind it. Only recently did I learn that these cloud trails are caused by vortexes of wind as it goes over the wings of the plane and creates lift. All things that fly have these vortexes in their wake, and depending on the flight pattern of the subject flying, the wake will have different vortexes. Because the hummingbird flies so different, they have a very unique type of wake vortexes.

Warrick, Tobalske, and Powers sampled the wake produced by rufous hummingbirds using something called the DPIV methodology. This technique involves putting a hummingbird into an idle wind tunnel with a  a feeder ( a 1-ml syringe containing a 20% sucrose solution). The bird has been trained to fly to the feeder and hover while it drinks the 'nectar'. This feeder has been positioned so that the bird will cross a light sheet of oil particles. The bird flaps its wings to hover in place, and the oil-particles are moved by the bird's wings and airflow. By using a laser, they illuminate the oil particals so they can be seen. They then can record the movement of the oil particles by using high-speed cameras, taking two photos about two hundred nanoseconds apart. By examining where the oil particles move, we can learn the structure of the wake that the hummingbird creates.

Figure 1 (Ref.1): DPIV methodology (Copyright, Nature Publishing Group, 2005)

The test was done by placing the laser light sheet at different angles so that they could observe different portions of the wake. Frontal-plane samples of the wake (1a, 1b) showed that vortexes were created at the tip of the wing. Parasagittal planes (1c, 1d) showed that the structure of the starting and stopping vortices of the downstroke, and the starting vorticies of the upstroke, and any leading-edge vorticity (LEV) shed at the end of the halfstroke. As the wing transitions from the downstroke to the upstroke, vortices are released spinning back towards the body.

Figure 2 (Ref. 1): Flow field vorticity (Copyright, Nature Publishing Group, 2005)

The DVIP method has shown to be a very effictive strategy of finding how vortexes form in the wake of a hummingbird's wing flapping. We can study different angles of the bird flying at to see how the vortexes are formed while it flies.

Works Consulted:

  1. Aerodynamics of hovering hummingbird. Warrick, Tobalske and Powers, Nature, Vol. 435, 1094-1097 (2005)
  2. Bird Flight II, class notes.

By Grace Ingalls | Posted in student post | Tagged , , , | Comments (1)

It’s a bird. No it’s an insect. No, wait, its a hummingbird!

October 21, 2011 at 6:24 pm

When we are young, we classify everything into very black and white catagories. What is a bird? Orginally, our response was a tiny flying animal with feathers and wings. However, that description is astronomically incorrect. Birds do not have to be small. The largest flying bird, the Argentavis, (extinct as of 2009) has a wingspan of 24 feet. Birds don't even need to fly. Take the penguin for example. Forever flightless but still called a bird. The hummingbird may seem like it fits a child's idea of 'bird', but it's actually more different from a bird than both the Argentavis and the penguin. The hummingbird resembles something between an insect and a bird.

Wing Shapes of various birds

The hummingbird has the amazing ability to hover in one place for up to 50 min, fly backwards, and upside-down. It can do these things because its body structure differs from other birds. One of the most important features is its wing structure. Most birds' wings have a bend in them so they can reduce the wing's surface area or fit through small spaces. The hummingbird, however, has wings that do not bend or fold in the middle but are straight out from the body. They, unlike other birds, use their upstroke to propel themselves forward with almost as much force as the down stroke. These differences are what make the hummingbird such a fast and agile flyer, like insects.

Flight pattern of Insect

Although the hummingbird's flight is similar to an insect's flight, it's not quite the same. When most birds fly, they create 100% of the lift on the downstroke and use the upstroke to 'recover'. Insects create lift evenly on the upstroke and downstroke, with a 50/50 ratio. The hummingbird creates 75% of the lift on the downstroke and 25% on the upstroke. Not quite like an insect, but also not like a bird. The wing pattern of the hummingbird is more closely related to insects than other birds. Hummingbird's flight pattern is almost horizontal and moves in a figure eight, just like an insect. Most birds flap their wings close to vertically in ovals, which makes it harder for them to hover like hummingbirds and insects do.

The hummingbird. It's not an insect, that's obvious. It flies like one and flaps like one, but it's not a bug. It's a bird, but because of its wing structure, flight pattern, and flapping pattern, it's like no other bird. Children have no idea just how strange a bird a hummingbird is.

Works Cited:

Altshuler and Dudley, The Ecological and Evolutionary Interface of Hummingbird Flight Physiology.

Hummingbird Flight

By Grace Ingalls | Posted in student post | Tagged , | Comments (1)

The Physics of Paper Planes

October 21, 2011 at 5:31 pm

We’ve seen many different gliders from the animal world throughout this course. But as children, our first experiences with gliding probably came from paper planes. As a kid, I remember learning lots of different designs for planes, in an effort to make them fly farther and stay in the air longer.

But there are actually people who study paper airplanes and compete for the world record for longest time in the air. In 1998, Ken Blackburn set a world record for a paper plane flight that lasted 27.6 seconds! In order to design the plane that did this, Blackburn studied the physics of real airplanes.

One of the keys to reducing drag on the paper plane is to have thin wings. This has to do with a paper plane’s Reynolds Number, which indicates the significance of the viscosity of the fluid (air) on flight. Paper planes have a very low Reynolds Number, which means the air’s viscosity has a much larger effect on a paper planes than on airplanes. With thick wings, the boundary layer of air tends to separate from the wing, resulting in large amounts of drag. So as far as reducing drag goes, the thinner the better. We see this in nature as well. Insects, which have low Reynolds numbers, have thin, flat wings (like flies or butterflies). Meanwhile birds, which have higher Reynolds numbers, have thicker, curved wings.

Another design key is to have dihedral wings, or wings that are angled upwards (pictured below). This is a technique many airplanes use to improve stability. During flight, disturbances may cause the plane to roll in one direction. This results in different angles of attack for the two different wings. With dihedral wings, the lower wing will have a greater angle of attack. Because lift is proportional to angle of attack, there will be more lift on the lower wing, so the plane will roll in the opposite direction, returning to level flight. This stops paper planes from spiraling out of control and quickly falling to the ground.

Dihedral wings

Dihedral wings

One more of Blackburn’s techniques is used to slow the plane horizontally, lengthening the flight. He calls it an “up elevator” on the end of the wings (shown below). He folds the back end of the wings upwards, causing the back end of the plane to pitch slightly downwards, and the nose to pitch upwards. With more of the bottom surface area of the plane exposed to the oncoming air, there is more horizontal drag. Incidentally, the larger angle of attack would lead to more lift as well. Airplanes also use this technique. Pilots are able to control the back edge of the tail wing to change the speed.

papere1c17e

Putting all these techniques together helped put Ken Blackburn in the record books. Maybe you can also use these the next time you have some spare paper lying around.

References

By Hersh Bendre | Posted in student post | Tagged , | Comments (1)

Fly Like a… Bar-Headed Goose?

October 21, 2011 at 6:38 am

Have you ever seen a movie where birds are flying directly next to an airplane? Probably, but obviously in the real world, it could never happen. However, there are some birds that do fly above those nimbus clouds.

The bar-headed goose. Magnificent and graceful, these geese migrate over Mount Everest every winter.

Mount Everest capped off at 29,028 feet has a very low density of air. Since the air is so thin, it is extremely difficult for birds to "flap". The bar-headed geese in fact have to flap harder and more vigorously to make sure that they stay afloat and balanced.

Wide Wingspan

Surprisingly, these geese manage to fly thousands of miles in a day, if they are lucky. Jet streams blow directly over the highest mountain on this planet at over 200 mph. Birds that fall into this jet stream are usually ripped apart and die from lack of oxygen or are blown off course by cross winds.

What makes the bar-headed goose so special?

Actually, they are special. Bar-headed geese have an incredibly large wingspan, which helps them "catch" the air at high altitudes. Bar-headed geese also have a special type of hemoglobin that allows them to absorb oxygen at alarming rates while they are at alarming altitudes. This combined with their extreme flapping vigor allows them to travel extremely far distances with the wind pushign them along their back.

The bar-headed geese is the highest flying bird and most likely the highest flying creature on the planet.

They fly so high, they make Far East Movement look bad.

Sources
1. The High Life

2. Bar-Headed Goose

By Chris Hui | Posted in student post | Tagged , , , | Comments (1)