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Begin the lesson by asking students how they think wings help birds fly. Then instruct each of them to take a piece of paper and roll it into a ball. Let it go and watch how it falls. Then take the same paper and smooth it out. Let it go and watch how it falls.
Ask the following questions: Based on your observations, does the weight
of the paper have any effect on whether the paper falls or floats? How
about surface area? Explain that the greater the amount of air hitting
the bottom of the paper, the more "lift" the air can give it. The wings
of birds are similar to those of gliders. Not only do they have a large
surface area, but they also have a special shape that helps keep them afloat.
Flying a sailplane is probably the closest thing any human will come to feeling like a bird. Powered only by gravity and air currents, these gliders move silently through the sky, often for hours at a time. Because they have no engines, gliders or sailplanes can be thought of as pure flying vehicles, staying aloft by balancing the forces of gravity, lift, drag, and thrust.
As you might suspect, if you want to stay airborne for a long time, the most important force to conquer is gravity. Lift, the force that directly opposes gravity, comes from the force of the air on the underside of the wing. In wings, lift is controlled by three factors: surface area, shape, and angle of attack.
To see how surface area works, roll a piece of paper into a ball. Drop it and the paper falls. Spread the paper out and drop it, and it will float. The greater the surface area, the greater the amount of air pushing up on the wing. The shape of the wing works because of something called Bernoulli's principle. Most wings are curved on the top and flat on the bottom. As the wing pushes through the air, the air on top of the wing must move a little faster than the air on the bottom. This creates slightly lower pressure on the top, which allows the greater air pressure beneath the wing to push the plane up.
The angle of attack is the orientation of the wing as it faces into the wind. Increasing the angle of attack means increasing the amount of air hitting directly on the bottom, which gives the wing more lift. Of course, if you make the angle of attack too big, the wing will blow backwards, and the plane will come crashing down!
In a sense, a sailplane is very similar to a roller coaster. Both are
towed up high and released. They begin to fall and the force of gravity
gets them going. Unlike a roller coaster, which continuously loses height,
a sailplane can also gain elevation by riding rising currents of air. Known
as thermals, these localized updrafts are caused by air being heated by
the warm ground below. When the sun shines down on a sandy beach, for example,
the sand heats up faster than the water. As the air in contact with the
sand begins to heat up, it expands and rises. This differential heating
is what causes thermals and when a glider hits one, it can fly for hours
at a time.
1. How is the flight of a bird similar to the flight of a sailplane?
2. How do birds get their thrust and how do they control their
direction of flight?
THE RIGHT FLIGHT
GLIDERS: Student Activity
Design a glider to achieve maximum glider efficiency.
If you could build your own sailplane, what would it look like?
Since sailplanes are pure flying vehicles, they have no engines to power
them. Instead, they depend on their wing structure and stability to maintain
lift. See what kind of aeronautical engineer you are by designing and building
your own glider. Try to set your own personal best for maximum flight time
with your plane.
2. Have each team member take a turn flying the plane and record all the flight times. Gently throw each plane from the same place. (It's essential to launch each trial the same way.) Compare your flight times with those of the other groups and discuss how the size and the shape of the wings may have affected the flight.
3. After you have evaluated the performance of your plane, try
modifying the design to maximize your time aloft. Test your plane again
to see if you improved on your flight time.
Extend the activity
How does your plane behave under different atmospheric conditions? Once you have perfected your glider, see how it will work when the air is in motion. Try flying it over a fan or maybe even a hot plate. Can your plane take advantage of thermal updrafts? Test it out and see.
1. What were some of the common features of the planes with the longest flight times?
2. How did the size and shape of the wings affect the way the planes flew?
3. What other materials besides paper might you use in constructing your plane to get an even longer flight time?
4. Gliders are often towed by airplanes and released at a relatively high horizontal speed. How could you perform this experiment to measure the effects of thrust on the glider's flight?
Hixson, B. (1991)
Hosking, W. (1990)
Vogt, G. (1996)
Casady and Greene (1991):
Virginia Air and Space Center
Sky Sailing: Frequently Asked
Research how engineers use wind tunnels to test the design of a wing, and then try building and testing one yourself. Start by building an experimental wind tunnel, using an ordinary house fan blowing through an empty cardboard box to focus the airflow. Try putting different-sized cardboard inserts in the box to deflect and focus the flow and see how it affects the lift on a standard paper glider.
Build a kite from a kit or from plans in a reference book. Try flying the kite on days with different weather conditions. (Watch out for thunderstorms!) You'll discover how wind, thermals, and changing barometric pressure have an effect on how gliders fly.
Research how engin-eers use wind tunnels to test the design of a wing, and then try building and testing one yourself. Start by building an experimental wind tunnel, using an ordinary house fan blowing through an empty cardboard box to focus the airflow. Try putting different-sized cardboard inserts in the box to deflect and focus the flow and see how it affects the lift on a standard paper glider.