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Gliders

 

Overview

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.

Activity

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. Materials
  • standard 8 1/2" x 11" sheet of copy paper
  • paper airplane book (can be found in most hobby or bookstores)
  • stopwatch
1. Discuss how the forces of lift, drag, gravity, and thrust all work together to keep a glider in flight. Look at one of the basic glider designs pictured above (or get a paper airplane book with other designs) and construct the one you think will have the longest timed flight, based on wing shape and structure. 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. Questions 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?

Resources

    Hosking, W. (1990) Flights of imagination: An introduction to aerodynamics. Washington, DC: National Science Teachers Association.
    Vogt, G. (1996) The space shuttle
    glider kit. NASA Johnson Space Flight Center-Educational Publication
    349 8-96.

    Computer software:
    Casady and Greene (1991):Glider 4.0 for Macintosh. (408) 484-9228

    Organizations
    Virginia Air and Space Center
    NASA Langley Research Center
    600 Settlers Landing Road
    Hampton, VA 23669-4033
    (804) 727-0900, ext. 757
    Web sites
    Sailplane Construction and Cost
    www.wiltonct.com/soaringadv/gliderbook5.html
    Learning to Fly Sailplanes
    www.wiltonct.com/soaringadv/gliderbook3.html
    Sky Sailing: Frequently Asked Questions
    groupgenesis.com/soar.html