Usually, when somebody tells you to "take a flying leap," they don't really expect you to fly. But you can if you know what you're doing and have a hang glider strapped on. Hang gliders work on the same principle as any winged aircraft. As the wings move forward, air is deflected above and below. The air traveling over the curved wing must travel farther--and faster--than the air below. When air speeds up, it drops in pressure, creating a low-pressure zone above the wing. This in turn gives the air traveling below greater relative pressure and the strength to push up the wing. To become airborne, a hang glider's airspeedmust equal about 20 mph. Airspeed is a combination of the pilot's running speed and the speed of the wind coming toward the pilot. Many different combinations are possible. If a pilot is running at 20 mph, no wind is necessary. (But this is unlikely, since the world's fastest sprinters run only about 23 mph--without carrying hang gliders.) If the wind is blowing at 15 mph, the pilot need only run at 5 mph. A combination of a 10 mph wind with a 10mph run is considered ideal. As a wing lifts a glider up, gravity pulls it down. The two forces combine to create the gliding action, which is measured by the lift to drag (L/D) ratio. For instance, the average glider L/D ratio of 13:1 means that for every foot of drop, the glider sails 13 feet forward. However, pilots need to find constant upward forces to stay in the air for longer periods. Thermals, huge masses of rising warm air, are what hang gliders ride to stay aloft. These thermals are formed near Earth's surface and depend on warmth from the ground. Flat fields, dark pavement, and low lying towns create heat early in the day, while wooded areas heat up more slowly and stay warm longer. Ridge lift is another power source. When a ridge or a hill deflects wind upward, gliders can "catch the wave!" While suspended in the harness system, the pilot steers a hang glider by shifting his or her center of balance. Leaning forward and backward causes the glider to dive or climb. This motion changes the >angle of attack and is used to take off and land, as well as to control speed during the flight. Shifting from side to side causes the glider to bank into turns. Pilots use a control bar to move their weight in relation to the wings. Other recommended equipment includes a helmet, parachute, variometer, and altimeter. Breaking gliding records is difficult, but designers are always trying to glide farther, faster, and higher. The new boomerang shaped SWIFT (swept wing with inboard flap) is capable of an incredible L/D ratio of 25:1 and has a top airspeed of 80 mph.
- How would the altitude of your launching point affect your flight and the equipment you would need?
- What sort of research could a pilot do before a flight to identify possible thermal locations?
- You've seen hang glider pilots running into the wind to take off. Hang gliders need a total airspeed of 20 mph to take off. How fast would a pilot have to run if there was no wind to run into?
- How fast if there was a 10 mph wind to run into?
- If the pilot was running 20 mph with a 10 mph wind coming from behind, would the glider take off?
In this activity, you'll make a model of the countryside and a simple glider. Then, using a blow dryer as a wind source, you'll recreate the air currents and thermals that a hang glider would encounter.
- pieces of paper about 20 cm x 25 cm (about 8" x 10"). Get a few different thicknesses.
- a room with open floor space
- blow dryer with extension cord
- measuring tape
- items to create different structures and landmarks. Be creative! You can use books standing on end to be buildings or lying propped open for cliffs. Cereal boxes, milk cartons, and teapots can be city skyscrapers. Pillows can be mountains, and so on.
- Drop the pieces of paper just as they are to the floor from a height even with your chest. What happens? The air pressure below the paper is being released in random spurts. Now fold the pieces of paper in half to be 20 cm x 12.5 cm (8" x 5") and spread them open again. What happens now when you drop them, with the fold crease down? The fold is dividing air pressure equally on either side of it.
- Open the creased pieces of paper on a table and fold a long edge back about 2 cm (3/4"), creating a sort of lip. What happens when you drop the paper again? Keep folding this edge over onto itself, 2 cm at a time, and keep dropping it until you get a smooth gliding action. Why does this cause a forward motion? Can you figure out a way to calculate the lift?to?drag ratio, using the measuring tape? (Remember the L/D definition--as the glider drops, how far does it move forward?) What thickness of paper has the best L/D ratio? Why?
- Using the blow dryer on a low, cool setting, turn your floor into a hang glider's paradise. Have a friend sit on the floor and hold the blow dryer even with a structure, while you fly your gliders over, around, and through your buildings and mountains. Create turbulence by shaking the blow dryer. Does a warmer setting cause different flight patterns? Create the ultimate thermal by pointing the blow dryer straight at the ceiling. Experiment using two blow dryers at once. Have fun! (Hint: If you have trouble keeping the paper gliders aloft long enough to see the flight effects caused by your classroom countryside, try using air-filled balloons instead.)
Monitor, p. 10.
- On the wing. (1993, Mar 20) The Economist, p. 100.
- Plimpton, G. (1991, Dec) Four thousand feet over Africa. Esquire, pp. 87-89.
- Skow, J. (1993, July 19) Sailing seas of air. Time, pp. 56-57.
Additional sources of information
General Aviation News & Flyer
8415 Steilacoom Boulevard
Tacoma, WA 98498
Soaring Society of America, Inc.
PO Box E
Hobbs, NM 88241-1308
(publishes Soaring magazine, Technical
Soaring quarterly, and a directory of
U.S. Hang Gliding Association
PO Box 8300
Colorado Springs, CO
(publishes Hang Gliding magazine.
Can help you locate a hang gliding
school or club in your area.)