Experiments in a Wind Tunnel

The Swiss mathematician-scientist Daniel Bernoulli (1700–1782) is famous for discovering that as the speed of a liquid or gas increases, its pressure decreases. This is known as Bernoulli's principle, and it is the scientific foundation of modern airplane flight. The wing of a modern aircraft is rounded at the top. This increases the distance that the rushing air must travel relative to the lower edge of the wing. The air must move faster over the top of the wing than at the bottom, so there is lower pressure on top of the wing. The greater air pressure from below causes an upward force called lift, which lifts the wing—and the plane with it.

If you hang two strips of paper parallel to each other and blow between them, they will move together. This works with hanging balloons as well. By blowing between the paper or balloons, you increase the speed of the air in between, causing the air pressure to decrease. The higher air pressure on the outside of each piece of paper or balloon pushes the object inward.

A wind tunnel is a device used to study the aerodynamics (airflow across surfaces) of objects. You can build a simple wind tunnel and conduct engineering tests on model planes and airfoils (wing samples).

Orville Wright (1871–1948) took off on the world's first human flight on December 17, 1903, at Kitty Hawk, North Carolina. He would not have gotten very far if he and his brother had not done wind tunnel tests.

Materials

• heavy cardboard box (such as one in which a computer has been shipped)
• pencil
• safety razor (requires adult help)
• air-conditioning filter or thin foam pad
• duct tape
• Plexiglas or clear plastic sheet, same size as largest wall of box
• electric fan (small enough to fit in one end of the box)
• balsa wood, hard packaging foam, or cardboard
• glue
• digital postal scale
• notebook
• different shaped objects, paper airplanes, model airplanes

Procedure

Balloons do not rely upon Bernoulli's principle for aerodynamic lift. They rise because the internal gas–helium or hot air–is less dense than the surrounding atmosphere.

1. Construct the shell of your wind tunnel out of a cardboard box: Mark the cutting lines on the cardboard with a pencil first, then have an adult C1llt the sections out using a razor with a safety handle. Both ends of the box should be open. Divide the box into two unequal compartments by mounting an air-conditioning filter, secured with duct tape, approximately one-third of the way in from one end. The purpose of the filter is to help even out the airflow. Make a window in one side of the larger compartment, leaving a 2-cm border. Insert a sheet of Plexiglas inside the border to serve as a window pane, and secure it with duct tape.
2. Place a fan in the smaller compartment so that air blows through the filter into the larger compartment.
3. Make a variety of airfoils in the test shapes shown above, using balsa wood and glue. If balsa is not available, you can use hard packaging foam or cardboard. If you can, visit an airport and study the wing shapes on aircraft.
4. Mount one of your airfoil samples on the postal scale. Since the scale measures downward force, mount your sample upside down for purposes of gathering lift data. Tape the sample down using duct tape, with a small balsa cube separator holding it above the surface of the scale. (rape the balsa cube to the scale.) The purpose of the separator is to allow airflow both above and below the airfoil. Keep in mind that when you place the scale in step 5, the front of the airfoil (or plane) should face the fan and the readout on the scale should face the open end of the wind tunnel.
5. Make sure the postal scale is set to measure weight (grams), not dollar postage values, then place it in the larger compartment.
6. Read the scale, then turn the fan on and read the scale again after a few seconds. How do the numbers compare? Can you explain the difference? If the fan has variable settings, you can take readings at each setting (high, medium, low).
7. Test several different shaped objects and record their lift forces in a table. Analyze your results. Which shapes work best? You can also test the lift on paper airplanes and model airplanes, and relate your wind tunnel data to their actual performance when you fly them. Is there a correlation between the lift on a toy plane and the distance it flies?

References

Airplanes (What If … series) by Steve Parker (East Grinstead, Engl.: Copper Beech Books, 1995).

The Great International Paper Airplane Book (publication of the First International Paper Airplane Competition conducted by Scientific American) by Jerry Mander, George Dippel, and Howard Luck Gossage (New York: Galahad Books, 1998).

The Paper Airplane Book by Seymour Simon (Burnsville, Minn.: Econo-Clad Books, 1998).

Science Book of Air by Neil Ardley (San Diego: Harcourt, 1991).

"Caught in a Wind Tunnel" by Shawn Carlson: www.sciam.com/1197issue/1197amsci.html

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