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Test-Flying Planes Underwater

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Author: Marc Rosner

Here we study the hydrodynamic properties of planes underwater.

Hydro means "water". Hydrodynamics is the study of the flow of water across surfaces, just as aerodynamics is the study of airflow.

The behavior of planes underwater is related to their flight ability. Although water is much denser than air, a plane that "flies" poorly underwater will probably have similar problems in the air.

Common flight problems include sharp turns left or right, nosedives, and tailspins.

Materials

  • plastic model plane
  • water
  • drill and bits (optional—requires adult help)
  • about 0.5 kg wax (Candles will work.)
  • Pyrex measuring cup and shallow pan (if using wax)
  • stove or hot plate (if using wax—requires adult help)
  • lard or petroleum jelly (optional)
  • bathtub
  • food coloring or nontoxic, water-soluble paint (optional)
  • meterstick
  • wax pencil or adhesive tape
  • notebook and pencil

Procedure

Test-Flying Planes Underwater

In wind tunnels, engineers use smoke to see the aerodynamic effects of wings.
The glide ratio tells you how fast the plane descends. A plane with a low glide ratio drops quickly; a plane with a high glide ratio drops slowly.
A thermal results from different air densities. Air that is cold relative to its surroundings is denser and sinks; hot air is less dense and rises, creating a thermal. Turbulence arises when the atmosphere has air pockets with differing temperatures. The eddying of these air masses pushes planes around, much to the discomfort of passengers.
  1. Prepare your plane for underwater flight. It is important that your plane does not float. You will have to fill it with water to give it ballast. You may need to ask an adult to make a couple of holes in the plane with a small drill bit. Sometimes it works better to fill the plane with wax. Have an adult melt the wax by heating it slowly in a Pyrex measuring cup nested in a shallow pan of water on a stove on low heat. When the wax is liquefied, ask an adult to pour some into your model plane. Allow the wax to cool. Alternatively, you can fill your plane with other substances, such as lard or petroleum jelly.
  2. Fill your bathtub with warm water. (The reason for using warm water is simply to make it more comfortable for your hands, which will be underwater for a while.)
  3. Do a test flight of your plane. Give it a gentle push underwater and study its behavior. Does it fly nicely, at an even speed, and nearly level? Or does it dive or turn?
  4. Drop a few drops of food coloring or water-soluble paint from about 1 m above the tub. After the colors have passed through the water, fly the plane through the colored trail to see how the water moves. You should see small eddies, or whirling currents, as the plane passes through the water.
  5. You can measure the glide ratio of your plane as follows:
    1. Make marks 1 m apart on the bottom of the tub, with either a wax pencil or strips of adhesive tape.
    2. Release the model from a point in the water above the first mark so that it glides to the bottom at the second mark. (This may take a few trials.)
    3. Measure the distance from the bottom of the tub to the place where you released the model plane.
    4. The glide ratio is easy to calculate—it is 100 divided by the altitude in centimeters at which the model was released.
  6. You can make adjustments to your plane to help it compensate for poor flight.

    Test-Flying Planes Underwater

    1. To manage balance, add a few thumbtacks stuck with putty or soap to the nose of the model. Plastic model planes (or paper airplanes for that matter) are sometimes too heavy in the rear.
    2. To straighten the flight path (keep the model from veering to the left or right), make adjustments to the vertical tail (or create one if it is missing), using thumbtacks and putty.
    3. Spiral dives indicate the plane is slightly tail heavy. Lighten the tail by filing away material, or weight the nose some more with tacks and putty.
  7. Create thermals by letting the faucet run. A column of water moves up around the falling stream. When your model hits this gust of water, it will react exactly as a plane hitting a thermal in the air.
  8. Create turbulence by pushing water toward the plane sharply with your submerged hand. This models how the plane would react to a gust of wind.
  9. Can you modify your plane to reduce the dangerous effects of thermals and turbulence? How does the mass of a plane determine its behavior in rough weather? What changes can you make to stabilize it?

References

Air and Space: The National Air and Space Museum Story of Flight by Andrew L. Chaikin and Sonian Smith (Boston: Bulfinch Press/Little, Brown, 2000).

100 Planes 100 Years: The First Century of Aviation by Fredric Winkowski and Frank D. Sullivan (New York: Smithmark, 1998).

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