Key concepts
Vehicles
Air
Friction
Engineering
Aerodynamics

Introduction
Have you ever ridden on a hovercraft? It is like gliding on a cushion of air! In fact, that's exactly what you're doing—a hovercraft is a vehicle that glides over a smooth surface on a cushion of air. Because a hovercraft can travel over flat land or water, it is an amphibious vehicle. In this activity, you'll get to build your own mini hovercraft using a CD or DVD, a pop-top lid from a plastic bottle, some glue and a balloon. How will different amounts of air in the balloon affect how long the hovercraft hovers?

Background
Because a hovercraft is a vehicle that glides over a smooth surface by riding on a cushion of air, it is also called an air-cushion vehicle, or ACV. How is the air cushion made, and how does it allow the vehicle to glide over it so freely? Vents or currents of slow-moving, low-pressure air are ejected downward against the surface below the hovercraft. Modern ACVs often have propellers on top that create the air currents. These currents are pushed beneath the vehicle with the use of fans. Surrounding the base of the ACV is a flexible skirt, also called the curtain, which traps the air currents, keeping them underneath the hovercraft. The trapped air currents can create an air cushion on any smooth surface, land or water! The cushion greatly reduces friction, allowing the vehicle to glide freely over the smooth surface below.

Materials
•     Pop-top lid from a plastic drinking bottle. (Reusable plastic drinking bottles sometimes use these kinds of lids.)
•     An old CD or DVD that can be made unusable
•     Craft glue or Super Glue
•     A medium-size balloon (should be able to inflate up to at least 28 centimeters, or 11 inches)
•     Stopwatch or timer
•     Large flat surface for testing the hovercraft

Preparation
•     Remove a pop-top lid from a plastic drinking bottle.
•     Glue the base of the lid to the CD (or DVD) so that the lid covers the hole in the center. If you use Super Glue, have an adult help, use caution, and follow all of the instructions and safety warnings on the packaging.
•     Allow the glue to dry completely.

Procedure
•     Make sure the pop-top lid is closed.
•     Blow the balloon up as large as you safely can without popping it, then pinch the neck so that no air can escape.
•     Stretch the neck of the balloon over the pop-top lid, being careful not to let any air escape. Carefully center the balloon's opening above the pop-top lid opening. Your completed hovercraft should have CD flat on the bottom, pop-tip lid above and the inflated balloon's neck stretched snugly around the closed lid. Your hovercraft is now ready to do some hovering!
•     Tip: If the pop-top lid detaches from the CD and you used craft glue to glue them together, carefully re-glue them using Super Glue.
•     Place the hovercraft on a flat surface. Start your stopwatch or timer, open the pop-top lid and push the hovercraft. Stop the stopwatch when the hovercraft stops hovering. How long did the hovercraft hover? Detach the balloon from the pop-top lid.
•     Repeat this process two more times, inflating the balloon as large as you safely can, reattaching it to the pop-top lid, and timing how long the hovercraft hovers. Did the CD hover for the same amount of time each flight, or was there some variation?
•     Repeat this process three more times, but this time only inflate the balloon to medium size. For example, if it took three breaths to blow the balloon up as large as you safely could, use only two breaths or a little less to inflate it this time. How long did the CD hover when the balloon was only a medium size? Did the hovercraft hover for about the same amount of time for each of the three flights using a medium-size balloon?
•     Repeat this process three more times, but this time only blow the balloon up to a small size. For example, if it took three breaths to fill the balloon to maximum size, use only one breath or less to blow it up now. How long did the CD hover using a small-size balloon? Did the craft hover for about the same amount of time each of the three times you tested it using a small balloon?
•     Overall, which size balloon allowed the CD hovercraft to hover for the longest amount of time? Which allowed it to hover for the shortest duration? Why do you think this happened?
•     Extra: In this activity, you made the balloon approximately different sizes. However, you could more accurately measure the different balloon sizes and see how this correlates to the time the hovercraft is suspended. Repeat this activity, but in this version, measure the circumference of the balloon each time you inflate it and estimate the volume of air inside by calculating the volume of a sphere. Alternatively, you can use either a water-displacement method: dunk the filled balloon in a large metric measuring container and determine the change in height of the water; or an air-displacement method: fill a large graduated cylinder with water and place it in a large tub of water, invert the cylinder underwater so that the liquid inside does not escape, release the air from the balloon into the cylinder opening so that the air travels up inside, and then look at how much air is trapped in the cylinder. How does the volume of the balloon correlate with how long the your CD craft hovers?
•     Extra: Test the CD hovercraft on different surfaces. On which type of surface does it work best?
•     Extra: Can you improve on this hovercraft design? Think about features of this craft that you can change, such as the construction materials, the lid's opening size, the hovercraft's shape or its size. Could you add a skirt like that used on real ACVs to better contain the air cushion? Tip: Foam board cut into different shapes may work well instead of a CD or DVD. What modifications make the hovercraft stay suspended even longer? Can you use the action of a modified hovercraft for any practical purpose, or to solve a problem?

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