Launch It: the Improved CD Hovercraft.

The CD hovercraft is a tried-and-tested Make, but technological improvements have tended to be in the method of connecting the balloon to the CD. I have attempted to apply some of Sir Christopher Cockerell's original developmental research to this old favourite.

In addition to the basic materials (an old CD, a balloon and a plastic bottle cap and hot glue), you will need a sheet of paper or light card, a thin sheet of expanded polystyrene, ordinary glue and a sharp knife.

The type of glue is not vitally important, since it will not be under any strain greater than the weight of the polystyrene and paper. If you use a water-based glue, though, try and be very sparing with it, as excess water will make the paper "cockle" and cut the vehicle's efficiency.

Hovercraft gain lift from pressure-drops. More rapid changes in pressure and downward air-velocity give rise to greater lift.

Early attempts at a hovercraft simply blew air down an open shaft towards the ground. However, when the bulk of the shaft was closed off, and the air-flow restricted to the edges of the craft, the increase in lift was startling.

In the traditional CD hovercraft, the drop in pressure is gradual, spread over the whole surface of the CD. We are going to use the polystyrene and paper to create channels that will restrict the airflow so that the pressure-drop occurs more rapidly near the edge of the disc.

The air is restricted by using channels cut in a layer of polystyrene, capped by a layer of paper.

Using the attached templates, cut pieces from the polystyrene and the paper. If you are doing it by eye, remember that the CD has a diameter of 12cm.

It is easier to glue the polystyrene to the paper, and then both to the CD. Use the "normal" glue to do this, being careful not to clean off any "ooze" before it dries: blobs of glue on the bottom of the hovercraft will catch on the surface below the craft and slow it down or even stop it completely.

Drill a hole in the middle of the bottle cap, as large as the hole in the CD.

Hot-glue the cap to the CD, upside down. As you press the cap in place, gently twist it to ensure the glue forms an air-tight seal. You must be very careful to avoid glue oozing through the hole in the CD, as it will block the channels you have just made.

When all the gluing you did has dried and/or cooled, blow up your balloon and twist the neck to keep the air in.

Stretch the neck over the cap, place the hovercraft on a smooth surface and let the neck untwist. Air will rush down the channels, under the craft and lift it on a cushion of air. The slightest nudge should send the hovercraft scudding away.

Time to be honest - things did not go as well as expected.

The hovercraft worked, but the large hole in the cap meant that it only ran for a couple of seconds.

Restricting the air-flow seemed to cure this, but I left things a bit late to finish this Instructable, and did not wait until the glue dried before testing. (Bad Kiteman, naughty Kiteman!)

This meant that the glue oozed through the paper and stuck to the mat while I was holding the hovercraft still.

Have a look at the video to see the results so far, but the deadline is too close to be able to try it with a dry, non-adhesive base.

If your smooth surface is also level, the hovercraft will not move unless you nudge it.

You may want to add one or two narrow extra channels through the outer ring of polystyrene to the edge of the disc.

The portion of air that leaks through these channels should provide enough thrust to propel the hovercraft across the desk.

Build two or more CD Hovercraft with thrust channels, and you can race them across the kitchen floor - the loser washes up!