Hovercrafts ride suspended on a thin bubble of air. The main technical challenge in building a hovercraft is to control the shape, flexibility and leakage of that bubble. Too much air leaking out of the hovercraft will empty the balloon air supply before the craft has a chance to slide along the surface. Too little air, and  the hovercraft will fail to rise above the surface.

Simple balloon hovercrafts are nothing more than a flat disk with a hole in the center fed by the air from a balloon. As long as the disk sits on a very flat surface it will skid along quite nicely on a very thin skin of air. However, even a small piece of debris is sufficient to hold the disk in place. Likewise, a dent in the table will bleed air out from under the disk so quickly that the bubble of air collapses and the hovercraft stops hovering.

Thus,  most real hovercrafts employ a flexible skirt along the bottom edge of the air bubble. This skirt will yield when encountering debris, and flows into dents and grooves to prevent air leakage. Even a simple balloon hovercraft toy benefits from a flexible skirt.

Materials list:

1. A plastic funnel, about 4" wide at the top. Find one with a flat lip around the openning (as shown in this cross-sectional sketch)
2. A thin flexible sheet of plastic larger than the funnel. A vinyl or rubber sheet works well, as will a thick piece of polyethylene sheeting or even waxed paper.
3. A 1" square piece of heavy duty aluminum foil.
4. A large balloon- the bigger the better (one foot inflated does quite well).
5. Double stick tape
6. Regular tape

Instructions:

1. Place the aluminum foil on the neck of the funnel (if the neck has an angled edge, rather than a flat tip perpendicular to the openning as shown, use a sharp razor blade to re-cut it flat and perpendicular to the neck. Needless to say, sharp knives can be dangerous (DUH!) so please be carefull and ask for help if you are unsure of your guillotining skills) Fold the edges of the foil down around the neck, sealing off the funnel.
2. Using regular cellophane tape, wrap the sides of the neck to hold the foil in place. Make sure this is an air tight seal. Also, keep wrapping layers of tape around the diameter of the neck until it is large enough that the balloon´s lip will fit tighly over the neck.
3. Now make the hovercraft skirt. Place the funnel on the sheet of plastic. Trace around the funnel. Then, draw two circles around this ring - the first about a 1/2" larger in diameter, and the second about 1/2" smaller in diameter. Cut along these two new circles, forming a washer shaped gasket.
4. Cover the bottom lip of the funnel with double stick tape. Use many 1´´ segments of tape. Make sure the tape is flat, and that each piece of tape overlaps its neighbor to prevent leaks.
5. Carefully center the bottom of the hovercraft on the skirt. Press onto the skirt and rub to adhere tightly. Trim off any double stick tape that extends over the sides of the skirt.
6. Using a pencil, punch a small 1/16" hole in the center of the aluminum foil. This small hole is called a "bleed valve", and controls the rate of air leakage from the balloon.
7. Inflate the balloon. Stretch the balloon over the lip of the neck. And you´re done!

Flying Instructions:

1. Place the hovercraft on a flat, smooth surface (like a table or vinyl floor). It should skim along effortlessly.

If the hovercraft doesn´t hover:

1. Sometimes the skirt is sticky or wet. Make sure it is dry, or powder the bottom of the skirt with flour.
2. Make sure the skirt is centered- if not one side may act as a pivot and the hovercraft will spin in circles.
3. If the craft is too light, air will leak from under the skirt. Try adding a few weights symetrically around the perimeter of the funnel - for example, glue on a dozen pennies. If it is too heavy, open the hole in the "bleed valve". You may find it easier to control the size of the hole if you first put a strip of tape over the aluminum fail.
4. Check for leaks. Hold the hovercraft firmly against a smooth surface- no air should leave the balloon.