INSTRUCTIONS:
MAKE THE WOOD DISK
Cut out your plywood disk. You can leave it square, or experiment with
other shapes instead of round, but the sharp corners can hurt people.
Round is best for safety.
Drill a 5/16in hole in the exact center, and
make sure that the 2in bolt easily passes through it.
NOTE: people tell me that you can avoid using a big bolt. Instead,
fasten down the small plastic disk with several short wood screws. This
is a big improvement! Kids sitting on the hovercraft won't get poked in
the butt anymore by that big bolt sticking up.
Make a hole in the plywood which exactly fits the end of your leaf blower
or vacuum cleaner blower hose.
This hole must be placed half way between the center of the disk and the
edge, as shown below. It's a good idea to trace the hole in pencil on
the wood (place the mouth of the vacuum cleaner on the wood and trace
around it.) It DOES NOT have to fit perfectly. Later you can seal
any leaks with duct tape. Or just let it leak. The hose should be flush
with the bottom surface (don't let it stick out or the floor will
block the air flow.)
![[four foot wood disk with two inch hole off-center]](/graphics/hov1.gif)
MAKE THE PLASTIC SHEET
Next, lay your plywood disk on the center of your large plastic sheet.
Fold the edges of the sheet up over the plywood, then use the staplegun to
staple it to the top of the plywood disk. Put a staple about every 4
inches. The plastic should be tight against the wood, but don't pull it
TOO tight or the plastic will tear loose when inflated. When finished,
you can cut off the excess plastic. If you wish, used duct tape to tape
the edge of the plastic down to make it look nice. From above, it should
look like this:
![[plastic sheet on disk bottom, edges folded over disk edge and attached]](/graphics/hov2.gif)
ADD THE "SKIRT LIFTER"
Poke a hole in the center of the coffee can lid. Attach it to the
bottom of the hovercraft as shown below. It goes over the plastic sheet.
It pins
the plastic sheet firmly against the plywood. (The coffee can lid forms
the "donut hole" when the vacuum cleaner slightly inflates the plastic
into a
"donut" shape.)
CUT THE CENTER HOLES
Use your razor knife to cut six vent holes in the plastic as shown below.
They should be about 2in diameter. They must be placed within a few
inches of the coffee can lid. Space them out so that there is plenty of
plastic between each of them. But if they are too far away from the
center, they will become plugged when the plastic sheet lays flat against
the floor. If the plastic between the holes is too narrow, it will tear.
If you wish, reinforce the thin necks of plastic between the holes using a
couple of layers of duct tape.
![[cut six two-inch holes in the plastic sheet]](/graphics/hov4.gif)
DONE!
Flip your hovercraft over so the plastic sheet is on the bottom. Place it
on a smooth floor. Stick the vacuum cleaner hose into the hole and turn
it on.
The plastic on the bottom should inflate. If it does not, lift the
plywood up a bit to let the air get in and inflate the "skirt." The
hovercraft will lift up slightly and start gliding around.
DEBUGGING
If it doesn't work, first read the above article again to see if you
missed anything.
Always make sure to lift the wood disk up a few inches while the blower is
running. If the disk stays flat against the floor, then the air can't get
out of the blower, and the plastic sheet will never inflate.
When you turn it on for the very first time, run it on a smooth shiney
floor such as linoleum or a wooden Gym floor. Try other kinds of rough
floors and rugs later, after you know it works. Smooth dirt works
OK, but it makes a big cloud of dust. I've never tried it on
water.
If too much air is leaking from the around the vacuum cleaner hose, add
some
temporary duct
tape at the point where the nozzle goes into the hole in the wood.
Still no luck? Maybe your plastic sheet is too loose. Turn the
hovercraft up on edge and turn on the blower. The plastic sheet should
inflate, and it should lift up from the wood by only three or four inches.
If the sheet is too tight then it will tear loose from the staples. But
if it's too loose it will form a floppy bag and won't glide around.
DON'T use a hair blow-dryer, it won't work. You need either a
a lawn leaf blower or a vacuum cleaner which has a blower outlet. If you
use a
vacuum, remove the bag and use
the blower outlet (the
hovercraft needs pressure, not suction!)
HOW IT WORKS
The air inflates the plastic which pushes upon the floor and provides a
ground-hugging "skirt." This lifts the entire hovercraft. The coffee-can
lid provides "strain relief" for the plastic sheet, so that the inflated
plastic doesn't tear loose from the center. The coffee can lid also lifts
up the plastic so air can escape through the vent holes and pressurize the
center donut-hole. The air then leaks along the floor and out from the
edges of the hovercraft. This creates an "Air Film Bearing" which has very
low friction. The plastic isn't touching the floor. Instead it is riding
on a thin layer or "film" which is made out of air. Climb aboard!
This hovercraft can support many hundreds of pounds. It works best on
very smooth surfaces (linoleum, or school gym floor.)
PS
if you want to experiment,
maybe you could try four or five blow-dryers with their heat turned off.
I know that a single blow-dryer doesn't supply enough wind, but several
blow-dryers might do the job. Also, think of ways to drive your
hovercraft forwards. Maybe a big fan would work. (I KNOW that a couple
of CO2 fire extinquishers work well as rocket propulsion, but they're too
dangerous for kids to use. )
PPS
Where did this hovercraft idea come from? The device is called
an "air film bearing" and has been used to move heavy loads on flat
floors for many decades. Physics teachers used them for classroom
demonstrations, and we had a compressor-driven one which lifted a granite
block, back when I worked at the Museum of Science in
Boston. I first saw the home-built plastic sheet version in THE PHYSICS TEACHER magazine in
late 1989, called "human air hockey puck." I built one in 1990
and found that the holes in the plans didn't work right. The hovercraft
would inflate like a beachball and fling the rider onto the floor! I
moved
them closer to the center, as shown here. We used hovercrafts for
outreach classes at The Science Club for years; giving out
plans to kids so they could build one and amaze their friends with the
secret.
Finally I realized that people on internet didn't know about this cool
device, so I wrote my own hovercraft article in 1997 (first one online, as
far as I know.)
Here's a set of photocopied plans
for "Human Air-Hockey Puck," from
1990 from a college physics teacher. It appears that B. Saur in 1987 may
be the original inventor.
![[]](/graphics/hov6.gif)
|
|
Lift it up at the start,
otherwise the bag might not inflate. |
|
SAFETY ISSUES
ADULT SUPERVISION REQUIRED! Don't let little kids play alone with this
device.
- To be safest, operate the hovercraft in a small classroom. (In the
school gym adults must control it, since it can get going too fast!)
- Climb aboard carefully, since the thing is darned slippery. Or climb
aboard BEFORE turning on the blower
- Bystanders should watch their feet, since the edge of the moving board
can give stubbed toes. (WEAR SHOES!!)
- Don't push it too fast, and remember how to stop it quickly: yank the
vacuum cleaner hose out of its hole.
- Don't jump off it while it's moving, or you'll launch it at high speed
in an unexpected direction (it could crash into somebody's feet.)
- Don't remove the vacuum cleaner suddenly while it's moving fast,
because
the hovercraft will stop, but the passengers will keep going... fast!
- SUGGESTION: attach a long rope to the hovercraft, and let it trail
behind. That way it can be grabbed if it starts going too fast in the
wrong direction. Even better: use an AC-powered leaf blower or vacuum
cleaner,
and have an adult remain near the wall outlet. If the hovercraft moves
too fast, yank the power plug to put on the "brakes." Use this only in
emergencies, since the passengers will fly off when the hovercraft
suddenly halts.
HOVERCRAFT LINKS:
MORE SCIENCE PROJECTS HERE:
![[diagram of layers: nut,washer, wood plate, plastic sheet, small disk, washer, bolt]](/graphics/hov3.gif)
![[attach a small plastic disk to the bottom]](/graphics/hov5.gif)
