I am doing a science project for school.  I actually made a HoverCraft
as a hobby activity, I would now like to turn it into a science fair
project.  The question I would like to invesitigate: How does forced
air assist in making a HoverCraft float over a variety of surfaces?
(For lack of a better question) I would like to know how do I go about
measuring the amount of forced air pumped into a HoverCraft by a leaf
blower.  I need data for my board.  I am willing to reconstruct my
HoverCraft if I can find out what I should be measuring and how to go
about measuring. The HoverCraft is made from a disk of 1/2 inch
plywood, 4' in diameter.  The bottom is constructed out of heavy
plastic sheeting.  It is powered by a leaf blower.  I am in the 5th
grade at Anna M. Joyce School in Detroit, MI.  *I need the answer ASAP!

Hello,

Forced air assists in lifting the hovercraft because it creates a
higher pressure underneath the hovercraft than above the hovercraft.

So, if you take the difference in the pressure between under the
machine, and above it, and divide that by the area of the top surface,
you should come up with the minimum pressure required to lift the
machine off the ground.

Say for example you've got a hovercraft that weighs 35 pounds, and
that you've got a 4' diameter top for it.  The area of the surface is
pi x (radius in inches squared) = 3.14 x 24 x 24 = 1808 sq inches. 
Since the craft weighs 35 pounds, you need 35 / 1808 = 0.019 pounds
per square inch (PSI) as a theoretical minimum.  If you put a person
on it (35 + 120) / 1808 = 0.086 PSI.

If you take a very sensitive pressure gauge, put it through a plywood
plug for your leaf blower, you can measure the pressure that it will
produce under a similar situation to your hovercraft.  Another way is
to plug it with a loose fitting plywood plug, and figure out how much
force is required to hold it in.   Divide that force by the area of
the plug, and you've got the developed PSI for the leaf blower.  If
this reaches 0.086 psi, you will just barely be able to lift a person,
*if* your hovercraft was sealed to the ground.

Since the point of a hover craft is to move around, and not be sealed
to the ground, you'll have to compensate for the volume of air that
escapes under the skirt.  This is going to be tremendously variable,
since it's largely affected by the space under the skirt, which is
going to vary when the craft tips, or goes over uneven ground.

This is going to be the tough part, since I suspect that a leaf blower
isn't going to be able to keep up with the amount of air that will be
required to maintain that 0.086 psi that you'll need under the
machine.

There's a relationship between pressure of a gas and it's volume.  So
to relate two pressures and volumes together:

P1 x V1 = P2 x V2

If you measured your blocked leaf blower pressure, and can find some
information about how much air it's pushing (check the manual with the
leaf blower) you can calculate how much air volume is leaving under
the machine at the higher pressure.

One way to calculate the amount of air is to get or make a large
plastic bag.  Then place it over the end of the exit leaf blower, and
turn it on.  Time how long it takes to just fill it, and calculate the
volume of bag when it's full.  Divide the volume in cubic feet by the
time it took to fill it.  That will tell you what volume per second is
produced by the blower.  You can make a bag out of a roll of plastic
sheeting, and some silicone sealant.

Hope this helps,
kpcal