Measuring static pressure
People,
I know I'm about a year too late but here goes.
Static pressure will be a function of blower characteristics and is hard to know. When you start restricting the blower, the pressure and air mass will change due to increased resistance. Blowers are not designed to handle serious backpressure.
You can measure static pressure. You need to make a water filled U-tube manometer. This can be as simple as a clear piece of tubing ($10 at the hardware store). Figure out what the pressure inside will be (approximately) and determine water column height from that. Remember that if pressure inside is too great, it will shoot the water column out of tubing. You can easily make an 7 foot water column in your garage by attaching the tubing to a 2x4. Add a bit of red food coloring to the water to see it easily. You need to see through both tubes so you can measure the difference in water height. 1 PSI = 2.3067 ft for water, so a seven foot tube will get you just over 3 PSI inside the hovercraft skirt. you should only need about 0.25 PSI (~7 inches of water colum) if my calculations and assumptions below are correct.
The next thing to do is stick the tube into the air chamber under the skirt. Drill a hole in the top of the overcraft or just stick a tube under the plastic. Start up the motor and put your standard weight on the deck, then measure the water height difference. Make sure you don't point the end at any directional air vector flowing in/out or the reading will vary. You want the static pressure. You might be able to put a sponge tip over the sampling tube to break up any gusts of turbulance and average out the water level.
Once things are set up, take your measurements. Use bricks or something for the weight as people tend to be unsteady and it throws the balance off. Five or six concrete blocks or a few tool boxes should be enough. Keep someone with his thumb close to the top of the tube to cap it quickly if water squirts out, or you will have red water all over everything.
As disk diameter decreases, you should see the pressure inside go up. I think the best idea is to pick a diameter that will maximize lift for a given weight. If you assume 230 pounds is max for a grown man, you should be able to make a disk that will lift him to max height possible for a given leafblower. The problem with blowers is not so much the air velocity as it is the mass of air created. You need lower velocity and more air mass to get best effect. Ideally, the pressure coming from the blower would only be somewhat greater than skirt pressure, and that would be only somewhat greater than velocity pressure of the air escaping from underneath.
Keep in mind a few things. Assuming your hovercraft doesn't grow and shrink area due to skirt changes, the disk size is always the same. The weight on the disk is always the same. Thus pounds of weight per square inch of disk (PSI, or pressure) will always be the same. I think if you do this experiment and can measure the real static pressure under the skirt but not around the lip, you will find the pressure stays about the same on high and low settings. What changes? The hover height! More air means you can widen the gap and let more air rush out. Here is an example:
32 inch disk = 804 square inches
200 pound person = 200 pounds / 804 Sq inches = 0.25 PSI
The other thing to note is that blowers will produce much less air if there is too much backpressure. This means make the holes in your skirt generous. The skirt must inflate but it should only be slightly more pressure than what is underneath the disk. This will rob air mass and cut your height. I know it is only a fraction of millimeter or so, but more height is better.
-d
