There's more to life
than work, sleep, family, and prayer.
There's also . . . hovercraft.
It all started when my 12-year-old son Peter wanted to get an airplane that could really fly. I told him I'd help him in his new interest, but he would have to be the main man. We got the plane, we did the flying, and how he's on to bigger and better things (gameboy, mainly). And I'm stuck with . . . hovercraft. Model hovercraft and kid-sized hovercraft so far. [Click on images to enlarge. Keep clicking.]
Initially, I tried a variety of construction materials, motors, and skirt designs, with varying success. A shoebox lid and duct tape with a motor mount carved out of a plastic margarine container constituted a typical attempt. Early attempts buzzed and skittered and flitted and zippered around the kitchen table. One kinda sorta worked -- it was a 6" disk of white foam with a 3" ducted fan lift motor from the (now-crashed) free-flight model airplane. It had a duct-tape "skirt" and it...hovered! We played hockey with it on the kitchen floor. Sadly, it disappeared after a herd of neighborhood kids moved through the house. Remnants were later found on the road, squarshed by a car.
An early attempt carved out of a sheet of 1" white foam fell on the floor and broke in two. So I cut out a piece of plywood and glued the foam to the plywood. (It hadn't yet occured to me that weight was an important consideration). "Woodie" was the result. Its skirt was constructed by taping a strip of plastic trashbag into a tube shape and gluing that to the foam. A not wholly successful design. However, it did hover -- kind of. If enough current were available. I had to carry along a 5 lb 12 volt battery designed to run a weed whacker. That white strip in the middle serves to hold a battery (which didn't have enough power) and to support the Batman figure my 5 year old son David contributed.
I ordered a set of plans for model hovercraft from Universal Hovercraft. I haven't built any of their models yet, but I did learn how to make a good skirt from plastic trash bag pieces welded with a soldering iron. From the fuselage of the free-flight airplane, two Radio-Shack "1.5-3v" motors, some 2" pink foam, some popsickle sticks, and some gold spray paint, "Goldie" arose. Goldie is a demo hovercraft. It works well, but only goes for about 30 seconds on a two minute charge of its tiny batteries. Just enough time to impress (?) your friends.
Something more was needed to impress the kids. Something . . . bigger. And noisier. Based on things I'd seen on the Internet, I thought maybe a leaf blower would be able to hover a kid. Leafy's parts fell together in a couple of days. It has a 4" skirt made of plastic trash bags pieces taped together and glued to a plywood disk a bit over 3 feet in diameter. A leaf blower is mounted in a block of pink styrofoam and held down with a Super Soaker strap. An old vacuum cleaner hose channels some of the air into the skirt.
And it worked! Well, it could lift a kid. But there really wasn't much gap between the skirt and the ground, so you had to balance perfectly to reduce friction to near zero. The idea was to lean to one side a bit, so that air escaping on the other side would push you along, but with so little margin for error, it was tricky. Anna, 9, did succeed in moving, an inch or two.
Later I learned about air volume and power requirements. With a radius of about 11 ft, a nice air gap of 1/2 inch would require 27.5 ft3/sec of air (escaping at 60 feet per second). An advertisement for a gas-powered leaf blower claimed 6.8 ft3/sec -- which would yield an air gap of only 0.2 in., less with less than 100% efficiency. Leaf blowers have more than enough power but too little volume per second. (Anyone for cutting open the case and attaching a big propeller?)
Back to model hovercrafting. The next big problem was making a hovercraft that could actually be controlled. A radio-controlled car sacrificed its life to the cause, and Zippy floated off the workbench. It's an 18" disk carved out of pink foam. It has two ducted thrust motors (from the R/C car) and a lift motor. It goes like a bat out of . . . the Netherlands, with an acceleration of about 20 ft/sec2. Of course, it sucks amperes like there's no tomorrow. But it only partially solves the "controllable" problem. It's . . . wild. It accelerates and turns quickly, it spins out of control, it bounces here and there. Just not where you had intended it to go, and not in a straight line. Fortunately for my shins, it weighs only 16 oz. Anna contributed the pilot.
More information is available on motors and propellers.