Goldie is a minimal hovercraft. From its weight (3 oz) to its power source (two 1.5-3v $1.49 Radio Shack motors) to its battery (3.6v 110 mAh) to its flight time on a battery charge (30 seconds). To its construction -- popsicle sticks as engine mounts. It charges in 2 minutes with a hand-held 9v power source. It hovers -- barely -- at 3.1 volts, and will zip across the kitchen floor a few times on a charge. (It's pretty fast!)
The skirt is made by welding pieces of a plastic trash bag together
and attaching them to the foam with 5/8" wire nails spaced every inch
or so. The pieces of plastic are cut and welded by drawing a
soldering iron across two pieces on top of each other. You have to
get the shape of the weld right, though. (To imagine it, imagine
cutting a plane through a cylinder at an angle, then unwrapping the
cylinder.) I learned how to do skirts in this way from the Universal
Hovercraft model plans. This was a test run.
Here's the completed skirt. The prop is spinning (at 3.1v) but the
lift motor
doesn't have enough oomph to inflate the skirt tightly. Oh, and
plastic trash bag skirts really don't like being bent along such a curve.
I guess it could be adjusted a bit better. But what matters is that
the distance of the skirt from the ground when the craft is hovering
is relatively uniform.
Left is a top view of the thrust duct, in which you can see the skirt
attach method. Note that the thrust propeller should come to within
1/16 inch of the duct on all sides. On the right you can
see it hovering at 4.8 volts -- what's that, about a 1/4" hover
height?
To make the thrust duct, saw it out a bit small and sand it
to the right size. The motor is mounted by gluing it to a pair of tongue
depressor-type sticks.
The motors really are quite wimpy, so it's a good thing the craft is so light. The packaging says that they run on 1.5 to 3 volts and draw up to 1 amp; with a 3.6 volt battery the actual voltage at a motor (with both running) is 3.1v. They ought to draw just over 1 amp, for a total of about 3.2 watts each. But they seem to generate less lift than I would expect for 3 watts, which really ought to be sufficient for a craft weighing 6 ounces. Still, on the order of 1/2 oz thrust for a 3 oz craft gives decent acceleration. Running at 4.8 volts, each motor draws about 8 watts, and there is enough lift to support the craft at 8 ounces. This craft might work better on four AA Ni-Cads -- with battery life approaching 10 minutes.
Efficiency computation. How much of the electricity going into the motor is converted to air power used for lift? Hmmm, let's see here. The hovercraft is about 15" by 12", boat shaped, with an area of about 0.78 ft2. The perimeter, where air escapes from underneath, is 3.7 feet. Hovering on 4.8 volts, the air gap is about 1/4", and if air escapes at 40 fps, that's 2.9 ft3/sec. The craft weighs 3 ounces, so the air is under a pressure of 0.24 lbs/ft2. That's 0.7 ft lbs/sec or 0.95 watts of air power. It's drawing 7.7 watts, so efficiency is 12%. Better than woodie (9%) but worse than zippy (15 or 20%). The lift duct, prop, and skirt are good on this one, so I'd guess that the $1.49 Radio Shack motor is the limitation.
So what's with the hull shaped to deflect water, when the craft is actually going through air? Got me.