We’ve been talking about using hovercraft as the ground vehicle in this year’s Cyber-Physical Systems Challenge, so I spend an hour in the shop tonight making a very rough prototype from scrap foam insulation.
The main thruster is a Turnigy 2213 brushless motor, with a ten inch propeller, blowing air downward for lift. It’s driven by a 30 amp electronic speed controller (ESC), which takes a gentle control signal from my radio control receiver (“R/C RX”, in the lingo) and big beefy amps from my 12 volt lithium polymer battery (or “3S lipo”).
The frame is thick white extruded foam insulation board, which comes with a cool silvery radiant barrier on one side. I bandsawed the outside of the foam, and used a scroll saw to cut the inside holes, and finally a router to mill out the groove that supports the thruster’s aluminum beam. There are three layers: a one inch thick “skirt” layer on the bottom, a two inch thick motor layer on top of that, and on top a two inch thick prop guard and electronics protector. The layers are held together with wood screws, which I very gingerly thread in by hand (the foam is very easy to strip out, but if you’re gentle a wood screw will hold). The all-in weight of the frame is 400g, plus 300g for the battery, which is heavier than I’d like, but the vehicle is two feet long by one foot wide, so it doesn’t feel too dense.
In theory, this thruster is rated for 1kg of thrust in clear air at maximum power, which is *incredible* for a 56 gram motor (thrust-to-weight ratio: 17+). In practice, it’s not only powerful enough to fill the “skirt” with air and float nicely on the ground, at 70% power it’s enough to lift the entire front end off the ground and flip the whole vehicle over!
The ingot balanced on top weighs 2kg. At about 85% power, I can still lift off the ground even carrying this ingot–so I could strap a netbook on this thing for an onboard controller!
As Steven and Mike both predicted, with a single prop the entire vehicle tends to counterrotate, moving in the opposite direction that the prop is spinning. I tried to counteract this with a little tilted trim tab pointing down into the prop wash, but that wasn’t enough. With the single lift prop, and the battery not centered over the prop, the back of the vehicle tends to drag along the ground.
Finally, my plan for horizontal motion was to rig up some PC case fans, but even my fastest 120mm case fan only produced 30 grams of thrust. Since the fan weighs 300 grams, this is not only a disappointing thrust-to-weight ratio of 0.1, it’s not enough to overcome the drag on the back end. Since my other brushless motor is currently part of the aero balance beam experiment, I’m considering making a new chassis using a case fan for lift, and the much lighter but more powerful brushless motor pushing horizontally.