Ducted-Fan Flying Hoverboard
by Bill Butler

Conventional hovercraft use ducted-fans for levitation and propulsion, so it makes sense that a scaled-down version would also benefit from the same proven technology. Yet for many of us, fans aren't the first thing that leaps to mind when we think about hoverboards. Maybe it's an innate fear of whirling blades, or the misconception that a ducted-fan hoverboard, like a hovercraft, would never get more than a fraction of an inch off the ground. Either way, there are many good reasons for designing a hoverboard with ducted-fans.

If getting more than a few inches off the ground is a priority, then fans are currently the best solution available. Rockets, as used in jetpacks, only offer about 30 seconds of flight time before running out of fuel- although turbojets promise to extend that to twenty minutes. Fans are still the propulsion mechanism of choice for many existing aircraft and concept vehicles including the V-22 Osprey, the Moller Skycar and many other VTOL concepts still on the drawing board.

Another benefit of fans is their high efficiency. So far, no novel system of generating thrust can compare to the efficiency of a fan. Case in point is the Lifter which produces ion wind without moving parts, but only delivers a thrust to weight ratio of 1g per watt. In comparison, a typical helicopter produces upwards of 8.5g per watt. Now with today's lightweight lithium-ion batteries and high-output electric motors, we can build model electric helicopters that outperform the real ones. And once scientists figure out how to build large lithium-ion cells or other lightweight energy storage devices, electric VTOL (vertical take-off and landing) aircraft will take off in a big way.

One such energy storage device may be a flywheel. Flywheels store kinetic energy which may be converted into electrical energy by a generator, or tapped mechanically to drive say a ducted-fan. The round-trip energy efficiency of flywheels can be as high as 90%, and their energy densities can exceed 130 W·h/kg. Some of the more advanced flywheel designs are made from carbon composite or diamond fiber and rotate at speeds over 50,000 rpm. By using magnetic bearings and housing the flywheel assembly inside a vacuum chamber, friction and drag can be virtually eliminated.

Flywheel energy storage has many advantages over chemical batteries. Recharging, for instance, can be accomplished in minutes rather than hours. In addition, flywheels have no charge/discharge cycle limitations giving them extremely long-life. They are not affected by temperature changes as with chemical batteries, nor do they suffer from memory effect. They are also less damaging to the environment due to their composition of largely inert or benign materials. The biggest drawback to flywheels is their tendancy to explode if the tensile strength of the flywheel is exceeded. This risk, however, has been minimized by the use of better materials and tighter quality control over the assembly process.

Perhaps what makes the flywheel best suited for powering a ducted-fan has to do with energy conversion. Since the fan can extract kinetic energy directly from the flywheel, there would be no need for a separate motor. Instead, coils around the edge of the fan could act like a magnetic clutch to draw kinetic energy from the flywheel as needed. And with the fan also supported on magnetic bearings, there would be nothing to break or wear out.

For highly detailed plans on how to build a flywheel/ducted-fan, please see our recently updated e-book Personal Flying Machines.

As for the airworthiness of the Green Goblin board, the physics are mostly valid. Though judging by the size of the twin ducted-fans, turbojets, or perhaps a flywheel/fan combination would be required to produce the necessary thrust along with a way to ensure their speeds are perfectly matched to prevent spinning out of control.

Stability could be a major issue due to the high cg of the rider standing on the board, although there appears to be several rocket thrusters on the bottom of the board for attitude control. Gyros could provide the same effect for less energy, although at the expense of additional weight. Interesting enough, a pair of counter-rotating flywheels could also provide stability, though turning would become more difficult. Finally, for propulsion, the Goblin board uses a large centrally-located rocket. While it indeed makes for a more menacing machine on the screen, it's use is total overkill. Simply tilting the board will vector the thrust from the ducted fans to generate enough speed for even the most demanding supervillian.

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