Here's an idea that could make you a billionaire. In the 1950's, Hiller Aircraft Company made a prototype flying device (for the US Navy) known as the Hiller Flying Platform. It's straight out of science fiction... but it worked! You stand on this small round platform and hold onto the handles and you can fly around like you're on a magic carpet. It uses two counter-rotating blades for stability and it's remarkably maneuverable. You can get the patent online and that's probably enough information to build your own version. If you could create a working consumer version of this device, you could easily sell 100,000 units for $29,900.
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From @sultanmr