(via Futurismic) Alright, this is cool. Hunt Aviation, a Nevada-based aviation company, is working on an aircraft that could carry passengers and cargo arbitrarily long distances, all without fuel. They're calling it the GravityPlane.

Getting lift from helium, and using compressed air to add a bit of propulsion, once the GravityPlane tops out at ten miles or so, it deploys wings and uses the pull of gravity to drive turbines that refill the compressed air tanks. It can glide at high speeds, in a slow descent, for 400 to 600 miles. When it gets too low, it shoots out the compressed air again, rising back up to 10 miles, and starts the process over again.

I lack the math and engineering fu to say whether this is feasible or not, but it seems workable to me. The GravityPlane is still on the drawing board, so it remains to be seen whether they can build a working prototype, but wouldn't it be cool if they did?