It might look like some kind of crazy machination that adorned the cover of Popular Aviation back in the '60s, the flying machine pictured above is the real deal. Developed by NASA, GL-10 Greased Lightning is an unmanned hybrid-electric aircraft that can swivel its wings and engines -- into the vertical position for vertical takeoff and landing (VTOL), and then horizontal for conventional flight. The fairly recent advent of electric propulsion -- thanks in large to the growing power and energy density of batteries -- allows for some very efficient hybrid-electric aircraft designs that could finally replace the current (massively inefficient) king of VTOL: the helicopter.