I'm not an engineer, but try this... A Massive Wing Area (think feather?) and very little weight. Without some greater weight, ambient winds and other weather will control that 'feather' no matter how much thrust is applied. The control surfaces can do only so much, especially at low speeds. While 'forcing' a super-light 747 off the ground may be possible, controlling an even lighter one for landing in anything but a perfect headwind must be a significant challenge. I don't know how to fly feathers.
