There's an article over on
Damn Interesting about a physicist named Robert Hunt who's proposing a "gravity powered" airplane. He's formed a company, Hunt Aviation to pursue it.
The basic concept is that the plane has two nacelles containing helium bladders. The bladders can be inflated or deflated at will. The plane starts on the ground and the bladders are inflated, making it bouyant. It rises to the desired altitude and then the bladders are deflated and the plane glides to a lower altitude on its wings. Once at lower altitude, the helium bladders are re-inflated and the process repeats.
I see a couple of problems with this idea. The first is that, in order to carry any appreciable amount of payload, the helium bladders and their associated nacelles would have to be huge. The wings and associated structure of this vehicle make for a lot of added weight that conventional lighter-than-air ships do not have, and those ships already need an immense volume of helium to carry any appreciable payload. It would be interesting to run the math.
The second problem I forsee is that the constantly changing altitude would cause pressurization problems. The pressurization of the cabin space of the vehicle would need to be changing constantly, and would need to be designed to withstand a large number of pressurization cycles.
Am I missing something?
(Hunt also has a design for a vertical axis wind turbine on the Hunt Aviation web site. It uses "blades" that expand when they're positioned to provide power and collapse when they're not. Interesting but very complex mechanically, which is not a recipe for long-term reliability).