this is what i've been thinking about...didn't know such a thing was
already being explored.

i've been trying to think about just how to calculate the minimum size
blimp needed to carry a recumbent...turning a propeller.

http://www.treehugger.com/files/2008/06/pedal-powered-blimp-attempts-english-cha\
nnel.php

i was also thinking back to my hang-gliding days. consider a stiff
wing, filled with helium; transparent on the top side, and black on
the bottom side. lift from both the helium and the hot helium, no? how
big would the wing have to be?