Todd Pattist wrote in message . ..
"Marc" wrote:

2) Where is the power source? Conservation of energy says that the plane
could not continue to ascend and descend without some input of energy.

A gas balloon can oscillate up and down by harnessing the
temperature differential with altitude. As it rises, it
cools, the gas contracts and the balloon then falls to
lower, warmer, altitudes, where the gas expands and the
cycle repeats. It could in theory have wings that allow it
to glide somewhat during each cycle.

I understood that as the balloon rises, the decreasing atmospheric
pressure allows the balloon to expand, displacing more air and
increasing lift to compensate. Some guy invented a black balloon some
years ago, really big, that was entirely solar powered. It worked.

Dan