"Louis L. Perley III" wrote in message
...
Saw an article today about an aircraft that would change it's buoyancy, so
it would float up like a balloon and then change buoyancy and glide down.
Interesting stuff, although I doubt we'll see it anytime soon. Is
something
like this even practical? The article says they take the aircraft up 10
miles to get 400 miles range, I would think ATC wouldn't like the idea of
a
free balloon (which is what it would be at that point) up in the flight
levels.


http://www.machinedesign.com/ASP/vie...PACE&catId=379



1) I wonder why a typical blimp has to use such a large volume (the ballon)
to provide the lift and a relatively tiny cabin attached underneath, whereas
this craft seems to use a much smaller volume (the wings). Has the inventor
calculated this correctly?

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. The
initial ascent would be powered by the energy required to separate helium
from the atmosphere, which is accomplished on the ground, but it should not
be true that the plane would "always land with its tanks fully pressurized",
since theoretically the amount of energy generated by the turbines during
the descent could not be enough to compress the gas used for the ascent.
Esecially if the efficiency of the turbines is 20%.

On the other hand I still think that Boeing 747s get off the ground by some
sort of magic. There is no way such a huge vehicle could be lifted by thin
air.