On Tue, 06 Jan 2004 09:37:45 -0500, nafod40
wrote:

E. A. Grens wrote:
As Bob notes, the "second law" says you can't win; the "third law" says you
can't break even. It is very hard to break these laws. Here, the energy
required to change buoyancy, considered in the underwater version, is
apparently ignored. Just another "perpetual motion" machine.

the wiley dirigible designer would hang some solar cells on the back of
that beast, and be able to pump air in and out to his heart's content.
You could heat and cool the air using the day/night cycle to give you
the positive and negative bouyancy. You could probably figure out some
way to make it switchable between reflecting and absorbing to allow
multiple positive and negative bouyancy cycles in a day.

The things big enough that you might be able to use the differential
wind speeds on its surfaces for energy, ala dynamic soaring.

The point is there is plenty of energy swirling around out there for the
grabbing, so no laws of physics need be violated.

Of course. Bear in mind that the only winged submersible proposals
I've seen detail in (the one man deep-diver) involved positive
buoyancy at all times for safety. It could glide upwards, but used a
combo of electric propulsion and its wings to dive.

As to this dirigi-glider thing: of course it *could* use solar power
to run its buoyancy change system but as described on the website it
doesn't. Instead it is planning to use wind turbines to extract that
energy during descent (and also wrecking its glide ratio), so its is,
as described, just another perpetual motion device.

Also note that, unless the buoyancy is reduced virtually to zero
during descent and raised to about twice the flying weight during
ascent its glide will be very slow due to the minimal potential energy
available to drive it forward and at some point as the weight tends to
neutral the drag forces will prevent ANY forward motion. Add on the
benefit (?) of the symmetric or symmetric flapped wing that's needed
for equal efficiency during climb and dive and I think the idea is
pretty much a turkey.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :