Re: Underwater questions [ot]

Excerpts from mail: 3-Nov-99 Re: Underwater questions [ot] by Beth
Fulton@marine.csiro 
> >Maybe.  Also, a helicopter type vehicle might be good - no wings. 
You'd
> >need a secondary (water) propulsion mechanism, though. 
>  
> What if you just had the blades slip behind you instead to give a
propeller
> kind of set up?
>  
Well, then you've got to have space for the blades (meaning the tail of
the 'copter has to move out of the way or something).  And while air and
water are both fluids, the density difference means that you need
different kinds of rotors to move through different fluids.  

Reconfiguration in general is a hideously difficult task.  We all see
transformable robots, etc. in the media and think that it's plausible;
but it's really not easy to make things reconfigurable and robust
simultaneously.  It's especially difficult when you're dealing with
something moving at the speeds (and experiencing the stresses) that an
airplane does.	As an example, there are several projects at Carnegie
Mellon (my school) dealing with reconfigurable or modular robots.  They
haven't gotten very far, because what you gain in articulation you lose
in strength, so modular robots of any size have a tendancy to fall apart
under loading (or even under their own weight).  

I guess what I'm stressing here is the incompatability of rigid
structural members and reconfigurability.  If you had exotic materials
(pneumatically inflated wings made of efficient and cheap shape-memory
alloys, maybe), then you might be able to do a lot more reconfiguration.
 But the basic tradeoff is still there.

A manta-shaped lifting body with some "flapping" ability is probably
your best bet if you don't want to have two unrelated movement setups
(helicopter rotor plus submarine screws, for example).

Again - this *is* science fiction, so you *can* just wave your hands :)