Re: Underwater questions [ot]

Steven M Goode wrote:

> > >5)  What is the best shape for a lander desgined to move though
the air and 
> > >go into the water?
> >  
> > Hydrodynamics and aerodynamics share an awful lot in common due to
the
> > basics of fluid dynamics. Just remember to give it stabilisers so
it
> > doesn't go into a spin once in the water.
> >  
> 
> Yes and no.
> 
> Water differs from air in two major ways that impact what you're
discussing:
> -Buoyancy
> -Density/viscosity
> 
> In air, a plane must support itself through lift.  This means that
the
> plane must be light, strong, and have a large wing surface.

...while balloons and airships are supported vertically through
buoyancy in air. They're not particularly fast or able to survive
under-water pressure levels, of course.

> In water, a vehicle is supported vertically through buoyancy.

...or flies through it, eg with hydrofoils. Vertical attitude fins on
submarines and torps tend to generate a non-negligible amount of lift
as well.

'Course, buoyancy and lift both come down to the difference in pressure
between the upper and the lower surfaces of the body in question; if
the difference is large enough to overcome the gravitic pull on the
body it will fly/float. The main difference is the physical mechanism
which creates the pressure difference <g>
 
> In order
> to move through the water at a reasonable speed, it needs to have a
> small cross-section with nothing that will snap off due to the drag
on
> the vehicle (like wings).

The cross section isn't as important as the general shape of the body,
though. Yes, low-drag bodies are fairly slender, but a thin wire
perpendicular to the flow has a rather high drag :-/
 
> Thus airplanes and submersibles are fundamentally different, and I
doubt
> you could make a very effective vehicle that operated both in the air
> and the water unless you've got exotic technology (like anti-grav).

Most vehicles we have today which operate both in the air and under
water are single-journey, but I strongly doubt if they have anti-grav
technology...

There have been some projects on swing-wing submersible flying boats,
too; AFAIK none of them got beyond the "working prototype" stage, but
IIRC at least one of them actually both flew and dived - and survived
<G>

Regards,

Oerjan Ohlson
MSc Fluid Dynamics
oerjan.ohlson@telia.com

"Life is like a sewer.
  What you get out of it, depends on what you put into it."
- Hen3ry