Re: GEV on other worlds

Hi Allan,

Thanks for the detailed input!

agoodall@att.net wrote:

> No, that's not true. Atmospheric pressure on Earth, at sea level, is
> roughly 14.7 pounds per square inch. A square metre of surface has a
> column of air on it that weighs about 10 metric tonnes. This does not
> mean, though, that something that's a square metre in size weighs 10
> tonnes! The reason has to do with buoyancy.

Ah, yes, I agree; I was sloppy in my terminology: I meant the pressure 
generated by the GEV cushion fans depends on the atmospheric pressure, 
not that the vehicle itself was exerting greater pressure on the ground.

  My bad.

> This is where the heavier atmosphere comes in. You're moving more gas
> molecules with each rotation of the propellor blade in a heavier
> atmosphere than you are in a thinner atmosphere. The propellor
> doesn't have to move as fast in order to lift the GEV in a dense
> atmosphere compared to thin atmosphere. Aircraft see the same thing,
> but they also have to deal with the heavier atmosphere causing drag,
> which is less of an issue with hovercraft.

Right.	The same volume rate can be achieved with fewer revolutions in a

thicker atmosphere.  But doesn't the engine have to work just as hard 
because it is after all pushing the same amount of atmosphere?	I.e. 
energy is expended to move air -- in both cases it's the same amount of 
air, so by conservation of energy arguments you don't get any advantage 
either way.  Or should that more appropriately be conservation of 
momentum...?

> So, no, the heavier atmosphere does _not_ offset the gravity effect.
> I'd imagine that a GEV with vacuum sealed cabin would work better on
> Venus (with less gravity but a much thicker atmosphere) than on the
> Earth (though, of course, you add weight to the GEV in order to make
> it air tight, and if the cabin of the GEV is ever punctured the crew
> is toast).

Again, though, isn't this sort of like running in a swimming pool?  The 
GEV fans have to push against very thick atmosphere (requiring more 
energy), but they don't have to turn as fast to push the same volume 
rate (requiring less energy) -- so it comes out even in the end.

Or does it?  ;)

> Wrong. If you are spinning your propellor (turbine, whatever) twice
> as fast, you're using up more fuel in the same period of time. Even
> in a 100% efficient system, it stands to reason that you'll use up
> more fuel spinning a propellor 1000 revolutions per second than if
> you were spinning it 500 revolutions per second.

See above...

Thanks again.

Cheers,
--Jerry

-- 
Jerry Acord [+] acord@imagiware.com [+] http://imagiware.com/acord/