Re: GEVs, ACVs, ground effect, etc etc etc

OK, a bunch of comments about disparate parts of this thread:

1.  Down hill thrust vector:

>You are making the mistake of thinking of this as a fan blowing air
>down.	It
>is not, it is compressed air in a plenum.  It doesn't press harder at
>any point,
>it has a standard 1 or 3 or 5 or whater psi.  All the fans do is put
>more air
>in to replace that escaping under the bottom of the plenum.  The only
>way you
>get an affect of lifting one part more than another is if the design is
>poor
>and one part weighs less than another, and in this case it will do this
>on
>level ground.
>

OK, hang on a second.  We're actually mixing parts of different threads
here.  I originally posted the bit about the down-hill component of the
thrust vector.	At that point in the thread, what we were actually
discussing was using the fans on the bottom of the vehicle on
"supercharge"
to lift the vehicle over hills, obstacles, etc by aerodynamic lift.  The
conversation turned to the size of rotor blades necessary to do this
(they
would be BIG by necessity, and unsuitable for the bottom of a vehicle).
Then I tossed in the bit about the thrust vector.  This didn't really
have
anything to do with air escaping from the skirt providing some kind of
thrust, though that would have SOME effect (I don't think too much, as
it
would be escaping around the whole skirt at more-or-less the same
amount).

What I was referring to is the idea that if you have a
fan/rotor/propeller
fixed in the frame of the vehicle, it produces thrust downwards.  When
the
plenum is closed (ie vehicle is sitting on flat-ish surface) it is
acting
as a pump, forcing air into a mostly-closed volume, and the "thrust" it
provides is by no means holding up the vehicle.  But it IS moving a mass
of
air.  When it goes to "supercharge" mode to hop the vehicle over
something,
the fan is using aerodynamically created "thrust" only (ie the air
cushion
is no longer relevent).  Either way, there is a thrust vector acting
away
from the force of gravity (ie downwards).  If the fan is tilted (say,
because the vehicle is travelling UP a hill, and the vehicle tilts
backwards), the thrust vector is now not acting directly against the
force
of gravity.  It is "split" as it were, and though part is acting against
gravity, part is by necessity acting in the direction perpendicular to
the
surface of the hill - and thereby part of THAT part is acting to push
the
vehicle in a direction opposite to that in which it is travelling...  I
could DRAW this in a vector diagram simply, but my ascii-graphics skill
isn't up to it...

If the fan/rotor/prop was in a gimbal mounted housing so it always
pointed
down relative to gravity, this wouldn't be a problem, but as to the
ORIGINAL idea, which was to overpower the fans to counteract the losses
'cause of the vehicle tipping, etc my comment still stands - your
increased
fan power would actually be at least partially working against the
vehicle
getting up the hill, and you would THEN have to have an even MORE
powerful
thrust fan/device pushing you up the hill - and all of this seemed
really
inefficient.

As to the point that "if we can posit Grav vehicles then why get hung op
over hovercraft going up hills" - well, yes.  I buy into the idea that
the
future-hovercraft has a limited duration VTOL capability using a
different
type of thrust unit (ducted fan, jets, rockets, etc) but I don't like
the
idea of grav units at all.  If you can use grav units, why not just make
a
grav vehicle.  I know, one could make lots of arguments against that
(maybe
grav in small units is cheaper, maybe it's easier to make in small
units,
maybe etc etc).  But I see hovercraft as being a "lower tech"
alternative
in the Tuffleyverse, anyway.  It's easy to make, and doesn't require too
high an industry base tech level.  Hovercraft are things that could be
manufactured and supported on distant colony worlds with relatively
lower
tech.  Grav is so revolutionary, if it were common it would change
everything - and in my incarnation of the Tuffleyverse is subsequently
very
rare.  We're debating hovercraft 'cause we see them as more plausible on
a
common basis in the Tuffleyverse.

2.  GEV vs. ACV  

This is kind of a fiddly point here, but there is a BIG difference
between
"Ground Effect Vehicles" and "Air Cushion Vehicles".  Ground effect is
an
aerodynamic effect encountered when an aerofoil is creating lift (ie
flying) close to a surface (ground, water, etc) - usually within a
distance
roughly equivalent to the length of the aerofoil (well, ish).  Roughly
speaking, air is compressed between the aerofoil and the surface
creating a
higher-than-normal pressure zone, and the aerofoil is "artificially"
more
effective.  It creates more lift for a given speed, or to put it another
way, you can use a much smaller aerofoil to create a given amount of
lift
than you would need to fly freely.  This makes it really interesting to
land gliders, with their low weight but really long wings, 'cause when
you
are just about to touch down, the wings suddenly become a lot more
effective and you have a tendancy to float along for a much greater
distance than you would think the glider was able to.  Gotta be careful
when planning the landing to account for this.	Less of a problem with
powered aircraft, 'cause their wings are a lot shorter relatively...   

Anyway, the Russian GEVs that someone else mentioned recently are
examples
of vehicles designed to exploit this.  They are huge great big machines
(the biggest version was 707 sized, and could carry several hundred tons
of
cargo) that can travel at a few hundred miles per hour, within about 10
feet of the surface.  Great for ferrying HEAVY cargo over long distances
quickly - provided you don't hit rough surfaces.  There is a US company
building a small civilian vehicle (appx 4 seats) that does the same
thing -
I dont' know if they ever got it certified.

A hover craft is an Air Cushion Vehicle.  It does not use the
aerodynamic
effect called "ground effect" but, as we all know, rides along on a
compressed cushion of air contained in a chamber - like an air hockey
puck.
 Similar SORT of idea.	Completely different practical effect and
technological requirements to use it, though...

The Russian GEVs are cool - but rather limited.  And you can't go SLOWLY
in
them - which limits their effectiveness for over-land military use.

Air cushion vehicles, on the other hand, are combat proven.  I'm sure
there
could be some kind of VTOL/ACV combination that, using simple (non-grav)
tech could lick the "how do I get up a hill" problem, but I only see
them
operating in the light-armour, transport, scout, type roles - not as
front-line MBT's.  That "sinking into the lake/swamp/river/soft earth"
problem is a big one :)

Now, the "put auxilliary tracks on them for climbing hills" idea makes a
lot of sense, but they'd be UGLY (if the tracks were on the outside -
offends my aesthetic sensibilities...).  On the other hand, the tracks
could be retractable, and reside in housings up underneath the skirts.
When you get to a hill, down come the tracks and they pull you up.
Wouldn't be necessarily fast, but would work - and you wouldn't suffer
the
complete loss of the air cushion - which would happen if you hopped it
over
the hill.  AND would require a LOT less lifting power than the VTOL
option.

Anyway - there you go...  

We don't all have to race out to put tracks on the sides of our GZG
Hover
Jeeps, 'cause they're tucked up underneath... :)

Adrian Johnson
ajohnson@idirect.com