Some time ago we had a discussion about heavy hover tanks over water. The
conclusion was
that if the tank massed enough that the pressure/area (PSI in US terms)
was too great the tank would sink while furiously blowing bubbles from under
its' skirt. Basicly a Hammer's Slammer
hover-tank would probably sink.
The thing we didn't discuss was how movement would affect this. If the tank
were moving at 100KPH would this affectively give it a greater area under the
skirts?
Any ideas?
> On Wed, May 25, 2005 at 11:53:07AM -0400, Roger Books wrote:
The
> conclusion was
was
> too great the tank
Basicly a
> Hammer's Slammer
ISTR that this does indeed happen in one of the stories.
> The thing we didn't discuss was how movement would affect this. If the
Depends on the design, but generally the skirts are designed to keep outside
air away from the cushion as much as possible. I very much doubt that
compression under the leading edge will be significant compared with
compression from the main lift fan.
On the other hand, a vehicle could be _designed_ for this mode of
motion; googling for "ekranoplan" will show a lot of vehicles that were quite
specifically intended to operate over water.
R
> Roger Books wrote:
> >Some time ago we had a discussion about heavy hover tanks over water.
The
> >conclusion was that if the tank massed enough that the pressure/area
Not just "probably"...
> >The thing we didn't discuss was how movement would affect this. If
Don't think so, no. If it is really lucky it might plane on the water
surface much like a fast-moving snow-mobile can do - but that requires
that it comes into actual physical contact with the water (as opposed to being
separated from the water by its air cushion), so it'd run a major risk of
damaging at least the rear wall of its plenum chamber.
> Roger B-W wrote:
> Depends on the design, but generally the skirts are designed to keep
But such a vehicle would not be a hovercraft. Ekranoplans - aka WIGEs or
GEVs (the proper use of the term "GEV", as opposed to the incorrect SF use
of the term to mean "hovercraft") - have no plenum chamber; they are
essentially aircraft with wings too stubby to allow them to fly very high,
instead relying on the lift-increasing Wing-In-Ground-Effect (or just
"Ground Effect" for short) to keep them flying.
Regards,
> On Wed, May 25, 2005 at 06:24:45PM +0200, Oerjan Ariander wrote:
> But such a vehicle would not be a hovercraft.
Oh, certainly.
> Ekranoplans - aka WIGEs or
On the other hand, there's no technical reason why one couldn't hybridise the
designs; the Orlyonok, for example, relied on blowing air under the wings at
least to break free of the water surface (I haven't been able to confirm
whether it was needed in cruise mode as well), which is a hovercraft
characteristic much more than an aircraft one.
What I'm suggesting is that the designs are in a continuum, and if one
wanted to build a hovercraft with WIGE-like characteristics it would
perhaps not be the huge engineering compromise that, say, a
hovercraft/conventional aircraft would be.
R
The Ekranoplane uses a slightly different ground-effect technique to
generate lift. The downflow of air from the wings rebounds from the ground and
back up to provide additional lift to the craft. This effect
is limited to a height roughly 1/2 the wingspan. In addition, the lift
effect only kicks in at speed, so at low speeds (i.e. taking off) the
Ekranoplane is just a fast boat and sits in the water. It has no capability to
hover or move slowly over terrain.
As for a heavy GEV craft being able to get across water - think skipping
stones. Obviously rocks are denser than water, but it is possible to skip them
right across a pond. Don't know if you can do that with an
air-cushion (not enough substance to redirect all that momentum quickly
upward) but in theory a fast moving object with constant propulsion should be
able to skip indefinitely across water.
Perhaps tanks could be equipped with a small set of retractable hydroplanes to
provide additional lift?
--Binhan
[quoted original message omitted]
> At 10:45 AM -0600 5/25/05, B Lin wrote:
Like the new USMC AAAV?
http://www.fas.org/man/dod-101/sys/land/aaav.htm
Ok, but despite the fact that you theoretically *could* build a hybrid design
that would hover over both land and water, that is NOT what the original
poster asked about. They wanted to know if a HEAVY hovercraft which is NOT
capable of crossing water, could skip across it like a stone given sufficient
velocity.
Granted, with the right shape, you *might* be able to do this, but as OA
(formerly known as OO) pointed out, your plenum chamber is going to at *least*
be damaged. And the crew is NOT going to want to be skipped across a pond in a
brick.
If the vehicle (hybrid or not) is capable of crossing water, then it's a
hovercraft which is capable of crossing water, and NOT what the poster asked
about.:)
J
John K. Lerchey Computer and Network Security Coordinator Computing Services
Carnegie Mellon University
> On Wed, 25 May 2005, Roger Burton West wrote:
> On Wed, May 25, 2005 at 06:24:45PM +0200, Oerjan Ariander wrote:
So unless you have some sort of hybrid hovercraft/hydrofoil your tank is
going to sink even if moving at a high rate of speed. The hydrofoil mechanism
won't work through a highly compressed and contained column of air.
For some reason that doesn't "feel" right. Too bad I don't have time to build
a radio controlled hovercraft.
> Ryan Gill wrote:
> >Perhaps tanks could be equipped with a small set of retractable
Something like that, except that the AAAV floats even when stationary whereas
the tank would need to hit the water at high speed and never slow down in
order to avoid sinking. (The AAAV also doesn't have anything on
the bottom which can be ripped off when moving fast over water :-/ )
> RBW wrote:
> >Ekranoplans - aka WIGEs or
Doesn't make it a hovercraft-ekranoplan hybrid, sorry. Without a plenum
chamber there's no trace of any hovercraft heritage.
> (I haven't been able to confirm whether it was needed in cruise mode as
Almost certainly not - just like normal flying boats, WIGEs need far
more
power during take-off than during flight.
> which is a hovercraft characteristic much more than an aircraft one.
IMO it is about as much a "hovercraft" characteristic as booster rockets
for carrier-launched aircraft are, or the underwing exhausts on a
Harrier... or the engines of just about every flying boat in history :-/
> What I'm suggesting is that the designs are in a continuum,
And what I'm suggesting is that they aren't! Sure, you can build a hybrid
hover/WIGE in the same way as you can build a hybrid boat/car (not just
an
amphibious car, but an amphibious car with boat-shaped hull/boat with
four
wheels) or van/aircraft, or why not a van/spaceship like Lone Star's
Space Winnebago... but all these craft have two *separate* sets of propulsion
equipment; they're not sliding on some sort of "continuum" between their
different propulsion types.
Regards,
The hover ships / tanks that are landing craft still exert less than 8
pounds per square inches to go over the water. That means you have to blow the
skirt to hell and back and then most have water tight hulls underside to float
if power shuts off. If we do this today, why would that change radicly in the
future. Most engineers aren't brave enough to
go against proven techniques. :-> Does this help any? ;-P
Roger Books <roger.books@gmail.com> wrote:Some time ago we had a discussion
about heavy hover tanks over water. The conclusion was
that if the tank massed enough that the pressure/area (PSI in US terms)
was too great the tank would sink while furiously blowing bubbles from under
its' skirt. Basicly a Hammer's Slammer
hover-tank would probably sink.
The thing we didn't discuss was how movement would affect this. If the tank
were moving at 100KPH would this affectively give it a greater area under the
skirts?
Any ideas?
Roger Books (offlist for some time)
A hydrofoil would work in conjunction with an air cushion, but would only be
useful at higher speeds to generate sufficient lift. A hydrofoil is
essentially wing that is submerged in the water (thus the requirement for it
to be retractable, otherwise you will be plowing dirt the rest of the time).
Hydrofoils (airfoils) generate lift partially based on speed of the material
over and under the foil to generate a density differential that appears to be
"lift". The faster you go, in general the more "lift" you get. The downside to
hydrofoils is that they have to be immersed in the water and at relatively
high speed to be effective (i.e. hydrofoil ships don't rise out of the water
until they
reach speeds of 40+ knots) so if a hover tank was dependent on
hydrofoils to provide additional lift to keep it out of the water, as soon as
it slowed down, it would sink into the water.
--Binhan