Skirt information help!!
Troy Alan
How does the skirt of a hovercraft work? Is it completely underneath the
hovercraft and have holes in the bottom or is it just a peice of material
that is taunt at the bottom as to not let air outside of the underneath of
the craft? I have read alot about the skirts and how the craft floats on
air, but I can't seem to find any info on exactly how a skirt works and what
it looks like on the underside? Any assistance on this would be VERY VERY
helpful.
Troy
Ken Roberts
no, the skirt does not go all the way under, no matter what type of skirt it
is.
there are four major types of skirt. there are several combinations that are
used. there are various methods of feeding the skirts.
the skirt's purpose is to provide a barrier which keeps air under the
hovercraft. the lift system pumps air under the craft somehow, (lots of
different methods) and the skirt prevents its easy egress from the bottom of the
craft. the resulting high pressure area lifts you up. note that the
ground/water is ALWAYS the bottom barrier to the egress of the air. no matter
what type of skirt you use.
your first skirt is an air curtain. this is no longer used so far as i know.
it's a high pressure jet of air shooting down from the rim towards the ground,
and towards the center of the craft slightly. this takes huge amounts of power,
which is why nobody does it anymore. my opinion is that this method's press in
the early days is why lots of people think hovercrafts take huge amounts of
power.
your second skirt is a "c" skirt. this is a single layer of flexible material
with its top attached to a round hovercraft's outside edge. the skirt "hangs
down" if you will, and comes close to the ground. this is a dramatic
improvement over the air curtain in that not as much air needs to be pumped
under the craft. it's not commonly used because your craft must be round or
nearly so, and it collapses easily.
next (in complexity) is the bag skirt. imagine a c skirt that's maybe 3 times
as high as you want it. pull the bottom edge up and inward and attach it to the
hull so that the distance between the inside and outside attach lines are maybe
1.5 times the desired hover height. that's the basic idea of a bag skirt, but
the oversimplification is going to start a big argument here. suffice it to say
that the bag is pressurized and so is the air enclosed by the bag, the hull, and
the ground. the bag is usually pressurized to slightly more than the inside
air.
next is the finger skirt. to get the basic idea of a finger skirt, take a
square sheet of paper and bend (not fold!) two opposite corners together so they
would form a triangle if you were to crease the bend. now imagine that that
paper is skirt material instead, and that the two corners you're holding are
attached some distance from the outside edge of the craft. the top corner that
is loose is attached to the outside rim of the hull. there is usually a hole in
the hull that feeds the finger. the open space that is hanging down is truly
open. this also is an oversimplification that may start a war. some finger
skirt systems are fed as i described, each finger having a hole in the hull
through which air is pumped. others are passively fed from the area enclosed by
the skirt.
some crafts use variations on these schemes, and others use combinations of
them. sevtec, for example, uses a bag skirt around the sides and back but uses
a double c skirt in front, with a partition between the left and right halves of
the front. this allows for selective plow-in control. some racing crafts have
experimented with a bag that has a very tiny c skirt on the bottom of it to
reduce drag. some very large commercial crafts such as the srn-4 that used to
ferry cars across the english channel use a bag skirt that feeds a finger skirt.
in short, there are lots of ways you can do this. most recreational designs are
either a bag skirt or a finger skirt. also as a general rule, homebuilts tend
to be bag skirts more often than not, and "turnkey" crafts tend towards the
finger skirt. each has advantages and each has proponents and opponents. as
with anything else in life, none is perfect for every application.
Troy Alan
next question, is air then pumped into the bag skirt and into the center of
the haul? how is the pressure on in the bag skirt higher then the pressure
in the center of the haul?
"Ken Roberts" <k...@roberts.9ci.com> wrote in message
news:slrn9ekmo...@roberts.9ci.com...
Ken Roberts
i'll ignore commercial crafts for the most part because i have little or no
experience with them directly. i've seen pictures and that's about it.
recreational hovercrafts which use bag skirts tend to fill the bag and the
central area in one of three ways i've seen:
1 a single fan supplies both lift and thrust.
2 a dedicated lift fan feeds both the bag and the central area directly.
3 the bag is fed from the pressure of the central area.
i've also heard of a recreational craft which feeds the bag and that air feeds
the central area. this is called a serial feed and is usually not used on
recreational crafts. the third item in the list is what i call reverse serial
feed, which i've only seen on sevtec crafts. this feed, by its very nature,
can't have higher pressures in the bag than in the central area. this is by
design.
whatever way you do it, (and there are probably others i didn't think of) you
feed air into the lift system. on a bag skirt, (we're only talking about bag
skirts on this post) the bag has a tiny exit hole for air, water, dirt and
sticks, kittens, gloves and anything else that might go through your lift prop.
actually, kittens will almost certainly not be anywhere near a running lift
engine, so you can probably rule that out. kids and adults, though, aren't
nearly as smart as kittens.
it's so easy to get sidetracked! ok, the exit hole on the bag. most of the
crafts i've seen have that exit hole in the back, and the lift fan in the front.
i'm not sure how the single-engine single-duct people do it, though. i think
they still go out the back. when you're buzzing along, the incidental wave/dirt
contact will work any of the items you may have collected towards the rear of
the craft. these eventually find the exit hole (maybe 2" diameter on my craft)
and leave. since the exit hole is significantly smaller than the air entry
hole, the pressure is dictated by the size of the exit hole.
the central area, for which i'll have no name, i'll call the plenum chamber.
technically, a plenum chamber is a chamber inside the hull of a hovercraft which
transports lift air. crafts with finger skirts usually have them. don't
confuse that with the thing i'm talking about, which is a space UNDER the
hovercraft's lowest surface and above only the ground/water. let's call that
the "lower plenum chamber" for clarity. somebody really needs to either invent
a word for this place or tell me what it is. i certainly haven't run across it.
on a parallel feed skirt (item 2 above) the lower plenum chamber is fed from the
same fan that the bag is, only it gets a far greater percentage of that air.
the decision about the amount of air that goes to each place is made at design
time and is not usually very adjustable. there is a splitter, which is simply a
partition, which divides the air. on my craft, that constitutes a board to
which the the front portion of the skirt's inside edge is attached. anything
that goes between that board and the duct is bag air. the majority goes in
behind the board, though, and feeds the lower plenum chamber.
the lower plenum chamber's egress point is underneath the skirt. air is
constantly pumped in at extremely high volume, and it constantly leaves under
the edges of the skirt. this space is as big as it has to be, and never bigger.
if the skirt is too low to the ground for all the supplied lift air to exit, the
pressure in the lower plenum chamber increases. the craft goes up because of
this, and the opening between the skirt and the water/ground increases. the
air escapes, the pressure decreases, and the craft goes back down. it's an
inherently stable arrangement.
anyway, since the lower plenum chamber's exit air is essentially arranged "as
needed" it has a lower pressure than the bag air on this type of system.
the first item in the list is single engine, single duct. it is usually exactly
the same as item 2, which i've just described, only all the lift air is supplied
by the thrust fan, by use of another splitter.
Zardos
> the bag is usually pressurized to slightly more than the inside
>air.
I worked that one out on my own, go me !
i'm usually so dumb
Zardos.
May our bits collide, somewhere out there
=======================================
WebPage at www.hispc.demon.co.uk
=======================================
Gordon McAndrew
;>ok.
;>
;>i'll ignore commercial crafts for the most part because i have little or no
;>experience with them directly. i've seen pictures and that's about it.
;>
;>recreational hovercrafts which use bag skirts tend to fill the bag and the
;>central area in one of three ways i've seen:
;> 1 a single fan supplies both lift and thrust.
;> 2 a dedicated lift fan feeds both the bag and the central area directly.
;> 3 the bag is fed from the pressure of the central area.
;>
;>i've also heard of a recreational craft which feeds the bag and that air feeds
;>the central area. this is called a serial feed and is usually not used on
;>recreational crafts. the third item in the list is what i call reverse serial
;>feed, which i've only seen on sevtec crafts. this feed, by its very nature,
;>can't have higher pressures in the bag than in the central area. this is by
;>design.
Ken I think you had better look at a SEVTEC craft or at least some of the
build pictures more closely. Barry in one place that I have seen says that
the bag to under craft pressure ratio is 1.1 to 1. In some of the pictures I
have seen there is a hole on each side of the fan chamber that feeds the bag.
I do not think it is even possible to feed the bag from the under skirt area
because the bag would just be blown out of the way and not fill with air.
Gordon
Gordon McAndrew
Ken Roberts
gordon,
i'd be more than happy to retract, but unless my memory is faulty, i got that
information from barry on this newsgroup some time back. i distinctly recall
having a huge amount of skepticism that a skirt could stay down when the bag to
central chamber pressure ratio is 1:1. the answer supplied was that the venturi
effect between the surface and the skirt pulled the skirt down. when the skirt
makes contact with the ground, the airflow stops. venturi effect disappears,
and the skirt rises again. feeding from the central chamber was the way i
understood his description of the system, but i could very well be wrong there.
i do not think i'm wrong about the bag pressure ratio. i've heard other
designers wonder how he could keep the skirt down with that ratio as well.
no doubt barry will be on sometime soon to let us know the truth of the matter.
Gordon McAndrew
to 1 but there was no mention on Barry's part that the skirt was fed from the
central under craft area.
This site has a bunch of pictures of the Prospector being built.
SEVTEC Prospector
http://users.neca.com/cummings/sev.html
Look at the pictures that are near this text:
Fan Bay Assembly
The fan bays serve to direct air from two composite fans into the skirts and
bottom partitions.
Gordon
k...@roberts.9ci.com (Ken Roberts) wrote:
;>
Gordon McAndrew
SEVTEC
>From: gor...@arc.ab.ca (Gordon McAndrew)
>Date: 4/29/2001 7:17 AM Pacific Daylight Time
>Message-id: <3aec1a21....@news.ab.tac.net>
>
>I remember that discussion and the talk was that his ratio was very close to
>1
>to 1 but there was no mention on Barry's part that the skirt was fed from the
>central under craft area.
>
>This site has a bunch of pictures of the Prospector being built.
>
>SEVTEC Prospector
>
>http://users.neca.com/cummings/sev.html
>
>Look at the pictures that are near this text:
>
>Fan Bay Assembly
>The fan bays serve to direct air from two composite fans into the skirts and
>bottom partitions.
>
>Gordon
>
All of the Sevtec craft skirt feeds are from the area just under the fan.
little to no attempt is made to generate added skirt pressure by using the ram
pressure from the fan to add to the skirt internal pressure and get it above
cushion pressure, as is done in Universal Hovercraft and other designs. A pair
of skirt bleed holes are provided at the bow to vent skirt air into the forward
cushion (s) which allows air to escape when the craft is operating on
roughness.
If one doubts if the skirt can be held down by the venturi effect alone, just
examine a "C" skirt, where the entire interior of the skirt is open to the
cushion. In this case, the pressures have to equalize, but the skirt system is
unworkable because water boards it too easily, and in too great quantity to get
drained away.
The result of using such low pressure tactics is lower drag and most important,
much lower skirt wear.
Finger skirts, while frequently fed through holes in a hard structure or a bag
must by their nature be at cushion pressure, except for the ram pressure from
the air blasts from the feed holes.
I have seen small craft, such as the old Alaska Hovercraft "Mustang? Pony?,
Stallion?" I forget what it was called do fine without the distribution and
hole system, which appears to be a mistake, anyway, to this designer due to
high power costs.
Barry Palmer, for <A
HREF="http://members.aol.com/sevtec/sev/skmr.html">Sevtec</A>
Andrew Barber
skirt where it then goes under the craft. There are 2 output ducts on the
Fan that vent the air out the sides and directly into the skirt. We have yet
to get enough running in on it to see how it is going to behave but it has
not been all that bad thus far.
I also did some updating to the page if anyone is interested in looking at
it. More pictures and what not.
http://hovercraft.go.to/Leo
Andrew Barber
"SEVTEC" <sev...@aol.com> wrote in message
news:20010429112259...@ng-mg1.aol.com...
Indi
I recently completed an RC Hovercraft from www.Kinetics.com. Its the
LCAC-1.
OK, so now I've got the bug!
Anyhow, the system they use is a tubular skirt surrounding the craft.
it has two openings on each side which fills the skirt by directed air
from a semi-taught air dam made of the same skirt material. The air
dam is not sealed to the openings in the skirt, so it allows the
excess flow of air from the two lift fans to get into the central
chamber.
Is THIS a practical method, or only feasible for an RC Hovercraft?
Sometime in the future I'd like to build my own 2~4 passenger HC and
use a similar concept with one lift and one prop engine. The RC
version has 4 motors.
If it feasible, I hope I helped.
On Fri, 27 Apr 2001 22:58:41 -0400, "Troy Alan"
<tbl...@netexecutive.com> wrote:
*Okay,
*How does the skirt of a hovercraft work? Is it completely underneath
the
*hovercraft and have holes in the bottom or is it just a peice of
material
*that is taunt at the bottom as to not let air outside of the
underneath of
*the craft? I have read alot about the skirts and how the craft
floats on
*air, but I can't seem to find any info on exactly how a skirt works
and what
*it looks like on the underside? Any assistance on this would be VERY
VERY
*helpful.
*
*Troy
*
Ken Roberts
>All of the Sevtec craft skirt feeds are from the area just under the fan.
>little to no attempt is made to generate added skirt pressure by using the ram
>pressure from the fan to add to the skirt internal pressure and get it above
>cushion pressure, as is done in Universal Hovercraft and other designs. A pair
>of skirt bleed holes are provided at the bow to vent skirt air into the forward
>cushion (s) which allows air to escape when the craft is operating on
>roughness.
so your feeds come in from just under the fan, but the feeds are perpendicular
to the air flow? it seems to me that this must generate a vacuum relative to
the regular cushion pressure. or am i wrong here? i realize that the mass of
the skirt has some effect in keeping it down, and for the sake of discussion i
can agree that the venturi effects will help too. my biggest problem is
thinking that it's enough. if your pressure ratio is truly 1:1, then i have to
believe it. or at least that something else is making it work.
>The result of using such low pressure tactics is lower drag and most important,
>much lower skirt wear.
yes, but how does it work at higher speeds? won't the low pressure be overcome
by the wind pressure? also, what about stability in "exciting" conditions?
i've fiddled with the bag pressure ratio (my term) on my craft and realized that
when my bag pressure gets lower my stability drops too. obviously there is a
happy medium for any particular application, just like with anything else. also
like everything else, no particular value is going to be perfect for every
application.
-- Ken Roberts
Ken Roberts
get. i'm not sure, but it seems to me that serial skirts would have a very high
bag pressure ratio.
by your post, i assume you're just finishing things up and haven't had it out
much. is this correct? if so, (or anyway!) please post any figures on
performance or specs.
Andrew Barber
with Thrust engines.. we're on our third, and we had some problems with too
much movement with the whole thrust assemble, when we tried to come up to
high RPM the belts would slip off. We also had the problem that we did not
have enough Pitch on the fans we're using for thrust. So with a "new" fuel
injected Subaru (spelling?) and some structural modifications we're alsmost
ready to go.
"Ken Roberts" <k...@roberts.9ci.com> wrote in message
news:slrn9epig...@roberts.9ci.com...
Staltus
>
> hmm, that's a serial skirt. i'm interested to see what kind of performance you
> get. i'm not sure, but it seems to me that serial skirts would have a very high
> bag pressure ratio.
I would think that the bag pressure ratio would be determined by the
size and placement of the holes that pass the air to the central
chamber. If you get big enough holes, I would think that you could get
very close to a 1:1 ratio. The question is can you get the holes that
big and still have a stable bag.
Steve
Staltus
>
> Please excuse my ignorance for the following comments.
>
> I recently completed an RC Hovercraft from www.Kinetics.com. Its the
> LCAC-1.
> OK, so now I've got the bug!
> Anyhow, the system they use is a tubular skirt surrounding the craft.
> it has two openings on each side which fills the skirt by directed air
> from a semi-taught air dam made of the same skirt material. The air
> dam is not sealed to the openings in the skirt, so it allows the
> excess flow of air from the two lift fans to get into the central
> chamber.
> Is THIS a practical method, or only feasible for an RC Hovercraft?
My craft, a UH13-T has a wooden air spliter rather than the semi taught
air dam, but yes, that is how at least some of the rec crafts are made.
I have a 26" duct for lift and a split off a 20" chord for use in
pressurizing the bag. The remainder goes into the center sction of the
craft.
Ken Roberts
belts stuck together) can handle quite a bit of misalignment. the timing belts
(grooves go across the belt to prevent slippage) don't tolerate that so well.
the things that come to mind immediately are pulley misalignment or an
insufficiently stiff mounting assembly. that includes the entire structure
holding the engine, pulleys and props.
Ken Roberts
when i look at my craft, i still can't see how you could do it with anything
like a normal ratio. my bag makes a duct, if you will, less than a foot in
diameter. my lift fan is 30 inches diameter. frankly, i don't think there's
any way i could pump that much air through the bag without having a brutally
high bag pressure. if at all. i don't know how the serial skirted crafts do
it, but i can't imagine any system that would work on my craft.
i'm at somewhat of a disadvantage here, because i've never seen a craft that i
knew to use a serial skirt. i've never seen one in real life, and i can't
recall having seen pictures of how one works.
SEVTEC
>Date: 4/30/2001 6:50 AM Pacific Daylight Time
>Message-id: <3AED6D...@tamu.edu>
You can get a stable bag, and indeed, avoid bounce problems associated with
this setup, but you may also have problems with water splashing into the bag,
which is bad news for performance of the craft.
Barry Palmer, for Sevtec
Ken Roberts
yeah, having a bag full of water is a real drag!
heh.
Andrew Barber
this problem by putting two holes in the back corners to force the water
out. (it sometimes takes a while)
"Ken Roberts" <k...@roberts.9ci.com> wrote in message
news:slrn9er21...@roberts.9ci.com...
Andrew Barber
reasently changed the lift engine and added an Idler pully to tighten things
up.
"Ken Roberts" <k...@roberts.9ci.com> wrote in message
news:slrn9equf...@roberts.9ci.com...
Ken Roberts
>We had the Skirt fill up with water one of our first runs out, we solved
>this problem by putting two holes in the back corners to force the water
>out. (it sometimes takes a while)
i have one in the middle in the back.
SEVTEC
>Date: 4/30/2001 4:37 PM Pacific Daylight Time
>Message-id: <2ImH6.232883$Z2.24...@nnrp1.uunet.ca>
>
>We had the Skirt fill up with water one of our first runs out, we solved
>this problem by putting two holes in the back corners to force the water
>out. (it sometimes takes a while)
>
You probably flooded the skirt statically. A drain is essential in this case,
and skirt material flotation makes emptying even easier. However, in even
slightly rough water, skirt openings into the cushion can allow water to board
faster than you can get rid of it via the drain.