Bag skirt pressure
jarrob
Sorry this is a bit of a long post but I thought I'd better get all of it in
the same place!
I've been doing some winter work on my UH18 and have finally got around to
checking the bag pressure. I used a home-made manometer to compare the
pressure under the craft with the pressure in the bag skirt. I've noticed
several things - some of which are obvious - others aren't (well - they
weren't obvious to me :-)!
The test conditions were:
Static tethered UH18 hover on a smooth level surface. The craft was put onto
full hover (i.e. minimum power needed to raise the craft until there was a
discernible skirt to ground gap. The minimum hover was with the engine at
about 1800rpm with the variator set to full lift. The lift fan skirt
splitters are set at a 1.5" maximum gap to the duct wall (as per plans).
Result:
1. The cushion pressure under the craft does NOT increase with lift
fan/engine speed after the craft is on full hover. This makes sense as the
increasing amount of lift air is just being pushed out of the gap between
the skirt and the ground. The pressure is being automatically stabilised
via the skirt gap. The cushion pressure is directly proportional to the
weight of the craft - nothing else!
2. The skirt to ground clearance does not increase with increased lift power
to any significant extent. The average gap increased from about 1/4" to 1/2"
between minimum lift and maximum power (4300rpm on my craft).
2. The bag skirt internal pressure increases dramatically with lift fan
speed. At minimum hover it is about 15% higher than the cushion pressure.
At full lift power it is 60% higher. This makes the 'optimum' skirt
pressure ratio of 1 to 1.1: 1 unachievable
3. There is no significant difference in bag skirt pressure at various
points around the craft. This is slightly surprising as I expected the
pressure to be higher next to the lift fan splitters.
These results lead me to one question - maybe you guys already have the
answers :-).
Why would you need to increase lift power above the minimum? The usual
reason given is to compensate for increased 'leakage' from under the skirt
as you travel over less than smooth surfaces. The problem that I see is
that increasing lift power also increases skirt pressure. This then
decreases the skirt to surface seal efficiency therefore needing more lift
power. This seems like a bad thing to me - over rough surfaces what you
actually need is a softer skirt to provide a better seal rather than a
harder skirt.
From experience of using the hover over land I have found that you do indeed
need large amount of lift to get over rough surfaces - the craft seems to
get 'stuck' and you need to increase the lift to a large degree to get back
onto hover. I think that what is happening is that as you increase lift
power you are initially making the ground seal worse by increasing the skirt
pressure - then, as the lift increases further, you eventually reach the
point where you are pumping enough air to overcome the poor ground seal
(caused by the surface AND the high skirt pressure) and build cushion
pressure again.
The above conclusions make it obvious to me why a segmented skirt seems
'better' than a bag skirt on rough ground - it is at the same pressure as
the cushion. If a simple way could be found of controlling bag skirt
pressure then it may perform much better on rough surfaces.
Has anyone tried feeding the skirt from the cushion instead of directly from
the fan?
Has anyone experimented with opening up the skirt at the inside attach
points to make the pressure equalise with the cushion?
High bag skirt pressure does also have some advantages but the above IMHO
seems like a serious disadvantage as directly affects the primary function
of the skirt - to 'seal' the cushion.
Any comments anyone?
Ken Roberts
thanks for the information. i don't think anyone i know has made those tests
exactly, though the universal guys might have done direct-to-atmosphere tests
specifically on that hull, they do lots of stuff i don't know about.
one question: did you use one manometer with one end in the bag and one in the
cushion, or did you use two manometers both vented to the outside? if you have
more details in your test, i would really like to see them, including any detail
of pressure change even though it might be minute.
i'm not entirely sure if they intended the bag pressure to be that high or not.
i'll ask them when i get a chance.
if they did intend for it to be that high, then it is probably for high speed
stability and in sacrifice of low speed obstacle clearance. the folks i know
that have these crafts drive them up the ramp but don't do much on land.
regarding your minimal lift question, yes you have hit the nail on the head.
lift should be the minimum you need to get adequate lubrication in the current
conditions. the extra is there for abnormal conditions. it seems to me that
the extra bag pressure at full lift is probably a bad thing at low speed but
maybe a good thing at high speed.
nobody i know of has tried feeding the skirt from the cushion on a high speed
craft. i think high speed stability would suffer a lot, but you never know
till you try, right? i think graham spencer's heron craft does some
cushion-feed fingers though.
i've seen people with smaller crafts open the bag on the inside to lower bag
pressure. it makes a huge difference on a low-powered craft with, say, 10 hp.
the most common way i've heard of to adjust a larger craft is by varying the
splitter gap. at 60+ mph you really don't want the bag to be loose anywhere on
the bottom, so the only way to adjust it would be to adjust the input or the
output aperture.
you could hook up a third fan that is run at a constant speed (separate
throttle) and that would soften the bag as the lift increases, or at least keep
it constant. You could no doubt also invent a contraption that would regulate
the bag pressure as a separate system, which would probably be a lot easier and
cheaper than using a separate fan.
that could be a flap in the splitter, below the fan but above the water line.
it would dump air from the bag (inside the splitter) to the cushion area. hmmm.
that really gets the imagination going...
Fanman_UK
Congratulations Jarrob, you have just discovered why few people in the
UK use no-flow bag skirts :o)
Your conclusions 1 & 2 are spot on. The cushion pressure is determined
by the fan speed and will increase with the square of the speed. i.e
p2 = p1 x (speed2/speed1)^2
Obtaining the desired skirt/cushion ratio partially depends of course,
on what your designed cushion pressure is.
You will not find *significant* differences in cushion pressure on a
closed skirt because there is almost no velocity and the measured
pressure is therefor static pressure. i.e. there are virtually no
losses.
Increasing lift power will give greater fan pressure (and therefore
more skirt pressure) and more volume (in the cushion only), which will
give an increased hover gap, although as you have found this increase
will not be large unless the increased volume is *very* large. This
can and does lead to the problems you have found. It is important when
designing a skirt system to design the hull, fan and skirt together to
get the optimum design you require. Changing any one element can have
marked effects.
You cannot feed the bag from the cushion. The bag must be above
cushion pressure to maintain its shape and air cannot travel from low
pressure to high pressure areas. Although there are some designs which
use very low bag pressures. If you want to lower the bag pressure, you
can achieve this simply, with the same method used for full flow bag
skirts. Cut a hole(s) in the internal wall of the bag allowing air to
spill into the cushion. The pressure will vary with the size of the
opening. You will need to experiment until you find the best size for
your craft and the use to which you want to put it.
Good Luck.
Fanman_UK
To reply by email remove the "fanman"
Terry
monometer with the 18 just never got around to it. It seems to me what
comes to mind on the bag pressure is that when I'm driving on water I
keep the lift at a minimum by adjusting the var. and throttle. Just
enough to stay on bubble. But when coming up to an unseen wake I crank
the lift (60% higher sounds right) and the bag gets good and stiff and
pops right over the wake, so it's nice to have at command. So part of
the high lift might also be what pulley size you are useing to drive
your lift fan. When I was running on a 5.5 inch pulley (for a short
time) I was getting way too much spray at speed, so much so that it
took the glass off of the prop at the tips (big peices) and my top
speed suffered big time. I use a 4.5 inch pulley now and it works
great, best speed and lift control, for what I do. Like ken said most
of the time it's water, ice, sand bar, or boat launch. But a 3rd fan it
must have been late when ya wrote that.... :-)
Terry
www.angelfire.com/tv2/tvthover
Ken Roberts
Correct me if I'm wrong, but doesn't the full flow design suffer from the same
issue? Here you have a bunch of fixed diameter holes with the entire lift
volume going through them, and as you increase lift volume the pressure on the
upstream of those holes is going to increase, while the cushion pressure will
remain constant for that entire duration.
On top of that, how do you take care of sticks or snow or water or whatever
getting into the bag? With a no flow skirt, there's a good chance that
anything going through the fan will hit the ground or water, rather than the
100% chance that it's gonna have to take a trip through the bag on a full flow
skirt.
Besides, the stability of a bag skirt is one of the advantages of using it. I
doubt you'd get the righting forces if the bag pressure were too low.
The more I think about current skirt designs as I understand them, the less
convinced I am that there is any design that stands noticeably above all the
others for recreational hovercraft applications. I can readily accept that the
no-flow bag skirt is not optimal, but as yet I see no alternatives that are more
attractive to me. Admittedly I have only seen most of these designs from afar,
and I'm trying to look at them with an open mind.
Talk to me folks. I'm not trying to start an argument, I'm just trying to
figure out how the world works.
rdu...@pdq.net
been small gas powered RC toys. That, plus some observation of real
hovercraft, leaves me willing (enough) to stick my neck out.
1. Bag skirts are more degraded by large scale damage that segments.
2. Finger skirts have often been more susceptible to minor/temporary
damage than bags. That is the good and bad part of relying on weak-link
cable ties.
3. Bags provide more stability than one really needs and sometimes more
than one really wants.
4. Segments can provide enough stability if done right.
5.Not needing to maintain pressure in a cylinderical bag gives a basic
efficiency advantage to segments. The need to distribute lift air all
around the periphery tends to take that efficiency away.
6. The small radius UH type of pointy skirt has quite a bit less top
side air drag that an extended,lumpy Scat-type full-sized segment
system. Above 30-40 mph this begins to matter.
7. Small fingers-with minimum size for cushion height(and low drag) do
not work with my "reverse step" type anti-plowin tricks which bothers
me a lot.
8. The loop and finger design has all of the advantages but is tricky
to design and build, and the few examples I have seen in competition
have not made much of an impression.
The advantages include: Replaceable segments and smooth sides.
Zero bag-cushion pressure gradient. Excellent flexibilty. I think I
see a way to make serious anti-plow fit into the picture. But nothing
is ready to test.
Kelley Jernigan
I am curious about this statement.
It seems that a Sevtec Proespector and Explorer does exactly this.
The air fed into the bag skirt is done through one large hole on each side
of the craft and the air comes straight from under the craft, without
splitter plates. It is fed 1st under the craft and from there it goes into
the skirt.
You can see this here:
http://members.aol.com/hiitec/kite/assem.html
or more specifically here:
http://members.aol.com/hiitec/kite/assem.html
Kelley
"Fanman_UK" <pa...@commark.freeserve.co.uk> wrote in message
news:leggu0h2fhdok1asg...@4ax.com...
jarrob
I'm not trying to start yet another bag versus segment skirt discussion. I
personally don't think any of the current skirt designs are particularly
great - they all have significant disadvantages. All I'm trying to do is
to get a better understanding of what is actually happening under and around
a hovercraft.
"Fanman_UK" <pa...@commark.freeserve.co.uk> wrote in message
news:leggu0h2fhdok1asg...@4ax.com...
>>
> Congratulations Jarrob, you have just discovered why few people in the
> UK use no-flow bag skirts :o)
Maybe there is less difference than you think between segmented and bag
skirts. Personally I would define a 'pressure segment' as a small bag skirt
with a hole into the cushion ;-).
>
> Your conclusions 1 & 2 are spot on. The cushion pressure is determined
> by the fan speed and will increase with the square of the speed. i.e
> p2 = p1 x (speed2/speed1)^2
Sorry, but I didn't think that was my conclusion ;-) My understanding is
that (once on hover) the cushion pressure is constant and depends only on
the mass of the craft. Any extra air volume pushed into the cushion by
increasing the lift fan speed is simply expelled out of the (increasing)
skirt to surface gap. This varying surface gap effectively acts as a
pressure regulator for the cushion. The same thing has to happen with any
type of skirt.
Maybe the problem here is with terminology. When people talk about
increasing lift to get over rough terrain or obstacles they are *not*
actually increasing the hover height - they are increasing the air volume
being pushed into the cushion to replace the air lost due to the poor skirt
to surface seal (caused by the rough terrain).
>
> Obtaining the desired skirt/cushion ratio partially depends of course,
> on what your designed cushion pressure is.
>
> You will not find *significant* differences in cushion pressure on a
> closed skirt because there is almost no velocity and the measured
> pressure is therefor static pressure. i.e. there are virtually no
> losses.
>
> Increasing lift power will give greater fan pressure (and therefore
> more skirt pressure) and more volume (in the cushion only), which will
> give an increased hover gap, although as you have found this increase
> will not be large unless the increased volume is *very* large. This
> can and does lead to the problems you have found. It is important when
> designing a skirt system to design the hull, fan and skirt together to
> get the optimum design you require. Changing any one element can have
> marked effects.
>
> You cannot feed the bag from the cushion. The bag must be above
> cushion pressure to maintain its shape and air cannot travel from low
> pressure to high pressure areas. Although there are some designs which
> use very low bag pressures. If you want to lower the bag pressure, you
> can achieve this simply, with the same method used for full flow bag
> skirts. Cut a hole(s) in the internal wall of the bag allowing air to
> spill into the cushion. The pressure will vary with the size of the
> opening. You will need to experiment until you find the best size for
> your craft and the use to which you want to put it.
I do understand that a bag skirt must be at a slightly higher pressure than
the cushion - but it only needs to be slightly higher. Segmented skirts (at
least the standard open C segments) also have a slightly higher pressure
than the cushion (because the lift air is directed at the top part of the
finger from the plenum).
Maybe we need to step back and focus on what the skirt actually needs to do
rather than on what current designs do. That way we may be able to come up
with a better skirt solution.
Regards
John
rdu...@pdq.net
skirt designs?
jarrob
<rdu...@pdq.net> wrote in message
news:1105804119.1...@c13g2000cwb.googlegroups.com...
> What do you see as the most serious shortcomings of current available
> skirt designs?
You listed quite a few of them in your previous post.
They all contain different compromises that directly affect the primary
function - bag skirts are better than segmented skirts on smooth surfaces,
seg skirts are better on rough surfaces. Bag skirts are more stable, seg
skirts are less stable, etc, etc.
The primary function of the skirt is to prevent (most) of the cushion air
from escaping from under the craft on a variety of surfaces. The other
functions (stability, durability, repairability, etc) are secondary issues -
although they are also important. I don't think that any of the current
skirt designs are flexible(?) enough to be used on any type of surface at a
wide range of speeds.
Although there has been development on skirt design (particularly seg
skirts) the basic concept and materials hasn't changed very much in a long
time. I just think it may be time for a fresh look at the issue. This may
mean re-defining the design problem without all of the baggage of previous
'solutions'.
Maybe the solutions we have *are* the best - we won't know unless we are
willing to try!
My 'perfect' skirt would be:
Soft enough to deform easily on rough surfaces
Firm enough not to deform at high speed
Firm enough to provide stability
Wear resistant
The first two look like direct opposites and therefore impossible to
achieve. However, if you think about it the 'softness' needs to be the
vertical axis and the 'firmness' in the horizontal axis so maybe it's not
impossible after all - we just need some type of material or structure that
behaves this way.
As you can probably tell it's a wet and windy Saturday afternoon and I can't
get out hovering so I've been spending too much time thinking ;-)
John
Barry Palmer
>Subject: Re: Bag skirt pressure
>From: "jarrob" jo...@cirtech.co.uk
>Date: 1/15/2005 10:33 AM Pacific Standard Time
>Message-id: <SldGd.268$7j4...@newsfe3-gui.ntli.net>
One of the major considerations is how much material is needed in a skirt
design. The loop-segment skirt will take about double the material of an
equivalent bag skirt, for a single compartment cushion, and will also expose
more than that to direct wear. However, that wear is on the tips of the
fingers, which should not rapidly harm the performance of the craft, while if
the bag skirt is allowed to wear through, it will result in extreme
deterioration of performance. It should be noted that I am referring to
uncompartmented cushions, where the finger skirt does not provide the stability
of a bag, in this area it is hard to get "equivalent" skirts.
Saying that, I have never worn through a bag skirt, but have applied infrequent
local patches to extend the skirt life, while in the case of a finger skirt,
much more material is used up replacing fingers. The Sevtec skirts run at a
pressure almost immeasurably above cushion pressure and this results in a very
flexible skirt and one that actually will tend to lift off in places where it
contacts the ground, minimizing wear. Also, Sevtec craft tend to run higher
cushion volume flow rates than other contemporary craft of similar size, which
lessens the problem of the skirt conforming to rough surfaces.
There is a drag problem with all skirted craft, and that is thought to be due
to skirt inflexibility, but is really a lack of the cushon air volume to
accommodate a rapid change in volume as the craft passes over roughness. This
problem can be solved (and was solved by myself on a working craft) but is
really for large craft. It is strange, to this writer, that the well funded
commercial interests have not figured this out.
In very rough areas, with sharp rocks, it is unrealistic to pull up on dry
land! A vinyl 18 oz skirt can be trashed in a few operations, while it will
last for years if the operator just ties the craft off by its painter, where
the rocks are sharp.
Barry Palmer, for <A
HREF="http://members.aol.com/sevtec/sev/skmr.html">Sevtec</A>
Fanman_UK
>Paul,
>
>I'm not trying to start yet another bag versus segment skirt discussion. I
>personally don't think any of the current skirt designs are particularly
>great - they all have significant disadvantages. All I'm trying to do is
>to get a better understanding of what is actually happening under and around
>a hovercraft.
>
Nor am I. I didn't mention segmented skirts :o)
>>>
>> Congratulations Jarrob, you have just discovered why few people in the
>> UK use no-flow bag skirts :o)
>
>Maybe there is less difference than you think between segmented and bag
>skirts. Personally I would define a 'pressure segment' as a small bag skirt
>with a hole into the cushion ;-).
>
Tonge in cheek I know, but if you were correct I might grow to like
bag skirts. :o)
>>
>> Your conclusions 1 & 2 are spot on. The cushion pressure is determined
>> by the fan speed and will increase with the square of the speed. i.e
>> p2 = p1 x (speed2/speed1)^2
>
>
>Sorry, but I didn't think that was my conclusion ;-) My understanding is
>that (once on hover) the cushion pressure is constant and depends only on
>the mass of the craft.
My mistake, I meant to say skirt pressure!! The cushion pressure is
dependant on the total mass and does not change significantly.
>Any extra air volume pushed into the cushion by
>increasing the lift fan speed is simply expelled out of the (increasing)
>skirt to surface gap. This varying surface gap effectively acts as a
>pressure regulator for the cushion.
Glad you said effectively :o)
> The same thing has to happen with any
>type of skirt.
>
>Maybe the problem here is with terminology. When people talk about
>increasing lift to get over rough terrain or obstacles they are *not*
>actually increasing the hover height - they are increasing the air volume
>being pushed into the cushion to replace the air lost due to the poor skirt
>to surface seal (caused by the rough terrain).
Correct. You appear to have a good grasp of the subject.
>> You will not find *significant* differences in cushion pressure on a
>> closed skirt because there is almost no velocity and the measured
>> pressure is therefor static pressure. i.e. there are virtually no
>> losses.
There I go again !!! I meant skirt pressure.
>I do understand that a bag skirt must be at a slightly higher pressure than
>the cushion - but it only needs to be slightly higher. Segmented skirts (at
>least the standard open C segments) also have a slightly higher pressure
>than the cushion (because the lift air is directed at the top part of the
>finger from the plenum).
>
>
>Maybe we need to step back and focus on what the skirt actually needs to do
>rather than on what current designs do. That way we may be able to come up
>with a better skirt solution.
I quite agree John, unfortunately the skirt requirement varies with
use, conditions and personal preference. Any answer found is likely to
remain a compromise. :o(
>
>
>Regards
>
>John
Fanman_UK
<hover...@cyberjernigan.com> wrote:
>Fanman,
>
>I am curious about this statement.
>It seems that a Sevtec Proespector and Explorer does exactly this.
>The air fed into the bag skirt is done through one large hole on each side
>of the craft and the air comes straight from under the craft, without
>splitter plates. It is fed 1st under the craft and from there it goes into
>the skirt.
>You can see this here:
>http://members.aol.com/hiitec/kite/assem.html
>or more specifically here:
>http://members.aol.com/hiitec/kite/assem.html
>
>
>Kelley
From those images it doesn't look that way to me. It appears that some
of the air from the fan plenum is directed into the skirt via the side
holes whilst the rest passes through a hole in the floor to the
cushion. Perhaps Barry can clarify the air paths taken.
Fanman_UK
<k...@kroberts.in.9ci.com> wrote:
>Correct me if I'm wrong, but doesn't the full flow design suffer from the same
>issue? Here you have a bunch of fixed diameter holes with the entire lift
>volume going through them, and as you increase lift volume the pressure on the
>upstream of those holes is going to increase, while the cushion pressure will
>remain constant for that entire duration.
In practical term No. Whereas the no-flow skirt usually derives its
pressure from the fan alone, having no other form of adjustment (not
all types) the full flow has means of altering the pressure in its
design. I should point out that the term 'full flow' is a bit of a
misnomer, as most of the air going to the cushion takes a *very* short
route though the skirt.
>
>On top of that, how do you take care of sticks or snow or water or whatever
>getting into the bag? With a no flow skirt, there's a good chance that
>anything going through the fan will hit the ground or water, rather than the
>100% chance that it's gonna have to take a trip through the bag on a full flow
>skirt.
Water is not a problem, nor are small stones etc. which can pass
through the lubrication holes or drains. Sticks, bricks etc. would,
granted be more of a problem. :o)
No attempt is usually made to keep water out of the skirt, as it
drains very quickly.
>Besides, the stability of a bag skirt is one of the advantages of using it. I
>doubt you'd get the righting forces if the bag pressure were too low.
I agree!
>
>The more I think about current skirt designs as I understand them, the less
>convinced I am that there is any design that stands noticeably above all the
>others for recreational hovercraft applications. I can readily accept that the
>no-flow bag skirt is not optimal, but as yet I see no alternatives that are more
>attractive to me. Admittedly I have only seen most of these designs from afar,
>and I'm trying to look at them with an open mind.
>
>Talk to me folks. I'm not trying to start an argument, I'm just trying to
>figure out how the world works.
Give up on the world Ken, its far too complicated.....stick to
hovercraft :o)
I think that skirt choice is one of those subjects for which there
will never be a definitive answer. It really does come down to
preference, and the conditions in which the craft is to be
predominantly used. It is interesting however that certain types gain
favour in different parts of the world and also on different classes
of craft.
Ken Roberts
> Ralph,
>
><rdu...@pdq.net> wrote in message
> news:1105804119.1...@c13g2000cwb.googlegroups.com...
>> What do you see as the most serious shortcomings of current available
>> skirt designs?
>
>
> You listed quite a few of them in your previous post.
>
> They all contain different compromises that directly affect the primary
> function - bag skirts are better than segmented skirts on smooth surfaces,
> seg skirts are better on rough surfaces. Bag skirts are more stable, seg
> skirts are less stable, etc, etc.
Some of this is cause and effect, to a certain degree. Bag skirts are more
stable BECAUSE the bag pressure is higher, which means the skirt is stiffer
because of that pressure. Which means that the rough surface performance is not
as good.
I cut down the bag pressure on my 12r quite a bit at one point, which made the
craft a lot more tippy but I just glided over the rough stuff.
> The primary function of the skirt is to prevent (most) of the cushion air
> from escaping from under the craft on a variety of surfaces. The other
> functions (stability, durability, repairability, etc) are secondary issues -
> although they are also important. I don't think that any of the current
> skirt designs are flexible(?) enough to be used on any type of surface at a
> wide range of speeds.
>
> Although there has been development on skirt design (particularly seg
> skirts) the basic concept and materials hasn't changed very much in a long
> time. I just think it may be time for a fresh look at the issue. This may
> mean re-defining the design problem without all of the baggage of previous
> 'solutions'.
I'm intensely interested in this. My own opinion is that we could have a very
good lift system design if we don't oversimplify it.
> Maybe the solutions we have *are* the best - we won't know unless we are
> willing to try!
>
> My 'perfect' skirt would be:
> Soft enough to deform easily on rough surfaces
> Firm enough not to deform at high speed
> Firm enough to provide stability
> Wear resistant
Why not a context-sensitive skirt? Cars have suspensions and power trains that
adjust to the conditions: antilock brakes, traction control, all-wheel-drive,
you start taking too many g's and your car squats down to perform better. Why
can't we make something that can be a cruising skirt one minute or an obstacle
clearing skirt the next, and then keep the craft from tipping or whatever?
Ken Roberts
> On Fri, 14 Jan 2005 23:14:36 GMT, Ken Roberts
><k...@kroberts.in.9ci.com> wrote:
>
>>Correct me if I'm wrong, but doesn't the full flow design suffer from the same
>>issue? Here you have a bunch of fixed diameter holes with the entire lift
>>volume going through them, and as you increase lift volume the pressure on the
>>upstream of those holes is going to increase, while the cushion pressure will
>>remain constant for that entire duration.
>
> In practical term No. Whereas the no-flow skirt usually derives its
> pressure from the fan alone, having no other form of adjustment (not
> all types) the full flow has means of altering the pressure in its
> design. I should point out that the term 'full flow' is a bit of a
> misnomer, as most of the air going to the cushion takes a *very* short
> route though the skirt.
Then there's a difference between the documentation I've seen on full flow
skirts and the reality then, because I see no really short route. It seems to
be one of two things: A plenum craft with a bag and in turn holes in the bag;
or a bag which acts as a plenum, which seems to not be enough to transfer the
large volume of air needed for lift.
The discrepancy between my understanding of a full flow lift system and the fact
that they are actually used on recreational sized crafts proves I don't know
much about them. With my understanding of them, they are a non-choice.
>>On top of that, how do you take care of sticks or snow or water or whatever
>>getting into the bag? With a no flow skirt, there's a good chance that
>>anything going through the fan will hit the ground or water, rather than the
>>100% chance that it's gonna have to take a trip through the bag on a full flow
>>skirt.
>
> Water is not a problem, nor are small stones etc. which can pass
> through the lubrication holes or drains. Sticks, bricks etc. would,
> granted be more of a problem. :o)
> No attempt is usually made to keep water out of the skirt, as it
> drains very quickly.
So it's not a problem if you take a wave into the lift system? On my 12r it
just falls through, maybe stalls the engine or maybe just slows down the fan,
but the water mostly just lands on more water. You get a quart or so in the
bag, which eventually works its way to the drain hole and out. I can see where
the drain holes would be a lot closer and bigger, but you still have a lot of
water hitting what is effectively the inside of your hull, and you have to
support it for a while.
>>Besides, the stability of a bag skirt is one of the advantages of using it. I
>>doubt you'd get the righting forces if the bag pressure were too low.
>
> I agree!
>>
>>The more I think about current skirt designs as I understand them, the less
>>convinced I am that there is any design that stands noticeably above all the
>>others for recreational hovercraft applications. I can readily accept that the
>>no-flow bag skirt is not optimal, but as yet I see no alternatives that are more
>>attractive to me. Admittedly I have only seen most of these designs from afar,
>>and I'm trying to look at them with an open mind.
>>
>>Talk to me folks. I'm not trying to start an argument, I'm just trying to
>>figure out how the world works.
>
> Give up on the world Ken, its far too complicated.....stick to
> hovercraft :o)
> I think that skirt choice is one of those subjects for which there
> will never be a definitive answer. It really does come down to
> preference, and the conditions in which the craft is to be
> predominantly used. It is interesting however that certain types gain
> favour in different parts of the world and also on different classes
> of craft.
I don't think that's confusing at all. There are lots of choices everywhere and
definitely different schools of thought. There are also different applications
and different environments.
However, if you take the USA for example, we have never-ending rivers and lots
of places to play. Very few people in the US would consider having a hovercraft
just for racing. Most buy for a cruising application or helling around, and
then just happen to race it every now and then.
You guys are more restricted by which waterways you can use, and the racing
scene is so huge that if you want to race it's a full time hobby. Buying a
machine that is not a practical cruiser makes a lot of sense there.
Just the cruising vs the racing aspect changes a huge amount of design
parameters. Our most successful class is one you are considering dropping. F25
is huge here, and it's our fastest growing class for actual races because there
are so many cruisers out there that perform really well, and they make great
racing machines too for that class.
Fanman_UK
<k...@kroberts.in.9ci.com> wrote:
<snip>
>
>Then there's a difference between the documentation I've seen on full flow
>skirts and the reality then, because I see no really short route. It seems to
>be one of two things: A plenum craft with a bag and in turn holes in the bag;
>or a bag which acts as a plenum, which seems to not be enough to transfer the
>large volume of air needed for lift.
Perhaps my statement needs clarifying :o)
If the craft has a front mounted lift fan, then the cushion air route
is short, as the vents to the cushion are usually located in the
inside skin at the front of the skirt. This allows some of the air to
leave the fan and enter almost directly into the cushion.
If the craft is an integrated type, then the air has to travel via the
bag to the holes at the front of the skirt. A proportion of the total
volume will be past through the lubrication holes.
I cannot remember seeing a craft which used both an annular plenum and
a bag skirt, except where the design was originally intended for
segmented skirt.
<snip>
>So it's not a problem if you take a wave into the lift system? On my 12r it
>just falls through, maybe stalls the engine or maybe just slows down the fan,
>but the water mostly just lands on more water. You get a quart or so in the
>bag, which eventually works its way to the drain hole and out. I can see where
>the drain holes would be a lot closer and bigger, but you still have a lot of
>water hitting what is effectively the inside of your hull, and you have to
>support it for a while.
A potential problem exists for *any* craft taking a wave through the
lift fan. The craft design dictates how it will behave in such an
event. If the fan is mounted such that its discharge is into the hull
(above a front planing surface) Then this area will flood temporarilly
until it drains. The dain holes (which would also double as feed holes
to the cushion) would be large enough to disperse this water quickly.
If the air/water goes straight into the bag, it will be distributed
along the length of the skirt rapidly and blown out via the
lubrication holes. In practice it does not stay long enough to 'pool'.
<snip>
>You guys are more restricted by which waterways you can use, and the racing
>scene is so huge that if you want to race it's a full time hobby. Buying a
>machine that is not a practical cruiser makes a lot of sense there.
>
>Just the cruising vs the racing aspect changes a huge amount of design
>parameters. Our most successful class is one you are considering dropping. F25
>is huge here, and it's our fastest growing class for actual races because there
>are so many cruisers out there that perform really well, and they make great
>racing machines too for that class.
It is certainly true that many of our waterways are either protected
areas or used heavily commercially, which makes the choice of cruising
grounds difficult, although not impossible. Some areas specifically
ban hovercraft use in the bylaws, not because thay have had problems,
but simply because the local legislators regard them as noisy and
damaging based on hearsay evidence of the old designs of the 60's.
Many people here do not support the change to F35. Quite frankly I am
not sure why it is happening. It seems to have more to do with racing
than cruising, which is a bit strange when the original concept was
for a cruising craft that can be raced, not the other way round.
Ken Roberts
><k...@kroberts.in.9ci.com> wrote:
>
><snip>
>>
>>Then there's a difference between the documentation I've seen on full flow
>>skirts and the reality then, because I see no really short route. It seems to
>>be one of two things: A plenum craft with a bag and in turn holes in the bag;
>>or a bag which acts as a plenum, which seems to not be enough to transfer the
>>large volume of air needed for lift.
>
> Perhaps my statement needs clarifying :o)
> If the craft has a front mounted lift fan, then the cushion air route
> is short, as the vents to the cushion are usually located in the
> inside skin at the front of the skirt. This allows some of the air to
> leave the fan and enter almost directly into the cushion.
> If the craft is an integrated type, then the air has to travel via the
> bag to the holes at the front of the skirt. A proportion of the total
> volume will be past through the lubrication holes.
> I cannot remember seeing a craft which used both an annular plenum and
> a bag skirt, except where the design was originally intended for
> segmented skirt.
>
><snip>
So the distribution of the holes is uneven? There are more vents in front than
in back or on the sides, or are there no vents on the back or sides? The
concept drawings I've seen seem to imply that the holes are evenly distributed.
The pictures I saw of an actual hovercraft which was partially fitted with a
full flow skirt showed a plenum. It had a look that might have been a finger
skirt craft, but I latched onto it as a means of supplying enough air to take
the full output of a lift fan.
How does the entire output of a lift fan go into that bag? The designs I saw
with bag skirts in WHC2002 did not seem to have bags any bigger than mine. If I
had to put the entire output of my lift fan into my bag, there would be a huge
restriction. If I were to design such a system, I would tend to make a "partial
flow" system instead, where about half the lift air went directly into the
cushion.
>>So it's not a problem if you take a wave into the lift system? On my 12r it
>>just falls through, maybe stalls the engine or maybe just slows down the fan,
>>but the water mostly just lands on more water. You get a quart or so in the
>>bag, which eventually works its way to the drain hole and out. I can see where
>>the drain holes would be a lot closer and bigger, but you still have a lot of
>>water hitting what is effectively the inside of your hull, and you have to
>>support it for a while.
>
> A potential problem exists for *any* craft taking a wave through the
> lift fan. The craft design dictates how it will behave in such an
> event. If the fan is mounted such that its discharge is into the hull
> (above a front planing surface) Then this area will flood temporarilly
> until it drains. The dain holes (which would also double as feed holes
> to the cushion) would be large enough to disperse this water quickly.
> If the air/water goes straight into the bag, it will be distributed
> along the length of the skirt rapidly and blown out via the
> lubrication holes. In practice it does not stay long enough to 'pool'.
><snip>
Describe the bag on a system where the fan output goes directly into the bag. I
can't visualize a system like this. Does the bag cross section get really long
and flat at that point?
>>You guys are more restricted by which waterways you can use, and the racing
>>scene is so huge that if you want to race it's a full time hobby. Buying a
>>machine that is not a practical cruiser makes a lot of sense there.
>>
>>Just the cruising vs the racing aspect changes a huge amount of design
>>parameters. Our most successful class is one you are considering dropping. F25
>>is huge here, and it's our fastest growing class for actual races because there
>>are so many cruisers out there that perform really well, and they make great
>>racing machines too for that class.
>
> It is certainly true that many of our waterways are either protected
> areas or used heavily commercially, which makes the choice of cruising
> grounds difficult, although not impossible. Some areas specifically
> ban hovercraft use in the bylaws, not because thay have had problems,
> but simply because the local legislators regard them as noisy and
> damaging based on hearsay evidence of the old designs of the 60's.
So maybe you can understand how the environment to be hovered in dictates a lot
about which design a person finds most appealing.
> Many people here do not support the change to F35. Quite frankly I am
> not sure why it is happening. It seems to have more to do with racing
> than cruising, which is a bit strange when the original concept was
> for a cruising craft that can be raced, not the other way round.
I think it's because somebody who doesn't understand the goals of F25 assumes
that everything must get bigger in order to get better. There's a huge number
of UH-13p's in the US, and there are a lot of them that show up in races.
If the push for F35 were in the US, I would say it's because some folks build
with a 10 hp lift engine and a 25 hp thrust engine. That's a really popular
configuration, in fact. The thing I've noticed about it is that a really good
25 hp craft will outrun an above-average craft with 10 and 25 hp.
That fact, in and of itself, tells me that the F25 design goals are working.
There are several distinct designs that race in the US, and they are all very
competitive. Some folks build a racing craft that they also cruise, and others
race their cruising crafts. The difference is that these are the same model of
hovercraft, just with a different focus during the build process. Shane Wilke's
UH-13p was built from the ground up to save every ounce, and everything was
geared towards racing. It is what seems to be a particularly crappy cruising
craft, but it accelerates like an F1 craft almost. It cruises just fine too, by
the way.
jarrob
"Barry Palmer" <sev...@aol.com> wrote in message
news:20050115154151...@mb-m26.aol.com...
< snip >
>
> The Sevtec skirts run at a
> pressure almost immeasurably above cushion pressure and this results in a
> very
> flexible skirt and one that actually will tend to lift off in places where
> it
> contacts the ground, minimizing wear. Also, Sevtec craft tend to run
> higher
> cushion volume flow rates than other contemporary craft of similar size,
> which
> lessens the problem of the skirt conforming to rough surfaces.
Barry,
You've obviously done a lot of work in this area (from reading some of the
papers you've written) and I'm very interested in your results. The
pictures I've seen of the Sevtec craft show pretty large side feed holes
into the skirt - I would have thought the skirt pressure would have been
significantly higher than the cushion pressure (certainly at higher engine
rpm). Or do you have other skirt feed or cushion dump holes?
>
> There is a drag problem with all skirted craft, and that is thought to be
> due
> to skirt inflexibility, but is really a lack of the cushon air volume to
> accommodate a rapid change in volume as the craft passes over roughness.
I suppose the optimum solution is to have an air 'reservoir' that can be
dumped into the cushion quickly. Couldn't the bag be used as the reservoir?
Maybe this is one reason why Sevtec craft have large bag skirts?
> This
> problem can be solved (and was solved by myself on a working craft) but is
> really for large craft. It is strange, to this writer, that the well
> funded
> commercial interests have not figured this out.
OK Barry you've got to come clean at tell us how you did this ;-)
John
rdu...@pdq.net
Does the contemplated rule change allow for a wider selection of
engine sources? I remember some conversation about using 2 CV motors
which I am told are plentiful and cheap on your side of the pond.
Barry Palmer
>Message-id: <pAzGd.669$4S1...@newsfe1-win.ntli.net>
>
>
>"Barry Palmer" <sev...@aol.com> wrote in message
>news:20050115154151...@mb-m26.aol.com...
>
>< snip >
>>
>> The Sevtec skirts run at a
>> pressure almost immeasurably above cushion pressure and this results in a
>> very
>> flexible skirt and one that actually will tend to lift off in places where
>> it
>> contacts the ground, minimizing wear. Also, Sevtec craft tend to run
>> higher
>> cushion volume flow rates than other contemporary craft of similar size,
>> which
>> lessens the problem of the skirt conforming to rough surfaces.
>
>
>Barry,
>
>You've obviously done a lot of work in this area (from reading some of the
>papers you've written) and I'm very interested in your results. The
>pictures I've seen of the Sevtec craft show pretty large side feed holes
>into the skirt - I would have thought the skirt pressure would have been
>significantly higher than the cushion pressure (certainly at higher engine
>rpm). Or do you have other skirt feed or cushion dump holes?
>
forward cushion, and the drain hole. The holes help the skirt get rid of extra
volume when compressed by an obstacle.
>>
>> There is a drag problem with all skirted craft, and that is thought to be
>> due
>> to skirt inflexibility, but is really a lack of the cushon air volume to
>> accommodate a rapid change in volume as the craft passes over roughness.
>
>I suppose the optimum solution is to have an air 'reservoir' that can be
>dumped into the cushion quickly. Couldn't the bag be used as the reservoir?
>Maybe this is one reason why Sevtec craft have large bag skirts?
The air volume must be controlled by a variable volume of air that can be fed
in and out of the cushion. I used the skirt volume for this on my tests. This
is not used on the Sevtec craft, as another side effect was my first case of
sea sickness on one of my surface skimmers, so I would leave it to much larger
craft as a necessity it they are to compete with boats in the real world.
>> This
>> problem can be solved (and was solved by myself on a working craft) but is
>> really for large craft. It is strange, to this writer, that the well
>> funded
>> commercial interests have not figured this out.
>
>
>OK Barry you've got to come clean at tell us how you did this ;-)
>
>
>John
>
Put it this way, did you ever see a hydraulic system without an accumulator?
That is really all one has to do, considering air is uncompressible for all
practical reasons at the pressures we use.