Skirt material
Paul Kenna
integrated Hovercraft.
What is the state of the art material for skirts, and what can I
source and afford. I'm looking for durability for a cruising craft.
From the wear and damage on the existing skirts I'm also interested in
whether you can use double skins, composite materials or any method of
wear protection to extend the skirt life.
Any help would be appreciated
Paul Kenna
Melbourne
Ken Roberts
of the two common types, there's vinyl coated nylon and neoprene coated nylon.
i've always used vinyl, but neoprene is supposedly much better in terms of wear
and for cold weather. it's also about 3x the cost, i think.
i heard the military used some special kind of material for hovercrafts going
through desert, it's supposed to be super wear resistant. don't recall what
it's called, though. probably costs as much as a new car.
Externet
I got lost in so many replies for the subject, but just in case this
site was not mentioned, it could be of help. One of the posters was
asking for canadian skirt material; here is a place I have seen, with
no comments about its performance:
http://www.rushstar.com/AirCushion/TeflonSegment/index.htm
Miguel
Barry Palmer
has virtually nothing to do with performance of a skirt, bag or finger. As
surface skimmers are used in and near water, the
test example should be lubricated with water, as well as air, finger or bag
skirt, and contact pressure must be kept at to just a little above cushion
pressure. And still, the main drag problem (for high speed craft, excluding
planeout drag) is the lack of some sort of volume continuity (accumulation) of
the cushion as the craft negotiates roughness.
A Teflon surface will cause some interesting problems in bonding properties of
the skirt material, probably limiting the material fabrication to rivets and
stitches. Fingers could be made from a single piece of material, mechanically
fastened to the craft.
While fingers work well on large craft, as well as racing craft, (which
generally operate in very short duration races in golf course and lily pond
environments) they do cause problems in "real world" circumstances, when
compared to bag skirts.
The primary reason is that in small craft, the finger skirt can be pushed aside
more easily due to ts smaller "basic radius of curvature", for a given cushion
pressure, and cushion air is lost more easily, a problem aggravated by the
separation of the fingers, collapsing the skirt.
I have operated alongside the finger skirt machines in the real world, and find
that not only do they shed fingers regularly (usually the finger is retained by
the outer attachment, but not always) they require much more material, maybe
twice as much as an equivalent bag skirt) . Also, this is the reason that you
see some with finger skirts will start out in deep grass on some sort of
tarpaulin, as with speed, some of the finger problem dissappears.
Very Large craft, where fingers are invariably used, do not go as fast,
relative to their size as small craft, and they operate on smoother terrain,
relative to their size. These facts, and the fact that the basic radius of
curvature of the fingers is large, and a higher cushion pressure is used,
allows for good function of the fingers.
Skirt drag is an area that needs to be explored further, and maybe some sort of
stiff finger, or large finger might be a solution for small craft (the Sevtec
bow and partition skirts could be thought of as a large finger system)
<< Subject: Re: Skirt material
From: exte...@inorbit.com (Externet)
Date: Wed, Nov 19, 2003 9:55 AM
Message-id: <6a725fe7.0311...@posting.google.com>
http://www.rushstar.com/AirCushion/TeflonSegment/index.htm
Miguel
>><BR><BR>
Barry Palmer, for <A
HREF="http://members.aol.com/sevtec/sev/skmr.html">Sevtec</A>
Ken Roberts
when you hop a wave? if so, then the "small wave" solution is to go fast enough
that the ram effect picks the whole nose of the craft up, and you just skip over
the crests. i call that "tigger mode" and it can be terrifying to get there
unless you have smooth water to get up to speed before you hit the waves. oddly
enough, though, it's a pretty smooth ride once you're there.
and what would be the ramifications of increasing cushion pressure on small
crafts, really? i've wondered about that, for racing and high speed cruising
purposes.
in my current understanding of things, i think that as the hull gets larger, the
higher cushion pressure is required to get the same performance for a
realistically proportional load. in other words, the cushion area goes up as a
square, but the volume and therefore the mass of the load goes up as a cube
function. in my understanding, the cushion pressure is bumped up in order to
compensate for that, but the scale of things makes the higher cushion pressure
more reasonable.
on the other hand, certain things about a high cushion pressure would make for a
faster racer. i just wonder what the downfalls of that would be. i can think
of plow-in issues and flotation issues, and getting over the hump. but getting
over the hump for an f1 or f2 racer is not an issue.
the fact that two strokes are tending to dwindle in the light of ecological
issues means that four strokes are going to wind up in racers eventually, not as
the exception but as the norm. that in turn means that perhaps the cushion
pressures might go up a bit in newer racers, unless there's some compelling
reason to keep them low.
Barry Palmer
pumping" where the entering wave from the front and exit wave from the rear
present a variation of volume in a rigid volume cushion, which is nearly what
an air cushion is. I once corrected this problem, the drag dropped
dramatically, but it is the only time I have ever been sea sick on a surface
skimmer. So more work is needed here, and it is really more applicable to
large skimmers. I am puzzled as to why the large machine people have never
seemed to investigate this, at least in the literature except noting its
existence in a (US naval report I would have to dig up), much recognize it as a
big problem.
Air cushion vehicles are very low drag, right now, so long as the water is
rippled or smoother, but when there is a seaway, drag rises above that of an
equivalent boat, and this is perhaps the number one reason why surface skimmers
have not made stronger inroads in commercial use.
What you are talking about is only in the case of a small, underhulled and
overpowered craft, is not an operating mode useful for a larger vehicle as
impractical power levels are required, and the driver weight shift is now a
major control function. You would need a dancing elephant to drive a large
machine in that mode, or effective computer controls, as you are now
approaching a ram wing mode of operation.
Ralph DuBose
> by "volume continuity" in the first paragraph, do you reference loss of air from
> when you hop a wave? if so, then the "small wave" solution is to go fast enough
> that the ram effect picks the whole nose of the craft up, and you just skip over
> the crests. i call that "tigger mode" and it can be terrifying to get there
> unless you have smooth water to get up to speed before you hit the waves. oddly
> enough, though, it's a pretty smooth ride once you're there.
>
> and what would be the ramifications of increasing cushion pressure on small
> crafts, really? i've wondered about that, for racing and high speed cruising
> purposes.
>
Cushion pressure is just craft weight divided by cushion area. Bag
pressure or bag to cushion pressure ratios are a different matter, as
you well know.
There was a time when higher cushion pressure raised the speed at
which the front skirts blew inward and the craft began to fly.
Nowadays, that problem has much better answers available and I do not
see any advantage to higher cushion pressure per se.
At the high performance end of the spectrum, I do not see much
problem with 4 stroke power keeping hovercraft fully interesting.
Motorcycle engine powered F3 craft(gearbox and all) have had lap times
right there with 2 stroke craft with 100+ horsepower. If craft are
built with sufficient attention to weight control, it only takes about
60-70hp to make them comprehensively over-powered and therefore
interesting to race, in my humble opinion. The weight of the pilot
will have an effect obviously.
Your point about ram air effects is an important one. I get the
impression that early hovercraft theorists did not think all the way
thru to the notion that air-cushion support would start to blend with
surface effect lift as craft got better thrust to weight ratios. I
well remember the exhileration I felt the first time I learned that
the way to cope with wave chop can be to fly over it.
> in my current understanding of things, i think that as the hull gets larger, the
> higher cushion pressure is required to get the same performance for a
> realistically proportional load. in other words, the cushion area goes up as a
> square, but the volume and therefore the mass of the load goes up as a cube
> function. in my understanding, the cushion pressure is bumped up in order to
> compensate for that, but the scale of things makes the higher cushion pressure
> more reasonable.
>
The area avaialable to generate lift increases much faster than the
circumference around it--where air leak occurs. So larger crafe more
easily contain higher cushion pressures.
Ken Roberts
in terms of volume continuity, i understand what you're talking about now.
in terms of the ram effect, we were talking apples and oranges. when you
discuss "hovercaft" my mind conjures up what i have, which is pretty much what
you described, though i would use different terms. i don't really see
commercial vessels. the ram effect, if you have "interesting" (my term) or
"overpowered" (your term) engines, is viable for the sort of thing i'm
interested in driving.
back to volume continuity, you're describing a cushion which has, at least
mostly, a good seal and no major gaps under the skirt and the wave action is
changing the volume of your cushion.
that means that the air is rushing in and out of your lift duct, and that the
craft is bouncing on the waves.
it seems to me that the only way to really fix this would be to have a variable
volume chamber for your lift that would accept the too-high pressure air and
then release it when the pressure drops. some sort of elastic bag? but it
seems to me that it would have to have a fairly large capacity, like 1/2 of the
total cushion volume, maybe. and then it would need some sort of damper to
prevent oscillation.
how did you fix the problem?
Ken Roberts
Ralph DuBose wrote:
> Ken Roberts <k...@kroberts.in.9ci.com> wrote in message news:<slrnbrnr8...@kroberts.in.9ci.com>...
>> by "volume continuity" in the first paragraph, do you reference loss of air from
>> when you hop a wave? if so, then the "small wave" solution is to go fast enough
>> that the ram effect picks the whole nose of the craft up, and you just skip over
>> the crests. i call that "tigger mode" and it can be terrifying to get there
>> unless you have smooth water to get up to speed before you hit the waves. oddly
>> enough, though, it's a pretty smooth ride once you're there.
>>
>> and what would be the ramifications of increasing cushion pressure on small
>> crafts, really? i've wondered about that, for racing and high speed cruising
>> purposes.
>>
>
> Cushion pressure is just craft weight divided by cushion area. Bag
> pressure or bag to cushion pressure ratios are a different matter, as
> you well know.
> There was a time when higher cushion pressure raised the speed at
> which the front skirts blew inward and the craft began to fly.
> Nowadays, that problem has much better answers available and I do not
> see any advantage to higher cushion pressure per se.
come at it from the opposite side: putting a larger engine on the same sized
craft. to make, for example, an f1 racer with an automotive engine on it. what
problems would there be to tolerate a large engine on the same sized racer?
i notice that the racers that were brought to whc2002 could barely float, if
they could float at all. i'm under the impression that the average f1 hull for
a visitor was significantly heavier than the f1 hulls we use in the us. or it
could also be that with the plenum chambers they just didn't displace as much
water when not under power. certainly, the hulls were smaller than what we
figured were the norm.
> At the high performance end of the spectrum, I do not see much
> problem with 4 stroke power keeping hovercraft fully interesting.
> Motorcycle engine powered F3 craft(gearbox and all) have had lap times
> right there with 2 stroke craft with 100+ horsepower. If craft are
> built with sufficient attention to weight control, it only takes about
> 60-70hp to make them comprehensively over-powered and therefore
> interesting to race, in my humble opinion. The weight of the pilot
> will have an effect obviously.
> Your point about ram air effects is an important one. I get the
> impression that early hovercraft theorists did not think all the way
> thru to the notion that air-cushion support would start to blend with
> surface effect lift as craft got better thrust to weight ratios. I
> well remember the exhileration I felt the first time I learned that
> the way to cope with wave chop can be to fly over it.
>
>
>> in my current understanding of things, i think that as the hull gets larger, the
>> higher cushion pressure is required to get the same performance for a
>> realistically proportional load. in other words, the cushion area goes up as a
>> square, but the volume and therefore the mass of the load goes up as a cube
>> function. in my understanding, the cushion pressure is bumped up in order to
>> compensate for that, but the scale of things makes the higher cushion pressure
>> more reasonable.
>>
>
>
> The area avaialable to generate lift increases much faster than the
> circumference around it--where air leak occurs. So larger crafe more
> easily contain higher cushion pressures.
you're talking about supply and demand, right? i'm not sure exactly how that
applies. the cushion pressure is imposed by the mass and the surface area
covered. i don't really see how circumference is a major player here.
certainly if you have a craft of x amount of area that's round and one that has
the same area which is long and skinny, the round craft is easier to lift
because of the better area-per-feet-of-skirt ratio.
but if you have two other crafts of similar shape but one is much larger, the
larger craft can still tolerate a larger cushion pressure.
Barry Palmer
around 1973.
Suspension was via rubber bands stretched between the outboard line of the
bag skirt to the hull. You do not have to accommodate the entire volume error
to go a long ways in fixing the problem. Elastic suspension of the mid lines
of the fingers on larger craft would possibly solve this problem, as well as a
fan that is driven up and down its shaft via computer controls or pneumatics,
as well as a "trampoline" in the craft hull, and on and on. (record this
message so no one can patent this stuff, please. I have posted on this matter
earlier.)
The cushion really does not backflow through the fan duct, especially for
typical bag finger setups as backflow impedance is too high. The US Navy did
some work in trying to solve the problem by merely venting the extra cushion
air through pressure restrictive vents, a design mindset caused by not having
to pay for the craft fuel, I guess. To make an effective system, someone with
some competence in the area would have to be used, I do not see anyone in the
industry of large air cushion craft that has noticed this obvious (to me)
solution.
<< Subject: Re: Skirt material
From: Ken Roberts k...@kroberts.in.9ci.com
Date: Wed, Nov 19, 2003 7:38 PM
Message-id: <slrnbrodp...@kroberts.in.9ci.com>
barry,
>><BR><BR>
Barry Palmer
large a hull, like a hull that has reasonable sea keeping ability, it will blow
over at a lower speed. You can figure on a blowover when the craft ram
pressure approaches cushion pressure. Some of the problem is suppressed just
by moving forward to hold the bow down, which would require that dancing
elephant or well coordinated and dependable passengers in a large craft.
<< Subject: Re: Skirt material
From: Ken Roberts k...@kroberts.in.9ci.com
Date: Wed, Nov 19, 2003 7:52 PM
Message-id: <slrnbroej...@kroberts.in.9ci.com>
>> >
Tiberius
"Barry Palmer" <sev...@aol.com> wrote in message
news:20031119163309...@mb-m16.aol.com...
> While fingers work well on large craft, as well as racing craft, (which
> generally operate in very short duration races in golf course and lily
pond
> environments)
Hi Barry,
You don't go to many races do you?
Seriously though, my point is that the engineering challenges in making a
racing craft go are not necessarily the same as making a large craft go
faster, but they are still interesting and relevant. It might be quite
interesting to work out whether an attempt on the ultimate speed record is
better started from a small craft or a large craft basis.
The second point I would make is that there is far more involved in getting
a low lap time in a race than operating in a straight line on a glassy
surface.
Regards,
Nick
Ken Roberts
regarding "record this message please" you'll have to do better than that.
there's no central repository of all posts made to newsgroups, ever. part of
nntp is an expiration time which is variable per server, but still as a
practical matter they all expire.
if you want credit for an idea, do this:
- document the idea thoroughly on paper.
- notarize the document.
- use the usps to mail it to yourself, and make it a registered letter.
- the post office will post mark it, and it will be recorded by them as well i
think.
- do not EVER open that letter. the fact that it's still sealed, has the
postmark on it, and was registered is legal proof that the document came
from you (the notarized signature, etc) and can be admissible in court.
- if somebody else patents your idea, you can drag this unopened letter to
court as proof. so long as the other side has no earlier proof, you win.
Rick
> barry,
>
> regarding "record this message please" you'll have to do better than
> that. there's no central repository of all posts made to newsgroups,
> ever. part of nntp is an expiration time which is variable per server,
> but still as a practical matter they all expire.
>
> if you want credit for an idea, do this:
> - document the idea thoroughly on paper. - notarize the document.
> - use the usps to mail it to yourself, and make it a registered
> letter. - the post office will post mark it, and it will be recorded
> by them as well i
> think.
> - do not EVER open that letter. the fact that it's still sealed, has
> the
> postmark on it, and was registered is legal proof that the document
> came from you (the notarized signature, etc) and can be admissible
> in court.
> - if somebody else patents your idea, you can drag this unopened
> letter to
> court as proof. so long as the other side has no earlier proof, you
> win.
Hey, you use the poor-man copyright and patent registration method too! I
keep my un-opened envelopes in a safety deposit box - that way it's safe
from fire, flood or theft. But it still doesn't help provide the funds
needed to start the legal action to defend your patent so if the other guy
is bigger and richer than you are, you've lost anyway . . . sorry for the
negative tone - it's raining and I feel depressed today :o(
--
Regards,
Rick
to reply by email remove the "-mapson2"
Barry Palmer
driver weight shift avoids a lot of the problems of surface skimmer design,
and over powering with unrealistically high powered frantic engines (other
people's technology, not hovercraft technology, anyhow) "solves" some of the
rest of the problems.
As far as speed goes, Speeds are quite "Milquetoast" in hovercraft racing when
compared to racing boats. This is because the racing boats are largely air
cushion vehicles, except for the skeg which allows turning well beyond
hovercraft, as maybe 85% of the boat weight is airborne anyway, and if they
could gain propulsion on land with an air drive (and a retractable skeg
certainly would help) they would be quite capable of competing with a skirted
hovercraft, especially with UMHW plastic available for low friction operation.
(Yes, I have done land operations with airboats.)
In short, racing hovercraft as we know them now are trying to operate in a
speed realm beyond a reasonable level for the type of craft. Until the race
regulating bodies see fit to put realistic configuration limits on this type of
vehicle, such as noise restraint, reasonable vehicle load carrying capacity,
realistic powering, racing will cause no improvements in the technology of the
surface skimmer any more than, say, dragster technology can improve automobile
technology. I could see a simple foam rubber edge that is semi rigid and
compartmented put on a racing hovercraft that could possibly beat a skirted
hovercraft, but be of little use for the real world.
Another example of this is the application of "unlimited" hydroplane racing,
where they used to take an engine from an aircraft design capable of doing
about 400-450Kts, and carry all sorts of nasty stuff and lots of fuel at the
same time, (P51) put it in a boat that can go all of 175kts. with nothing but
the driver and 10 gallons of fuel aboard. (Miss Bud.) This is an
accomplishment??
<< Subject: Re: Skirt material
From: "Tiberius" sp...@mania-n.info
Date: Thu, Nov 20, 2003 1:40 AM
Message-id: <bpi27p$1nnqde$1...@ID-130298.news.uni-berlin.de>
Hi Barry,
Regards,
Nick
Barry Palmer
court. Stuff stored on servers probably lasts about 2 years. However, if we
talk about the subject enough, then it becomes "prior art", with lots of
witnesses, which cannot be patented if you can find the witnesses. So, lets
talk about it. Maybe I should post it in the Sevtec website, which is
approaching a decade in age, perhaps, along with a few dozen other people.
It is really strange that the hovercraft community, especially moneyed research
programs, have not become involved (as seen by lack of published literature)
explored this concept, as it is basic to a hydraulic system that some sort of
accumulator be in the circuit, and that is what the air cushion problem is.
<< Subject: Re: Skirt material
From: Ken Roberts k...@kroberts.in.9ci.com
Date: Thu, Nov 20, 2003 8:53 AM
Message-id: <slrnbrpsb...@kroberts.in.9ci.com>
Charles K. Dixon
"Paul Kenna" <pkenna...@vicnet.net.au> wrote in message
news:7femrv0doks293ac3...@4ax.com...
> I'm looking to replace some of my segmented skirts for my Turbo
> integrated Hovercraft.
>
> What is the state of the art material for skirts, and what can I
> source and afford. I'm looking for durability for a cruising craft.
I built a "Viper" which uses the same basic skirt design that you are
using... I'm not sure it's "State of the art" material, but I sewed my
skirts using Automotive "Air bag" material from a commercial fabric supply
shop in Chicago (Mauritzon). I have 2 years on the craft, and very little -
if any- wear.
As far as I know I'm the only one who has used this material, but there
are a few bag skirted Formula 25 racers that are planning on switching to it
after experimenting a bit with some of my material.
Ken Roberts
i've never actually used this method, but heard about it and thought it was
worth passing on. a post on a newsgroup legally counts as much as hearsay, so
expressing a new idea here is not going to win a patent lawsuit.
and yes, even genuine patents do not actually do anything for you. if someone
infringes on your idea, you still must defend your patent in court. he who has
the most bucks wins, pretty much.
Ken Roberts
you've stated the point without understanding it yourself.
racing ANYTHING as we know that thing now is trying to operate in a speed realm
beyond the reasonable level for that thing. by definition. that is the entire
point of racing--to see who can push the envelope furthest outside the
reasonable level and be reliable enough to make it across the finish line first.
that describes the f1, f2 and f3 classes extremely well. however, f25 can
hardly be described as frantic. it is by definition limited to stock engines,
and getting the best performance possible from a given powerplant. that sort of
contest can be weighted in any direction, and those contests can be used to
declare superiority for any purpose. including getting the maximum payload from
a to b with the least fuel, if that's how the contest is set up.
a manufacturer can make any claim in the world. however that doesn't mean jack
unless you can prove that yours is better in a neck-and-neck contest. even if a
third party comes along and makes his own tests to publish them, it only works
if everyone recognizes the neutrality of that party. if you really want to
prove the superiority of a machine, get a contest going at a public event (a
race event, for example, even if it's not a speed contest) and let anyone who is
interested compete. then you know, at least for those crafts which showed up,
which is superior for that thing.
some of the contests/races don't make much sense in the real world, but others
make a lot of sense. if you want to prove something about your designs, you
should push to get a realistic contest started and then show up.
Ken Roberts
you might find some server somewhere that keeps things for two years, but each
server's configuration is going to be up to the whim of the IT staff that
maintains it, probably not even company policy. you may "rely" on that server's
back log and then discover that somebody flushed the history on a whim. there
is no legal requirement to keep that log, and all the servers i've tried keep
history for something like 3 months or less. that's not even remotely enough
time to finish the thing and have word of that trickle back to the guy with the
original idea.
whether it would work in court:
registered letters are admissible in court. that's the whole purpose. if
you're going to evict a tenant you are legally required to serve notice a
specific number of days prior to the eviction, and registered letters count as
proof that such notice was sent. they are admissible in court.
similarly, notarized documents also function in a similar way, even though you
could technically come up with a crooked notary, that person is legally
responsible for the accuracy of the documents they notarize: in other words, if
a notary notarizes something which does not represent the truth, that notary can
be sued for the dishonesty and for damages related to that.
if you post the information, it had better be a digitally signed document. get
a VeriSign certificate or similar, and then you get to keep it current for
however many years it takes. it's cheaper to patent it.
Barry Palmer
<< Subject: Re: Skirt material
From: Ken Roberts k...@kroberts.in.9ci.com
Date: Thu, Nov 20, 2003 11:35 AM
Message-id: <slrnbrq5r...@kroberts.in.9ci.com>
barry,
>
Ken Roberts
what is that stuff called? how thin is it? how much does it cost?
Ken Roberts
however, most manufacturers who want to prove the superiority of their product
are willing to put their products on the line and see what happens. until such
a contest happens, whether you yourself are part of it or not, your claims (and
anyone else's who does not compete some way) are pure myth.
Fanman Uk
>A racer may have to have a small hull, as if the craft is too light on too
>large a hull, like a hull that has reasonable sea keeping ability, it will blow
>over at a lower speed. You can figure on a blowover when the craft ram
>pressure approaches cushion pressure. Some of the problem is suppressed just
>by moving forward to hold the bow down, which would require that dancing
>elephant or well coordinated and dependable passengers in a large craft.
Barry, do you have a URL where I can see this dancing elephant :o)
In general, I agree with almost everything that you have said
regarding the problems caused by pumping losses, particularly on
medium to large sized craft. However... as I understand the
development of the commercial hc skirt systems, these problems were
recognised long ago. Moving from the bag skirt to the loop and segment
type was one of the results. This type helped reduce the losses,
although it obviously did not overcome the cushion effects of wave
action. If cushion volume is reduced then some losses must occur
unless a variable volume system is employed. In general terms this has
been found to be impracticable due to the speed at which these changes
occur.
With regard to racing craft and the idea of suppressing lift by
weight-shift, yes it can be and is a solution, but it is a dangerous
one. As I am sure you will agree, shifting weight in the craft alters
the centers of gravity and balance, but does not have significant
effect on controlling cushion pressure or volume unless the cushion is
divided. I couldn't quite understand your idea of sliding the fan on
the shaft unless you mean to alter the volume downstream of the fan.
Currently I can see no solution to the problem except variable volume
supplies, unless the principles of resonance chambers (as in
carburettor variable volume ) has any merrit. I doubt that any change
to skirt geometry can have significant effect other than reducing the
losses but probably at the cost of increased wetted drag.
With regard to your comments on 'finger skirts' I assume you are
refering to extended segment skirts which now dominate the segmented
skirt types. I can't accept the idea that these suffer higher losses
than bag skirts in rough terrain at lower speeds, unless the cushion
pressure is unacceptably low. UK craft operate without problem and
rarely would you see significant volumes of air escaping from between
segments in normal conditions, as the cushion pressure holds the
segments open and pressed together. Agreed that loss of segments can
be a problem, but if the lower locating points are designed correctly
this is actually a rare event and many people would prefer to replace
a segment in the field than repair a torn bag skirt.
Fanman Uk.
To reply by email remove the "fanman."
Tiberius
Here in the UK, the preliminary application is free. You can download the
form off the net, fill it in and post it off. You then have a year before
you have to decide whether to pursue it or not. If you want to pursue it,
you will have established the original date that will apply for other
countries too. If you drop it, at least you have prevented anyone from
patenting it against you.
The downside against posting it to yourself is that you have to reveal it
publicly - but in reality it is unlikely that anyone will notice for the
first year.
OK, if you want a watertight patent then you might want to employ an agent.
But if your concern (which mine was when I did it) was preventing someone
else claiming it, then just do it yourself.
Nick
Ken Roberts
when you're referring to "loop and segment" you mean the bag and finger combo,
right? the way that lots of commercial crafts do it, ala srn-4?
Ken Roberts
the process i described was actually given to me by someone who had submitted a
patent and then someone else snuck an application in under the wire that caused
the one i knew of to fail.
if you believe the guy i heard all this from, it's evidently not unheard of for
patent applications to mysteriously be superceded during the application
process. this could just be the way things work, where several people see the
same sort of problems and invent the same logical solution, or it could be that
somebody at the patent office is scanning for likely ways to make some cash and
puts the original application on hold for "research" so they can get their own
version of the idea into an application. or, for that matter, some company who
is inclined toward such behavior could plant somebody in the patent office.
whether you believe the conspiracy theory or not, if a patent you sent in is
rejected you can contest that, in which case it goes to court and then you need
to present proof that you had the idea first. the registered letter to yourself
provides you with that proof. you will never get the original drawings back or
anything else that you sent to the patent office, but if you send multiple
identical copies to the patent office, yourself and some other trusted party,
then you cover most of the bases for required proof.
so this process would be considered "pre-patent" insurance.
Ralph DuBose
> nick,
>
> the process i described was actually given to me by someone who had submitted a
> patent and then someone else snuck an application in under the wire that caused
> the one i knew of to fail.
>
Ah, patents. The patent office files are now on-line, at least the
text is, not yet graphics. Look under US Gov. Anyone can do a
preliminary patent search/review from your pc.
Last time I checked, there were 400+ hovercraft related patents.
90+% are pure crap... stupid ideas from the start, would not work if
tried. Some of the old ones from BHC and others make interesting
reading as a kind of historical review of development. I have heard,
on good authority, that money has changed hands on about 2 or 3 of
them, always when large companies use techniques developed by other
large companies. In the history of the world, no small hovercraft
inventor has been paid anything, as far as I know. The UK designer of
the SCAT did get licence fees for its US manufacture but had to endure
watching his design get managled by that South Florida entity.
Actually, I agree with Barry in that early public disclosure is the
best way to establish priority and ownership. I went to a lot of
trouble to make an early public presentation of my recent anti-plow
technology, not because I wanted to patent it but to establish (
Something, exactly what is hard to say. I mean, it works really well
but no one but me will ever use it, most likely. It is hard to build
the first time and if one is convinced that plowin is not their
problem, it will not happen)
Barry, darn it. Your comments about race craft and extended finger
design would carry a little more weight if you had actually seen a
hovercraft race since Ronald Reagan was president. I have been
involved for about ten years now and the progress has been delightful.
Blown out skirt segments are rare events nowadays. At the last whc
event in Terre Haute, I race and cavorted 4-5 days and had exactly one
skirt segment come loose. My experience was typical. And almost
everything is much quieter these days. Nothing out there was nearly as
loud as an old Scat 1, for example. F 25s are inaudible from across a
pond.
All of your comments about what might or might not work better than
finger skirted craft on a typical race course would, again-with all
due respect, mean more if you had a better mental image of what the
rest of us are talking about.
Ralph DuBose
> I fixed the problem by elastically suspending the side bags of the Fan-tastic I
> around 1973.
>
> Suspension was via rubber bands stretched between the outboard line of the
> bag skirt to the hull. You do not have to accommodate the entire volume error
> to go a long ways in fixing the problem. Elastic suspension of the mid lines
> of the fingers on larger craft would possibly solve this problem, as well as a
> fan that is driven up and down its shaft via computer controls or pneumatics,
> as well as a "trampoline" in the craft hull, and on and on. (record this
> message so no one can patent this stuff, please. I have posted on this matter
> earlier.)
>
The impact of wave pumping depends somewhat on how lift air is
delivered. If, for example, all the lift air goes in at the stern,
wave pumping can momentarily deprive the front of the cushion of its
supply. Whereas designs which put air into the cushion all along the
periphery or selectively at the bow would have less problem with this
aspect of the issue.
A variation of your elastic attachment of a bag skirt is even
easier to do with extended finger skirts. The lower skirt attachment
line of a finger skirted craft can be made quite springy and still do
its job. Cushion volumn can rise quite a bit as the skirts move
outward, then to spring back. Skirt ties are protected from shock
loads as well.
Barry Palmer
Message-id: <slrnbrqds...@kroberts.in.9ci.com>
I have yet to hear about a large surface skimmer with a bag skirt without
fingers. I would like to find out about it if you can point me at a name that
can be googled.
Some may consider the SRN-5 early versions to have a bag skirt, but this was
entirely different from the conventional bag skirt. A peripheral jet was
attempted on the early craft, (which doesn't work, analytically, if one
examines the whole craft as a system) and the peripheral jet was a failure, as
the craft got bag-fingers later. (There is a possibility that someone sealed
the slot, but I have not heard of it.) I suspect that the slot in the bottom
of the bag was a big scooper in any small seaway, and some of the design (and
problems) caused by the peripheral jet philosophy still remain on the SRN-5 and
-6.
paul,
Ken Roberts
that doesn't even approach the question, let alone provide an answer. what post
were you responding to?
i was asking for clarification of the term "loop and segment". i think i know
what paul means by that, but i want to be sure.
this thing you wrote implies that i was stating something about large
hovercrafts with bag skirts. i have no idea how you got that from my question.
Barry Palmer
craft with bag skirts. "Loop segment" and "bag finger" are the same, as far as
I know. I still am curious as to if a large machine has been operated with the
typical bag skirt that is now used for small machines. The largest one I know
about is on a Sevtec Mariner.
<< Subject: Re: Skirt material
From: Ken Roberts k...@kroberts.in.9ci.com
Date: Fri, Nov 21, 2003 9:28 AM
Message-id: <slrnbrsip...@kroberts.in.9ci.com>
barry,
Ken Roberts
amazon. not sure if it's exactly a bag skirt, but i'm pretty sure there were no
fingers on it.
the uh-26s is a pure bag skirt. 16 passengers, 26 feet long, 12 feet wide,
powered by a v8. i have no idea how big a mariner is. i've never seen a 26s in
real life, but i've seen photos of one in operation besides those on the uh web
site.
Charles K. Dixon
> hey charles,
>
> what is that stuff called?
I'm looking at the sales invoice...
Mauritzon called it just called it "Black Air Bag Material"
>how thin is it?
It's about the same as the neoprene coated fabric that is usually used for
finger skirts.
>how much does it cost?
Cheap! It ran me an even $100 for 30 yards back in August of 2001... 64"
wide material. That was enough to make 96 finger skirts.. 68 for the craft
the rest for "spares". It's good material for finger skirts, but not the
typical type of material (or stiffness) you would expect to see on a bag
skirted craft.
J. Kelley Jernigan
Check this out:
http://www.geocities.com/crabola36/Craigs_Hovercraft.html
There is a link on this site to UH26 construction photos as well as a CAD
model.
Ken Roberts wrote:
>
> the uh-26s is a pure bag skirt. 16 passengers, 26 feet long, 12 feet
> wide,
> powered by a v8. i have no idea how big a mariner is. i've never seen a
> 26s in real life, but i've seen photos of one in operation besides those
> on the uh web site.
>
--
J. Kelley Jernigan
-
Registered Linux User # 282143
Mandrake 9.1
http://www.hovercraftersresource.com
-
"If what you believe does not reflect truth, then what you feel does
not reflect reality." Dr. Neil Anderson
Ken Roberts
craft long enough to notice?
i'm obviously trying to figure out how well it would work for a bag skirt. you
say it's not the typical type or stiffness you'd use on a bag skirt, but what's
the deviation? HOW is it different? thinner, evidently, and maybe more
flexible? that's not a disadvantage for a bag skirt by itself, unless it's easy
to cut, tear or abrade.
Ken Roberts
this is the same craig that made the aluminum 17t, right?
i wonder if he's been asked for a building permit. that thing's bigger than the
garage.
i wonder where he's going to put it, and how he's going to get it there. i
suspect you'd need a permit to trailer it each time, for a 12-foot wide craft.
In article <Xqtvb.2985$dl.3...@twister.southeast.rr.com>, J. Kelley Jernigan
wrote:
Alistair Furnell
Alistair,
It is Canadian $ 65.00 per square meter, Available in Clear and Red colour.
Canadian Dollar is much cheaper than Euro.
Email your Order, we'll send you a PayPal invoice, you pay with any credit
card
We accept international bank transfer
Lead time from 6~8 weeks
Free shipping to the nearest standard seaport any where around the world.
RushStar International Team.
www.rushstar.com
Canada.
Charles K. Dixon
"Alistair Furnell" <mr...@orcon.net.nz> wrote in message
news:3fbe91dd$1...@news.orcon.net.nz...
Lets see now.....
If I calculate it all out I used about 480 square feet of material to
make 96 skirts- the material cost me $100 (US). If my math is right you
would need about 45 square meters of material to have the same amount of
material I used- $2925 Canadien / $2246.64 (US), and $3110.79 (AUD)......
OUCH!
Hehehe... you should probably order enough for an extra full set of skirts!
Charles K. Dixon
> is it easy to slice or tear? and what does it wear like, if you've run
your
> craft long enough to notice?
The craft has been up and running for almost 2 years now... 2 different
motors, of course... Very little wear, and very difficult to tear. Graham
Spencer "tested" one of my spare skirts at Terre Haute and could not tear
it.. and he was REALLY trying. He and Kevin Pratt are planning on using it
on their "Herons" because of the toughness weight savings. I cut the stuff
with a "Hot Knife" which sealed the edges so there was no fraying. As far as
toughness goes I have only ripped one when I railed a log, and it ripped the
seam out not the material.
>
> i'm obviously trying to figure out how well it would work for a bag skirt.
you
> say it's not the typical type or stiffness you'd use on a bag skirt, but
what's
> the deviation? HOW is it different? thinner, evidently, and maybe more
> flexible? that's not a disadvantage for a bag skirt by itself, unless
it's easy
> to cut, tear or abrade.
The stuff you UH guys are using is usually "Coverlight" (Vinyl Coated
Nylon) type material which is less flexible, and thicker... in other words
if I made a finger skirt from the Coverlight type material and I wadded it
up like a piece of paper I probably couldn't wad it tight enough for it to
fit it in my hand, and when I let it go it's going to spring back to it's
general shape.... The air bag material for a similar skirt could be held
wadded up in your hand and when you drop it it stays wadded for a while....
It stays flexible in the cold too- shock tested it (and me!) on the
DesPlaines last winter on a 5 degree (F) day....
Craig
My plan is to build it and permanently keep it in the backyard so
that I can look at it every once in a while and say "Holy shit....Look
at that thing!"
Actually I'm gonna build a 102inch wide trailer that carries the craft
by the skids. It will have pvc rollers so that I can water launch and
load with relative ease. A land launch trailer would be a hydraulic
design and I just don't want to spend the money.
The wide load permit in Fla. is $300 per year for unlimited use or 10
bucks for 2 days at a time. I have a buddy that hauled a 12ft wide
boat without a permit for 2 years in Fla. and didn't get harrased but
I won't take that chance.
I'm trying to imagine what it's gonna be like to drive this thing. I
imagine it will be like driving in slow motion. I can't wait to take
all of my friends and family out.....in one shot!
Craig
Ken Roberts <k...@kroberts.in.9ci.com> wrote in message news:<slrnbrss0...@kroberts.in.9ci.com>...
Alistair Furnell
in NZ$ it is going to set me back $1862.64 for 24m of it just going by the
Viper hand book this is a bit more than the stuf Neil has but at the same
time it will go tho hell and back where the other stuf will not and need
more to replace I wount to try to get a way from changing to meany skirts in
the first place but ho knows ?
" But then you did have to make 96 of them A" see things tend to be chepper
in the good old USA than any where alse, I am out of luck any way.
How is your Viper any way Charles, put any more holes in it lately
Hehehe....
Alistair
NZ
"Charles K. Dixon" <speed...@speedjunky.net> wrote in message
news:bpm7h2$dr7$1...@news.netins.net...
J. Kelley Jernigan
Can you take it across state lines with the permit?
Craig wrote:
>
> The wide load permit in Fla. is $300 per year for unlimited use or 10
> bucks for 2 days at a time. I have a buddy that hauled a 12ft wide
> boat without a permit for 2 years in Fla. and didn't get harrased but
> I won't take that chance.
--
Fanman Uk
><< Subject: Re: Skirt material
>From: Ken Roberts k...@kroberts.in.9ci.com
>Date: Thu, Nov 20, 2003 1:52 PM
>Message-id: <slrnbrqds...@kroberts.in.9ci.com>
>
>I have yet to hear about a large surface skimmer with a bag skirt without
>fingers. I would like to find out about it if you can point me at a name that
>can be googled.
>
>Some may consider the SRN-5 early versions to have a bag skirt, but this was
>entirely different from the conventional bag skirt. A peripheral jet was
>attempted on the early craft, (which doesn't work, analytically, if one
>examines the whole craft as a system) and the peripheral jet was a failure, as
>the craft got bag-fingers later. (There is a possibility that someone sealed
>the slot, but I have not heard of it.) I suspect that the slot in the bottom
>of the bag was a big scooper in any small seaway, and some of the design (and
>problems) caused by the peripheral jet philosophy still remain on the SRN-5 and
>-6.
>
You're testing my memory now Barry. I think that you are probably
correct in your assertion that your craft is the largest "commercial"
craft (commercial implying here, several built)to use a bag skirt as
we recognise them today.
The earliest british craft were fitted with a simple curtain skirt
(would you say drape ??). Saunders Roe then tried a variation of this
which used two curtain skirts one inside the other, with the air jets
blowing in the space between. (I can only assume that this originated
from the original concept of Cristopher Cockerill which used an
annular feed, as per his coffee can experiment. Air inflated the
skirts which were held in place by straps or chains. This skirt design
was patented by SR or Westland.
The dual Curtain quickly gave way to a "Tubular design" (Loop)which we
would now call a bag skirt. Air from the tube was fed into the cushion
(full flow type). Before this skirt was used in full service it was
modified to include "flaps" on the inner surface which directed the
cushion air downward toward the contact point to give some pretence of
peripheral feed.
This is the modified bag skirt which eventually saw full service with
the SRN2 craft ( a 20m craft I think). It did not have segmented
fingers as is common on larger craft today. A good photo of this skirt
fitted can be seen at:
(Wrapped)
(There are some good photo's on this site of early craft, inluding a
beautiful image of an SRN1 with clear air bebeath it :o> )
This skirt type did not use segments (fingers). These appeared on
later craft and by 1964 approx. was the only type to be seen on 20m +
craft including the SRN5.
It is unfortunate that the early information gleaned from experiments
and patents etc ( and in particular their reasons for some of the
design changes )in the UK was classified by the Ministry Of Defence.
(some of this information may now have been released by the MOD under
our 30 year rule)
Regarding the use of the peripheral Jet, I have to disagree with you.
It is true that in general this system uses more power than the
conventional skirt types (any type) but this is because the fan system
has to provide higher pressures than they do. Interestingly however a
jet system uses a lower volume of air than the (now) conventional
system, hence the losses are not much higher. This then gives the jet
some advantage because of lower volume.
It tends not to suffer from pumping losses in the same way
the fans can be smaller.
Personally I think that there is some potential development to be had
by re-examining the priciples of controlling cushions by jet. I have
personally made some small inroads into this, but there is still much
work to do before real figures can be generated from this research.
Barry Palmer
craft, and if you examine the fluid dynamics of the jet and cushion alone, you
see some benefit in that power, and therefore weight should be reduced. But,
when you consider absolutely most optimistic minimal diffusion losses in
getting the air to the periphery of the craft the peripheral jet fails to
compete with the plenum system.
Then, when you start to examine other aspects, such as added weight, it becomes
a wonder why anyone ever even tried the peripheral jet. (Except the government
and military, where they do not care about operating costs.
Then you get downright practical, you have to make the machine. The
complexity, and added weight of the extra flow passages, and fan size, (the
peripheral fan may have a smaller eye or inlet diameter, but is certainly
vastly heavier than a fan that can be used on a plenum system (Remember, it is
not just rotor weight, but housing weight, and flow passage weight, and skirt
weight.
Just look at the SRN-6 mixed flow fan, it probably would have been a one stage
axial fan with a slightly larger diameter than the inlet diameter of the mixed
flow fan if the lower pressures of plenum systems or current SRN-5 plenum
pressure were looked at) These factors should further resolve that the
peripheral jet configuration is a waste of time. Couple that with "real world
problems of a seaway, and the exposure of the jet passages to scooping water,
it becomes beyond consideration.
I suspect people looking at these early concepts did not use basic fluid
mechanics to examine the problem before building hardware, while not looking at
real possibilities such as the elastic suspension I have mentioned in previous
posts.
>Subject: Re: Skirt material
>From: Pa...@fanman.commark.freeserve.co.uk (Fanman Uk)
>Date: 11/22/03 8:52 AM Pacific Standard Time
>Message-id: <3fbf919f...@news.tiscali.co.uk>
Charles K. Dixon
> Hehehe....
> Alistair
> NZ
No more unscheduled holes! It's far cheaper to go around the logs that are
in your path than it is going through them!
The Viper is running VERY well- still too wimpy to run it to top
speed....I've had it to 52 mph with a LOT of throttle left over. Plenty fast
enough! I haven't been in a big enough area on a calm enough day to push it
any harder- strange things tend to happen when your running over 50......I
get to "play" with the Viper pretty much any time I want-. I built a 1 mile
"course" on part of my farm in and around my lake. It's very similar to some
of the UK Hovercraft race tracks that I have seen on video. I'll do a video
of it one of these days......
Charles...
Fanman Uk
>I examined the peripheral jet concept before I knew it was on some early
>craft, and if you examine the fluid dynamics of the jet and cushion alone, you
>see some benefit in that power, and therefore weight should be reduced. But,
>when you consider absolutely most optimistic minimal diffusion losses in
>getting the air to the periphery of the craft the peripheral jet fails to
>compete with the plenum system.
>
>Then, when you start to examine other aspects, such as added weight, it becomes
>a wonder why anyone ever even tried the peripheral jet. (Except the government
>and military, where they do not care about operating costs.
>
>Then you get downright practical, you have to make the machine. The
>complexity, and added weight of the extra flow passages, and fan size, (the
>peripheral fan may have a smaller eye or inlet diameter, but is certainly
>vastly heavier than a fan that can be used on a plenum system (Remember, it is
>not just rotor weight, but housing weight, and flow passage weight, and skirt
>weight.
>
This may be true for heavy weight craft, particularly those such as
the early craft of '59- '62 vintage. I too have run through the
calculations comparing 2 like systems for a lightweight craft (2-3
seat size) and found that a small benifit should theoretically result
from using the peripheral jet. I agree that if you use the same
approach as Saunders Roe did in 1959 you end up with a problem almost
as big as the SRN1 :o)
It is possible however to marry the simplicity of the integral plenum
hull with segmented skirt and the ideals of a simple PJ craft and
produce a lightweight practical design which can still use the same
fans that most recreational craft use today. The result is a slightly
higher pressure requirement and lower volume requirement. The power
consumption is similar, possibly very slightly higher than would be,
the same craft fitted with a simple segment or extended pressure
segment.
>Just look at the SRN-6 mixed flow fan, it probably would have been a one stage
>axial fan with a slightly larger diameter than the inlet diameter of the mixed
>flow fan if the lower pressures of plenum systems or current SRN-5 plenum
>pressure were looked at) These factors should further resolve that the
>peripheral jet configuration is a waste of time. Couple that with "real world
>problems of a seaway, and the exposure of the jet passages to scooping water,
>it becomes beyond consideration.
The effectiveness of the PJ principle cannot IMO be directly linked
with the fan type used. In many instances it would still be possible
to use an axial arrangement unless the pressure requirements was very
high (requiring a 2 stage fan setup). A PJ system is possible without
using extreme pressures, at least in a lightweight craft. As to
scooping water, this is one of the advantages of a Jet. Due to the
high velocity of air leaving the jet, it is virtually impossible for
water to enter and the high velocity air stream also forms an
*effective* curtain which retains the cushion better than a standard
segment with peripheral feed does. Experiments conducted here in the
UK have shown that using this principle at the front of a craft has
eradicated spray from the front of the cushion entirely, showing
extremely good cushion containment.
I think that it is important to evaluate individual benifits in any
skirt design and not simply strive to reduce the total power
consumption. It is afterall possible to build a small craft using just
2HP for lift, but it won't get you anywhere :o)
> I suspect people looking at these early concepts did not use basic fluid
>mechanics to examine the problem before building hardware, while not looking at
>real possibilities such as the elastic suspension I have mentioned in previous
>posts.
Most probably not. In 1959 I think that the first craft was designed
about experiments based on models, and was more about proving a theory
than trying to arrive at a truly "commercial" design. We should not
dismiss the work out of hand tho' because it did produce a working
craft, albeit one that was for most purposes, impractical and had to
have a skirt fitted to it a couple of months later. Is this not what
pioneering is all about?
Barry Palmer
Message-id: <3fbff0c5...@news.tiscali.co.uk>
The idea is to get the skirt as high above the water surface as is practical,
with a reasonable use of power. At the same calculated height, vehicle weight,
cushion area, the simple plenum comes out lower powered then the peripheral jet
machine, and as a result, the craft is lighter, and the gap is further
increased due to the lighter weight. Then, to make the two systems comparable,
again, we have to reduce the engine power of the simple plenum machine to
re-establish the same hover height as the peripheral jet machine we are
comparing to. When we do that, the craft now has even less engine, so the
hover height gets more, and we have to further reduce the power on the plenum
craft. And on and on. This is the way systems work. What may seem to be a
small difference in performance of some element of a design can affect the
system very greatly.
The cushion power requirement per cubic unit of cushion air increases as the
3/2 power of the pressure at the fan discharge for the same final cushion size
and hovergap, so it really pays to get fan discharge pressure down. For
instance, if we overpressure the jet by just 30% for a single fan pressure
machine, not one with separate jet and cushion charging fans, I have never seen
such) we would require 1.3^1.5=1.5 times the power, and still possibly not get
enough pressure to the skirt discharge nozzles to get a reversal of momentum of
the discharging jet containing the cushion due to internal ducting pressure
losses. Probably for the peripheral jet to work at significantly upping the
hover height, a more realistic number would be at least 1.5 and maybe 2,
corresponding to 1.5^1.5=1.8 to 2^1.5=2.8 times the power per cubic unit of
cushion air, obviously perpheral jet momentum would start containing the
cushion, so reduce these powers for equivalent craft by maybe 20% to 30% to
roughly 1.2 to 2.0. (I'm guessing from calc experience, too lengthy to
calculate while writing this post.
>Just look at the SRN-6 mixed flow fan, it probably would have been a one stage
>axial fan with a slightly larger diameter than the inlet diameter of the
mixed
>flow fan if the lower pressures of plenum systems or current SRN-5 plenum
>pressure were looked at) These factors should further resolve that the
>peripheral jet configuration is a waste of time. Couple that with "real
world
>problems of a seaway, and the exposure of the jet passages to scooping water,
>it becomes beyond consideration.
The effectiveness of the PJ principle cannot IMO be directly linked
with the fan type used. In many instances it would still be possible
to use an axial arrangement unless the pressure requirements was very
high (requiring a 2 stage fan setup). A PJ system is possible without
using extreme pressures, at least in a lightweight craft. As to
scooping water, this is one of the advantages of a Jet. Due to the
high velocity of air leaving the jet, it is virtually impossible for
water to enter and the high velocity air stream also forms an
*effective* curtain which retains the cushion better than a standard
segment with peripheral feed does. Experiments conducted here in the
UK have shown that using this principle at the front of a craft has
eradicated spray from the front of the cushion entirely, showing
extremely good cushion containment.
I am sure either axial or mixed flow fans could be used. I just finished
operating my 26hp craft 83 feet ^2 cushion, at a weight of 1450 lb gross, (made
a deep water planeout in that state in 11 seconds no less) for 17 psf cushion
pressure, with an axial fan blade tip speed of 300ft per second max, liftoff
fan tip speed of 200ft per second, which means that I could easily handle the
(23psf, I guess) of an SRN-6 cushion at a modest maximum tip speed of 350 ft
per second. Even a big fan built like the Multi-Wing I am using, (or 6 or so
of the fans I am using) which can work up to 395 ft per second tip speed, would
handle the SRN-5 with a simple plenum. (That would be a little noisy, but I am
sure the propeller noise would mask the sound.)
I think that it is important to evaluate individual benifits in any
skirt design and not simply strive to reduce the total power
consumption. It is afterall possible to build a small craft using just
2HP for lift, but it won't get you anywhere :o)
2hp, on a polished marble surface. 2 hp will lift a person easily, but just
barely generate a hovergap for travel over roughness. ( I built a 4hp craft,
see Sevtec website, old photo section, that could plane me out, 160#, with a
140# craft) with about 2hp to the cushion. The slightest breeze on the nose
would prevent planeout, however.) I would suspect you could go a lot lower in
hp if you stuck to very smooth surfaces. I could put 3 people in my 12Hp
Tecumseh 1 cylinder Sevtec Scout and fly them on concrete, (45 ft ^2 cushion)
maybe 750lb gross the cushion gets only 4.5hp of that 12. While the propeller
got 5.5. (Yeah, I know, it does not add up but that is probably real hp I am
getting.
> I suspect people looking at these early concepts did not use basic fluid
>mechanics to examine the problem before building hardware, while not looking
at
>real possibilities such as the elastic suspension I have mentioned in previous
>posts.
Most probably not. In 1959 I think that the first craft was designed
about experiments based on models, and was more about proving a theory
than trying to arrive at a truly "commercial" design. We should not
dismiss the work out of hand tho' because it did produce a working
craft, albeit one that was for most purposes, impractical and had to
have a skirt fitted to it a couple of months later. Is this not what
pioneering is all about?
Pioneers do head in the wrong direction if they do not read the trail markers.
> >><BR><BR>
Paul Kenna
I'm trying to track down the material in Australia.
What type of firm repairs airbags.
Paul
Melbourne
Tiberius
If I understand your discussion properly, is there not another factor to
take into account in the peripheral jet versus direct discharge debate?
Does not the PJ system give a higher (effective) skirt pressure? This will
increase the speed you can go before ram air presure gives skirt collapse.
Incidentally, it seems to me, watching races, that craft are routinely going
faster than the speed at which simple theory would predict collapse of the
front skirt. I assume this is because the skirt pressure is higher than it
actually needs to be and this is either because of pressure head due to
pumping losses or because the finger segment skirt sections are showing some
of the properties of the peripheral jet. Comments please.
Nick
Fanman Uk
<snip>
>>It is possible however to marry the simplicity of the integral plenum
>>hull with segmented skirt and the ideals of a simple PJ craft and
>>produce a lightweight practical design which can still use the same
>>fans that most recreational craft use today.
<snip>
>
>The idea is to get the skirt as high above the water surface as is practical,
>with a reasonable use of power. At the same calculated height, vehicle weight,
>cushion area, the simple plenum comes out lower powered then the peripheral jet
>machine, and as a result, the craft is lighter, and the gap is further
>increased due to the lighter weight. Then, to make the two systems comparable,
>again, we have to reduce the engine power of the simple plenum machine to
>re-establish the same hover height as the peripheral jet machine we are
>comparing to. When we do that, the craft now has even less engine, so the
>hover height gets more, and we have to further reduce the power on the plenum
>craft. And on and on. This is the way systems work. What may seem to be a
>small difference in performance of some element of a design can affect the
>system very greatly.
This makes no sense to me when applied to small craft. It assumes that
the differences in power between the two systems will be sufficient to
require an engine change thereby reducing/gaining weight. In the UK we
have conducted experiments with small craft of F3 and F2 types, which
have been adapted to make use of the PJ principle without change to
the power supplies, or major adjustment to the fan(s) or control
systems. The weight remained the same. The hover height is unaltered
and the hovergap is adequate for the task.
>
>The cushion power requirement per cubic unit of cushion air increases as the
>3/2 power of the pressure at the fan discharge for the same final cushion size
>and hovergap, so it really pays to get fan discharge pressure down.
Yes, but if the PJ craft uses a lower volume of air then the power
increase is kept to a managable figure. 2-3 Hp maximum in the craft
to which I have refered above.
> For
>instance, if we overpressure the jet by just 30% for a single fan pressure
>machine, not one with separate jet and cushion charging fans, I have never seen
>such) we would require 1.3^1.5=1.5 times the power, and still possibly not get
>enough pressure to the skirt discharge nozzles to get a reversal of momentum of
>the discharging jet containing the cushion due to internal ducting pressure
>losses. Probably for the peripheral jet to work at significantly upping the
>hover height, a more realistic number would be at least 1.5 and maybe 2,
>corresponding to 1.5^1.5=1.8 to 2^1.5=2.8 times the power per cubic unit of
>cushion air, obviously perpheral jet momentum would start containing the
>cushion, so reduce these powers for equivalent craft by maybe 20% to 30% to
>roughly 1.2 to 2.0. (I'm guessing from calc experience, too lengthy to
>calculate while writing this post.
This is an extention of the argument you put above. It may well be
true for those '59 - '63 vintage craft, but for a modern lightweight
which retains a skirt but combines the principles of the PJ it does
not reflect the reality of the situation bourne out by testing. It is
not necessary to use large amounts of power to create adequate jets,
providing that energy losses are minimised within the plenum feeding
the jets. The object of our experiments is not to replace the modern
standard of skirt, merely to enhance it by combining advantages of
both systems
I suspect that the opinions of many American designers/builders are
coloured by their preference toward the direct supply of air to the
cushion. A major European small craft manufacturer recently produced
a small racing craft with a segmented skirt based on this premise. It
was extremely unstable and remained so until some peripheral feed was
supplied to the skirt. Vitually all European craft use peripheral
feed.
In the experiments conducted with PJ's on an F3 craft the stability
was considered to be better than the same craft fitted with simple
segments and arguably better than when the craft was fitted with
pressure segments.
<snip>
>I am sure either axial or mixed flow fans could be used. I just finished
>operating my 26hp craft 83 feet ^2 cushion, at a weight of 1450 lb gross, (made
>a deep water planeout in that state in 11 seconds no less) for 17 psf cushion
>pressure, with an axial fan blade tip speed of 300ft per second max, liftoff
>fan tip speed of 200ft per second, which means that I could easily handle the
>(23psf, I guess) of an SRN-6 cushion at a modest maximum tip speed of 350 ft
>per second. Even a big fan built like the Multi-Wing I am using, (or 6 or so
>of the fans I am using) which can work up to 395 ft per second tip speed, would
>handle the SRN-5 with a simple plenum. (That would be a little noisy, but I am
>sure the propeller noise would mask the sound.)
Mixed and Centrifugal flow fans can have advantages. In most
hovercraft, the axial fan (which provides quite high air velocities)
is mounted into a plenum, which forces the air to turn through 90 deg
immediately after the fan. this causes considerable energy losses. The
mixed/cent flow fans reduce these losses naturally by discharging air
at 90 deg. Axials can be mounted to avoid these losses too, but it
usually means having a clear length of duct after the fan which
effectively makes the fan as large or larger than an equivalent
mixed/centrifugal type.
>
<snip>
>2hp, on a polished marble surface. 2 hp will lift a person easily, but just
>barely generate a hovergap for travel over roughness. ( I built a 4hp craft,
>see Sevtec website, old photo section, that could plane me out, 160#, with a
>140# craft) with about 2hp to the cushion. The slightest breeze on the nose
>would prevent planeout, however.) I would suspect you could go a lot lower in
>hp if you stuck to very smooth surfaces. I could put 3 people in my 12Hp
>Tecumseh 1 cylinder Sevtec Scout and fly them on concrete, (45 ft ^2 cushion)
>maybe 750lb gross the cushion gets only 4.5hp of that 12. While the propeller
>got 5.5. (Yeah, I know, it does not add up but that is probably real hp I am
>getting.
My point exactly. There is little point in chasing lower and lower
power consumption at the expense of practicality. The craft must be
capable of performing over the intended terrain.
>> Is this not what pioneering is all about?
>
>Pioneers do head in the wrong direction if they do not read the trail markers.
>
Real pioneers do not follow trail markers, they lay them :o)
If we are to improve the overall effeciency and/or effectiveness of
skirt systems on small hovercraft we need to start thinking laterally.
Simply re-itterating designs of the past will not provide the advances
that are needed.
Barry Palmer
craft may be going slightly above their blowover speed not because of the
hardened skirt but because the driver moves forward as the craft starts to
become airborne and this suppresses the bow lift as airodynamic lift builds.
Aerodynamic lift acts maybe 1/3 of the way back on the hull, while cushion lift
acts at the center of cushion area, (halfway back) and diminishes as a
supporting factor as blowover ensues.
Obviously suppression of blowover by moving forward, requires little skill,
but does require a definite awareness that the blowover condition is being
approached, and this would be unacceptible for a recreational craft, operating
on its own over long periods, far from rescue, and subjected over time to the
good possibility of wind gusts.
Since the fingers have a small radius of curvature compared to a bag, they are
more easily pushed aside, and a little jet of air at their inner surfaces will
have some slight effect in holding them down. This is in no way a periphral
jet, however. If the driver were in a fixed position, such as being seated
normally as in Sevtec sevs, blowover would occur with or without a pressurized
bag at about the same time as it would on an equivalent finger skirted craft.
You should note that since there is very little pressure difference across the
forward finger skirt elements when ram pressure is approached, they are
practically just dangling off the front of the craft, (and also losing their
finger to finger seal), so any little bit of help at holding the fingers down
is welcome.
A big help would be to used a pressurized bag, which would behave like a big
inner tube, or small flexible foam nubbins rather than a skirt might be the way
to go for operation of the nature of hovercraft racing.
It should be noted that even in low powered craft such as the 16hp to 20hp
Sevtec Vanguard, where a real payload is the objective, not the highest speed
possible, one must consider the case where a light weight driver operates the
craft solo you can approach blowover speeds. On ice, I have had a blowover
reported from a builder operating on ice with a 7hp older sev of my design,
when I was Palmer Aerosystems, a Fan-Tastic I.
If you put a Vanguard airdrive on a Sevtec Scout hull, while acceleration,
which is really important for the short closed course races of hovercraft
races, would not be as great, the Scout would possibly reach the speeds of the
typical racers, with attendant blowover suppression problems, as cushion
pressure would be up as the hull area would be small, but there would be no
payload beyond the driver.
<< Subject: Re: Skirt material
From: "Tiberius" sp...@mania-n.info
Date: Sun, Nov 23, 2003 7:21 AM
Message-id: <bpqjc8$1rjgek$1...@ID-130298.news.uni-berlin.de>
Barry, Paul,
Nick
Fanman Uk
wrote:
>Barry, Paul,
>
>If I understand your discussion properly, is there not another factor to
>take into account in the peripheral jet versus direct discharge debate?
>
>Does not the PJ system give a higher (effective) skirt pressure? This will
>increase the speed you can go before ram air presure gives skirt collapse.
In a pure PJ system with no skirt, the cushion pressure is that set by
the craft mass and cushion area. some protection of the cushion is
given by the jet curtain which is unlikely to be disrupted by
impinging air, but this air curtain can be shifted by the impinging
air causing variance in cushion pressure and changes in CoG although
these are likely to be very small and not be problematic.
In a combined system, skirt pressure is raised (as it would be in a
pressure segment) thereby stiffening the envelope, offering greater
resistance to impinging air. The action of a jet system forming an
effective air curtain retains the cushion more effectively within the
hovergap and may therefor also resists better the influx of impinging
air into the cushion.
>
>Incidentally, it seems to me, watching races, that craft are routinely going
>faster than the speed at which simple theory would predict collapse of the
>front skirt. I assume this is because the skirt pressure is higher than it
>actually needs to be and this is either because of pressure head due to
>pumping losses or because the finger segment skirt sections are showing some
>of the properties of the peripheral jet. Comments please.
>
>Nick
This is not completely true. If a craft is ultra light and very fast
the velocity pressure may come closer to the cushion pressure but it
is yet to exceed it in normal conditions. The problems occur when such
a craft is "thrusted" by another leading craft and has to contend with
its fan discharge velocity. A 60 mph craft hitting a 100mph wind meets
a velocity pressure of 303 Pascals (6.3 Lbf/sqft), a light Formula 3
craft using simple segments, could be using a cushion pressure of this
value thereby making the skirt unstable. Using a pressurised segment
(or bag) improves the skirt stability in this situation. A standard
segment although directing air downward toward the contact point,
exhibits none of the characteristics of a Peripheral Jet in the true
sense.
Charles K. Dixon
"Paul Kenna" <pkenna...@vicnet.net.au> wrote in message
news:rh61sv8vcf5qr1v2e...@4ax.com...
> Charles
>
> I'm trying to track down the material in Australia.
>
> What type of firm repairs airbags.
>
> Paul
> Melbourne
As far as I know they don't repair airbags... Once they deploy they are
replaced with new ones.
The place I bought the material from is a wholesale fabric supplier-
they supply fabric to commercial industries- they would be the fabric
supplier to the company that physically makes the air bags for the car
manufacturers. Not sure where to begin looking in Australia, but I imagine
there are similar types of supply houses... the trick is to find one that
will sell to the general public- a lot of them over here will not. Finding
the right company is 75% of the battle and can be very helpful in finding
new materials to try once they understand what you are looking for.
Ralph DuBose
Nick
Some thoughts: The state of art for race craft, notably the pintail
design, features semi-enclosed segments along the front that operate
at plenum pressure and so do not collapse when ram air pressure
reaches cushion pressure. That combined with zero lift topside means
that they do not fly away as long as the bow stays close to the
surface.
My guess is that a typical extended segment design does have some
of the features of a peripheral jet design except that most of the
time, the actual flow thru the skirts is very inconsistent - higher
where there is an airgap, less where there is not.
The whole question of a p.jet design seems to me a historical
footnote. Stabilty has got to be terrible compared to a typical modern
skirted craft. The first rule of efficiency in aerodynamics, in my
humble opinion, is to avoid accelerating or compressing air any more
than necessary. A p. jet design depends on putting a lot of energy
into the lift air which is then dissippated.
Ideally, that diffusion efficiently makes pressure under the craft but
the losses from cramming airflow all the way round the periphery have
to be significant, regardless of what the theory is about how well the
system works once the air is being jetted under the craft.
I bet that p.j. systems had good anti plow-in characteristics
because there would be no skirt drag at all.
Tiberius
"Ralph DuBose" <rdu...@pdq.net> wrote in message
news:cb5b2d4e.03112...@posting.google.com...
> I bet that p.j. systems had good anti plow-in characteristics
> because there would be no skirt drag at all.
Interesting observation. In my earlier post, I had forgotten that PJ could
be done without a skirt. I was just assuming that the final part of the air
guide would be flexible. But whatever it is, until the ram air pressure
gets too large, or until it contacts something, surely it doesn't make any
difference whether it is a flexible skirt or part of a rigid hull.
But what about the incipient plough in? Maybe a rigid PJ system is
different. Does a rigid PJ system then behave the same over water as over
land? Anyone know?
Nick
Ralph DuBose
> It is possible for racers to operate just above blowover speed, but the racing
> craft may be going slightly above their blowover speed not because of the
> hardened skirt but because the driver moves forward as the craft starts to
> become airborne and this suppresses the bow lift as airodynamic lift builds.
Where is it written that the topside must generate lift? If the
topside is configured to produce extremely dirty airflow from the bow
rearward, why should there be any lift? The pintail craft have an
uncowled lift engine right on the bow and no windscreen or anything
shaped to allow for significant high velocity flow.
The reason for putting weight forward is to limit the amount of ram
air getting under the front skirts into the cushion space. Otherwise,
the air-dam effect of more rigid front skirts would be circumvented.
All of this is exactly the same as race car aerodynamics. Notice
the front airdams as close to the ground as possible.
> Aerodynamic lift acts maybe 1/3 of the way back on the hull, while cushion lift
> acts at the center of cushion area, (halfway back) and diminishes as a
> supporting factor as blowover ensues.
>
> Obviously suppression of blowover by moving forward, requires little skill,
> but does require a definite awareness that the blowover condition is being
> approached, and this would be unacceptible for a recreational craft, operating
> on its own over long periods, far from rescue, and subjected over time to the
> good possibility of wind gusts.
>
I agree completely. My standard advice for budding hovercraft
pilots is to develop the habit of always going as slow as possible all
but the most predictable circumstances.
> Since the fingers have a small radius of curvature compared to a bag, they are
> more easily pushed aside, and a little jet of air at their inner surfaces will
> have some slight effect in holding them down. This is in no way a periphral
> jet, however. If the driver were in a fixed position, such as being seated
> normally as in Sevtec sevs, blowover would occur with or without a pressurized
> bag at about the same time as it would on an equivalent finger skirted craft.
>
>
> You should note that since there is very little pressure difference across the
> forward finger skirt elements when ram pressure is approached, they are
> practically just dangling off the front of the craft, (and also losing their
> finger to finger seal), so any little bit of help at holding the fingers down
> is welcome.
>
But in this situation, a lack of seal is irrelevant because no air
is going to escape into the blast of oncomimg air. Air is trying to
get inside the cushion from the outside.
> A big help would be to used a pressurized bag, which would behave like a big
> inner tube, or small flexible foam nubbins rather than a skirt might be the way
> to go for operation of the nature of hovercraft racing.
>
Virtually every race course I have been on has had more rough and
uneven ground than anything I would choose to encounter while on a fun
cruise. That old golf course event in Terre Haute was an extreme
anomaly. Most racers find themselves nowadays wanting all of the
cushion height they can get, especially when confronting a 30-40
degree pond bank that must be flown up at speed.
Craig
Craig
"J. Kelley Jernigan" <hover...@nc.rr.com> wrote in message news:<VbKvb.14946$I53.5...@twister.southeast.rr.com>...
Alistair Furnell
VIPER HOVERCRAFT
A Division of NELL FABRICATION
ABN - 93 536 383 210
2/288 Princess Highway
Sth Nowra. NSW. 2541 AUSTRALIA
Ph/fax: 02-44230383
Barry Palmer
From: rdu...@pdq.net (Ralph DuBose)
Date: Sun, Nov 23, 2003 5:44 PM
Message-id: <cb5b2d4e.03112...@posting.google.com>
sev...@aol.com (Barry Palmer) wrote in message
news:<20031123121404...@mb-m14.aol.com>...
> It is possible for racers to operate just above blowover speed, but the
racing
> craft may be going slightly above their blowover speed not because of the
> hardened skirt but because the driver moves forward as the craft starts to
> become airborne and this suppresses the bow lift as airodynamic lift
builds.
Where is it written that the topside must generate lift? If the
topside is configured to produce extremely dirty airflow from the bow
rearward, why should there be any lift? The pintail craft have an
uncowled lift engine right on the bow and no windscreen or anything
shaped to allow for significant high velocity flow.
The reason for putting weight forward is to limit the amount of ram
air getting under the front skirts into the cushion space. Otherwise,
the air-dam effect of more rigid front skirts would be circumvented.
All of this is exactly the same as race car aerodynamics. Notice
the front airdams as close to the ground as possible.
I am afraid the topside must produce lift, as the craft represents a moving
lump on a plane surface, rough or not, and the displacement of air near a plane
alone will produce lift, if the lump is sealed or just near to the surface.
Look at the exhaust of a car. If the exhaust pipe is near the side, it swirls
out from the underside, the same phenomenon as wingtip vortices of an airplane,
indicating the car has net aerodynamic lift.
In the case of the race car, the aft is raised and the front airdam does work,
as the underside pressure is pulled down more than the upper surface, and now
you have a download (at least while the air dam is near the surface. You have
an upload as soon as the front lifts or the car gets sideways.). There happen
to be four wheels that hold the whole thing up that are not usually found on a
hovercraft . If the same principal were to be applied to the hovercraft, it
would just go splat!
It is not irrelevant when the bow slams back down and the skirt is not a little
deployed to reinflate the cushion. Usually the skirt will deploy, but the
cushion has to get a small handhold for this to happen.
> A big help would be to used a pressurized bag, which would behave like a big
> inner tube, or small flexible foam nubbins rather than a skirt might be the
way
> to go for operation of the nature of hovercraft racing.
>
Virtually every race course I have been on has had more rough and
uneven ground than anything I would choose to encounter while on a fun
cruise. That old golf course event in Terre Haute was an extreme
anomaly. Most racers find themselves nowadays wanting all of the
cushion height they can get, especially when confronting a 30-40
degree pond bank that must be flown up at speed.
I have yet to see any really challenging terrain on a race, as the craft are so
noisy I suspect the only place you can race them is on private land maybe in
someone's large back garden, if not some private golf course?. Transitions
might be helped by a pressurized bag up front, especially if backflow from the
bag were restricted by a one way air valve, (which would be terrible for
operating in a seaway but might work for single transitions).
>
Ralph DuBose
In a mis-spent youth, working with small gas powered models, I
tried out a lot of really bad ideas concerning skirt design. One thing
I learned is that anything will work on a flat, solid surface. On
water, there are challenges because the self-righting effects needed
when a skirt touches or approaches the surface must have a broad
contact footprint on the surface to work. That is why a simple "drape"
skirt is so unstable. The edge just cuts into the water surface
whereas a round bag exerts a lot of upward force with surface contact.
Also, on water, the cushion pressures acts as a boat hull in every
respect; below plane-out it displaces water of equal wt. as the craft
and going on plane it flys over its twin bow waves into
non-displacement mode. (The second wave is ahead of the rear skirts.)
Reece
Paul Kenna wrote:
> I'm looking to replace some of my segmented skirts for my Turbo
> integrated Hovercraft.
I'm in the same boat err hovercraft as you and would also like to know.
>
> What is the state of the art material for skirts, and what can I
> source and afford. I'm looking for durability for a cruising craft.
There's a producer in Footscray who sells material specifically made for
the Turbo he even calls it 245 (something like that) prob. is $18 a
metre and only by the roll, that's 25m. At least the roll is near 2m wide.
If you can bargain him down a little I'd go halves with you.
Isn't it amazing how this thread has been high jacked.
I've just beenwading though the replys and there has been about 2 replys
that actually addressed the content of your original message.
Talk about soap boxes and hobby horses!
Anyway let me know what you think.
BFN
Reece.
Fanman_UK
wrote:
<snip>
> In my earlier post, I had forgotten that PJ could
>be done without a skirt.
Nick, AFAIK all peripheral jet craft were skirtless until a FJ craft
was fitted with a simplified version of the 'progressive pressure jet'
of which I know you are aware. The craft in question was driven by
Kristie Scales at Weston Park approx 10 years ago.
Fanman_UK
Ken Roberts
just got your whc2002 race video, btw. haven't watched it yet.
anyway, how hard it it to cut? read: if i run over a submerged bit of sharp
metal that sticks up in the air, am i going to cut my skirt? i'm sure you have
a scrap left over, please picture a bag skirt coming around a corner at 50mph
and a sharp bit of something sticking up. do i then spend another $1000 for
another skirt, or will it survive, or can i patch it at all?
Ken Roberts
that's just physics. if you have a jet of air running across a surface, it
produces lift. it doesn't matter the smoothness of the surface, but rather has
to do with the properties of an air stream. the high speed air tends to pull
low speed air (such as air trapped in your cockpit and moving the same speed as
you) towards the high speed air and take it along.
and it tends to pull solids along too.
take a piece of foam and an air hose. put the foam on the table, blow across
the surface with the air hose. the foam picks up and blows away. now make the
surface rough, like your intended cockpit. it'll do the same thing.
In article <cb5b2d4e.03112...@posting.google.com>, Ralph DuBose
wrote:
Charles K. Dixon
> charles,
>
> just got your whc2002 race video, btw. haven't watched it yet.
Cool! You get it from Universal Hovercraft? They are supposed to start
selling them.
Have fun watching it.
I just got a copy of Paul Taylor's (Paulz Videos) newest 2003 DVD. Paul
does fantastic video work and it's worth every penny- definetly a "must
have" in my book Paul's website is at: http://www.hovercraftvideo.co.uk/ -
ordering information is on his site and he offers a NTSC version for us
North Americans.
>
> anyway, how hard it it to cut? read: if i run over a submerged bit of
sharp
> metal that sticks up in the air, am i going to cut my skirt?
Geez... You can cut ANYTHING if you try hard enough.. so far I have not
torn one- even when slamming sideways into a log and punching it into the
cockpit- the skirt didn't tear, but the seams ripped out of the skirt making
it useless.Like I said- you can cut anything, but I think you would probably
stand just as great a risk of maybe hooking or puncturing your bag skirt and
having the skirt ripped off of the craft- or creating one hell of a splash
when it stops the craft dead in it's tracks!
> i'm sure you have
> a scrap left over, please picture a bag skirt coming around a corner at
50mph
> and a sharp bit of something sticking up. do i then spend another $1000
for
> another skirt, or will it survive, or can i patch it at all?
Patching it shouldn't be a problem.. the rest of your question? I don't
know.. thats one of the reasons I have a craft with finger skirts.... I can
easily repalce the ones I destroy for a couple of dollars and some time
sitting in front of the "Speed" channel while sewing the new ones.
Probably your best bet at this point is to give Mauritzon a call and see
if they can send you a sample to play with- It was $3.?? a yard and 62" wide
which is more than enough to test with. Use a hot knife to cut it....
Ken Roberts
you to get the temporary every time.
Ken Roberts
>
> "Ken Roberts" <k...@kroberts.in.9ci.com> wrote in message
> news:slrnbs4eo...@kroberts.in.9ci.com...
>> charles,
>>
>> just got your whc2002 race video, btw. haven't watched it yet.
>
> Cool! You get it from Universal Hovercraft? They are supposed to start
> selling them.
> Have fun watching it.
that's exactly where i got it. i'm sure it will bring back memories.
> I just got a copy of Paul Taylor's (Paulz Videos) newest 2003 DVD. Paul
> does fantastic video work and it's worth every penny- definetly a "must
> have" in my book Paul's website is at: http://www.hovercraftvideo.co.uk/ -
> ordering information is on his site and he offers a NTSC version for us
> North Americans.
>
>
>> anyway, how hard it it to cut? read: if i run over a submerged bit of
> sharp
>> metal that sticks up in the air, am i going to cut my skirt?
>
> Geez... You can cut ANYTHING if you try hard enough.. so far I have not
> torn one- even when slamming sideways into a log and punching it into the
> cockpit- the skirt didn't tear, but the seams ripped out of the skirt making
> it useless.Like I said- you can cut anything, but I think you would probably
> stand just as great a risk of maybe hooking or puncturing your bag skirt and
> having the skirt ripped off of the craft- or creating one hell of a splash
> when it stops the craft dead in it's tracks!
i get your point, i'm just thinking that there are some materials which are very
hard to tear but not hard at all to cut. if we get some very expensive material
that wears forever and tears not at all, then drive over a sharp nasty bit of
metal and our skirt is totally destroyed, that wouldn't be fun at all, would it?
i'm just trying to figure out if it's worth it or not.
>> i'm sure you have
>> a scrap left over, please picture a bag skirt coming around a corner at
> 50mph
>> and a sharp bit of something sticking up. do i then spend another $1000
> for
>> another skirt, or will it survive, or can i patch it at all?
>
> Patching it shouldn't be a problem.. the rest of your question? I don't
> know.. thats one of the reasons I have a craft with finger skirts.... I can
> easily repalce the ones I destroy for a couple of dollars and some time
> sitting in front of the "Speed" channel while sewing the new ones.
> Probably your best bet at this point is to give Mauritzon a call and see
> if they can send you a sample to play with- It was $3.?? a yard and 62" wide
> which is more than enough to test with. Use a hot knife to cut it....
you know, i'm getting this mixed up with the teflon skirt. i forgot your air
bag material is cheaper than a regular skirt. never mind all that. if i were
making a teflon skirt that would be something to test. this might just be worth
a try.
Ken Roberts
experience with any of this, and for that matter the only peripheral jet
documentation i've seen is from the 60's, where it also says no hovercraft can
manage more than a 10% grade. i had thrown all the peripheral jet info that was
in my head out and dismissed it as antique lore.
it sounds as though you're saying someone over there tried a combination
peripheral jet and something else. i assume a pressure or finger skirt?
it also seems that the amount of power required for a pj skirt would relate
directly to the height of the gap being generated by the jet.
the thing that comes to mind for me is a "loop jet" skirt, where you have a bag
skirt with a second air feed at a much higher pressure, and a tiny peripheral
jet of maybe 1-2 cm height and a low volume. this would allow a stiffer bag
for stability and maybe better wear characteristics, especially on concrete. i
would be inclined to place the jet slightly outboard of the bag contact line,
which would give the craft a wider bubble.
it intrigues me about the blowover issue you're talking about. using a jet on
the front to prevent skirt collapse, it sounds like just the thing.
Ralph DuBose
> ralph,
>
> that's just physics. if you have a jet of air running across a surface, it
> produces lift. it doesn't matter the smoothness of the surface, but rather has
> to do with the properties of an air stream. the high speed air tends to pull
> low speed air (such as air trapped in your cockpit and moving the same speed as
> you) towards the high speed air and take it along.
>
> and it tends to pull solids along too.
>
> take a piece of foam and an air hose. put the foam on the table, blow across
> the surface with the air hose. the foam picks up and blows away. now make the
> surface rough, like your intended cockpit. it'll do the same thing.
>
>
O K, Yeah, I see this point now -- something I had not thought thru.
In a brute force sort of way, the air across the top is always going
faster and so there is going to be some lift unless there is some
designed-in downforce.
On the other hand, the degree of lift produced by a smooth rounded
shape on top and a totally rough shape is the difference between an
airplane wing flying and one that is stalling and falling from the
sky. The same amount of air is going over the wing at the same speed,
right? Whether it is flying or stalling. Clean or iced over. If the
bottom of the wing were embedded in the ground and a hurricane were
blowing over it, some lift would be produced even if it were in a
stall mode. Yes, I overlooked that.
But it does seem to me that the strong forces of lift will be
associated with areas of attached flow over the topside of a
hovercraft or of a wing. And that one big advantage of having no
attached flow over the topside of a hovercraft is that there is
stability and predictability in the amount of lift that is generated
which makes it much easier to handle and compensate for.
In another posting in this thread an irritable comment was made
about the alleged "noise" that some say is generated by high
performance hovercraft. My thoughts inevitably turn to a time in the
late 1960s when the British rock band , the Who, were turing and
having all sorts of the kind of fun that one is not allowed to have
nowadays. Keith Moon, their drummer, loved to play tapes of their
music full bore and at all hours. Very late one night a hotel manager
came around and and told him to, "turn off that noise." Shortly
thereafter, Keith rigged the managers door with a species of very
powerful firecracker and blew the door to splinters. Whereupon he
yelled," That was noise. What you heard before was the WHO."
Barry Palmer
hovercraft than would be attracted by them.
If you want to make a lot of noise, fine, just do not inflict it on others
who may not be appreciative of the racket. After all, the noise may cause
legislation that may damage the sport by limiting use of these craft in public
areas.
I remember one time at a rally I was subjected to an attack TV Interview, (of
the type usually reserved for politicians and mass murderers and such) about
why the hovercraft concept was not really taking off. They were never able to
get the interview off, as when the talking head would open his mouth, someone
in one of those typical two stroke craft would hit the throttle and "whooooeee"
away the talk, and after several "whoooeeoooee"s, from well out in the lake,
the TV crew simply gave up. They may have had the answer right there, and
didn't recognize it.
Noise regulations usually "bend" around vehicle noise, but not of the
exceptional level of most all two stroke hovercraft. And, it is so
unnecessary!
Sevtec uses four cycle engines, and limits the maximum rpm for engine
installations to keep noise levels within reason, (and keep engine life and
reliability up) and wishes that the racers might give this a consideration,
also.
It is now occuring to me that some may not be familiar with the peripheral jet
concept. The idea is to surround the cushion, or plenum, with a high pressure
jet of air, higher pressure than the plenum, and directing the jet of air
inward to contain the cushion. As the high pressure jet has velocity, and
therefore momentum, the lower pressure in the cushion will eventually overcome
the momentum and push the high velocity jet outwards, so the daylight clearance
of the skirt can be enhanced by the air jet going in, turning around and being
pushed out, being used twice, roughly speaking.
The problem with the system at very low hover gaps between skirt and water is
that the jet uses up too much power and it is destroyed by the variability and
contact of the skirt with the water. Also, as the water dynamic pressure is
many orders of magnitude above the jet dynamic pressure, the water has no
problem of getting into the skirt, which causes lots of drag. At high hover
gaps the system simply uses up too much power, and unless there are jets inside
the plenum will not respond well to changes in the surface (waves, beaches,
etc.) over which the craft is travelling.
<< Subject: Re: Finger, bag and blowover, was Skirt Material
From: rdu...@pdq.net (Ralph DuBose)
Date: Mon, Nov 24, 2003 5:25 PM
Message-id: <cb5b2d4e.03112...@posting.google.com>
> > >><BR><BR>
<snip >
Paul Kenna
Do you have the name of the product or its item No in Mauritzon's
catelog?
http://www.mauritzononline.com/catalogindex.htm
Paul
Melbourne
On Mon, 24 Nov 2003 13:34:17 -0600, "Charles K. Dixon"
<speed...@speedjunky.net> wrote:
>Mauritzon
Ralph DuBose
> It might just occur to you that vastly more people are turned off by noisy
> hovercraft than would be attracted by them.
>
> If you want to make a lot of noise, fine, just do not inflict it on others
> who may not be appreciative of the racket. After all, the noise may cause
> legislation that may damage the sport by limiting use of these craft in public
> areas.
>
I was using that folk story of Keith Moons adventure to make the
point that there is a differnce between noise and music and the
difference is not merely a matter of decibels. Often, it is a matter
of taste.
But not always, which is why I am actually in sympathy with noise
reduction efforts. The obvious approach is to slow everything down.
Sevtecs and Webers and all the new UH designs are really quiet with 4
stroke engines and reduced tip speeds. Nothing wrong with that except
one ends up with a large, bulky hovercraft. To make something more
compact and nimble, other approaches are needed to have interesting
performance.
The SCAT brand did everything wrong. There was an aircooled engine
mounted high up driving a very noisy gearbox and a noisy fan. The
decibel number was high although that per se did not describe the
damage done. The gearbox made sounds like a dentists drill and the fan
needed a lot of rpms to do anything. I recently raced a very heavily
modified Scat which had a water cooled engine mounted low, V belt
drive, high pitch fan, a muffler built into the exhaust, and some
other things. It would fly well at 2500-3000 rpm and most of the sound
was from the exhaust. Whether or not that sound was perceived as music
or not I cannot say but it was a smooth non-raspy "inline 6" sound
that one gets from a 3 cyllinder 2 stroke. Another muffler on top of
that and I bet I could fly by the average river garden party without
giving any offense. I have noticed that, when playing around on my
home river, I can easily sneak up on people fishing without having
them look up until the last minute-- something unheard of with any
scat.
Some of the Australians at the last whc had carbon fiber multiwing
blades that were radically reshaped, somewhat "scrimitar" shaped and
much quieter than the usual type.
A few years ago, I went to a CART race and had a trackside seat.
All of the junior classes were unbearably loud. But the CART cars were
not loud at all and the sound they did make was a pleasant, though
very urgent, sort of "whoose". They had about 8-900 hp but they were
turbocarged with some kind of after muffler. Anyway, it worked. For
some of us, it was music for sure.
Ken Roberts
ralph, did you know that a stalled wing has more lift than does the same wing
just before stall? it also has way more drag, which slows the wing down more,
which is what actually reduces the lift you had. that much i remember from
taking flying lessons in high school.
and iced over wings have a huge amount of weight on them, and the wing shape is
changed by the ice.
unsteady flow only means unsteady flow, and probably higher drag than laminar
flow. at this point of the conversation we're stepping outside my knowledge and
into speculation, though.
the idea of predictability of a surface to which air will not "attach" as you
put it, that might hold some merit. but it would also be always higher drag, if
my own understanding is correct. on the other hand, the entire science which
has developed around unsteady flow in aircrafts is based on controlling and
predicting the forces on a surface which has unsteady flow--much higher forces
than what develops with laminar flow.
Ken Roberts
you're right that the noise is unnecessary. in fact, at whc2002 the noisiest
crafts were a few selected crafts which had less power than the front runners.
most of the crafts, including the most powerful/fastest crafts, could be run up
right next to your ear and caused you to shield your eyes, not your ears.
as has been mentioned before, your opinion of hovercraft racing seems to be
fairly antiquated.
yes, there are some very noisy hovercrafts out there. yes, a 200hp racer will
probably be noisier than a 25hp low-speed cruiser. but the simple air noise
will be higher. when the racers were just cruising by at what you would call a
reckless speed but what was achievable by a 25hp craft, some of them were, by my
ear, quieter than the 25hp craft going the same speed.
Charles K. Dixon
> Charles
>
> Do you have the name of the product or its item No in Mauritzon's
> catelog?
>
> http://www.mauritzononline.com/catalogindex.htm
>
> Paul
> Melbourne
Wow.. they have a catalog?
Show's you what I know! (or don't as the case may be!)
The only thing they wrote on the sales invoice is: Black Air Bag Material-
no stock reference number or anything. Sorry...
Barry Palmer
the increase in hull size so that realistic payloads can be carried, and in the
case of a Sevtec vehicle, room to do things, like fishing and hunting and
backcountry camping. Racing machines give no regard to this, with the driver
frequently sitting in a cramped cockpit in the midst and very close proximity
of all kinds of obnoxious machinery, just like other forms of racing.
One could easily get speed up merely by under-sizing the hull, on a Sevtec, as
is done on most all racing craft, eliminating such "unessentials" such as
enough positive flotation, enough hull to go out in big waters, far from
rescue. You will not get the acceleration at low horsepower, however.
Would the typical racer really go on 100 mile journeys, across big waters, out
in the unpopulated areas far from rescue, I think not, but capable machines for
big waters can certainly be raced, if someone places reasonable restrictions on
their design to maintain some degree of practicality, so the machines can be
used for fun, not just bragging rights.
>><BR><BR>
Ken Roberts
i have what might be called a "typical racer." i've gone on 100 mile journeys
with it. not on big water, but since i can't figure anything more boring than
going across some giant lake you'll probably not see me drive ANYTHING across
that sort of water--boat, hovercraft, whatever.
enough positive flotation, you'll have to take that up with the individual
hovercraft manufacturer. mine has enough to float the craft and myself and a
120 lbs passenger plus gas and tools, with zero power and having the water level
inside the cockpit equal the water level outside. i've proven it. my craft, in
the cockpit, is quieter than some 25-horse thrust/10hp lift engine four strokes
i've driven, at least at the same speeds they can go.
for that matter, if enough interested parties get together and want to have wig
races, i'll be up for that too. or for that matter, a uh-18 without the wings
can get up and haul faster than any of the racers that showed up at worlds. are
you saying you can't fish off of that?
put your money where your mouth is. prove some of this stuff by comparing
MODERN hovercrafts, using best of breed whether they be racers, cruisers or
cargo crafts. you're using generalizations that you made 20 years ago, not
evaluating first hand modern vehicles.
Paul Kenna
cost at $3.50USD per yd 100yd roll. (approx $5 AUD but shrinking on
a daily basis)
Given Nell charges $28 AUD per m for neoprene coated nylon I'm now
looking for some aussie builders interested in experimenting in half a
roll.
Paul
Melbourne