Sev Bow Skirt Flap
kach22i
http://www.bandits-stuff.com/pb/wp_9959f162/images/img20802451b34cfdffd4.jpg
I was fortunate enough at the national hovercraft rally to get a ride
an a Sevtec and have a question. How does the front air damper/brake
really work?
A: It provides reverse thrust braking.
B: It allows air to escape which pitches the nose down generating
skirt DRAG at the front skirt drape which slows the craft down.
The operator was able to pivot the craft at the nose and park it much
like a car, so I think it's "B", but I don't want to assume too much.
n
--
Ken Roberts
> It's "B".
>
> On Wed, 15 Aug 2007 04:47:45 -0700, kach22i wrote:
> > Image link:
>
> > I was fortunate enough at the national hovercraft rally to get a ride
> > an a Sevtec and have a question. How does the front air damper/brake
> > really work?
>
> > A: It provides reverse thrust braking.
>
> > B: It allows air to escape which pitches the nose down generating
> > skirt DRAG at the front skirt drape which slows the craft down.
>
> > The operator was able to pivot the craft at the nose and park it much
> > like a car, so I think it's "B", but I don't want to assume too much.
>
> --
George,
The Sevtec skirt has two or three compartments. The main chamber in
back is fed by the lift fan, and then there are two single-layer C
sections which could be viewed as a finger skirt which is the width of
the craft, only there is one inside the other. On the 3-compartment
designs there is also a boundary which divides the front chamber in
half, left to right.
The front sections are fed by air escaping from the main chamber.
They run at a lower pressure than the main chamber, and these chambers
provide additional stability. They provide a passive pitching
control, in that when the nose drops the escape out the front-most
partition cuts off (the visible front drape touches the water/land)
and the hidden one is still being fed by the main partition, which
increases the pressure. Alternately, both partitions get cut off and
the momentum of the craft increases pressure.
Likewise, if you crest a wave or a hump, air leaks out of the front
partitions faster and the nose drops.
The flaps you saw are valves, if you will, into the front partitions.
On the three partition designs, you have one in the left partition and
one in the right, and you can open them to drop the pressure on that
partition and thereby drag that corner of the skirt.
This system is designed on the premise that you will never want to
induce a plow-in.
Barry Palmer
>
> - Show quoted text -
You can execute and do need an occasional plow-in with the Sevtec
brake. I almost hit a deer. Full brake with 3 aboard, got within 4
feet of that sucker. Yes, this was mid river.
I would recommend not trying to run over a swimming deer no matter
how tempting. A time before when I encountered one it planted all
four on the bottom and shot straight up in front of me, coming clear
out of the water.
Barry Palmer, for Sevtec
Ken Roberts
My apologies. I didn't mean to misrepresent the capabilities of the
craft, it was my impression that your setup was aimed at preventing a
plow-in at all cost.
C. Bailey
In my opinion, there are three levels of braking. If you pull on the brake
gently, it takes the craft out of trim, and the front corners of the bag
skirt drive. This significantly enhances the authority of the rudders, but
you don't slow down very quickly. If you pull on the brake harder, you can
bring the hull down low enough that the plow plane is contacting the water.
You stop fairly quickly in this mode. The shallow plow plane allows you to
operate in this manner fairly easily without the entire hull "gluing" to the
water. If you release the break at any point, the skirt instantly inflates,
and you are operating as per usual. The third level of braking is
continuing to hold the ports open, and inducing a plow-in such that the
entire hull "sucks" into the water. I suspect this is the plow-in that most
crafts refer to? You stop very quickly. Quick enough that you want to be
holding onto something. On my craft, which isn't built to Sevtec spec's,
once this type of plow-in is induced, the hull will not "unglue" itself from
the water until I've lost significant speed - regardless of whether or not I
have the brake on. I don't know if this typical of all Sevtec's, but it is
of mine.
Chris
"Ken Roberts" <for...@hoverclub.net> wrote in message
news:1187292010....@d55g2000hsg.googlegroups.com...
Ken Roberts
Chris,
A plow-in is hard hull contact with the water. I hear a lot of folks
refer to the initial dragging of the front of the skirt as plow-in,
because on some models dragging the front of the skirt seems to be an
irreversible precursor to an inevitable plow-in, which is often as bad
as your "level three" plow-in. People assume because of that that a
plow-in cannot serve any good purpose, and go out of their way to
design a system which cannot plow in.
I think yours is the most concise description of Sevtec plow-in
characteristics I've heard yet. For some reason, hearing Barry
describe it made me think that hull contact was possible but almost
never used. Maybe he was trying to keep from scaring potential
customers? Not sure, and not trying to put words into anyone's mouth,
just saying what my impression of those words was.
I don't buy into the "plow-in is evil" school of thought. Half of
going fast is stopping fast. A hovercraft (or car or motorcycle, for
that matter) which can go at high speeds but can't turn or stop to the
same degree it can go is inherently unsafe. My main interest in
hovercraft at this point is focused on skirt design and bottom hull
design with respect to high speed maneuvering. From the hull's
perspective, that means safely plowing in at high speeds, from various
angles. From the controls perspective, that means inducing or
recovering from any degree of plow-in at any step in its evolution.
If you were to cut out the flat part of your hull bottom, pull it up 3
inches or so, and then glass it back on so that there were a 3" step
on the front and sides, but the back went straight back without the
step, you would find that you could recover from your "level three"
plow-in. Universal Hovercraft designs use this step, and it's
effective enough that I use it as a turning/control device. On my
UH-12r I can pull out of a hard plow-in. The step makes it not quite
so severe, and it causes the water to detach from the bottom leaving
an air pocket. You still get stopping similar to hard braking in a
car, right about where the tires start to complain, and enough where
if you're not braced you'll be hurting shortly, and anything which is
not tied down in the cockpit will slide forward rapidly.
David Bosworth
--
From the rocky shores of the Skykomish river
David Bosworth
http://www.premier1.net/~daveb/hovercrafts.html
"Barry Palmer" <sev...@aol.com> wrote in message
news:1187280972.1...@i38g2000prf.googlegroups.com...
> On Aug 16, 7:23?am, Ken Roberts <for...@hoverclub.net> wrote:
> > On Aug 15, 8:50 am, n <1...@1.net> wrote:
> >
Snip> I would recommend not trying to run over a swimming deer no matter
> how tempting. Snip>
> Barry Palmer, for Sevtec
>
Hahahahahahaha, I needed that!
kach22i
I will be installing some forward facing puff ports on the bow of my
old Scat II, even though I've been told many times that will do
nothing, I suspect that it will allow lift air at the bow to be
dirverted before it goes under the craft and there for slowly allow
the bow to dip and create friction which in turn will slow the craft
down while preserving some thrust and rudder control at the stern.
I just wanted to make sure I understood correctly the working example
of a Sev, and that it does not rely on reverse thrust to work.
David Bosworth
twice before attempting to use a plow brake on a scat, Oh, yes you will stop
alright! heh
--
From the rocky shores of the Skykomish river
David Bosworth
http://www.premier1.net/~daveb/hovercrafts.html
"kach22i" <gkacha...@yahoo.com> wrote in message
news:1187710049.5...@l22g2000prc.googlegroups.com...
Barry Palmer
Sevtec craft fly at a much higher relative hull to surface clearance,
and when the craft goes nose down, leaving the plow plane in a
planing attitude, the bottom panel, which is on an extreme angle,
providing relief from water attachment (similar to the relief provided
by a seaplane hull or float). The bottom panel should be as low as
practical as that is where one would place his feet for a comfortable
seating position without compromising center of gravity height.
Raising the bottom panel effectively raises the craft occupants. The
recessed bottom is usable for those who race and are on their knees so
they can use a lot of body English to maneuver a craft.
The plow plane to hull joint on a Sevtec should be relatively sharp,
and the current plans call for a fiberglass skin doubler over this
joint should not so good construction allow the plow plane to split
from the bottom panel.
Barry Palmer, for Sevtec
> not tied down in the cockpit will slide forward rapidly.- Hide quoted text -
Barry Palmer
immediate ejection of the operator, who would be able to get a face
plant on the handlebars and perhaps an orthodontist bill,unless he is
ready and willing to do a half gainer off the seat.
The Sevtec system requires the use of a partition skirt so the forward
cushion can run at a lower pressure than the main aft cushion, and
opening the flap only knocks the craft out of fore and aft trim,
without dumping air from the main cushion, while maintaining some
degree of pitch control.
The bathtub like forward of a Scat hull is about the worst shape for
easing a plow in I have ever seen on a surface skimmer.
Barry Palmer, for Sevtec
kach22i
> Use of the Sevtec brake flap on a Scat at speed will result in
>
> The bathtub like forward of a Scat hull is about the worst shape for
> easing a plow in I have ever seen on a surface skimmer.
I agree, which is why I will have forward facing puff ports feeding
off the lift ducts.
Bathtub shape it is, and it is a severe shortcoming. I have re-
engineered how the lift air is directed to the bow and a few other
things (major re-work) which should avoid plow-in or make it less
traumatic.
I should have something to post and report by late fall, a few other
projects managed to jump in line and take cuts.
Cheers, George/kach22i
Ken Roberts
George,
There are some issues with this puff port project with regards to your
specific changes.
First, are you adding a partition skirt? Adding puff ports will serve
to lower lift pressure, and if you're just using one partition then
you will essentially create a drain on your lift air.
Second, your modifications caused you to add weight in the back in
order to keep the nose up. The mechanism Barry uses depends on that
forward partition, which is at a lower pressure than the main
compartment. In other words, you'll have to trim back even further.
Barry's mechanism has nothing to do with puff ports. It is a drain of
pressure from the front partition, which unbalances the craft and
causes the nose to drag. The overall effect is the same as leaning
forward.
I agree with the other posters here. I think you're going to use this
port no more than once, after which you will seal it off.
For anyone who wants to see what George's project looks like, his
modifications are extremely well documented on the forum at
www.hoverclubofamerica.org.
kach22i
photos at the HCA site.
Try this for pictures:
http://www.carnuts.us/viewtopic.php?t=446
What if you happen to have a single lift chamber and you were able to
"dump" a portion of lift air at will with a lever?
Would not the skirt all the way around the whole craft cause drag at
the same time only more quickly than just letting off the gas in an
integrated design?
Of course there is no guarnetee that such an event would be as
controlable as a localized event designed to effect a single bow
chamber via pitch of craft.
I have a change in the craft which I have not posted. I discovered
that I was losing lift air because of a "divergent" lift air tunnel
condition I accidentally created. I've corrected this and will be
using this additional air to "upset" the pitch, lift air and bow drag.
Too many variables to explain here, I'm just going to give it a try.
Got nothing to lose.
kach22i
PRESSURE RECOVERY
Cheers, George/kach22i
Ken Roberts
Dumping a portion of your lift... depends on what the craft's flaws
are, and what the hull is like.
The UH-12r was designed specifically to dump ALL lift, instantly, at
the flip of a lever. That component is no longer in the plans, but it
had a mechanism to suck all the air out of the bag very rapidly. The
craft then landed on its hull while the lift engine was still
screaming flat out.
The thing is, that hull was designed for it, designed to handle high
speed plow-in and turning based on what amounts to a keel built in.
The step was that keel, and while it did work and did dramatically win
a race, the technique was immediately outlawed in HCA racing
regulations.
So getting back to the point. If your hull is designed to control a
lift dump, then dumping the lift all the way is relatively safe and
controlled. If, on the other hand, your hull is known for ejecting
its contents during a hard plow-in, then no matter what skirt
improvements were made I would be extremely cautious about doing
anything to dump any or all lift air.
You have dramatically changed the skirt, but you haven't done much in
the way of hydrodynamics of your hard hull. As far as I can see, you
have the same old Scat that you started with when it comes time for
the whole hull to hit the water. Considering your lift mods, and the
fact that you needed to add weight to the back in order to keep the
nose up, I think the likely outcome of opening your puff port is that
the nose will dive for the bottom.
The reason Barry's design works as well as it does is because the vast
majority of the lift is unchanged. A significant portion of the
entire cushion is maintained at normal pressure. That's the control.
Then the lightly pressurized front area is vented to the outside,
which causes a _gentle_ dragging of the skirt.
kach22i
(approx.) plow plane.
The 10 or 12 degrees of a UH and Sevtec was not possible.
I'll post pictures next week, maybe by then it will be fiber-glassed
back up.
The right blade on a reciprocating saw and anything is possible.
Ken Roberts
What method do you use to detach the flow from the hull at the bottom
of the plane? Most of the force stopping you is the hydraulic action
of water against the rest of the hull, sucking you down.
kach22i
> What method do you use to detach the flow from the hull at the bottom
> of the plane? Most of the force stopping you is the hydraulic action
> of water against the rest of the hull, sucking you down.
Progress pictures:
http://www.carnuts.us/viewtopic.php?t=446
I'm not sure if you mean "water flow" or "air flow" at the bottom of
the hull Ken. There will one or the other depending on the situation.
I might secure a garden hose to the hull and plow plane to help absorb
impact and create a landing skid length wise - another silly idea?
I'm fearful that my current skids stopped a little to shy of the bow,
a first flight will reveal much about the "at rest" balance and while
emergency stopping, and any additional undiscovered requirements.
I am considering adding a fabric flap along one of the new plow plane
lines. Either arched at the top plane or simple and flat along the
lower plane. I will have air lift feed supplying each side of this
proposed divider curtain. However I don't want any more drag at the
bow when trying to get up on cushion, so right now it's more thought
than intent.
I think I'll build it as a simple all open lift chamber and then
experiment with curtains or flaps and air filled dividers (sausages)
as time goes on.
Ken Roberts
I mean water flow. When you plow in, the hull hits the water. Your
modification was to the plow plane, to keep the hull from digging in,
to cause more force to lift the nose and less of it to decelerate the
craft. That was only half the problem.
You need some device which causes the water (during plow-in) to
unstick from the hull. John Windt had a homebrew craft that plowed in
like that and the hydraulic literally sucked the bottom off the hull.
He had to hold onto the cockpit walls in order to get back to shore,
his feet were in the water. The force sucking you down could amount
to tons of force.
I think your landing skids are getting a bit out of control. My
UH-18sp has less than 1 square foot of landing pads. You don't need
any sort of bumper, you just need decent skids which have a bevel on
all sides. The skids you have seem to be growing, and pretty soon
you'll need another layer of skids because your whole hull will be a
giant skid, and you'll need something extra to protect it. :)
Why don't you invest a tiny bit of cash on some small electric motors
and batteries, and make small foam models of what you intend? You
could spend a bit more and get a two or three channel remote control,
and test your ideas for pennies on the dollar. A lot of your ideas
could be modeled with a styrofoam meat tray and some plastic from a
garbage bag.
Better yet, work in 1/4 or 1/5 scale, use real electric motors from a
drill or something and the radio for control. You can make skirts
from a visquin or trash bags, cut and shape it with a soldering iron.
I've made some 1/4 scale models for $10, not including the reusable
electronics. If it fails, you either whittle out a new change or
start over. Meat tray models are far cheaper than that, but it's
harder to get any scale performance data from them.
I'm really NOT trying to be critical of you George. I know that
sometimes it seems like I really let you have it, I don't mean to.
You have the soul of an experimenter, and you have a lot of ideas and
they all step on each other on their way from vision to reality. I'm
convinced that more than half your experiments that fail do so because
of some prior failed experiment which never was adequately restored to
functionality.
When you play with huge skirt design changes, you need to model it on
a small scale. Not even the US government experiments in full scale.
It doesn't pay, either in time or money. Models are easily made, used
and either expanded on or trashed, depending on the results of the
experiment.
With the sort of thing you're doing, I'd be tempted to make a highly
detailed mold which accurately depicts a Scat of the same model you
have. Then make a few dozen foam models out of white spray foam.
Your inner finger feed ductwork could be implemented by gluing a
plastic strip down as plow planes. Get one of them functional as a
generic Scat, another should be modified to represent your full-sized
craft, and a third to represent what you want to accomplish at any
time.
I'd get three radios. Start with something like an Air Hogs RC toy,
you can get the helicopter for $30. Or go grab an RC car or boat
radio from Radio Shack. One is for your baseline Scat, one for the
model of your full-sized mods, and the third for your current
experiment. You could fairly easily swap out the radio gear, or you
could just use one transmitter and have separate receivers and servos
for each. Best of course is to have a separate radio for each and use
RC modeller components, because they're easier to work with.
kach22i
1. I mean water flow. When you plow in, the hull hits the water.
Your
> modification was to the plow plane, to keep the hull from digging in,
> to cause more force to lift the nose and less of it to decelerate the
> craft. That was only half the problem.
2. You need some device which causes the water (during plow-in) to
> unstick from the hull. .............
3. you just need decent skids which have a bevel on
> all sides. ..........
4. .A lot of your ideas could be modeled ............. Models are
easily made, used
> and either expanded on or trashed, depending on the results of the
> experiment.
A1: The rubber hose may break up the suction of the water on the bow
plane, another reason I'm considering it.
A2: See A1
A3: I'll look at the side bevel again, where I have it would be
similar to extending the side hull slope. I think this could work in
my favor somehow.
A4: The Scat II is a 10 foot long model. It's helped change my mindset
about the scale of forces I'm dealing with. It's my small scale
mindset which I want to change the most. Models will only reinforce
flaws in my thinking that I already have, spent my whole life building
models - it's a different world. I'm rewritting my rule book to full
scale, and that takes time/experience.
David Bosworth
anything but a means to satisfy curiosity and lord knows I have undertaken
similar activities in the past. What is it you like so much about this
particular craft/design that you are willing to put so much time and effort
into a redesign and build?
--
From the rocky shores of the Skykomish river
David Bosworth
http://www.premier1.net/~daveb/hovercrafts.html
"kach22i" <gkacha...@yahoo.com> wrote in message
news:1189009460....@o80g2000hse.googlegroups.com...
kach22i
What is it you like so much about this
> particular craft/design that you are willing to put so much time and effort
> into a redesign and build?
David; I have a lot of ideas, most of them are a little "out there"
and not worthy at this point of a dedicated "from scratch" effort or
hovercraft.
Model form just will not cut it anymore, full scale is what I need to
go to the next level of understanding in this self taught hover-
education.
The old Scat II as a "platform" happens to be an excellent tool to
experiment with. Different aspects or ideas are just a saw or patch
away. Yes, I'm just fooling around and having a good time. However off
in a secret and abandoned warehouse I'm building a 50 foot craft -
just kidding.
Seriously though, I am creeping up to making things work the way I
want. The things that do work get put into an on-paper design/drawing
I have hanging on the wall (that 50 footer). The 50 foot design came
first (started as 40 feet), but the Scat is too far a jump to have an
obvious connection. You would be hard pressed to see any connection at
all, however in my mind I can make that jump based on what I've
learned. Call it a dream, we all gotta have them.
Cheers, George/kach22i
David Bosworth
of the attraction in this sport for me.
--
From the rocky shores of the Skykomish river
David Bosworth
http://www.premier1.net/~daveb/hovercrafts.html
"kach22i" <gkacha...@yahoo.com> wrote in message
news:1189797633.2...@d55g2000hsg.googlegroups.com...
kach22i
David Bosworth wrote:
> Well as long as you are having some fun with it! Tinkering is certainly part
> of the attraction in this sport for me.
I've been to your website, I like the "Lil" hovercraft, definitely the
need to have fun expressing it's self.