Finger Segments feed by pressure under craft - no ductwork
kach22i
(semi-ductless, "flow through" & rigid included) which use finger skirt
segment tips.
Examples of such craft would be the last of Dr. Dr. Berlesen's
amphibious craft which used rigid inflated "wings" or tube. The CACTS
2002 print-out and presentation called the finger segments "closed" as
to not scoop up water at any angle (would love to see a detail of
this).
http://www.aeromobile.com/aeromobile_vers2/Amphbious/ACV_3/body_acv_3.html
There is also similar skit construction found on the Slider:
"The skirt is a permanently pressured, twin-cell loop and segment
design. Each segment is individually air-fed to provide reduced
friction and a smooth ride over irregular surfaces." (I'd like to see a
section diagram of this - if possible).
http://www.hovercraft.co.nz/slider/overview.htm
There are other examples which come to mind: Graham Spenser's F25 Heron
(side skirts feed by plenum - mostly). Stretching my requests a bit
would be Barry Palmer's single large finger segment at the bow of his
Sevtec craft. My question there might be; How did you ever figure out
it would fill (fully) from air pressure supplied at the rear of the
craft?
The main reason I'm looking into this aspect of hovercraft technology
is based on the old argument or statement that each finger segment must
be feed air individually and preferably from the top.
The examples mentioned above may comply with this general rule, however
it may be in varying degrees. In short I'm trying to determine a common
denominator in segment finger skirt supply air delivery.
kach22i
like the fingers on these are fed individualy.
Ken Roberts
them closely.
Slider poses a personal dilemma for me. It's a commercially built craft that
seems to be built for looks rather than performance, but it actually gets up and
scoots for a factory-made craft. I would love to be suspicious of it because of
the funky skirt but in fact, its performance forces me to acknowledge that the
guys seem to have done a good job. This is not just with speed, but evidently
plow-in is a non-issue with this craft too.
It's still slow relative to a homebuilt with the same power, but you have to
give a certain concession for factory made crafts and the liability issues their
manufacturers face.
Barry will no doubt answer with respect to his own crafts, and I will not step
on that. It seems to me though that simple geometry and logic make it simple to
see how it works. Coming up with the first idea was probably a lot harder.
I like Barry's idea in the basic premise of multiple chambers for the lift
system, but I can't say I like the implementation. Barry came up with a stable
system that maintains its own trim. As far as that goes, it is wonderful. It
does what he wants it to do, it is low maintenance and low cost. It serves the
folks who build his crafts very well. My problems with it, though, fall outside
Barry's original design spec, and he would probably say he designed those things
out deliberately:
1 You can't back slide, as in racing turns, without risking a torn skirt.
2 You can't deliberately and suddenly throw it out of trim, again as in
racing when you want the hull to touch in one spot or you want to spin the
craft.
3 I suspect it would suffer from blowover sooner because the front chambers
are at a lower pressure than the rest.
Note here that I'm not saying these things are result of bad design. They are
the result of designing for an environment that I don't find myself interested
in so much. My own tastes move toward active control and high speeds. Neither
environment is "better" than the other, it's just a different design goal. For
some applications one approach is better than the other, but for another
application the opposite is true.
My "item 2" above will probably cause some folks to mention the braking ports on
the front chambers. This is sort of what I'm after only not nearly aggressive
enough.
In short, I want something you can race, which means it is not inherently
stable. Barry is making an inherently stable platform. Racing involves
deliberately throwing the vehicle out of stability and then getting it back when
that instability has done what you wanted it to do.
Rick
shown at http://4wings.com.phtemp.com/crafts/a1.html. See
http://4wings.com.phtemp.com/tip/bfskirt.html for more details.
A hybrid (of sorts) where there is a permanently inflated section, plus
dynamically inflated bag and fingers, can be seen at
http://www.airlifthovercraft.com/HoverProducts%20frameset.htm
Rick
> Yes, they are on the Airlift machine, but they are not on the machine
> shown at http://4wings.com.phtemp.com/crafts/a1.html. See
> http://4wings.com.phtemp.com/tip/bfskirt.html for more details.
>
> A hybrid (of sorts) where there is a permanently inflated section, plus
> dynamically inflated bag and fingers, can be seen at
> http://www.airlifthovercraft.com/HoverProducts%20frameset.htm
and follow the "RIVAC 680Ri-ILP" link.
kach22i
Ken, before you pointed out earlier this week at the HCA forum, I did
not realize that "dig-in" if that's the right term, was an accepted way
to steer the craft while racing.
In my 35hp Scat-2 if I ever dug in, I would come to almost a complete
stop and it would take forever to get back up to speed - not the best
way to show off.
I don't know why Sevtec's don't race, perhaps it's for the resons you
describe.
Hey if a StarCrusier could win this year at the national rally, maybe
an adventerous Sevtec owner will win next year (but they have to
enter).
kach22i
the classic bag/finger combonation skirt here:
http://4wings.com.phtemp.com/tip/bfskirt.html
However I still don't have very detailed information on the rigid
inflated bag when used with fingers. It's no so much the attachment
point detail I'm interested in, it's the way the air is fed (not feed -
sorry) to each and every finger. If air is not fed to every finger,
them I'm even more curious as to how the perimeter feed pressure is
held somewhat constant.
I'm left pondering what would happen if the craft were to bank - how
the low side would ever be able to recover without direct air feed.
The F25 Heron solved this by giving some supply air to the sides and
back, plus of course direct feed to the critical bow
segments........but I'm looking at how others have addressed these same
issues.
Rick
actual skirt construction nearer the bottom of the page shows fingers
attached to a bag, with no holes in the bag to feed the fingers.
There is no need for a plenum or individual holes to feed a finger skirt.
The cushion pressure can keep them in proper shape.
Barry Palmer
"skirt" with a bag skirt attached below on the Sevtec website, in the
old images section, circa 1970's, so it is not a new idea.
There is no throughflow, and pressure was maintained at around 1 to 1
1/2 psi.
However, it leaked a little, so I provided with a tube so that it could
be re-pressurized occasionally during operation power bag was inflated
via the plenum.
Use of an air inflated structure is a realistic approach to a light
hull structure, but flowing air through such a structure, even when
pressurized to maybe only 125% of cushion pressure causes almost
prohibitively great losses in power, which can only be overcome by
making the craft extra large for its payload and expected speed.
Barry Palmer, for Sevtec
Ken Roberts
Not trying to bash your craft at all, but that's a stock Scat. It has
unfortunate plow characteristics combined with low power. Lots of folks race
Scats, but I think most of them are modified at least in power.
Take a look at Air Commanders, Revtecs and such. While they don't handle
plow-in as well as a UH craft does, their drivers do force a corner in
occasionally, because I've seen some of them do it. Whether they actually
intended hull contact or not I can't say, but it's just a matter of degree.
For the record, when a hull makes contact with whatever surface it's going over,
that's called plow-in. It's actually a bit more specific than that, because
dumping power and coasting is not plow-in, and a few other conditions that are
similar might also not be considered plow-in. Having the skirt make contact is
not, even if it is a virtually inevitable indication of a plow-in as it is on
some crafts.
Even on your Scat though, if you were to be in a turn and then touch the front
inside corner to the surface, you would not come to a complete stop. Just that
corner would, most of your mass would rotate around the contact point and you
would be facing another direction. If you have rudder authority for it, you can
then stop the rotation where you need to and your thrust is all going in a new
direction.
Every racer makes heavy use of weight distribution at every point on the course.
Sometimes they just drag a part of the skirt, sometimes they drag it so hard the
hull touches. The way it's used in turning, it almost makes no difference how
bad the craft handles plow-in. Where they seem to have problems is in head-on
plow, the way you describe coming to a full stop. IMHO, they should be able to
plow going forward, sideways or even backward without having any sort of
catastrophic incident.
A racing craft must have appropriate guarding, including a thrust duct. I
don't know if the duct is required by name, but the requirements for guarding
are stringent enough to almost preclude anything else. They include stopping a
driver who is flying through the air towards the prop, or stopping bolts from
coming out in the plane of the prop.
On 2005-10-12, kach22i <gkacha...@yahoo.com> wrote:
kach22i
Barry, you say the old design was 1 to 1-1/2 PSI............that must
of been much less than Pc (pressure under craft), right?
In your current designs, would you say the bag skirt pressure (Pb) is a
little less or a little more than what is caputured under the craft
(Pc)?
Side bar; see second to last photo:
http://members.aol.com/buftec/buf/ab.html
"inverted view of a paper model of a typical sev skirt."
Great to include this - I've been playing with some paper folding in my
study models lately........thought I was the only one.
Ken Roberts
about 0.2 psi.
On 2005-10-12, kach22i <gkacha...@yahoo.com> wrote:
Barry Palmer
foot)you find for cushion pressures. That is the sort of pressure used
on inflatable boats. ( I got tired of puffing through the tube to keep
that pressure up and the pressure sometimes did not reach 1 psi. )
I should say that an inflatable hull, including the floor is an
excellent way to build a sev, although it is difficult for the
homebuilder. Maybe I should look at this aspect again, as there has
been a lot of development in inflatable structure materials since my
efforts. Boats sort of need the rigid fiberglass floor if any speed is
to be obtained, while the rigidity beyond a couple of stringers as in
the example in the old Sevtec activity images is not needed for the
sev.
The internal pressure of Sevtec skirts is held very near plenum
pressure, undetectable by normal gauges, (and mathematically not needed
for shape and stability) and the forward compartment is about 1/2-3/4
inches water gauge (3-4 psf) below the main aft cushion. This forward
pressure automatically changes as the craft changes in pitch attitude,
and can be either partially of fully dumped as a primary control, along
with rudders and throttle, for control enhancement.
Barry Palmer, for Sevtec
The skirts were actually developed not in drawings, but with
calculations. Anyone can take a set of the Sevtec plans, run off two
extra copies (as there are no left and right side drawings, and
assemble a paper model of the skirt, along with a cardboard model of
the hull, with one to one scale to the plans.
I have even used a small electronics 110VAC muffin fan to fly a skirt
model, (so it did not make a very good bathtub toy.)
kach22i
Ken Roberts wrote:
> Look at your units, George. The Pc you're describing is 15-20 lbs/ft^2, or
> about 0.2 psi.
A: > > Barry, you say the old design was 1 to 1-1/2 PSI............that
must
> > of been much less than Pc (pressure under craft), right?
> >
(Pb) is a
> > little less or a little more than what is caputured under the craft
> > (Pc)?
I'll answer my own question if given enough time - I hope.
A: 1 or 1.5 psi is equal to let's say the average of the two (1.25)
muliplied by 144 (12x12) = 180 PSF. I get the strong feeling that is a
lot more pressure than what's under the craft.
B: Guess what I'm asking here is pretty straight up. However I'm asking
to see if there are any craft out there with lower pressure in the bag,
higher pressure under the craft. Conventional wisdom says NO. The bag
would just be pushed out of the way and all the air would escape before
any lift could take place. The clearence or air gap would be too large
to get up on cushion. However I've been doing some reading and it's
opened up a real can of worms for me. Conventional wisdom or not.
kach22i
"The internal pressure of Sevtec skirts is held very near plenum
pressure, undetectable by normal gauges, (and mathematically not needed
for shape and stability)"
Question: Barry, do you know of any hovercraft (world wide) that
operate with internal pressure (bag skirt) less than under craft
cushion pressure?
That is to say; lift cushion pressure is 1.2 times that of bag skirt
pressure.
Would that fly?
Ken Roberts
>
> A: 1 or 1.5 psi is equal to let's say the average of the two (1.25)
> muliplied by 144 (12x12) = 180 PSF. I get the strong feeling that is a
> lot more pressure than what's under the craft.
>
> B: Guess what I'm asking here is pretty straight up. However I'm asking
> to see if there are any craft out there with lower pressure in the bag,
> higher pressure under the craft. Conventional wisdom says NO. The bag
> would just be pushed out of the way and all the air would escape before
> any lift could take place. The clearence or air gap would be too large
> to get up on cushion. However I've been doing some reading and it's
> opened up a real can of worms for me. Conventional wisdom or not.
What sort of reading opened this can of worms? If it's this book you've talked
about on the HCA forum I think you misread some lines and are now suffering from
it. Having not seen the book firsthand though, I can't say for sure.
I guess that if you truly want to know if it's possible to have a hovercraft
whose bag pressure is lower than cushion pressure, then maybe you should build
one. Work at the model level and just see what works. Put a manometer in the
plenum and one in the bag, and start playing with the splitter. It can all be
made out of a shop vac, a piece of foam and a trash bag. If you feel ambitious,
put a servo on it to control the splitter so you don't have to keep tinkering
with it.
Realistically speaking, we have a lot of experience reading this forum but by no
means is it "all of hovercrafting." A huge amount of research and experience
either does not read this forum or does not post. Most of the expertise I
personally know does not read any internet forum. Asking questions here will
only get you answers from the folks who post here, so you can only prove one
half of your question by asking it here. If someone knows who has done it and
bothers to answer, then you can refine your research from there. However if
nobody has done it that does not mean it can't be done, no matter how many
people you ask, unless somebody is willing and able to prove that it can't be
done.
You're asking about something I take as a given, so maybe I'm not the guy to
ask. If your question is spawned because the books you're reading suggest that
such a thing is possible, then I would say the book is more right than I am.
If, on the other hand, you're just starting to question assumptions simply to
reevaluate everything you think you know, I guess you can go ahead and do that.
It's a free country.
rdu...@pdq.net
Things to remember about skirt elements. If they are enclosed, they
try to become as round as possible. If not fully enclosed, they are
shaped in a way similar to sails- according to how they are attached.
If there is abundant cushion height, air distributes itself easily all
thru-out the cushion space evenly and efficently no matter where the
lift air is inserted. The lower the cushion height and the less even
the surface, the more necessary it becomes to have multiple, peripheral
feed vents. It is all a matter of degree. In general, the faster you
want to go, the more you have to worry about where the lift air is
deployed.
The only way to have a peripheral bag with less pressure than the
interior would be to make it non-flexible. A sealed, high pressure
bag/bumper is actually designed like this. It works like a spring
rather than a air-containment system.
The sealed high pressure outer bag will always be cyllindrical in
cross section.
Barry Palmer
skirt material stiffness, but with the supply of cushion air being
available at a specific pressure, I fail to see any advantages. The
local curvature of the bag is not necessarily a circular cylinder, as
there are different regions that are exposed to different outside
pressures while the interior is at a given single pressure. The
visible outer portion of a bag skirt is very nearly circular as it is
exposed to cushion pressure on the inside, and ambient pressure on the
outside. Curvature of the running (lower) surface is variable,
dependent of the localized venturi action and resultant local static
pressure differential while the inner portion in Sevtec designs is
actually relatively uncurved (except for a diagonal sheer wrinkle that
supports the bow and partition skirts) as there is very little pressure
differential across the skirt in this area.
One can plainly see zero pressure differential working when examining a
craft with a "C" skirt, where thare is nothing but support shrouds
holding the skirt lower edge to the hull. Such a skirt, though
impractical for real world performance, does allow a surface skimmer
hull to lift.
Some designers may think internal pressure above cushion pressure is
necessary to hold the skirt down, but this is simply not necessary due
to the venturi effect of the cushion air blast passing under the edge
of the skirt. The real advantage of the low bag to cushion pressure
ratio is that when the skirt is driven into the surface, the venturi
effect holding the skirt down where contact is being made lets up,
which is conducive to minimizing skirt wear, while the portions of the
skirt that are not against the surface continue to provide craft
stability.
Pressurizing the skirt bag interior above cushion pressure guarantees
the skirt will drag and scrape and wear on the surface to a greater
degree than the zero pressure differential skirt. The advantage of
over pressure is greater stability, and one can have a smaller skirt
radius of curvature for the same stability (at additional cost of wear
potential.)
Barry Palmer, for Sevtec
kach22i
> Some designers may think internal pressure above cushion pressure is
> necessary to hold the skirt down, but this is simply not necessary due
> to the venturi effect of the cushion air blast passing under the edge
> of the skirt.
I'm going to assume the venturi effect will be upset (deflected down
and inward 45 degrees) by the addition of finger skirts, but an
exterior anti-spray flap will not have that same affect. Please correct
me if I'm wrong.
And once again, thanks to all for your input.
Ken, I've been trying to read sections of L. Yun & Alan Bliault's book;
Theory and Design of AIR CUSHION CRAFT. However as with any first read
of such a textbook many things are read over, overlooked and deserve a
second or third reading. One of the frustrating things is the simple
formula Pt/Pc and it's inverse Pc/Pt are constantly interchanged and
swapped chapter by chapter.
The Chinese craft discussed in this book use low pressure bag/finger
combinations. There is a great deal of discussion about the TENSION in
the bag, skirt shape, and finger tension. This "tension" has much
importance, some of which (it's value) is taking more than a week to
sink in to my head.
There is also language, which may have been translated and kind of
twists and turns on you. I'll copy this little part, and let you chew
on it - each time I read it, I read it differently:
"The responsive skirt with low natural frequency is widely applied to
modern ACV's/SES's....................This gives the advantages and
features of the responsive skirt. It may also lead to a reduction of
the tuck-under resistance of skirts. For this reason, it is necessary
to study the hydrodynamic characteristics of skirts, the rationale of
skirt tuck-under and study the measures for improving the tuck-under
resistance. The hydrodynamic characteristics of skirts and the skirt
tuck-under resistance are introduced as follows."
This phrase: tuck-under resistance
Does it mean-A: resistance TO tuck-under, (to keep it from happening).
Does it mean-B: resistance FOR tuck-under, (to make it happen more).
Add to this the font size which was written for people 20 years younger
than meyself, and I have to read everything many times over. Oh, and no
instructor to ask questions to.
PS; That figure 7.34 (bounce boundry diagram) we talked about at HCA is
Pc/Pgs...............I still have to find out what "gs" is (no guessing
this time).
rdu...@pdq.net
Thisd really sounds like it is related to plow-in; which is
sometimes referred to as a skirt "tuck under" phenomenon.
Google up plow in on this ng for a review of nearly endless
discussion.
Ken Roberts
I'm gonna top-post in order to keep the history but prevent people with slow
connections from having to read through the whole message. Also, I'm not trying
to be argumentative here but finally we get a discussion that is intensely
interesting to me again. All posters welcome!
Venturi effect:
I think that adding fingers changes quite a bit. First, let me remind everyone
that I don't have the math for this, I'm just speculating. But it seems to me
that once you add fingers to the bottom of a bag, you now have what amounts to a
finger-skirt craft with a flexible duct system to feed it. The duct may in fact
increase obstacle clearance, but what you have is a need to anchor the fingers,
and the bag must do this duty. Which means the bag must be pressurized
somewhat in order to give it stiffness.
Also, I don't think venturi effect or Bernoulli effect (same thing applied
differently, as far as I can figure) have as much to do with any downward force
on a finger as they do on a bag. The way I see it, the downward force is the
same force that makes airplanes fly, which means the downward force on a bag is
dictated by the shape of the surfaces and the speed of the air between them.
Tuck-under:
Tuck under would be a prelude to plow-in, if it goes that far. "...the
rationale of skirt tuck-under and study of the measures for improving the
tuck-under resistance." They want to improve tuck-under resistance, which means
it would be resistance to tucking under.
It's somewhat awkward usage compared to normal "street" English, but it's proper
as far as I can tell. My guess is that it was not translated, and that at least
one of the authors has a whole lot of education in the English language
specifically. That one little paragraph screams "scholarly work" like a siren
going off. I'm not sure how I can tell with just that little fragment, so don't
ask. :)
You mention a lot of talk about the tension in the bag, which would make a huge
amount of sense both in terms of air flow (pressure) and in terms of skin
tension, because the bag is a structural member for the support of the fingers.
I suspect that after you let it sink in enough, they'll be talking about both of
those things in detail, each as a separate thing.
kach22i
book). It shows both a high pressure bag skirt (w/fingers) and low
pressure bag skirt (w/fingers) reaction to a wave impact. Once you see
this, it will leave an impression.
Problem for me is that I have too many impressions going on at once,
and need to sort them out.
I'm going to use that credit card trick to plug the air supply fed
holes (in the Scat) and balance my side bag pressure to 1:1 with the
pressure under the craft.
I'm not going to deal with any fingers or the folded pockets which I
did in paper.
I need to keep seperate the theory/learning curve we do in the forums,
and my actual winter skirt project. I have to stick to the plan, and
need to define my plan at the same time.
Ken, feel free to start a new topic; very specific to what interest
you.
I'll be there to add my two cents, cheers.
Ken Roberts
I don't know if I'm just cranky or what, but I think you'll be a lot happier
with your experiments if you were to start from scratch, maybe with an RC model
at 1/4 scale or so. That way, you could put "stock" skirts on your Scat and
actually drive it, then go home and tinker with your magic skirts in something
that is actually designed for the purposes of experimentation.
Somehow, I think you're going to taint your experiment by using a hull designed
for a different application. I'm thinking of just a piece of foam with an
engine and a fan on it, nothing fancy. One of the cardinal rules of
experimentation is to only mess with one thing at a time. You're automatically
messing with several things, just by trying a nonstandard skirt on a Scat.
You're not even switching to a bag skirt, you're trying to cook up your own
thing.
If you wanted, you could make several platforms by getting a few sheets of 2"
pink foam. You could have just one engine, say a weedeater or something with a
fixed duct ala Bertelsen gimbal fan only without the gimbal, just plug it into a
hole and you're on your way. You wouldn't need much in the way of additional
work, you just add whatever skirt you want and figure a way to feed it. The
test craft will be small enough to not have to buy $100 of material each time
you change something, but big enough you can work on it without having to use RC
model widgets.
It is, of course, up to you on how you make your experiments, but that's my two
cents worth.
On 2005-10-13, kach22i <gkacha...@yahoo.com> wrote: