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markb - n/a
7-Jul-08, 09:41 PM
CAN ANYBODY GIVE ME SOME SKIRT DESIGN TIPS. NOW I HAVE MADE MY SEGMENTS FROM THE CONSTRUCTORS GUIDE. NOW I HAVE HEARD THAT THE CONSTRUCTORS GUIDE DESIGN IS QUITE POOR. MY SKIRT ONCE INFLATED SEEMS TO BE QUITE BAGGY AND ROUNDED AT THE TOP. WHEN I LOOK AT OTHER HOVERS SOME ARE QUITE FLAT AND MORE POINTY A BIT LIKE THE FLYING FISH DESIGN. IS THIS ANGLIA DESIGN BETTER AND MORE EFFITIANT.

HELP ME http://hovercraft.org.uk/images/icons/smiley_icons/icon_confused.gif

ANY IDEAS ARE GREATLY EXCEPTED.

THANKS MARK

jon_curtis - n/a
8-Jul-08, 06:12 AM
fold the lower attachment tags over pulling the skirt closer to the hull, it will reduce your foot print slightly all around, but the skirt will be less baggy and boucy! the back of the segment wants to be tight against the planning surface.



this worked wonders on my challenger! and its easy to do!

markb - n/a
8-Jul-08, 09:46 PM
thanks for the input sounds good.

but is it my imagination or am alone in thinking skirt design is shrouded in a bit of secretcy.I can find lots of information on fan design,air flow,formulars,engine choices,hull design ect,but never skirt design.

when people say the constructors guide to skirt design is poor they never say why it is poor.

any ideas anybody.

thanks mark

jon_curtis - n/a
9-Jul-08, 06:38 PM
its poor cos the lower skirt attachment tags are miles to long, and the skirt is baggy and bouncy, as it does not instruct to make the skirt so its tight against the planning surface.



not sure if its been updated recently but my old copy is like this, also it show pressure segements, which with the long lower tags, dont really do much. the geometry is not miles off tho.



i looked at segments of various different craft, before i finally modified the skirt, pulling it tight against the hull, is the major requirement to make it better!



index.php?t=getfile&id=1540&private=0



Baggy!



index.php?t=getfile&id=1541&private=0



not so baggy! http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif

markb - n/a
9-Jul-08, 07:44 PM
ahh i see now that sounds logical thanks.

in my constructors guide perchased in 2002/3 shows 100mm long tags from hull to skirt. would making the skirt sides bigger and reducing the tags to as short as possible do the same thing and that would keep the foot print the same ?

index.php?t=getfile&id=1542&private=0

index.php?t=getfile&id=1543&private=0

jon_curtis - n/a
10-Jul-08, 11:46 AM
hi



nice looking bit of ally http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif



i would just try shortening the tags first, its quick and simple and should only take a couple of days. it should pull the skirt in 30 to 50mm around around, which will give a higher cushion pressure, which will help anyway! i found the geometery of the skirt was better slightly inbound. if you dont get the desired effect, then unpick the folded over tags and try as you suggest. you might still find the bag lifting up at the top, as it needs to be pulled down towards the lower attachment point.



do you have pressure segments down the sides? if so i would remove the flaps in the side bags, which will get more air thru and under the skirt. the bouncy bag is more prone to plough in, but you have a front lift engine that will help, also the craft looks like it flies nice a level. Have you had it out on water yet?

Jonathan - n/a
10-Jul-08, 03:08 PM
do you have pressure segments down the sides? if so i would remove the flaps in the side bags, which will get more air thru and under the skirt. the bouncy bag is more prone to plough in, but you have a front lift engine that will help, also the craft looks like it flies nice a level. Have you had it out on water yet?




So Ideally you want pressure segments at the front of a craft and open along the sides?

Or would open segments at the front be better than pressure?



Am I right in thinking that open front segments would allow the air to go under the craft better, but be more likely to collapse at speed over water?



Jonathan

aka 'Mr Questions'

jon_curtis - n/a
10-Jul-08, 05:49 PM
So Ideally you want pressure segments at the front of a craft and open along the sides? YEP!

Or would open segments at the front be better than pressure? NOPE!



Am I right in thinking that open front segments would allow the air to go under the craft better, but be more likely to collapse at speed over water? MOST CERTAINLY



a pressure segment, jet segment or some form of high pressure segment at the front is best to limit plough! open down the sides gets more air under the craft as the pressure segment can limit airflow slightly! certinly the pressure flap has to be right for the craft for it to work properly!



i found a sealed bag, bit like a chip bag with a small opening at the bottom, so it fills and is firm on the front was better than a pressure segment, im sure others will disagree but a normal pressure segment is not that great at recovering plough, once it happens it happens, in my opinion its best to have a harder segment which limits it happening in the first place.



will see if i can dig out some pics of what i mean!

Ian Brooks - n/a
10-Jul-08, 06:13 PM
Skirt design is a black art and few if any people understand the subject well! Fortunately, Jon (Dangerman) has been through your situation so is well placed to advise.



The only thing I would add is on the subject of plough-resistance. Plough-in is caused by the collapse and tuck-under of the front segments, and can be a dangerous experience (Jon will tell you!). There has been some level of acceptance of plough-in as an inevitable thing that the driver has to learn to live with. Various solutions have been tried, including pressure segments that do help as they resist the tuck-under.



A better solution seems to be a compartmented cushion. This means that a divider skirt is provided across the craft about 1/3 of the way back from the bow. Should the bow tend to collapse downwards, then the divider contacts the water and allows the pressure in the front compartment to rise, thus preventing further skirt collapse.



Bryan White has pioneered a practical implementation of this technique using a simple stiff rubber sheet, that works incredibly well on his craft which has a similar skirt type to yours. It makes his plough-proof to in excess of 55mph (scary speed) in any conditions! I'm sure he will let you in on the secret if you ask!







Ian

bryan - n/a
10-Jul-08, 07:39 PM
I have only tryed this on my Mirage GP with great success.My craft is a twin machine so this might make a diffence. I have an intergrated craft powered by a Rotax 503 and i am about to try a similar easyer and hard waring solution. This should help many of the racing accidents caused by plough in!!. At presant racing craft use inflatable tubes in various positions and this works very well.However this is not very practical on a cruising craft. I have found a very hard waring material and use it in the same way a Sevtec front compartment works. So far it has worked a treat. With a 14hp lift engine i have been able to maintain a high speed over long distances up or down wind.

markb - n/a
10-Jul-08, 10:55 PM
reply to dangerman's question have i taken it on water yet?

no not yet i've still got that joy to come.

don't rush me it's only taken 6 years to get this far. http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif

and i have got pressure segments down the side.

in the constructors guide the top of the skirt has a "radias to suit" has anybody tried the idea of replacing the radias with a straight line ,short cutting the curve.And i assume looking more like the flying fish design ?

p.s thanks to don for the photos.

index.php?t=getfile&id=1546&private=0

index.php?t=getfile&id=1547&private=0

jon_curtis - n/a
11-Jul-08, 01:28 PM
http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif



the ally looks good, and fully welded, you must have a steady hand http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif



how is don?



oh, that little cable lever mounted on the rhs in the first pic, where did you get that? i could do with some of those.



i have been trying to dig out some drawings i had for those square skirts, but cant find the disk when i do will send you a copy. pm me with your email address.

kevinf - n/a
11-Jul-08, 08:46 PM
If you want a simple design guide to the square shaped segments, send me your email address by PM and I'll ping a copy back to you. They are a lot more economical on material and quicker to make than the rounded ones.



Kevin Foster

tonybroad - n/a
15-Jul-08, 09:45 PM
rather than being a dark art it really is a case of each craft needing a different skirt based on craft + driver weight, size of craft and output from the fan



some known guidelines which work are to keep the contact point at least 4" inside the outer edge and an angle of 45 degrees to the ground are good starting points



basically nobody has definitive answers for skirt design and we are all still learning



Tony