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  1. #1
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    Default Solid Skirt

    Hey all,



    I'm new here but have some questions when it comes to designing hovercrafts..



    Has anybody seen or looked into using a stiff skirt rather then a rubber flexable skirt?



    a stiff skirt that has a decent about of flex in it, like stiffer light weight rubber or something.



    Will hold you off the water(would be made to be extra boyant)/land making it quicker and easier to presurise the plenum to take off.



    Has anyone heard of anyone doing this sort of thing?

  2. #2
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    Default Re: Solid Skirt

    That packaging Dell send out with new computers (sorry I've been around it too much ) might be good. It's sort of closed cell plastic foam (possibly PE...? it has a waxy feel).



    In a sheet it's stiff and can support itself easily, but folds if enough pressure is put on it then springs back to its old shape.

    Dunno if that helps though



    Toby

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    Default Re: Solid Skirt

    Quote Originally Posted by loud'n'proud
    light weight rubber


    Is there such a thing?!



    I think a stiff skirt is ok if you are operating over a consistently flat and smooth surface but for anything with a slight lump or bump will cause a sudden increase in drag and cause very peculiar handling characteristics. For 'normal' operating environments the skirt needs to be compliant so that any obstacles and/or uneven terrain can be absorbed by the skirt and not be transmitted through a stiff skirt into the hull.



    Think of an F1 car with stiff suspension driving through a field!



    As for quicker lifting off times, most craft are up and moving in a few seconds so I'm not sure there is any need for trying to improve this?


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    Default Re: Solid Skirt

    There was a craft at Whittlesey last year with a solid skirt in FS, I will try and find a picture of it later. It was a French or Belgian craft from memory.



    Went like a rocket but looked uncomfortable as hell!!

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    Default Re: Solid Skirt

    kinda hard to describe.. and cant say too much since its all hush hush lol (i work for an R&D company in Australia)



    but the idea of a light weight yet rather stiff skirt has been bounced around abit and we are looking into the idea.



    on prototyping with our stiff skirt against the rubber skirt, the stiff skirt floated alot better,



    with rubber if air escaped out one corner, the air would lift that corner up more and then all the air would escape from 1 point rather then evenly around. on the stiff skirt we found that 4% of the skirt would touch the ground (slightly over balenced to 1 point) and the entire rest would be completely off the ground as the skirt holds its shape.



    our full size hovercraft will be 10m x 4m so its going to have the weight to plow through alot of things and the skirt will be long enough and have enough flex to go over rather large bumps. the maths works out that the hull will be 80cm above the deck




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    Default Re: Solid Skirt

    Hi



    Skirt design has evolved since the '60s to become more compliant (softer) for two main reasons:



    1 - wave drag. A firm skirt interracts with wave systems to cause very high levels of skirt drag. This is particularly important when taking off from a standing start in water ('going over hump'), where the hump wave interaction ('scooping') holds the craft back and prevents it from taking off.



    2 - choppy water. When operating in choppy water, wave drag increases therefor reducing the maximum speed. A compliant skirt is used to reduce this effect by allowing the waves to roll through the cushion with minimum drag, and a stiff skirt could make it worse.



    In an ideal world, the skirt would move up & down freely, to enable it to accomodate lumps, bumps and waves, but would not move laterally at all. This gives rise to large bulbous nose seen on the SRN series of craft (prevent plough-in) and the large side bags seen on SevTec craft.



    Which is not to say that your new concept cannot solve these issues and open up a whole new avenue of craft development - good luck! I would recommend Theory and Design of Air Cushion Craft by Yun and Bliault as one of the few up to date texts on the subject



    Regards

    Ian

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    Default Re: Solid Skirt

    As David and Ian have said it will depend very much on the surface you're trying to operate over. On a totally flat surface such as ice, calm water, tarmac etc it would be fine but on undulating ground the skirt would alternately ground, causing drag, or be high in the air causing air loss which ultimately would need more lift power to overcome. A particular example would be a transition from water up a steep slipway.

    Over water if the skirt is hard enough it could be wave piercing which could have advantages but not if the skirt is presented sideways on to the wave where it could cause high drag. Thus travelling sideways could cause the skirt to be snatched by a wave and with the crafts momentum higher up it could cause a rollover. Similarly the wide straight bow on most craft would present high drag to waves and a far more pointed shape is needed for a wave piercer - eg that seen on the UH18

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    Default Re: Solid Skirt

    the craft is not for crossing the English channel as well.. we dont have one...



    it will be used in open seas but only coast running never on huge swell.




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