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Thread: plowing in.

  1. #9
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    Default Re: plowing in.

    Don lighten up...! The fish boys were good enough to lend me a craft as a problem cropt up with my own. I spent around 3 hrs on and off mud banks on a tour of the Sheppy area. Yea the nose is a bit twitchy down wind but it was never a problem. I think i used about an egg cup of fuel all day....... made a nice change!!! An all round easy to use craft with no excess of spanners.

  2. #10
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    Default Re: plowing in.

    Bryan why tell me to lighten up I had only asked a simple question and recieved a good answer from Gavin with him saying that (reading between the lines) the SEV is the way to go for safety as he points out that a partitioned front skirt is the best way to avoid ploughing in.

    Quite why Russ Pullen can't accept what Gavin is saying without throwing his toys out of the pram is beyond me maybe he should lighten up !!!. I would just like to thank Gazza for pointing out the problem in the first place and even yourself Bryam say Quote "Yea the nose is a bit twitchy down wind" so to make that comment that must be a warning in its own right.

  3. #11
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    Default Re: plowing in.

    Quote (would love to race but I am sure as with most motor sport money wins)

    Do not be put of racing it’s not only the money that wins although it helps. It does not matter how much money you throw into it if 1 you can’t drive, 2 you don’t learn form your mistakes, there a quite a few drivers out there who do not throw loads of money at the sport myself included and remember the old saying IT’S NOT THE WINNING IT’S THE TAKING PART THAT COUNTS (although standing on the podium is a good feeling finishing a race in one piece and not having damaged any one else’s machine doing it is good enough for most people)


  4. #12
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    Default Re: plowing in.

    Quote Originally Posted by Don83000
    a good answer from Gavin with him saying that (reading between the lines) the SEV is the way to go for safety as he points out that a partitioned front skirt is the best way to avoid ploughing in.



    I meant a partitioned cushion area, not front skirt. i.e. such as many racers are now using with a divider to maintain the cushion in the front.

    Sevs can still plough in as can any craft in certain conditions.



    I'm waiting to test Hovpods claim that they don't plough in.

  5. #13
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    Default Re: plowing in.

    Quote Originally Posted by GavinParson
    <table border="0" align="center" width="90%" cellpadding="3" cellspacing="1"><tr><td class="SmallText">Don83000 wrote on Sun, 15 February 2009 18&#58;16</td></tr><tr><td class="quote">
    a good answer from Gavin with him saying that (reading between the lines) the SEV is the way to go for safety as he points out that a partitioned front skirt is the best way to avoid ploughing in.



    I meant a partitioned cushion area, not front skirt. i.e. such as many racers are now using with a divider to maintain the cushion in the front.

    Sevs can still plough in as can any craft in certain conditions.



    I'm waiting to test Hovpods claim that they don't plough in.
    </td></tr></table>



    I don't think Gav was intending anything "between the lines" Don.



    Plough in can happen with ANY hovercraft - the difference between craft is in the end result of the plough in. On some craft (as Gazza has found out) the deceleration can be very severe, on others it's a mild braking or slowing down. Large commercial craft "fixed" the plough in problem 30 years ago (ever wondered why large craft have a big bulbous front skirt?).



    Sevs have a partitioned cushion and not a partitioned skirt. The cushion is divided by a partition skirt roughly 75% rear/25% front with the forward cushion compartment operating at a slightly lower pressure than the main cushion. Although normally very stable, they can still plough-in if you get the wrong combination of wave depth and frequency. However, due to the raised boat-like underhull the event is pretty insignificant - the craft just slows down a bit, recovers and then carries on.



    Gazza, it may be worth contacting Ralph DuBose in the US - he developed an anti-plow modification for a Scat he had that apparently made the craft virtually plough-in-prof (Scat are/were notorious for unpredictable plough-in).



    I would be very surprised if HovPods don't plough-in, they appear to have attempted an underhull step as an anti-plough solution (as used in hydroplanes - and UH hovercraft for the last 30 years ) to prevent water becoming attached to the hull (water attachment is the cause of the sudden stop during a plough). Unfortunately, they rounded off the sharp step edge which has probably removed any effect it may have had.

  6. #14
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    Default Re: plowing in.

    Hi there,



    Keith adding his bit, somewhere on the north sea going to Denmark.....no its not rough.....yet..





    Despite what you read on various websites...all craft will plough in at some point..if yours doesn't you aren't going fast enough!!!



    Skirt pressure and skirt design determines when plough in will happen it could be at 30mph or 80mph......but it will happen!!!



    and no bag skirts don't stop it they mearly change when and how it happens



    Keith

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    Default Re: plowing in.

    I agree with all the above - all craft will do it sometime. But you might delay it by using a variable splitter plate. If you're trying to go along at 'half power' in an integrated craft the air feed to the front will be reduced and eventually a plough in will occur. Fitting a variable splitter, common on integrated racing craft, allows you to increase the ratio of air used for lift. Not easy on a marlin though because of the way the fan frame aligns with the splitter.

  8. #16
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    Default Re: plowing in.

    A bit of background...



    What is plough-in?



    Plough in is a phenomenon that occurs when the nose of a hovercraft suddenly drops. Often associated with sudden deceleration, uncontrolled swerving and in extreme cases the occupants can be thrown from the craft which might even roll over. It caused the death of several people in 1972 when an SRN6 overturned, so has to be taken seriously.



    What causes plough-in?



    Most people agree that plough-in follows the following sequence:



    1 - A hovercraft flying in straight and level flight before the event, with the bow skirt clear of the water



    2 - The bow drops slightly and the tip of the bow skirt touches the water causing hydrodynamic drag on the skirt - the water "tugs" at the skirt. Experienced pilots can feel this occurring at this stage and take avoiding action, but novices don't usually notice anything wrong.



    3 - The bow skirt begins to tuck under due to the tugging of the water, causing the nose to drop. As the skirt tucks under, it pulls back under the craft causing the center of pressure to move backwards.



    4 - The nose drops further, increasing the hydrodynamic tugging, and the skirt tucks under completely. Once this stage is reached, a full-on plough is inevitable.



    5 - The bow drops sharply until some hard structure hits the water. What happens next depends on the shape of the bit of hull that touches down. It might "dig in" causing the craft to stop like it hit a wall, or to "plane" over the water and decelerate more reasonably.



    6 - The final stage is sudden severe uncontrolled deceleration if the structure digs in, or sudden but survivable deceleration.



    When does this happen?



    Plough-in happens when the hydrodynamic forces can overcome the skirt tension & make them tuck-under - that is, at high speeds.



    It's worse when flying downwind for reasons that don't seem absolutely clear, with integrated craft the lift air might be reduced which makes it worse, and the aft-wind probably causes a more nose-down trim which helps to initiate the event, plus the craft may well be going faster than usual.



    It only happens on water.



    How can you stop it?



    If you have a craft that tends to plough, you learn to spot the "tugging" stage and react by increasing lift and trimming the nose up by moving weight backwards or altering the elevator.





    How do you prevent it by design?



    Plough-in can be designed out at step 3 or 4, and the end effect can be controlled at 5.



    At step 3:



    - Design the skirt so that it cannot tuck-under (Sevtec curtain).

    - Design so that the bow dropping forces the skirt to move forwards thus preventing further drop (Wouters "bulbous" bow skirt).

    - Increase the skirt tension (pressure) so that it overcomes the water drag (pressure segs)



    At step 4:



    Partition the cushion and feed the front partitition from a pressure higher than it "normal" pressure. As the bow drops, the skirt seals and allows the partition pressure to rise. This opposes the downward forces and lifts the bow back out of the plough-in. This can be done by various means:

    - Sevtec style "divider curtain"

    - Inflatable tubes

    - Rubber flexible dividers (Bryan's design)

    The typical segment skirt craft with air delivered via a plenum seems to be suited to either of these methods, and Bryan showed that the divider can be quite a loose fit under the craft.



    At step 5:



    Make sure the part of the craft that touches the water first does so at a shallow angle - about 10-15 degrees seems about right.



    Sideways plough-in

    All the above applies to forward travel. When operating sideways, the same can thing can happen. With small craft, it is effectively prevented by the pilot altering sideways trim. With a large craft, it has to be prevented by keeping speed down for large angles of yaw.



    Cheers

    Ian


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