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Thread: A question ...

  1. #1
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    Wink A question ...

    Sir Christopher Cockrel's invention of the perhpheral air jet hovercraft was made obsolete in 1960 by the development of C H Latimer-Needham's flexible skirt.

    Discuss.

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    Default Re: A question ...

    Not sure what the question is ... but I think the answer might be efficiency

    For the peripheral jets to be effective more than a few inches from the nozzle, the exit air velocity needs to be very high. Generating a high velocity air stream needs power (0.5 x air volume x velocity^2 - if I remember correctly?). Probably why the SRN1 needed a 450HP engine


    Which begs another question - why does the peripheral jet theory live on only in finger skirt craft? The theory I've seen is that the finger feed holes and segment shape generate an inward pointing "jet" at the segment tips. I find it hard to believe this as the "nozzle" hole is tiny compared to the exit area of each finger - the air leaving the segment tip must be at almost the same pressure as the main cushion.

    The plenum feed system on a typical small segment skirt craft can't be very efficient at all - the losses caused by pushing air around corners and through a tube ( the hull plenum) and then out small holes must be pretty large. Surely it would be far more efficient to just dump the lift air straight into the cushion plenum under the craft (as on commercial open loop/segment craft)? Some air may need to be used to initially "inflate" the fingers to get a decent seal but that could be done using very small holes and a small pipe/duct with the majority of the lift air directly feeding the cushion. I realise that some of this has probably already been tried in the dim and distant past but I can't find any examples - or the reason why it didn't work

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    Default Re: A question ...

    Precisely my thoughts John - I've been lying in bed thinking about it while I've had flu and decided to pose an exam style question for the gurus of the club to answer.

    After the VA1 and SRN 1 & 2 as originally built, I can't see the actual 'hovercraft' as developed today is the same thing as the design by Cockrel - and poor Latimer Needham sold his idea to Westland and got 'nowt in comparison.

    I can't see much evidence of a peripheral jet on modern craft - directed air possibly, ..... but a jet ????

    A sort of coffee break question :-)

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    Default Re: A question ...

    John.

    Many years ago my first craft, designed by Jeremy Kemp, did exactly as you suggest and dumped it's air straight out the bottom. It worked fine. However it was a two engine craft so the lift motor was right at the front. I believe with integrated craft it is necessary to direct the air to the front to avoid plough in problems at speed.

    Roger Drew

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    Default Re: A question ...

    Quote Originally Posted by John Robertson View Post
    Not sure what the question is ... but I think the answer might be efficiency

    For the peripheral jets to be effective more than a few inches from the nozzle, the exit air velocity needs to be very high. Generating a high velocity air stream needs power (0.5 x air volume x velocity^2 - if I remember correctly?). Probably why the SRN1 needed a 450HP engine


    Which begs another question - why does the peripheral jet theory live on only in finger skirt craft?

    not quite so, i believe vortex have small holes in the bag skirt, just inside the contact strip to give a peripheral jet effect.

    surely tho the air gap between skirt and surface, is kinda a peripheral jet on all craft, in that a uniformed stream of air exits from under the craft all around the skirt periphery?

    with regards a jet segment skirt, does this not have a curved panel inside which is designed to increase air pressure to an outlet point at the bottom, which should in theory give a higher pressure outlet compared to inlet?

    i put high pressure skirts on the front of my ally craft, where the outlet size was smaller than the inlet size and the skirt was sealed up the planning surface to the inlet, this increase the speed before plough in

    on a craft with skirts of segment or bag, or both does the peripheral jet/air blead/air gap blow out, not reduce surface friction, to therefore increase speed or decrease the amount of thrust needed for a given speed?
    Jon

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    Default Re: A question ...

    Think back to the original tin can experiment gents - I think the inward air jet ( although the original was vertical ? ) made a curtain that retained more air inside the plenum under the hull than would have been the case by blowing air in from the middle. That was the invention - retaining air with an inward pointing jet.

    I think we have moved a long way from this - indeed with the jet reaction effect, could you have a peripheral jet from a flexible skirt without the skirt collapsing?

    This came about as I have to give a talk to a local school - and then demo the craft I began to work out how to relate Cockrel to a BBV3 and decided it was difficult.

    Ross
    Last edited by Ross Floyd; 19-Feb-10 at 05:29 PM. Reason: typo

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    Default Re: A question ...

    Ross you are right, there is little or no air jet principle on a segmented skirt craft unless those skirts are tubular and effect would be slight compared with the containment of the fabric skirt, really only a curtain principle is common between SRN1 air curtain and a segmented or bag fabric curtain with lower snagging properties than a straight perimeter fabric curtain, which must have tried out at some point and logically discarded.

    There is a magnificent cut away drawing of the SRN1 which shows how the air was distributed under the skin to the curtain jets in individual channels, which is an important similarity between Cockerels SRN1 and a segmented skirt layout and the contact point air lubrication factor of a bag skirt. Air control with as few losses as possible is the game after Cockerel.

    best of luck with your school talk.

    Alan
    Last edited by yamah; 21-Feb-10 at 02:09 AM. Reason: tli

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    Default Re: A question ...

    On a segmented skirt craft, the inward pointing geometry of the skirt creates the peripheral jet effect. i.e. the airflow is curved inward onto the plenum to compress the trapped air.
    There are two reasons for the ducted hull design, particularly on integrated craft. 1.to give an angled planing surface and 2. to duct the air to the front of the craft rather than dump it all into the rear of the skirt.
    I have found that using pressure segments, which in theory give more of a jet effect and less airflow into the cushion, make the craft more "grabby" so it seems that having more air in the cushion is preferable to having a high pressure peripheral jet.

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