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Thread: skirt feed holes

  1. #1
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    Default skirt feed holes

    Dear people, is their a process that can determine the sive of feed holes to use on a craft to asure corect air flow from plemum, apart from trial and error.

  2. #2
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    Default Re: skirt feed holes

    Hi DG

    In my craft which is basically a challenger, i have small holes at the back and larger ones at the front. There are two reasons for this:

    <font size="2">1.[/COLOR]The fan will be giving more dynamic (moving) pressure at the rear of the craft than the front <font color="orangered">if it is an intergrated craft[/COLOR] so to get the same airflow around the craft the hole size needs to change. I alwase like to think back to the physics experiment with holes at different heights on a barrel filled with water. The hole at the bottom has a higher pressure so has a higher flow rate than at the top where there is less pressure so to get the same flow rate the hole at the top must be bigger.(im probably not exactly correct but it seems to be logical?)



    <font size="2">2.[/COLOR]If the rear of your hovercraft uses bag skirts it doesnt contribue to the lift of the craft. Bag skirts dont need much air flow, just enough to keep them inflated and at pressure so a smaller hole would suffice.

    On my craft i have used 32mm holes for the rear increasing to 40 on the corners 64mm on the sides and then 72 odd mm on the front.


  3. #3
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    Default Re: skirt feed holes

    I hope this helps and it might also dispel a few paddock myths



    plenum pressure is constant - there is no more pressure at the front than there is the back or sides - if you think of a balloon, you can't blow it up with more pressure at the top or bottom or sides and there's no difference in pressure where you are blowing the balloon up from.



    However, balloons aside, the size of the holes in the plenum can control the 'volume' of air that passes to each skirt segment so you can have a greater quantity of air going to different segments if you wish



    I have 2.5" smaller holes at the rear because the rears don't contribute much to lift and so you only need enough to keep them inflated - the rest on mine are 3" all round, that way i have even flow and air lubrication - i don't use pressure segments but that's another story



    as for the science experiment mentioned, it shows the relationship between height of fluid and pressure - good for making a simple manometer to measure your skirt/plenum pressures but not directly related to the holes in the plenum



    Sometimes it is trial and error though Mick



    Good luck








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    Default Re: skirt feed holes

    Tony, have you tried measuring your plenum pressure? Maybe your craft works differently from my old one but we did a fair bit of testing and found huge differences on the intergrated craft we were runing at the time.

    Bear in mind this is a flowing system, not a static one, there will always be losses in pressure the further you get from the source.

    Our measurements showed that the greatest pressure was at the left rear corner (where the fan was pushing all the lift air as it passes beneath the splitter), lowest was at the right front corner (furthest distance from the high pressure area). To maintain sensible airflow and bias flow to the front (where its usualy needed) we use smaller holes at the rear (1&1/2") increasing to huge ones at the front (3" by 5" oval). Carefull design and shaping of the plenum will help reduce the pressure drop, and skirt design will also affect the dynamics of the airflow.

    Ultimatly the best approach is to put a lift fan at the front to deliver the air where its needed most.

    dan

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    Default Re: skirt feed holes

    I have to agree with Dan on this one Tony. I Have measured the plenum pressures on a number of craft and found considerable differences in pressure around the plenum. it does depend on what you measure though )



    The system is dynamic. Close to the air source you have a large volume moving through a finite space giving high velocity therefore a high velocity pressure and low static. As air is lost to the cushion the volume (and velocity pressure) drops and the static pressure rises. This change is dependent on plenum length, section, shape, feed hole size and the fan duty (volume/pressure). If the front plenum is of too small a "cross section" it is possible to generate insufficient static pressure to generate the flow required through the feed holes to even lift the craft at low engine revs. This is evident when the craft rises evenly or rear first (integrated). I have seen this happen.



    I have seen situations where too much air passing through one side plenum has reduced static pressure to near zero and hence virtually no feed. (Ewan had this situation on one craft, which he had to correct with a vertical splitter in the duct.)



    I have also found situations where the velocity of air over the corner feed holes at the rear were so great that the static pressure was sometimes lower than the cushion pressure causing negative flow and requiring baffles in the plenum to move the faster flow inboard. This situation has been proven on a full sized model in Australia.



    As Dan stated, the highest pressure (Total pressure) is at the rear on an integrated craft and this will be reduced through losses as it travels toward the front. If too high a volume is used (or too small a plenum), the losses can be quite high. The method of terminating the duct and the shape of the rear corners of the plenum are absolutely critical in a good integrated design.



    I agree with Dan on the size of rear holes (typ 30mm nom.), the best size for the front holes usually has to be determined by experiment. Interestingly you will find quite a number of people in the paddock who firmly believe that the rear holes should be large.


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    Default Re: skirt feed holes

    my measurements were just static pressure i suppose and there seemed to be little variation in that



    i still think there's much further to go with integrated - this has spurned me to consider a system where you can slot in different size feed holes



    that's what i like about this sport - there's still so much not known or proven



    Tony

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    Default Re: skirt feed holes

    I've looked at this this theoretically, and agree with Dan and Paul. The key thing is that the velocity pressure can be very significant. On one large UK integrated craft I have looked at, the plenum velocity seems to be high enough to cause the reverse flow that Paul talks about.



    My calcs were stalled because I was unsure what happens as the flows slows down to the front of the craft - does some of the velocity pressure get recovered, in which case the static pressure would rise towards the front, or does the velocity pressure not get recovered. You seem to answer this question at least - the velocity pressure does not get recovered? I would be interested to see the results? This would make it possible to calculate the size of transfer hole, given a knowledge of the required flow rate. The calulation is iteritive - as the flow rate depends on the fan performance which depends on the plenum pressure which depends on the flow rate etc etc... Fortunately, Math~Cad deals with all that.



    The size of the hole may also depend on whether you think you are going to get some "air jet effect" in sealing the hover gap. If you think you are, you may wish to have a high plenum pressure, accepting the loss of static pressure across the (smaller) plenum holes that become crude "nozzles" forming the jets. If you think you are not, then you would want the largest plenum holes possible, as you want to minimise the loss of pressure through the holes. My guess would be the that "jet effect" is negligable.



    Some larger cruisers that I have looked at have a plenum pressure drop equal to the cushion pressure - this does seem wastelful to me, and increasing the area of the transfer holes can improve lift performance - maybe even permiting the splitter to be dropped a little.



    All this ignores any "secondary" effects - such as the segment type, or any pressure recovery effects intended to increase skirt stiffness locally - which may require mods to the hole sizes, and probably explains the disagreements on the subject, as craft may vary according to type.



    I havn't done much on this for a while now, as I lost interest once I had sorted out the Osprey, and the new craft uses a bag skirt anyway.



    Cheers

    Ian




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    Default Re: skirt feed holes

    thanks for all the replies,problem now solved and it was the skirt feed holes

    thanks rick

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