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Thread: WE NEED LIFT

  1. #17
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    Default Re: WE NEED LIFT

    My statement is anecdotal really, and I shouldn't have been specific regarding multi-wing, as I am sure that the evidence relates to a variety of manufacturers.



    I'm also sure that you are right when you say that most failures are due to external causes, with foriegn objects being one, and tip/duct collisions being another. Having said that, we should consider that these are 'likely' events in hovercraft use, and in a perfect world the fan/prop will have sufficient design margin (safety factor) to withstand them. In the same way, for an aeroplane, a birdstrike (bird sucked into the engine) is a likely event and an aero engine has to be tested to show that it is able to survive it.



    If we were trying to certify our craft to the aerospace authorities, we would certainly have to simulate and survive a blade strike, or otherwise prove that it could not happen.



    Having looked at the stress levels, compared to the manufacturers allowable levels, it is clear that many fan installations have much less design margin than equivelent propeller installations, therefore they will be more prone to failure due to some other factor that "triggers" the failure. If we were running more slowly, then an blade would be more likely to have enough reserve design margin to survive the addition stress of the tip strike.



    As I'm sure you guessed, I am bringing an Aerospace Engineers perspective to this one, and I could go on. These are some of the factors that influenced my decision to select a microlight propeller in preferance to a rraditional fan for my craft - as an example they have tested it for a "typical" foriegn object (a spark plug) ingestion at full speed - which it survived in one piece although it was damaged badly.



    So thats the background to my statement...



    Ian

  2. #18
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    Default Re: WE NEED LIFT

    i know of may damaged props that sucked stones of the strip on take of or landing, if hovercraft hovered at 2000 feet the fans would last a lot longer

  3. #19
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    Default Re: WE NEED LIFT

    I've been flying for close on thirty years now. In that time I cannot ever recall a prop failure. I've personally had gloves go through through the prop (twice!), a helmet visor go through (once) and a large crow hit the prop at take off (once). None of these even marked the prop, although small stones have caused a number of leading edge chips over the years.



    A microlight prop operates at full power just a few inches above the ground at take off, so frequently sucks in stones, bits of grass, twigs etc. A hovercraft prop is much better protected from such debris, both from being higher up and from the fact that it'll have some sort of guard to keep out the really big stuff.



    The only time I've ever had a prop badly damaged was when operating very low over water. I can't say more, but Tony's seen the damage(!). The wooden prop looked as if very large mice had been at it, with approximately 20 to 30% of the leading edge eaten away by high speed water damage, not surprising really, as it had experienced solid water thrown into it at a reasonably high power setting. Even then it all stayed together, despite having big splits down the prop that were just glued back together with a bit of araldite.



    You get to see it on telly in September, I think.



    Jeremy

  4. #20
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    Default Re: WE NEED LIFT

    i agree regards props and fans arent designed for the operational environment hovercraft put them thru.



    going back to the skirt design, and nick hope you havent made your skirt yet, even tho those pages do give you some more info regards the pressure segement design, the pressure flap don't do anything unless the skirt tags are very short and the rear of the segment is tight against the hull. a looped back bit at the end of the flap, an inch or so upwards which will be pushed against the planning surface and give a better pressure segement seal helps

    the bagyness of the constructors skirt, i.e. the long bottom tags are its major downfall. you can get away with bringing the contact point in a tiny bit too, it made my craft more stabile for some reason.


  5. #21
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    Default Re: WE NEED LIFT

    Thanks jon,

    No i havent made them yet .

    I have however redesigned them further so they seal the hull much much better ill take the looping back into serious concideration. By the way, how do you design skirts on Pro Engineer, it has been baffling me for weeks? you can do lofting through different profile shapes and i know you can set coorinate shape connection?

    Thanks

    Nick


  6. #22
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    Default Re: WE NEED LIFT

    its fairly pants with fabric, closest you can get is with some clever use of the sheet metal package.

    a bag skirt is really easy to generate, you can edge rip joints and create a flat pattern. fiddle with the y factor to get no bend allowance!

    segments would be hard to model accurately, but never really tried. create datum curves in multiple planes and extrude surfaces to them? or use the sheet metal thingy and shell solids.

  7. #23
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    Default Re: WE NEED LIFT

    Well, some intersting points made, I have heard from people that used to fly that the prop does form a little tornado under it that sucks stuff into it, another reason for the ducted fan as there is a lot of loose stuff where the craft is operated.



    The water damage sounds bad and would like to see a pic of the damage.



    Skirts, I learnt very early on that the craft workes better if the skirts are tight against the hull and so to fiddle the inner flap should be good enough, only time will tell.



    Will check in and let you know how I get on.

  8. #24
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    Default Re: WE NEED LIFT

    Right, I decided to have a look at the craft last night and determine how much it'l cost to fix, my biggest worry was the stearing console.



    However, the damage is not that bad, I held the console in with pop-rivetts and they just pulled out and broke the fibreglass in a few places, quick fix there.



    The same thing happened where the hull and deck come together and that too is a quicky.



    the duct can be mended but the damage is so little that I am just going to ignore that.



    The bottom of the craft has a hole in it about 20 X 20 and that I will just glass over, filled it with foam at the damb and will just sand down the edges to get the new stuff to stick.



    The back of the craft is the worst, all around the back corner for about six pockets the edging that the scirts are attached to is off, that will have to be remade of something, I am thinking of using the mold that I still have to maybe make that part over again and then just bond that to the deck, am thinking that this is the best way to go.



    Well only got till the 28th so must hurry, want to fix the front skirts too before then, not much time.

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