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Thread: Almost finished hovercraft but....big problem!

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  1. #1
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    Jan 2006

    Default Almost finished hovercraft but....big problem!

    Hello All,

    Thanks for your help over these last years, I have finally finished, excluding cosmetics, my hovercraft.

    One big problem....

    It don’t work 


    Well this is what I hope you could shed some light on.

    Apologies for this lengthy email but it should contain most of the information you need.

    My craft is based on a challenger; it has various mods that are not recommended in the constructors guide:

    • Adjustable splitter plate

    • Correction for duct circularity(by strengthening cord around duct)

    • As by the advice, pressure seggies at the front, open’s at the side and drag flap/chip bags at the back.

    • Sequentially larger holes as you get closer to the front: 34mm at back, 64 at side and 72 at the front.

    The specs of the craft are as follows:

    • Engine- RD250LC 1980, 35BHP @ 8500RPM

    • Fan- 6 Blade multiwing in 9 blade hub, 2up configuration, attack angle 45degs.

    • Gearing, 22 @ engine to 48 @ fan

    When I designed the skirt I changed it subtly to have the following:

    • Folded up corners to form loops instead of tabs.

    • Much longer tabs so they could join the top of the aluminium edging see figure 1 below.

    • Designed the “hull edge” part of the skirt so it is designed to actually hold against the hull rather than having a gap, see figure 2.

    I have also uploaded a video on YouTube of the event to help further, see:

    The few observations I have made is that by decreasing the blade angle I get a lot more static pressure and therefore lift which makes the craft easier to pull along but not enough thrust to push me along.

    There is a hell of a lot of drag at the back, i cannot even push it backwards at full power, but forwards it can easily be pulled.

    A lot of air is escaping from the top part of the skirt segment where it has the long tab, it forms a triangle shape where air freely escapes from. Does the skirt/hull need to be as air leak free as possible for crafts to work?

    I hope I have given you enough information to have a guess at the problems....

    Really hope you can help get me ready for next race season.

    Thanks Nick

    Attached Images Attached Images

  2. #2
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    Default Re: Almost finished hovercraft but....big problem!

    Gosh - dragging up my memories of racing the RD250LC 20 years ago and that engine just does not sound to be revving right.

    Did you start it up for the video from cold. If so, it is important to allow several minutes for it to warm up first before trying on full throttle.

    From the sound, the engine does not seem to be revving high enough. Assuming that the engine itself is working OK then the clue may be in the comment you make that you get better lift by decreasing the angle of the blades. Have you done a rev test to see whether the engine is turning over at the peak power setting on full throttle? If it isn't then you will need to trim back your blade angle until it is.

    As for the skirt, it looks good but if you know that you are losing air then this is not good as it will reduce the cushion pressure. Without the engine running, lift up the front of the craft and then drop it. If the skirt pops out and cushions the fall of the craft then you have a fair indication that the skirt is working. If on the other hand the hull just hits the ground then the skirt will need to be checked to try and close off all the parts where air is escaping where you do not want it to. You have mentioned that there are some places where this is happening.

    By the way. It is difficult to move an integrated hovercraft backwards with the engine on full power as you are working against the thrust air. But with no driver in the craft it should shoot away quickly forwards so make sure your kill switch lanyard is attached.

    You mention that the back is dragging but if everything else is working right then you being in the craft will help to balance it better so that the back should come up.

    Best of luck.

  3. #3
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    Default Re: Almost finished hovercraft but....big problem!

    looking at the video clip it does sound like the engine isn't pulling enough revs, any chance you can get a rev counter on it

    just a couple of mm on the blade pitch makes a significant difference so try different options

    as for the gap, unless they are really gaping it shouldn't stop you hovering but better to seal them. i used to have the same problem with my first craft and ran a length of skirt material 4-5" wide clipped to the edge before fitting the skirts

    one thought make sure you are actually getting full throttle when you open it up, take the air filters off and check the slides go right to the top

    good luck keep us posted


  4. #4
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    Default Re: Almost finished hovercraft but....big problem!

    the skirt looks pretty good nick, the front lifting like that with no weight in it is what you should expect. cant see enough detail tho, to see air leaking at the top of the bags! but the skirt to the floor looks good, the shape looks good too.

    nice duct lol, good to see it being used! its certainly sounds like an rpm issue there is no noise from the fan at all in the vid. would expect that 750 duct/fan to be rather loud at full chat.

    get a 20 quid HT lead tacho and put that on too see whats happening.

  5. #5
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    Default Re: Almost finished hovercraft but....big problem!

    It's quite common for the front to lift and not the back, especially without a driver on board!

    Remember without a driver, the centre of gravity moves a LONG way back, and your craft along with many others, will park her bum on the floor and let all the lift escape from the front.

    Start with Bobs idea of simply lifting the front of the craft & dropping it, (get used to it, I've found it's one of the best ways of checking your skirt between races )

    Do you have pressure segments? If so, the next thing I'd check is, can the air getting out of the skirt easily?

    I've spent a lot of time & energy over the last year trying to stop my craft 'ass-dragging' (very annoying after spending the previous year trying to stop her doing the opposite!) and one of the solutions was to allow more air into the rear skirts...

    Regarding your Airfeed hole sizes, (Don't I just know Someone's gonna jump on me if I get this wrong!! ) I've calculated the following:

    34mm diameter = 908 mm2 area

    64mm diameter =3,217 mm2 area

    72mm diameter =4,07 2mm2 area

    This gives you some idea of how limited the feed to your rear skirts are, then you have to remember that the ACTUAL flow is even more affected.

    IF you're engine is pulling the right revs, I would increase the size of the rear feed holes to at least 51mm (2043mm2 area), It's easy enough to decrease them afterwards if it doesn't work.

    Good Luck & hope to see you next year!

  6. #6
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    Default Re: Almost finished hovercraft but....big problem!

    Thanks all,

    Jon, your duct has been a great and it shall serve again so thanks for that!

    I do have a reasonaly good infra-red tacho and some reflective tape that i tried to bodge on the initial test but it didnt work so i will try again and find that RPM. To be honest i also think its running a bit slow, as Jon said they usually scream! I will investigate.

    How would the skirt sections cusion the blow of lifting the front of the skirt up and dropping it becuase surely the air will flow back up the holes into the plenum and out of the splitter plate(assuming engine is off)?

    I know as a fact that i have a leakage problem becuase when i gaffer taped every hole nook and cranny including a strip around the skirt, the craft lifted up much easier and was closer to moving. The only problem with the gaffer on the skirt leakage is the skirt inflates and pushes the gaffer out of place so as you said Tony I might add a strip all the way round or.... I can pull the skirt sections up higher on the edge, making the tab very small and reducing the gap that way.

    I have included a picture of my and the craft, note the skirt inflated and those triangle holes which are ALL aound the craft, it must add up to a lot of air leakage!

    Attached Images Attached Images

  7. #7
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    Default Re: Almost finished hovercraft but....big problem!

    Hi Nick

    It looks like a nice tidy job. As has already been said, the engine revs do sound very low, so check that the throttle slides are fully open. When adjusting the blade angle, just 1mm can make a whole world of difference.

    I do think a lot of your problems are to do with the skirt - if you can feel lots of air escaping from the triangular gaps then they aren't doing their job, you should have almost no leakage here. If you've lengthened the top fixing flaps and then pulled the cable ties really tight underneath you will get that puckered look. Try releasing a few segments and re-fixing them higher up, then replace the ties underneath, if this allows the segments to push closer together and keep the air in better, that could be the answer. The long edges do need to be pulled in close to the underside of the planing surfaces, but too much tension will just over-strain the fixings on your p-clips.

    Good luck with it, you are almost there so don't give up yet - it will fly. I know, I built 2 Challenger type craft!


  8. #8
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    Default Re: Almost finished hovercraft but....big problem!

    The problem is not the skirt Nick. 8500 rpm engine speed and 22/48 ratio will give a fan speed of 3895 rpm. at this speed a 5Z/6-9/45deg would absorb more than 90 hp.

    The fan is stopping the engine from reving. De-pitch the fan to 30 degrees where will absorb about 40 hp. that should get it on cushion.

    Originally you intended to use a ratio of about 2.4:1 why did you go with a higher fan speed?


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