ToolsnTrack
Posted a lot
Homebrew Raconteur
Posts: 4,117
Club RR Member Number: 134
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The Charity HoverCraftToolsnTrack
@overdrive
Club Retro Rides Member 134
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So I got bored waiting for the whole fundraiser aspect and decided to make a crack on this with my own credit card on the somewhat tenuous assumption I can settle it once the target is hit. First step, get stocked. I took a drastic step here and decided to build using MDF. Yes, I know the water resistant qualities of MDF resemble that of a sponge, but I have came to the following conclusions: 1. Its easier to work with for a prototype 2. I am unlikely to hit the water in hovercraft version 1. Once home, I started to get creative... This is the base. Both plys linked together with a length of wood either side. Spars on top side only, but a similar full length spine on the underside. This will be flanked by similar thinner "skids" underneath for support, but I left those off for now until I could know for sure the best structural place to mount them. Next up, I set the height of my "pressure chamber" (20cm in this instance). The base and the top of the hull, to be mounted above these blocks, will be sealed on the inside of the 4 posts, allowing for a place for the rider to locate. The fan will blow air around the outside of these posts against the skirt. The posts are approx 27" apart, giving me a safe gap for a 24" inflation fan blade if needed. From there on it gathered pace, and as per usual I forgot to use the camera for a lot of it. I took a breather here and documented a few things. Got the base up on the trolley, then it was time to put some strength into it. The 4 lengths of wood at the top of the 4 posts will be the upper part of the pressure chamber. This will be clearer in time, but they will help seal the upper section of the hull, whilst also helping to reduce front/rear flexing. This shot shows the ribs a bit clearer. These will bridge out to the upper spars and also add some lateral strength to the base. Flexing hulls are apparently a killer for these things staying afloat.You can also see I have doubled up the centre spine with some beefier wood. There will be a couple of engines, a saddle of some sorts, and a steering "device" tied into this one spine. At this point, I should point out I can still lift all of this by hand. Light!! Ribs sectioned and tapered in. There will be identical upper "tapers" to support the top of the craft, with the angled cuts allowing airflow. The gaps in the ribs will be filled with foam to reduce the area needing filled with air whilst also keeping the weight down. The ribs are secured here, but I still need to fasten down the spine, hence the gap... And that was as far as I got really. Will need to do a run for some more materials this week. Before I shut up shop, i did lay the thrust engine on for a cheeky pic... Thrust engine works a treat by the way, just a wee service and it should be good to go....
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I'm not being a killjoy here, but I used to work for a very well known hovercraft manufacturer....building it from mdf is a huge no no, the weight of the hull is going to be far too much without it being built up. Even if you managed to get it to lift and enough thrust to push it, good luck flying it. This screams unfinished project to me already. Sorry to sound negative, but it seems a waste of your time following this through. What are your plans for engine mounting? Fan frame? Ducting? Skirt design? Steering system? Fan and hub? Also do you know about setting the blade pitch etc, as just a few mm out with mean the difference between max thrust and no movement.
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few comments that should help you.
1, ditch all the frame work, it adds weight you don't want and is completely overkill. 2, chuck the MDF in the bin, to heavy and with no flex will shatter first time it lifts (if it lifts at all), go by exterior grade plywood, 12mm is a good place to start although most wood craft are built using thinner stock than this.
make your cockpit first. it's a box, big enough for you, a seat and the engine. normally 3ftx6ft ish. everything can hang from this. add some structural support to the outside of this, especially where it meets other panels.
now work out your top size, basically add about 18" to the sides, 24" to the front and rear. this you can frame out, not in anywhere near as heavy stock as you've used though, remember you will need to sit a duct in the rear so leave space in the frameing. take the corners off, something like 10" across the noise and 18" at the sides for the front and a 45 degree for the rear, you could get clever and round them if you wish. the top can be flat. now from this you can make your bottom, angle the sides back to around 3" from the base of the cockpit and skin under the box so you have 2 layers of ply for the base (it's going to get knocked about, it's also where everything hangs from, bond the sheets together) again frame it in thinish stock (best approach is a frame every 12-18"), you should now have a skeleton of a hull with just a bottom and cockpit built) you can now skin the sides out but leave the top deck open for now it'll make life easier in the long run. now you need to figure out what you are using for a duct (even wooden craft use a fibreglass duct normally) and work out your fan size but all that is for another day.
hopefully this should get you started on a path that will actually lift, what you have at the moment is more of a fan boat, every bit of weight you build has to be lifted, the heavier it is the harder it is to lift.
I guess you ignored me last time and didn't bother talking the HCGB, seriously, everything you are thinking has been tried before, even the engine you are using has been used before, someone will be happy to help you do it and do it so it works.
hay #volvo740 who did you work for if you don't mind me asking?
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Last Edit: Oct 15, 2015 9:34:46 GMT by novocaine
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ToolsnTrack
Posted a lot
Homebrew Raconteur
Posts: 4,117
Club RR Member Number: 134
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The Charity HoverCraftToolsnTrack
@overdrive
Club Retro Rides Member 134
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Oct 15, 2015 11:52:50 GMT
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Realistically I know the MDF is never going to be a long term solution on this (it what's been stated already), but it was easier and cheaper to use and make mistakes on, then rejig following inevitable changes. Granted, I'm obviously out my mind for wanting to do this and should just walk the proven route.
Yes, went to the website as listed. Found useful contacts, actually using them for some of the more critical parts such as blades etc.
I can see why you all think I'm wasting my time. Bear in mind though, the whole idea of this is to learn something. To have to alter things, go back to the drawing board, do it myself rather than stand on the shoulders of others. I do appreciate the advice given that is actually technical and useful, but really guys, the doubts and nae saying to my methods are not appreciated. I want to learn it first hand.
This version won't work great, but parts of it will. They will be used on the next version, or ditched if they need replaced.
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Oct 15, 2015 12:28:56 GMT
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not nae saying (and hope it doesn't come across that way) but why try to reinvent the wheel? i'm sure there is some rubbish statement about a wise man knowing when to ask or some such. it isn't that what you have will work but only just, as it stands right now it genuinely won't work at all.
go a different route by all means, I've seen craft built from all sorts of materials over the years, still remember a complete aluminium craft which was impressive to say none the least, it worked too, was a curse word to fix when you binned it though, but choosing a material because it's different is a bad idea when the material properties just aren't suitable in the slightest, like making a car from cheese (ok that bit is taking the micky), sometimes the proven route on material choice is because it's the right thing to use end of. I've built craft from timber (if I can find it I'll show you a picture of the first craft I every used, it was ply wood, flat topped with a segmented skirt).
let me put it this way, everybody who is in the hover club has started the same as you are doing, they have an idea, they build it and think they are the first to try something, then they turn up at a meeting and find out everybody else has already done it. the basic premise of a hovercraft hasn't changed since Christoper Cockerell built srn-1 in the 50s (after he fitted a skirt), this isn't because no one has tried, it's because he got it right.
anyway, back to how to, you don't need the spine, it will only twist, you don't need skids, thats what the floor is for. make your cockpit the strongest element and everything else is gravy. have you thought about making a scaled model first to test your ideas?
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ToolsnTrack
Posted a lot
Homebrew Raconteur
Posts: 4,117
Club RR Member Number: 134
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The Charity HoverCraftToolsnTrack
@overdrive
Club Retro Rides Member 134
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Oct 15, 2015 12:39:33 GMT
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I have. A 1:1 scale model as already done. Pick the snags from it and if necessary make lighter where needed.
I appreciate you are trying not to come over negative, so if we can drop me being likened to a man building a car from cheese that would certainly help matters.
Now I'm getting some snippets of use here. Dropping the ribs. A bit more of discussion on why would be useful...
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ToolsnTrack
Posted a lot
Homebrew Raconteur
Posts: 4,117
Club RR Member Number: 134
|
The Charity HoverCraftToolsnTrack
@overdrive
Club Retro Rides Member 134
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Oct 15, 2015 12:40:49 GMT
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See, in my mind I do need them. Lifting te base without them is surely going to cause the sides to bend up and lose the vision of air?
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ToolsnTrack
Posted a lot
Homebrew Raconteur
Posts: 4,117
Club RR Member Number: 134
|
The Charity HoverCraftToolsnTrack
@overdrive
Club Retro Rides Member 134
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Oct 15, 2015 12:41:18 GMT
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Also, I would be genuinely interested to see your timber craft. I would just have appreciated seeing it a page ago...
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Oct 15, 2015 13:04:21 GMT
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yer thats the problem, the images I have are in the real world it was so long ago, they are all at my parents house in Nottingham, I'm not, lol. the "ribs" you have are to heavy, which is why I'd suggest building the cockpit first then working out from there in at most 1" stock, you want something that can flex rather than be rigid else it will shake its self apart. also you aren't lifting the hull from the outside edge, which the ribs would be ideal for adding strength too, you are applying an equal pressure across the entire underside applying about 5psi across a large surface area. (pressure = force/area) so your ribs are more there to act additional structure not to be the structure as you have at the moment. heres a link with some inside images of a timber craft, note they made the skin then applied strengthing batons to it rather than making a frame the skinning it. hoverclub.org.uk/index.php?topic=700.0try here as well, this is an eagle craft built from plans available but the basics should be visable in the images, the cockpit is built then light framing made to support the top, the undeside is attached to that. website.lineone.net/~hovercraftnut/hovercraft_building_hull.htmlboth should give you a good start as to how heavy it needs to be to work.
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ToolsnTrack
Posted a lot
Homebrew Raconteur
Posts: 4,117
Club RR Member Number: 134
|
The Charity HoverCraftToolsnTrack
@overdrive
Club Retro Rides Member 134
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Oct 15, 2015 13:48:12 GMT
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Ah I see!!
Ok, I think I'm following now. Take your points in board, I'll grab a beer later and have a rethink...
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Oct 15, 2015 21:57:22 GMT
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hay #volvo740 who did you work for if you don't mind me asking? Flying fish, but i really dislike talking about that company.
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