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alpha3 - n/a
14-Dec-08, 11:30 AM
Hello to everbody!!!

Im a newbie and need help !

Im in the process of designing my first build and could do with a lot of tips and suggestions on ways to go ie (ducted fan vs unducted prop )hull height from floor to top of skirt (planing surface angle)if this is paramount in a design, power to weight ratio s, whats the best floatation medium expanding foam vs solid substance , etc etc a quick insight into my build plans are as follows .

Im using aluminium as the hull medium it will be built with some aircraft design in mind ie skeletel trusses etc for rigidity and i had thought of injecting the foam medium into the cavity so as i can finish the welding and fabrication without causing undo heat to the foam ! this also gives a good level of density to the craft when the foam medium drys, whats the best skirt/ material? ! the engines i have for lift and thrust are as follows ,

lift= a MZ 34 313CC 27HP AT 6250 RPM 2 STROKE

thrust= BMW R1OO WITH ROTAX DESIGN GEARBOX COMISSIONED BY SCRIMINGER ENGINES i think the engine is approx 90 hp

i did get a prop with this engine its a 3 bladed carbon fibre approx 2 metres dia .

im ok on the welding and fabrication side but could do with some help with wheres the best to mount lift fans , thrust aspects should i go with pulleys or hydrolics who is going to rip me off and who knows what there talking about(shops)etc

any help from those who know what there talking about would be a great help!!

thanks andy

sixpackpert - n/a
14-Dec-08, 11:53 AM
Hi Andy,



Welcome to the board. http://www.jonrb.com/emoticons/wavey.gif



Are you planning on racing or cruising with the craft?



If it's cruising then:



I see you have a 2-stroke paramotor engine for lift and a 4-stroke for thrust. The 2-stroke will be incredibly noisy for lift as you will be running it flat out nearly all the time. You would also have to have two fuel tanks in the craft and if you are going for a long journey you would have to take a spare jerry can of each type of fuel. Bit of a pain in the ass but if you have the room in the craft then fine. I would seriously contemplate changing the lift engine for a 4-stroke. It would make the build easier as you can have one fuel tank for both engines, it would also make the craft a lot quieter which is something that we as a club are actively trying to encourage. Quieter craft means less grief from nimby's.



I would also strongly recommend building to a known set of plans. Hovercraft are quite critical in many areas of the design, if you get it only slightly wrong it can be the difference between it working and it just sitting there in a ball of spray or dust!



I would say that a ducted fan would be the best option but spinning quite slowly in order to keep the noise down. Ian Brooks has done a lot of work with this and will probably be along to offer some words of advice now he's back in Blighty!



I have only ever built from kits and then fettled from there to varying degrees of success, so my knowledge of craft design is quite limited. Have a look on the links to the craft manufacturers and give them an email, Vortex, Bill Baker Vehicles and K & M Products offer parts off the shelf as well as machining services if needed.



If you need belts or bearings then give me a PM with the sizes and specs and I can get a mahoosive discount from a well know bearing supplier.



HTH.

alpha3 - n/a
14-Dec-08, 12:47 PM
Hi john and thanks for the info !!!!!!

im very lucky to live in leigh on sea (essex )and have an entire estuary to go at so the nimby problem is not that much of an issue , the vast quants of morons in very loud and high performing boats with zero experiance is more concerning but they tend to only turn up when its 75 degs and wearing ~~uck all ! my end aim is a WIG vehicle / hover/ground effect ! i can see a woried face lol dont worry im a fully educated engineer and ( apart from my spelling LOL) have 18 years craft expo under my belt alu , tit ,stst materials no prob , your right about the 2 stroke but its out of my paramotor and at hand so it will have to do for the moment , im not on a budget but would like to keep the outgoings to a minimum but wont be scrimping as the project i want to last for a about a year .

I managed to lay my hands on a hunter hawker canopy and attachments for the cockpit as well as some flight controls out of a cessna all beg borrowed and stole lol so ill uttilzes these in the craft obviosly keeping the weight to a minimum threw out the project, ive read different books on craft design (HOVERCRAFT THE CONSTRUCTORS GUIDEBY HCGB )COUPLE OF OTHERS

but there all conflicting in parts so any first hand information and experiance would be fantastic ive sent an email to kand m hovercrafts about purchasing the parts i need from them selves but i only want to make the journey a couple of times due to the 175 mile distance .

sixpackpert - n/a
14-Dec-08, 01:17 PM
Hi Andy,



The most important aspect of your craft IMHO will be the skirt design. For a WIG to actually lift off you would need to be doing at least 55-60mph (based on Universal Hovers figures for their WIGcraft). In order to get to that speed you will need a very good skirt design that will not plough-in.



Plough-in, if you are not aware, is when the front skirt section collapses causing the craft to stop very very quickly.



What weight do you envisage the craft to be?

Ian Brooks - n/a
14-Dec-08, 01:42 PM
Hello and welcome!



First - We're here to encourage & help! Sometimes, though, that means we have to be a little discouraging to begin with. Designing a hovercraft is hard - very hard. Which seems like an odd statement because it is patently very easy to get a few planks & a leafblower to hover, But a hovercraft that you will entrust with your life... now that's a different matter!



Having said that, designing a new hovercraft is within the grasp of an experienced Engineer who is comfortable with technical calculations, so here's a little help to get you going! The first thing you need to do is read up a bit - I recommend "Theory and Design of Air Cushion Craft" by Yun and Bliault (ISBN 0-340-67650-7).The other two references you'll need are "Homebuilt Hovercraft" by Williams, and "Practical Design Guide for Airboats and Air Cushion Vehicles" By Barry Palmer, both old but very relevant. The last one of these is perhaps the starting point. If you PM me an email address I can send copies.



If you prefer to just get on with the job, then I would recommend building to plans. This has a huge advantage - you know that it will work after you have spent hundreds of hours and several thousand pounds on it.



From your posting, I see that you want to build a cruiser and would caution that the 2 stroke is just not suitable as a lift motor for a cruiser, it'll be killed by the salt environment and leave you stranded. It would be a shame to compromise the design for a few hundred pounds worth of Briggs engine - about 12-14 HP for a mid-size cruiser. The R100, on the other hand, will be very suitable.



Here's some links you might want to review:



Wouter, designed and built an excellent cruiser in his ATTIC!!!!, here's how he did it:



http://www.xs4all.nl/~wcasten/hoverholic/



John, has built several excellent cruisers:



http://www.hovercruiser.org.uk/



Anyway, I hope that helps!



Ian

alpha3 - n/a
14-Dec-08, 02:01 PM
Jon i bought a set of universal plans from a very nice gentleman in scotland who s in the process of building a composite craft from plans, i had a good look at them in detail and there very confused and vague and the aero dynamics are rubbish its funny watching square boxs fly ! and yes speed is the key 53 mph rings a bell, over all weight im still calculating approx but when all material are in ill have more idea ill be doing a web blog prob anyway for my freind who lives abroad to follow so other peeps can see too!!

sixpackpert - n/a
14-Dec-08, 02:04 PM
Cool, keep us posted and of course any more questions, just ask.

alpha3 - n/a
14-Dec-08, 02:15 PM
Thanks great!!!!to you both ill get stuck in getting the books you have said and read them completely ill take on board the briggs and strat engines and maybe look online at getting prices its going to be a few weeks before anythings set in stone as i want my design turned into a 3d model first and iron out probs with the welding program if its ok id love to put ideas threw you both and get honest feed back ! look forward to getting a slating

andy

alpha3 - n/a
14-Dec-08, 02:20 PM
sorry here is my email

gregoryandy@hotmail.com

sixpackpert - n/a
14-Dec-08, 02:24 PM
It may worth looking into getting rid of the gearbox from the BM engine and using a belt reduction drive upto the fan/prop. This would enable you to keep the engine down low in the craft so helping with the centre of gravity.



The Universal craft are very good at what they do which, IMHO, is go quickly in a straight line on flat calm water. And the WIG version works.

tonybroad - n/a
14-Dec-08, 03:58 PM
Richard Evans succesfully built an aluminium hovercraft adapted from the constructors guide so it is possible



http://www.rhevans.co.uk/



it ended up a little heavy but was very well constructed



good luck with it



Tony

Jeremy - n/a
14-Dec-08, 04:26 PM
That BMW R100 will deliver around 65 to 70hp maximum, rather than 90hp, although I would have thought that this should be enough power for a fair sized cruising craft. I know of the conversion you have and know the chap, David Howell, that was hawking this engine around the microlight world a few years ago, and as far as I know it works OK. Curiously, I have a completely re-built R100, fitted with a Rotax C type gearbox and centrifugal clutch drive, sat unused in my workshop.



As has been already mentioned, that's a heavy lump of engine to have sat up in the air directly driving the fan/prop. I'd thought about using mine in my project but concluded that it was not ideal because of the weight/C of G issue.



Using a belt drive to the fan/prop would allow the engine to be fitted lower down, but would also mean doing some re-engineering to the conversion. It should be possible to fit a pulley to the Rotax prop flange easily enough. The gearbox is most probably a 2.62:1 reduction, which will give an output rpm of around 2500 at the engine max speed.



If opting to fit the engine lower down, then it will need some form of fan cooling, I think. When used in aircraft, these BMW R100 engines usually need a fairly nifty bit of air ducting to get enough air flowing over the heads. They are a bit fussy about air flow direction as well, they like the cooler incoming air to be directed at the exhaust port side of the heads. I fitted a reverse profile camshaft to mine so that I could change the heads from side to side and get the exhaust ports at the front, whilst also having the output shaft at the front (the opposite way around from when the engine is in the motorcycle).



Overall these engines are very reliable, even when run at high rpm for hours on end. I checked with BMW when converting mine, who confirmed that the engine would run continuously at 7000 rpm without having any reliability issues.



Good luck with this project, sounds interesting.



Jeremy

alpha3 - n/a
14-Dec-08, 08:31 PM
Thanks jeremy!

every bit of info is needed i realise theres alot to do your right it is a heavy lump but as bike engines go its not that bad http://hovercraft.org.uk/images/icons/smiley_icons/icon_eek.gif i can lift it so it has some good points ,whence i get into the workshop and have a proper look ill know more ! ive thought about making new components for the prop and fitting with a belt thus moving everything skyward but as you say cooling will be paramount, dont worry i have no intention of putting it up in the air its an engine not a keel after all lol.

Im pleased you have seen this engine and gear box before the bhp was not made clear when i picked it up but you have narrowed it down for me, an email or two to scriminger should get everything in focus then ill know def what im looking at , as for aluminium you have to work with what you know! i could of course work with foam board and fibre glass resin resin resin but im a metal worker and it will be a challenge to get the superstructure

both very light and very tough (if they can build wings fill them with aviation fuel and take to the sky im sure i can get my poxy craft to hover ha ha ha )as for making it to 53 mph to get into ground effect will have to see but thats why we do it isnt it ?long slog = failure drawing board more long slog and it might just come true you never know .

andy

alpha3 - n/a
15-Dec-08, 04:53 PM
the ali is ordered laser cutters at the ready !!!!!!! theres no going back now .

alpha3 - n/a
18-Dec-08, 07:18 PM
well ive been in touch with scriminger !!!

i spoke to steve scriminger about my lovelly http://hovercraft.org.uk/images/icons/smiley_icons/icon_rolleyes.gif tender engine

and he remebers building the two units for the effects people personally he says its basically a new engine now apart from the obvious ie shell and you were right 72hp running peak 7250rpm ! had a great chat with him and very helpful he was too .so the engines going on her holidays to newark !!! and shes gunna have a lovelly time with steve and team ah blessss on her return she will have twin spark skimmed heads new coils and a happy happy just over ten hp more!!!!! bit more fiddling and im told she will be running at 6500rpm and giving me the power i need !!!! praise the lord sorry steve lord steve yep!!

sixpackpert - n/a
19-Dec-08, 11:30 AM
...if they can build wings fill them with aviation fuel and take to the sky im sure i can get my poxy craft to hover ha ha ha...




Quoted for posterity. http://hovercraft.org.uk/images/icons/smiley_icons/icon_razz.gif



I would strongly recommend getting a ride in a fast craft before you go too much further in order for you to experience what it is like. You need to ask yourself some questions. What would happen at 50mph if you came across floating debris? A craft cannot turn as quick as a boat to avoid it. If the debris ripped off your skirt at 50mph you will stop very very quickly and probably very very painfully!



What experience of hovercraft have you got? Please note this is not meant to be a patronising question in any way, it is only asked to serve as a way to judge how to answer any further questions you have.



Please do not take this a being discouraging in any way, I am only trying to point out some issues you may come across. http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif



Look forward to seeing your initial design.

alpha3 - n/a
19-Dec-08, 06:17 PM
ITS ok im quite accustomed to craft in all shapes and sizes from planes to boats i know my threshold and that of the craft ill be flying ,plus the 3 litre cylinders and regulators which will be stored in the cockpit will give me a extra safety margin in case i flip it which will probably happen at some time !!! from hovercraft to submersible in one foul swoop cant be bad !! dont worry ill learn the hard way or no way . they say there are still fish in the thames maybe ill see one ?