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C/F tubing for fuselage structure

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Ady Hayward28/10/2012 01:32:21
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Hi, I'm considering building a large (8ft span) scale cabin plane and considering building the fuselage from carbon fibre tubing as per the fullsize steel tubing arrangements. Has anyone any experience of this type of structure whether it is too flexible or not.

Many thanks

Ady

Greybeard28/10/2012 08:03:48
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What an interesting idea, I've seen full size structre replicated with thin walled steel tubing with brazed joints, but not carbon fibre. I suggest that you make a section as a trial structure, perhaps best to add covering to the trial piece since that adds considerably to structual stiffness. I suspect that the issues to watch out for are security of the joints and radio reception.

Stefan Hafner28/10/2012 08:16:42
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Its something that i've thought about aswell as at some point i want do a large scale (read 4m span) super cub.

I think its doable, but joining the tubes will be tricky, as you would probably need to solder up a brass or steel connector where the tubes join. The other thing that was putting me off the idea is the type of tubes that are available in thinner sizes are pulltruded types, which i've always found the be rather prone to crushing when squeezed or take a knock from the side, and the much stronger roll wrapped types are not generally available in small sizes.

Flexibility wise, as long as you add cross bracing to the structure it will be very rigid, as carbon is a very stiff material, more so than steel when it comes down to the material properties

Ernie28/10/2012 09:32:18
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Hi Ady, I also reckon its a problem re. joining, I've built a fuz from dowelling, and it was difficult. I had to cut the ends of the tubes with a curve, so it would sit snug. Gluing was not strong enough, so I had to drill and pin the joints. I suppose you could do the same, you could fill the ends of the tube with epoxy, and drill into that. Also, I think it would be difficult to cut the ends to a curve

Here you are

ernie

ernie

Ernie28/10/2012 09:36:16
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woops sorry, It was very slow to load, so I did it again. I've got more pix, if they're any use

ernie

Stefan Hafner28/10/2012 10:59:40
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I would do it something like this

And would solder up a brass connector something like this

You probably wouldn't use as many tubes on all the joints i would think so it might be simpler to solder. This way would also prevent crushing of the ends of the tubes and it'll save you the hassle of cutting the ends of the tubes all to rounds as you could just glue a bit of balsa over the gap in tubes and you'll never know once its coveredyes

Radge28/10/2012 11:33:00
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This is an interesting thread Ady and some good viable solutions. So many good full size designs are steel tube, wood and fabric and that to replace the tube with carbon would seem a good modern day proposition, yet even that comes with it's problems.

Piers Bowlan28/10/2012 11:47:56
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The strongest way to join carbon tubes is to tack them together with epoxy or even cyano then wrap the joints very tightly with epoxy soaked carbon tows or thin strips of carbon cloth (round and round). Much stronger than any soldered brass or steel joint and quicker too. For strength it would be important to get a good fit between the tubes, perhaps use a jig and a sharp round file to get the angles exact and don't forget to use a good mask; carbon fibre dust is lethal if inhaled.

I don't think it would be at all flexable, very light and strong infact. I would be interested to see the build progress if you do go ahead.

An 8ft span model would be quite expensive to produce, what with the current price of carbon fibre. I think the Boeing B787 is responsible for that, as it is soaking up the world supply of carbon!

Edited By Piers Bowlan on 28/10/2012 11:50:53

Edited By Piers Bowlan on 28/10/2012 11:54:58

Ady Hayward28/10/2012 11:57:35
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Thanks for the swift replies folks. I'm glad I posted the question.Stefan has raised a crucial issue in the ease at which the tube can get crushed which really says the carbon tube is unsuitable in the form we normally see in the model shops or online shops. I guess spiral wound tube is the answer for that but at what cost?

It does still leave the possibility of carbon rod in the same fashion as Ernie described for his build. I had intended to sand the ends to conform to the join and reinforce the join with a plate in the inside so a small fillet of epoxy would tie them together nicely. There again would the wooden dowell be much heavier?

Hopefully others may add their experiences of this type of structure. Here's hoping.

Ady

PeD28/10/2012 14:07:35
28 forum posts

Could you not use a "spigot" joint as used on fishing rods. They're all fibre/carbon based these days and for conveniance most have gone fron 2 to 4 piece construction.

It's rare that a decent rod fails at a joint these days it's usually in the tube section a failure occurs.

 

Peter

Edited By PeD on 28/10/2012 14:08:15

Ady Hayward29/10/2012 17:19:06
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734 forum posts
1224 photos

Hi Peter,

Thank you for the idea I would use the spigot joint if needed to lengthen or join any section because as you say the joint is really good. The real issue as I see it would be the ease at which the available tubes can crush. The best tube I found so far is 6mm O/D and 2mm I/D which resisted some bending and failed at a the point where it can't bend naturally(in this case a simple T in the centre of the main length). It was interesting to see that it split length ways into lots of strands looking almost as if it was a dry layup, but definately gives no confidence in its use for fuselage longerons.

Ady

DB29/10/2012 18:03:56
42 forum posts

Hi Peter

Don't know whether this is of any use to you or not. I used to fly trick kites and some of the carbon spars in them were tapered and very strong. IIRC it was called Skyshark.

**Link**

Regards

DB

 

Edited By DB on 29/10/2012 18:04:44

Ady Hayward29/10/2012 19:08:07
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734 forum posts
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I just looked at the Kite link and their tubing is the pulltruded type which will be sensitive to crushing. This is the issue that if the inevitable object (Or plane bouncing in the car) strikes the longeron from the side it could cause crush type damage considerably weakening the longeron or cross brace. Maybe a balsa dowell inner is feasible but at 3mm or4mm diameter to fit may be troublesome. It is a possibility. My 1/4 scale project uses 1", 1 1/8", 1 1/4" tubing so will utilise tube with a 6mm or 7 mm diameter.

DB29/10/2012 23:01:12
42 forum posts

Hi Ady

Sorry about that got your name wrong in my post. Apologies.

DB

Ady Hayward30/10/2012 01:13:38
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734 forum posts
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No problem DB. Its loads better than some i've been called.

Ady

Greybeard30/10/2012 07:59:09
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There was an article on just such a building method in a recent edition of the Model Airplane News which may be worth looking at.

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