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Darren_O

Banana wing repair help

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Darren_O

This is the spar, I always figured it would be square section carbon, but it seems to be a complex carbon, glass and foam affair. Bottom is carbon, then glass then foam, with the sides glassed. One side as shown has snapped, the other side just delaminated.

Can I slide in some carbon square section with some carbon tow across the repair to join it? Or just put plenty of carbon tow across without tube?
 

SfV2YlwJ (1).jpg

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Brett82

Hi Darren 

I hope you don't mind me doing this but I'm going to post what I would do to see how far off I am from what the actual experienced builders here would do. Please understand I am not an experienced builder, almost everyone on here would give better advice which is why I want to see how far off I am.

I would put the carbon box in then do multiple layers of carbon on the sides with one on the top as to many layers on top would raise the wing skin. This is what I would use.

http://www.ecfibreglasssupplies.co.uk/p-3601-200-gsm-unifiber-carbon-12k-200mm-wide.aspx

I would laminate it to the bottom skin going up the side of the spar length ways in an L shape. Then do a layer of Kevlar tape just on the side of the spar. then use the carbon again to form an upside-down U shape going up one side, across the top, and down the other side of the spar, again carbon strands running length ways and covering the Kevlar on both sides. Do the carbon and Kevlar longer twice as long as the box on either side.

Would add a fair amount of weight though but I think it would be seriously strong.

Looking forward to seeing what others would do.

 

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isoaritfirst

Personally I would try to make the repair as close to the original construction as i could. No need to go overboard, or you may end up with a heavier wing.

Grind down the carbon tow for a few cm each side of the break, feathering the edge along another cm.  fit in a new piece of spar - either foam or vertical grain balsa spar.

Cap with wetted out carbon tows overlapping onto the edge that you feathered out.

Cut back to a clean edge the top and botton skin. 

Make new skins, by laying two or three layers of thin wetted out glass onto the wing where its a similar profile.

Cut them to fit into the space sand tack in to place with cyano , then finish with epoxy.

Done carefully it will be almost invisible and require no filling.

The spar strength will be fine as the top and bottom skin will also cover this break.

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Brett82

Nice method Mike, I'm glad I posted my thoughts because if I ever need to do this type of repair at least now I know I can be a lot more conservative and not have to go completely overboard. :)

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Scram

The strength in a spar or I-Beam (joist in steel) is in the top and bottom flanges.

If you took 2 carbon strips and clamped them together at the ends with a spacer in each end, you would be able to bend this quite easily, noting that the smaller radius piece would get closer to the outer piece in the middle of the bend.

So the purpose of the web in a beam (foam or vertical grain balsa) is to keep the 2 flanges apart.  Then when bent, the outer flange is in full tension and the inner is in compression. That's why it is vertical grain balsa - much harder to crush then horizontal.  I'm really not sure I would use foam for this purpose - too easy to crush.  

Thus a box section has the flanges held apart by the 2 sides but weight for weight it would not be a strong as an i-beam.

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Darren_O

Problem with making a mould is both sides are fubared in similar places. 
I have this in my basket, anyone know if it's suitable for the job or can point me in the right direction? http://www.ecfibreglasssupplies.co.uk/p-617-west-system-junior-pack.aspx

The other side of the wing is just delaminated, can I get some epoxy in without any support? 
Small vid here 

 

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isoaritfirst

Run some thin epoxy under that spar capping.

Run some penetrating cyano onto the wing skins - holding them to shape as you do.

Be careful of the spar foam as Cyano may dissolve it

 

 

Moulding off the wing doesn't need to be particularly accurate, just use the bit that is closest to what you want.  

Packing tape on the wing and lay onto that. Do a top and bottom rather than a complete LE wrap.

The glass will come as approximately correct and will take up the wing shape a little more when you tack it into place.

You could make a mould out of balsa or foam and lay onto that.

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satinet

you need to repair the spar cap with tows or unidirectional carbon.

You need high quality epoxy resin, which is expensive.

I would cut the carbon cap back a bit so you can replace the vertical grain balsa.

If the cap is only snapped on one side it's a lot easier repair.

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oipigface

That's what I use for most repairs with the 206 slow hardener, which gives a long pot life, so you don't have to rush anything. It also helps the adhesive to penetrate the surface of whatever you are gluing as far as possible. If you want to speed up the cure, you can put it somewhere warm (over a radiator or in an airing cupboard).

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Darren_O

Repaired the spar using 12k carbon tow. Now for the wing skins :)
Thanks for all the advice, very grateful.

0Q5zAKfi.jpg

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Darren_O

Quick question, peel ply, is it sided?

I used it to get a smooth finish and it's left anything but a smooth finish...

Released well enough, but no sign of the promised smooth as glass finish. 

Thanks in advance for all the help, I'd be grounded permanently without it.

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oipigface

No it's not sided. It is woven and one side is exactly the same as the other. The finish you've got is just like a weave. If you used good quality epoxy, you should be able to rub it down. And depending on how much time you've got you should be able to get a good result even now. If you want a true 'smooth as glass' finish you have to use glass and a PVA release agent.

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Darren_O

Made a very nice mould from other side of wing, fits like a glove only it's a bit soft. Think I didn't use enough hardener?

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satinet

Get yourself a cheap set of scales if you haven't already. Say off eBay. The ones that only got to a couple of hundred grams and measure in 0.01g increments.

I mix in plastic shot glasses. A hair dryer or heat gun is pretty essential to get the glue runny. Especially this time of year.

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oipigface

I've had bad experiences mixng epoxies in plastic containers. It melts some plastics and you wind up with a messy mixture of glue and melted plastic cup. I use the waxed paper cups that supermarkets sell some desserts in. Gooseberry fool for small quantities, Ben and Jerry's ice cream for larger ones. 

Maplin's sell a variety of scales. I use these at £9.99:  http://www.maplin.co.uk/p/satrue-micro-small-pocket-scales-100g-n95jl

 

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Jef Ott
On 20/12/2016 at 21:27, Darren_O said:

Made a very nice mould from other side of wing, fits like a glove only it's a bit soft. Think I didn't use enough hardener?

This is a popular problem. Many beginners put additional hardener in to make sure that it goes off, but actually, too much hardener will prevent the epoxy from setting. 

 

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Darren_O

Skins formed and ready for fitting.

Will trim the lhs wing into an easier shape before fitting. 

Happy with my progress is far, not perfect but it will fly again:)

IMG_20170103_130650.jpg

IMG_20170103_130639.jpg

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Darren_O

Thanks to everyone that contributed to this thread. Banana is repaired and ready for action. Unpainted as yet, just deciding if I should redo the far side. I've added 12k carbon tow under the skin on nearside and it's a much tougher repair that's as rigid as original wing. 

IMG_20170122_212936.jpg

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pete beadle

Hi Darren

It needs STRIPES mate, BIG stripes.....red or orange on top, dark blue or black underside .....so you can tell people it was deliberate "colour-mapping" for maximum visibility in the air, rather than covering up where you've put the carbon......:yes::whistle:

Go for it!

Pete

BARCS1702

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