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martynk

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martynk

Hi all

Brain picking time again if you don't mind. I am planning a (Scale) Glider with a 1.8m panel length. I want to use a similar construction that I used on the Bubble Dancer - Spar Caps with a balsa /web/spar/core.

 Can anyone suggest a method of splicing carbon unidirectional strip to make up the required length? The plan is to laminate the thickness of the caps so that they reduce in thickness towards the tip but I will have (hopefully) one continuous cap (at the surface) from root to tip so there is option to overlap the joint but I can see this being a problem :ermm:

Thanks

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oipigface

Why not make your own 'unidirectional carbon strip' by gluing tows directly to the spars? You would have to think about weighting or vacuum bagging the structure, but I can't think of any other downsides. You would be able to add more tows at the root to help the thinning toward the tip. One advantage is that you could do both (or all 4) spars at once, and separate them later. Make it all oversize, and finish the taper with a big sanding block.

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martynk

Would that work? I thought unidirectional C/F sheet was made under high pressure and relied on very close proximity between the fibres. Using Carbon Tows may work with a long splice joint as local reinforcement but I am very nervous. I am not looking for winch launch load strength but this will be a 4m (almost) wing for aerotowing, bungee launching and slope soaring. 

Appreciate the idea, I'll have a play with that in the shed.

Thanks 

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oipigface

You would need to use pressure. Vac bagging would be best. When you mention a splice joint is that in the wood?

Another thing you might try is to put the carbon either side of the spar, instead of top and bottom. That way the entire depth of the spar would take the stress, and if you wanted to use the preformed strip, you could splice the joints and stagger them.

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Jef Ott

Hi Martyn,

I have not done this on a scale as big as 1.8m panels, but have sandwiched carbon tows between 2mm spruce and 1.6mm balsa spar strips to produce composite spar caps, weighting them down onto the bench with the spruce on top while the epoxy resin set.

These were 10mm wide, and parallel, but would have been easy to taper in both dimensions, either before or after manufacture.

Made for some very strong wing tips, complete overkill!

If I can do it, it has to be easy.

Jef

 

 

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EssexBOF

On the Watts New, I use scarf joints which have not let me down as yet,. Angle each end for approx 20-25 mm, then join together with epoxy or cyno(carbon loves cyno), but you need to be accurate on positioning with the latter.

 

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