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Repair or Scrap? Advice please


BigT
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Well that could have gone better!  Not so strong as I thought it would be. 1 wrap of 20 oz fiberglass and one wind of carbon tow, wrapped up in gift tape until dry then sanded gently. Went out today in a genuine flat calm and experienced probably the best flights to date then on the 4 th flight it slipped out of my hand as I caught it and went nose in from about 4 feet and the tail boom cracked off again in exactly the same spot plus the leading edge of the port wing has de laminated and dented (no big deal as I can cover that bit with sticky tape.)  Disappointing. Perhaps the Light Weight Hawk was the wrong model for a newbie DLG.  I will have another go at repairing it using thicker materials and maybe a strip of solid carbon fiber on the top to act as a splint. Peed Off!!!!

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You say, 'one wind of carbon tow'.  If the carbon tow goes around the circumference of the boom, it will do nothing for the bending strength. the fibres must be along the length of the boom.

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Mikes' spot on regarding the direction of the tows.

Don't get to hung up on repairing it you'll be surprised how many you can get away with. I had a PCM 1 metre DLG, for the life of me I can't remember it's name, that I must have repaired a 1/2 dozen times at least. It didn't look the best at the end but it always amazed me how well it flew even when it was hugely overweight.  I don't think it's to much for you, just treat it as a not to expensive learning curve, keep repairing it and keep flying. When you go to full size you'll find them surprisingly easy to fly.

Tony

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4 hours ago, mikef said:

You say, 'one wind of carbon tow'.  If the carbon tow goes around the circumference of the boom, it will do nothing for the bending strength. the fibres must be along the length of the boom.

I was just following the video I was referred to, but I guess it makes sense. I will do the splint thing.

 

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1 hour ago, BigT said:

I was just following the video I was referred to, but I guess it makes sense. I will do the splint thing.

 

At 08:45 in the German video you’ll note that the uni-directional carbon he’s using is going on with fibres in line with the boom. I use this instead of tow. It’s easier to work with in my opinion:

https://www.easycomposites.co.uk/#!/fabric-and-reinforcement/carbon-fibre-reinforcement/carbon-fibre-tape/carbon-fibre-spread-tow-ribbon-80g-15mm.html

I also use their laminating resin:

https://www.easycomposites.co.uk/#!/resin-gel-silicone-adhesive/epoxy-resin/EL2-epoxy-laminating-resin.html

The 500g kit will last a while - I tend to mix 5g batches in McDonald’s ketchup cups and have plenty left over after a small repair like that. I use the “fast” Gardner to avoid waiting 24hrs. 

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1 hour ago, BigT said:

Materials arrived today so I am having another crack (pun) at it today. 

Just got to work out the ratio by weight for 10 grams of adhesive when the ratio is 100:30. 

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On 04/11/2019 at 18:01, PaulR said:

There are two repaired boom breaks in the photo. Both repaired with that uni-directional tape and wrapped with electrical tape. 

image.jpg

Hi Paul, did you do each repair in 2 stages? Do the lengthways strip first, remove electrical tape and sand then do the spiral wrap? Or did you do it one hit?

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1 hour ago, BigT said:

Hi Paul, did you do each repair in 2 stages? Do the lengthways strip first, remove electrical tape and sand then do the spiral wrap? Or did you do it one hit?

My repairs above are only lengthways strips.

The spiral pattern you can see is only the imprint of the shiny side of the electrical tape that I used to compress it all and squeeze out extra resin.

If you do want to use multiple layers then as Paul says above, do them all in one hit.

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I just took a few more pics of another model with the same repair approach. 

The tail end repair is detailed here: 

 

The other two repairs were done without any internal support. 

06B14B8F-1898-4BD0-9652-1A21E91EB157.jpeg

64CB4B25-1EE1-40BA-A235-E627CCEDD5AC.jpeg

99E605F3-2520-4024-B121-A3630F4B6D77.jpeg

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Thanks, most useful, my test piece went well, love the epoxy on the test piece it’s so much stronger than the Zpoxy I used before. 

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Well apart from the tail moving a bit out of line during curing I am happy with the repair. Those materials PaulR recommended are the best I have ever used in 50 years of modelling. Test flew and trimmed the Hawk this afternoon in very very light winds and even managed to get a teeny tiny bit of lift to give me a 2 min flight from a 70 foot launch, gives me a bit of encouragement to carry on. I am gaining respect for those among you that can stay up longer. Still got an enormous amount to learn with setting up different cambers.  

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On 04/11/2019 at 14:13, Tony said:

Mikes' spot on regarding the direction of the tows.

Don't get to hung up on repairing it you'll be surprised how many you can get away with. I had a PCM 1 metre DLG, for the life of me I can't remember it's name, that I must have repaired a 1/2 dozen times at least. It didn't look the best at the end but it always amazed me how well it flew even when it was hugely overweight.  I don't think it's to much for you, just treat it as a not to expensive learning curve, keep repairing it and keep flying. When you go to full size you'll find them surprisingly easy to fly.

Tony

That’s a very interesting statement. I agree it’s a not to expensive learning curve. But you are saying a 1.5 meter is easier to launch, fly and get better flight times ? So if that’s so what do you recommend next as a second model?

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