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Jun 07, 2021, 04:22 PM
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Siggy101's Avatar
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F3K - Repair or bin?


Alright then, I need some impetus.

I entered the Swiss F3K Nationals in about 2011.
In free practice before the comp started, a young gent kindly launched his Stobel into my Blaster 2.
I went home and put the Blaster in the hangar where it has sat for 10 years.
My sadness has subsided enough to have another look at the aftermath. It's not pretty.

It was the only layup available in the Blaster 2 meaning the wings are hollow moulded fiberglass with carbon/balsa sandwiched spars.
On the starboard wing, there is a full width crack from leading to trailing edge way out near the wingtip, including both main and sub spars as well as the aileron and it's spar, exposing the white foam inside the aileron. I see this side as less critical as the stresses out on the far wingtip during launch will be minimal.
On the port wing there is similar damage but right in close near the root (just outboard from the factory aileron hatches, which are not in use as my ailerons are in the pod). The crack runs from leading to trailing edge and bothe spars are fractured to the point that I can bend the wing upwards (accompanied by horrible splintering sounds). There are creases in the blue underside and splits in the orange top skin.

What do you think? Is this repairable?

I would only use it for sport flying, no comps. I am 6'3" and launch hard.
The fus is undamaged. Is it better to find a new wing? So far, I have not managed to find one though....
I don't mind a reduction in flight performance, within reason, but I would love to get her flying, if possible. I have plenty of spread tow if that would do the job for the spars. I have plenty of light fiberglass fabric. I have some foam that could be inserted to support the new fiberglass.
My concerns are the absolute failure of all spars (can this really be repaired?) and the super thin wing profile out at the starboard end (is that just too fiddly to get foam inside and re-glass it?)

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Jun 07, 2021, 04:25 PM
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Siggy101's Avatar
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(I am also going to post this in the crash/repair forum but thought it might get some F3K specific knowledge in here too)
Jun 07, 2021, 04:25 PM
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rdwoebke's Avatar
That is very fixable but I haven't ever repaired a broken spar on a hollow moulded DLG. I have thoughts on how I would fix that but I think we should wait until some folks that have fixed spar breaks on hollow moulded DLGs chime in.

Ryan
Latest blog entry: Supergee wing mount pylons
Jun 07, 2021, 04:31 PM
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Siggy101's Avatar
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Quote:
Originally Posted by rdwoebke
That is very fixable but I haven't ever repaired a broken spar on a hollow moulded DLG. I have thoughts on how I would fix that but I think we should wait until some folks that have fixed spar breaks on hollow moulded DLGs chime in.

Ryan
Thanks for the early start Ryan.
Indeed, F3K wings are fiddly little suckers and the stresses at launch are fairly enormous.
I am intrigued to hear what the F3K peeps think. If this were something bigger and that I didn't grab by one wingtip and chuck as hard as I can, I would tend to agree that it could be repaired without too much drama.
Jun 07, 2021, 04:45 PM
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flyinghedgehogs's Avatar
Yes it is repairable. Essentially, you need to make it a solid core plane in the damaged section. You can do this 2 ways. 1) remove the top skin, sand and insert a foam core. 2) inject some self expanding foam into the damaged area and let it expand down the span of the wing. Once you have a core, cut a slot in the core, insert some sheer web, and rebuild the spar cap. I would recommend digging through the old Stoebel and Concept 2,3, & 4 threads here on RCG to find the repair posts there.

EDIT: You can also do it this way: https://www.rcgroups.com/forums/show...gestions%2A%2A
Last edited by flyinghedgehogs; Jun 07, 2021 at 06:49 PM.
Jun 08, 2021, 03:11 AM
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Siggy101's Avatar
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Thanks @flyinghedgehogs. That thread is very useful.
In fact, it has given me an idea.
Before I built my Blaster 2, I bought a Predator 2 but it was destroyed in transit. I kept the pieces in case they came in handy. I just did a little research and it appears that the Blaster series and the Predator 2 both use AG45-46-47 airfoils! What a stroke of luck it would be if the part of the wing that I need (matching chord) could just be cut out and transplanted in, along with the intact skin and spar....
That might just work, if the stars and moon align (ie: the spar position, damage location etc).

It would be a hell of a frankenF3K but I love the idea of that fix.

Do you know if there is a reliable way to cut the spars cleanly (at a 45 degree angle?) and rejoin them with enough strength? It would mean having 2 joins instead of just 1 but I bet the foam core of the Predator 2 would make the transplant super simple.

Alternatively, I could cut away the skin to expose the spars, repair the spars with virgin material and then insert the Predator 2 foam and skin section in front of the spare and then another piece behind the spar. A thin layer of glass over the whole area would then cover the joins.

All of this is working on the assumption that the P2 transplant pieces could match up nicely, of course.
Even if it only worked out for the port wing, I feel it could be a good way to go. I can then bodge up the starboard wingtip with virgin foam and new glass because the stresses are much lower out there.
Jun 08, 2021, 05:18 AM
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pmackenzie's Avatar
I seem to recall that the spar in the B2 is blue foam covered in a carbon sock. It is more of a shear web, with the skins taking the tension/compression loads.
All you need to do is stabilize the existing spar with foam safe CA, then double it up on each side with balsa.

Tom Siler used to fly these a lot, he probably knows everything there is to know about fixing them
Jun 08, 2021, 08:12 AM
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Siggy101's Avatar
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Thanks P.
From reading wdwimbish's account of how he repaired his wing that appears to be extremely similar in construction (in the link that hedgehog posted above), I think I can do something extremely similar. He got an amazing result and it only weighed 2.5g more than stock.
I find that hugely encouraging as nothing he did there is hugely tricky and it doesn't need any special hardware.

I am going to have a go at opening the skins, replacing the shear web with balsa or foam, grafting in a new section of flat carbon capping, adding some carbon rod splints either side of the spar and in the leading edge, cleaning up the existing skins and then I'll do something similar with the reskinning. The outer finish is the part I am not sure of yet. I don't have much carbon lying about but I can get some. I guess for 50 bucks or so, I can have it all done (stuff is expensive here in Switzerland).

Now all I need is some time. Hmmmmmmmmmmm
Jun 08, 2021, 08:13 AM
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Siggy101's Avatar
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Silly question: do I need to use a Dremel and cutting wheel to get through the skins or do I just use a nice new exacto blade?
Jun 08, 2021, 08:15 AM
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tom43004's Avatar
You can use an xacto... just use a fresh blade.

As for fixing the spar, yes it's easily repairable. I've even done these at the field in a pinch. PM me if you get stuck on anything or post here.
Jun 08, 2021, 08:25 AM
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Siggy101's Avatar
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Quote:
Originally Posted by tom43004
You can use an xacto... just use a fresh blade.

As for fixing the spar, yes it's easily repairable. I've even done these at the field in a pinch. PM me if you get stuck on anything or post here.
Awesome. Thanks Tom.
I will find some time to begin the opening up and then will post some pics up of the internals.
I am fairly sure the spar is a sock of carbon around a foam core. I think injecting CA while adding 4 splints will most likely do the job. I'll be back soon with piccys for you to oggle.
Jun 08, 2021, 09:38 AM
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Siggy101's Avatar
Thread OP
Quote:
Originally Posted by tom43004
You can use an xacto... just use a fresh blade.

As for fixing the spar, yes it's easily repairable. I've even done these at the field in a pinch. PM me if you get stuck on anything or post here.
I have plenty of 1mm carbon rods in stock that I could group together if needed, or I can fairly easily buy thicker rods if that would be better. What size do you recommend to splint the spar?
Perhaps 1.5mm? https://shop.swiss-composite.ch/pi/H...0cm-x15mm.html (you might need Google translate on that site)

Do you recommend I get some rectangular profile carbon to cap the spar? I am thinking this sort of stuff: https://shop.swiss-composite.ch/pi/H...80-x-08mm.html

Finally, do you think that the glass fabric that I already have (see photo) would be adequate for the skin repair or do I need something heavier? The same company can supply loads of different fabrics: https://shop.swiss-composite.ch/ki/V...lasfasern.html I am not sure I want anything other than their standard 'tissue', right?
Perhaps the next weight up/down is better (mine is 49g) https://shop.swiss-composite.ch/ki/V...lasgewebe.html

To wet everything out, I was thinking of this epoxy as my stock is old and hard now: https://shop.swiss-composite.ch/pi/H...er-L-280g.html
Jun 08, 2021, 09:49 AM
the kitty litter of rcgroups
rdwoebke's Avatar
That glass is reasonable for skin repairs. That is what we in the States would call 1.5 ounce glass cloth. That is the cloth weight I used when I made bagged DLG wings. The lighter stuff you have available we would call 3/4 ounce glass cloth. I will let others who have fix Blasters chime in on if they would suggest the lighter stuff over what you have. I definitely wouldn't go heavier than what you have but folks may comment the lighter stuff is better (or maybe not, like I said I haven't ever fixed a hollow DLG wing).


Ryan
Latest blog entry: Supergee wing mount pylons
Jun 08, 2021, 10:06 AM
Barney Fife, Vigilante
tom43004's Avatar
There's no reason to go lighter. The weight savings is a rounding error.

1mm rods are enough for DLG spars. Most bagged DLGs back in the day used a pair of these top and bottom and rarely if ever failed due to bending. I would run the splints at least 30-40mm each side of the break to ensure you don't have a secondary failure.


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