Sep 18, 2012, 12:18 AM
Registered User

The Carbon Tube solution

OK I have worked out the Carbon Tube solution.

This is sized for the wing root bending moment and is way stronger than it needs to be outboard.
It is simple, light, available and I think cheap.

OK this carbon tube will run from the root to the end of the inbd panel, 66inches. a peice then extends into the tip panels a further 6 inches. (the offcut from the 72inch tube). The tube is nested up against the rear face of the front spar webs and passes through the ribs. The rib cut outs will need to be radiused at their front verticle to suit. (Dremel?). Where the tube passes through the ribs, glue on a rectable of 3/16 light ply with a 1 inch hole that the tube runs through and is glued to the side of the rib caps and rear face of the web. This should carry the air loads through to the tube spar OK.

The tube is from Rock West Composites, Cat # 25629, 1.0-72inch long-6p, and costs $73.99. You need two of them, one each side. The pair of them only weigh 14 ozs so no worry about weight.
The cross section is 1.07 inch outside diam and 1 inch inside diam and they are made of 6 plys (I think ) of uni directional prepreg Carbon fibre.

I recommend you use a steel joiner that slides into these tubes as the centre joiner. It is all you need. Use 4130 cond N Alloy Steel tube 1 inch Outside diam and 0.875 in Inside diam and about 27 inches long. This can be bent in a tube bender to get the dihedral angle. It will weigh 20ozs. You can buy this from in Seattle for just $18.69 for 3 feet.

Lastly you need to join the two wing panels at the outter joint so we need another tube to plug into the outter end of the inner panel carbon tube and into that short 6 inch tube you have fitted to the root end of the outter panel. I recommend a 6 inch long Aluminium Alloy tube to save weight out here where you don't want it.
The AL tube comes from too. It is 6061-T6 tube 1" OD to fit the inside of the carbon and is 0.035 wall thinkness. 1 foot will cost you the pricely sum of $4.14 and will weigh just 2ozs.

OK so there is my recommended Carbon tube reinforcement suggestion with cost, weight estimates and USA sources of supply. This should be least disruptive to the wing structure and may even require only one surface to be uncoved.
Use epoxy as your glue. This will give you a 5G capable wing. 2.5 times stronger than it is now and should allow mild aeros. The model will be just a couple of pounds heavier and most of that will be concentrated in the center where it will do no harm to the handling.

Good lucK everyone and keep us posted if you give this ago. Feel free to quiz me if you have questions.

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Sep 18, 2012, 08:27 AM
Registered User

I think we all owe Allan a big Thank You for taking time to do the analysis and even the sourcing of parts to fix the issues with this plane. I now know that if I do get a replacement plane how to go about fixing the plane before flying it. This of course if the Manufacturer does not take the work Allan has done to heart and fix the wing themselves. I really think these planes are worth the extra effort to fix them as they are really nice looking planes in the air and seem to fly really well to boot.

See Ya,

Sep 18, 2012, 09:00 AM
SZD16's Avatar
Thank you Allan for all your hard work with this issue.......I'll wait to see what the manufacture comes up with as far as a fix and then decide what to do.
Sep 18, 2012, 12:29 PM
Registered User
Allan, thanks very much! Any chance you can provide a simple sketch to show exactly what you have in mind?


Sep 18, 2012, 03:36 PM
Registered User
Originally Posted by SteveR View Post
Allan, thanks very much! Any chance you can provide a simple sketch to show exactly what you have in mind?


Sure Steve, a picture is worth a thousand words as they say.
I was pleased to be able to help and it was all just an extension of the hobby from my point of view. I had a number of spread sheets I can use from previous projects so it is pretty quick. This model is great value even if it needs some work after you buy it.
Now to get on with my own project. An SHK like this little model I built to get a feel for it. 5M span wing all in bagged carbon.

Sep 19, 2012, 08:26 PM
Team JR Air
SPasierb's Avatar
Sorry, couldn't resist. Substitute Vietnam...
Sep 20, 2012, 07:09 AM
Registered User
The layout of the cockpit on mine in not too bad, considering that this is not a museum scale model. However, if you look closer at the IAS-meter, you will see that it is totally wrong. Mine goes to some 700 MPH! Now, THAT would be an interesting view!
Sep 20, 2012, 05:35 PM
Registered User

Refinement of the tube soultion

I have been talking with Woo about the practicality of the my tube solution.
It has become obvious that the the location I have suggested would be too difficult.
The tube will have to go further back. It will have to pass through the ribs on a line just behind the spoiler box. This will also clear the front Al tube joiner.
This is less than ideal but OK.
That means that we need to make bigger ply plate rib reinforcements for the tube to pass through.
These need to be the shape of the rib and bridge from the front to the rear spar. I think 3/16 light ply 9Face sheet grain horizontal)would do for this for the first 24 inches from the root then you could go down to 1/8 for the next 24 inches and even thinner out to the outbd end of the panel.

TIP Joiners. The two hollow carbon joiners are good for 5G and they take bending loads well into the tip so no extra reinforcement should be needed in the tip panels. (Don't fit that 6 inches of new 1.07 carbon tube to the root end of the tip panel). I say that with some reservation because I'm unsure how they are secured in the ribs but loads are much lower out there so should be fine.

I have suggested that steel tube joiner as it carries the loads across the center right where you want it from one carbon tube to the other. With that in place, all the other joiners could be removed and just locator pins fitted fore and aft however..... The existing 0.496" carbon rods are also able to take the full 5G load. They will still be working long after the AL Alloy joiner tubes have yielded and given up. I suspect but can't prove that these will do the job as currently installed as long as the loads can transfer through the ribs to our new carbon tube.
Bottom line is you can probably leave the steel joiner out completely and run with what is there.
All this makes the beef up relatively limited and easy.
Any comments?

Last edited by AllanK1; Sep 20, 2012 at 09:47 PM.
Sep 25, 2012, 09:36 PM
Registered User

Tube location

The images attached aren't great but give an idea of how a tube fits and where.

OK unless we have questions that's me done.

Time to talk about other aspects of these big beauties.


Sep 30, 2012, 08:07 PM
SZD16's Avatar
Looks like another failure in Germany.....
Oct 01, 2012, 11:31 AM
Registered User
Slope P's Avatar
O poop!!!!!!

Wonder what serial number that one is or was.

Mine is #29.

Oct 01, 2012, 12:16 PM
SZD16's Avatar
The text says that it was just bought and one of the latest batch of planes built. I'm waiting on Hobbico to come out with thier finial "statement" on what is going to happen with this fiasco......if its up to me to do the repair I'll go with the carbon tube method as laid out by AllanK here..... It seems to be the "easiest" way to retrofit the wing with a real spar.
Last edited by SZD16; Oct 01, 2012 at 04:26 PM.
Oct 01, 2012, 12:43 PM
Electric Airplane Junkie
bhchan's Avatar
The photo shows the whole top is broken off the fuselage, does not look like the wing is folded??

Brian, an EAJ
Oct 01, 2012, 12:47 PM
Team JR Air
SPasierb's Avatar
Originally Posted by bhchan View Post
The photo shows the whole top is broken off the fuselage, does not look like the wing is folded??

Brian, an EAJ
That likely has much more to do with the "sudden deacceleration syndrome" and resulting "earth poisoning" that follows a spar break.

It's funny how people won't take folks at their word. I'm sure the person posting in Germany is not trying to explain away an exploding fuselage top as a wing fold?

Oct 01, 2012, 02:06 PM
Registered User
The owner of this latest crashed K8 states clearly that his K8 was from the latest batch, bought only three weeks ago. The wing broke off directly outside the joiners at a flying height of about 500 feet during its fourth flight. The damage you see is of the totally shredded plane after hitting the ground.

Somewhere else in this thread the anouncement was made that the soon to be expected next batch of K8 entering Germany will have strengthened wings.

I was at a tow meet yesterday and spoke to a guy who lost his K8 too because of wing failure, the second case I know of in Holland. His fuselage went up in flames after ground impact because of LiPo-fire.
I was so lucky to get from him his totally undamaged right outer wing part so I can replace my old one witch has a badly warped aileron and dented leading edge(landing misshap).

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