Wooden structure reinforced with carbon fibre

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AncientAviation

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Greetings all... I'm building a Mitchell wing B10 which is a wooden structure. I know one builder reported issues with the trailing edge of the wing as the fabric caused it to bow. He had to place additional wooden support from the rib to the centre point of the TE to prevent this happening.

I recently spoke to an old time B10 hang glider pilot that said he put a 1" carbon fibre strip over the TE, the rib upper and lower surfaces and over the plywood nose section but just above and below the spar and basically where the ribs attached. He said for very little weight it added a lot of strength. The hang glider version was very, very light.

I have tried to read up on how composites are added to/used with wood - but haven't come across much. I wonder about expansion with temperature, surface preparation and assume just a standard wet layup (no vacuum bagging)?

I think some of Jim Maske's Graphlite on the TE would be a good plan - although see in a thread here there are supply issues. I'm UK based so the Netherlands supplier (DPP | Carbon and glass-fibre profiles | Applications | Aerospace) I could use if need be...

I half considered if going to reinforce the TE as above, doing a similar thing with the spars? If I'm doing the TE anyway this might be easy enought to do although they are probably fine as designed and a waste of time and money...

Just before waste time and money thought I'd see if the gods on here could comment on this...

Pictures are of end product and give you an idea of the TE situation :)
 

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TFF

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To such a small trailing edge, it’s not a bad idea. Mostly you are helping it not be so delicate. Spar is different. Actual reinforcement is a hard balance of two completely different materials. The issue is the fight between them on who is in control. If you add so much carbon the carbon is, the wood is just along for a ride and is really just extra weight now. If the wood is dominant, the carbon will probably break or break free because of flexing that carbon not expecting to do. If you glue some on, more likely than not it’s just extra weight again. Can it be done correctly, sure, but not like a bandaid.
 

Hot Wings

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For the TE the triangulated sticks are going to be the lightest - and the most time consuming. An alternate method I've considered is a saw slot in the TE and then glue in a "T" strip made from thin ply and a small cap stick. these would be easy and quick to make:

TE support.jpg
Unfortunately the stabilator/elevon supports, as originally drawn, prevent this being a one piece operation.

IMHO DM should have put that mount extending from the bottom of the wing, not the top. Would keep a little more dirt and rain out of the wing if the hole were on the underside.
 

wsimpso1

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We have had this discussion before. A search will show it.

When the graphite fraction is small, it is not doing much. When the graphite is carrying a big fraction of the load , the wood is just along for the ride, being a heavy core. This is particularly true in beams and skins.

Now reinforcing a fabric covered trailing edge, it has been done spanwise with wood, steel wire, brake bent aluminum, etc. I expect that a suitable sized graphite rod inset in the ribs would work.

Billski
 

BBerson

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Greetings all... I'm building a Mitchell wing B10 which is a wooden structure. I know one builder reported issues with the trailing edge of the wing as the fabric caused it to bow. He had to place additional wooden support from the rib to the centre point of the TE to prevent this happening.
The additional wood braces could attach at one third instead of the center point to prevent bowing. Fabric bowing is a cosmetic issue, not airworthiness.
 

AncientAviation

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Just a quick thanks to all of you for your input! It is much apprecaited.

I did try a search for this before as I know I can't be the first to ask about it - but with no luck. Good thing I am not trying to do anything technical or complex :)

Great comments...and all make sense.

I like HotWings idea as I have a thin kerf blade that might work. Alternatively, the braces to 1/3 would be easy enough.

I'll be sure to pay more attention when it comes to sealling the hinge mount. Underside would have been nicer I agree. Thanks again all...
 

sming

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I like the second page that state, in english "keep calm and carry on" (to carry, spar, french humor?)

To summarize: the spruce sandwiching the carbon pultrusion protect them and add surface for glueing the shear web. Carbon over the shear web add stiffness.
 

Dusan

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I'm no structural engineer, but wouldn't carbon stiffness have all the loading transferred to carbon and the wood act only as 'foam'?
 

Victor Bravo

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Which type of bowing are you trying to protect against... the trailing edge being pulled forward (toward the middle of the wing) or the trailing edge becoming "wobbly" and "wavy" when sighting spanwise along the edge of the TE?
 

WonderousMountain

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Will be doing this on Flaps TE closeout, orthagonal, no sharp triangle shim. Basically adding weight equivalent to 1/2 cm in wood. So just a wee bit.
 

karmarepair

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Oh, this is GOOD!

At least in my Browser Of Choice, Chrome, linking to this document gets an English translation. The figures in the original French document are juicy. The short story is they reviewed the design while discussing their analysis method, then built a spar and a whiffletree to load it and tested it to destruction. It failed at nearly 9g.
 

cluttonfred

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I did take a look at the article and there isn't really a clear conclusion or summary that I can translate and post, it's more of an exploration of the structure and the methodology of the analysis and testing. Sorry, Google Translate or Deepl will have to do.
 

wsimpso1

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I'm no structural engineer, but wouldn't carbon stiffness have all the loading transferred to carbon and the wood act only as 'foam'?
If you are working with wood and carbon fiber combined as tensile or compressive members, the axial stiffness is EA of an element. And load is distributed according to the fraction of stiffness for each element in the structure. Bond two elements together or laminate them is alternating plies, and EA = E1*A1 + E2*A2. Doing a couple examples, E of spruce is about 1.57Mpsi and bidirectional graphite-epoxy is about 12 Mpsi:
  • If wood is 1 in^2 and graphite is 0.01 in^2, EA = 1.57e6*1 +12E6*0.01 = 1.57e6 + 0.12e6 = 1.69e6. It is only a little stiffer than the wood alone and the wood is carrying 93% of the load;
  • If wood is 1 in^2 and graphite is 0.1 in^2, EA = 1.57e6*1 +12E6*0.1 = 1.57e6 + 1.2e6 = 2.77e6. This thing is now 76% stiffer than the wood alone, and the wood is now carrying 57% of the load;
  • If wood is 1 in^2 and graphite is 0.25 in^2, EA = 1.57e6*1 +12E6*0.25 = 1.57e6 + 3e6 = 4.57 e6. This thing is now 291% stiffer than the wood alone, and the wood is now carrying 34% of the load;
You can look at strengths the same way. The two materials will strain together, and the first one to hit its failure strain will limit the assembly. Spruce gets to about 3%, graphite fiber usually to around 1%, so the part will fail when the graphite does, and your parts will only get to the loads the graphite can carry.

Similar things happen in bending, except the metric then becomes EI, where is E is the same, but I is second area moment of inertia...

Now if instead of laminating wood and graphite fiber, you slit the trailing edge of the ribs and put in a strip of carbon fiber to keep the trailing edge straighter than otherwise, it could be a fine fix. Lots of folks have strung a steel cable this way...

Billski
 
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