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Carbon Fiber wings

#1
Hey guys, I'm just new around here and I have never actually built a plane before. Today I got to watching some of the videos on the DIY foamies from FT and Experimental Airlines (The Armin wing). I am learning a lot about how models are built, and it doesn't look nearly as intimidating anymore. One thing I thought was pretty cool is that in one of EA's videos he compares the different styles of foam formers, and how much weight they can carry before deflecting and eventually failing.

I've been reading a lot about models in the past 2 weeks, and it seems quite common in the with balsa wood builds to wrap the wing in fibreglass after? I am supposing that this is where the wing gets a lot of its strength, and the balsa is there more or less to give a shape to wrap the fibreglass around. Somebody please correct me if I am wrong.

Anyways, this is why I posted in the 'Mad' corner:
What if you were to build a foam wing, (a-la Armin wing or FT style, doesn't matter), coat it in carbon fiber, and then fill the wing with a solvent to dissolve the foam? From the look of those foam wings you would shed a lot of weight, and the wing would probably be stronger than the original foam only shape. If anyone has the resources to try this out on a small 2-3 inch wide section of wing just to see if it works, you'd be super awesome in my book forever.

If this doesn't work, I have also given thought to wrapping a foam wing, then applying a PVA mould release, then wrapping it again and just tapping the wing former out with a mallet. The neat thing about this is that if you only tap it out 85% of the way you can rewrap the exposed part again, rinse and repeat, effectively allowing you to build an 'infinitely' long wing that appears like 1 solid piece.

At first I was really scared of CF partsmaking but WJP004 on youtube: https://www.youtube.com/playlist?list=PLh5B6MCXRHiEFl6VaERQKJUGFRj4Coh4H
makes it look so easy that I think most people who already scratch build their own planes should be able to do something like this. Especially if they already have learned how to fibreglass a wing.

Let me know what you guys think!
Andrew
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#2
Whilst on the surface a FGlass or CF hollow tube wing might seem like the answer to all prayers It does have some structural issues.
Try getting a carbon tube as used for wing spars and stepping on it and you will find that it is very brittle and fractures easily because it is thin and not designed to withstand a crushing force across its smallest dimension.

As for FGlass coated Balsa wings, do not underestimate the strength of the internal balsa structure. The glass adds additional strength but the Balsa is a required element structurally.

Foam core wings which are/were FGlassed in the past still relied on the foam to provide structural support to the FGlass except where the FGlass was applied thickly. Mind you self supporting FGlass wings are generally far heavier than glassed foam wings of the same strength.

Finally envisage the impact resistance of a hollow wing skin Vs a wing with internal structural support and also consider the flex and flutter you might expect again from a hollow tube wing.

A Glass or CF wing with internal structural support and possibly some form of crash/crush resistance, (along the LE especially), and that would be something to consider.

I am not against CF or Fibreglass wings or other structures but rather I am trying to point out that an airfoil shape without proper internal support or structure whilst feasible to some is an accident waiting to happen or far too too heavy.

Have fun!
 
#3
I'm looking to try some wire cut / glassed builds soon.

Every video I watch about glassing says that if you are relying on it for strength, you need to redesign your wing.
 
#4
Hai-Lee, you're absolutely correct about how brittle carbon fiber is. A CF composite can only handle a stretch of about 1.5% its own length before it breaks. It is a very rigid material. Fibreglass is at least 5X more flexible, which means that when you glass a wing, the glass allows the wing to flex, which passes some of the strain from the deformation inwards to the other structural elements.

The situation is a bit different when laminating with carbon fiber however, especially over a foam core. The foam is on the order of 10X less brittle than CF, which means that when a CF laminated foam structure is stressed right to the edge of breaking there is hardly any deformation (slightly less than the 1.5%) This means the foam is only deformed ~10% of its total capacity, so therefore only utilising 10% of its total strength.

The gist of this is any structure where you want to share loading between carbon and foam, things get complicated and you have to keep in mind that you must limit maximum designed deflection to that of the carbon, or else it will break and become useless.

I'm currently thinking about a EA-style wing with a CF tube in the leading edge, or as the leading edge, and a thin layer of CF for the top and bottom of the wing, with no foam at all. I am aware that the failure modes of this type of structure would pretty much turn them into junk, so I guess I'll just have to not crash :cool: I'm still brainstorming the exact layup and I'm hoping someone in here has done this type of thing before and will let me know what works reasonably well. I think with some adequate planning it could be almost as easy as making one out of foam, but I have a history of being way too optimistic about these things.

Thanks for the input guys!
Andrew
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#5
I do not question your figures or properties for CF but you seem to be missing the use of the foam. As a long structure of parallel surfaces is flexed the 2 surfaces tend to try to migrate inwards. the surface on the inside of the bend tends to bulge away from the centre of the bend, (under compression), whereas the outer surface is pulled towards the centre of the bend because it is trying to be stretched.

The use of a material between the two parallel surfaces increases the resistance of both parallel surfaces because the two surfaces push into the supporting central layer and transfer opposing forces from each surface to the other and thereby supply some reinforcing opposition to the bending forces. Yes, centre material compression can limit the effect but regardless of the central material it is always stronger than the 2 parallel surfaces alone.

As an example of such reinforcement examine the increase in structural strength in the Mosquito which used Balsa between two layers of light plywood.

Material selection is a very important but structural design is where most strength is realised.

Try it and see for yourself!

Would love to see your CF wings BTW!

Have fun!
 
#6
The use of a material between the two parallel surfaces increases the resistance of both parallel surfaces because the two surfaces push into the supporting central layer and transfer opposing forces from each surface to the other and thereby supply some reinforcing opposition to the bending forces. Yes, centre material compression can limit the effect but regardless of the central material it is always stronger than the 2 parallel surfaces alone.

As an example of such reinforcement examine the increase in structural strength in the Mosquito which used Balsa between two layers of light plywood.
OHHH Now I see what you mean! I'm currently In conversations with a couple of friends of mine, one of whom is an engineer and the other is an engineering student. We're brainstorming the best way to reinforce the aerofoil from inside without a huge weight penalty. One of the most attractive ideas is to insert a CF sheet/plate at the thickest part in the wing, and epoxy it to the top on bottom halves of the wing so that it acts like the webbing in an I-beam. This is most attractive because it requires the least amount of work, and adds a really negligible amount of weight, but we are a bit concerned it won't handle the torsion well enough, especially if the wing skin is thin, so a circular tube is also being considered.

Probably the least fun option is carbon fiber ribs with a tube through them, I'd probably rather just fill it with foam than do all that work. :D

Originally I was hoping to just wrap some CF tape or cloth around a foam die, extract the die and go, but as I've often found in life, if it were that easy somebody would be doing it already. It seems that the best way to get a structurally sound wing is to produce the aerofoil in 2 halves, wait for them to cure, add reinforcement, epoxy the top onto the bottom, and then wrap it with one more layer afterwards.

But it is still pretty cool that you don't really need any super expensive tools to use really high-tech(or high tech looking :p) materials.
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#7
As for increased torsion resistance try a second, thin, "I" beam type spar to make a box spar. The box spar can have a arrangement which is similar to triangular, (truncated) depending on the profile and can also be non-parallel from wing root to tip to ease manufacture.

With the main spar and sub spar not sharing geometry it is possible to link them with the occasional thin web (mini beam or even a "X" shaped double cross brace) to transfer differential torsional forces between spars and cancel out a lot of the stresses, or an oval holed sheet type spar cross brace which links spar lateral twisting movement directly back to the wing skin, up to the failure of the cross webbing/bracing or "I" beam spar that is.

Mind you the last step is for use only if the dual spar arrangement is still too flexible due to thin materials being used to minimise overall weight.

I will be watching for your wing build post!

Have fun!
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#8
As for you using a foam blank to cover with CF and then dissolve the foam it is still a valid methodology as long as you do not seal any area within the wing structure with CF or other none CF elements.

The spars could be embedded into the foam along with any crass bracing and then sanded to be flush with the foam or blank's profile.

Also the cross bracing if fitted could be attached to each spar independently and inserted into the foam through knife cut lines.

To avoid cutting the foam into a multi piece 3D jigsaw you could could give the spar a series of square notches along their edges, (top and bottom).

Just a few thoughts on construction!

Have fun!