I want to build my own Extra 330L out of carbon fiber. Where to start?  
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I want to build my own Extra 330L out of carbon fiber. ... - 1/22/2008 2:22:17 AM   
NebulaDDS


 

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Joined: 12/11/2002
From: Gainesville, FL, USA
Status: offline 		I'm looking to make my own Extra 330L, and make it a near-perfect scale model of the actual plane. I have access to plenty of photos and diagrams of the real plane.

I want to make it about 75" to 80" wingspan. AND, I'm looking to make the airframe out of carbon fiber.

What are some things I need to know about carbon-fiber relative to ply-wood? Is it heavier? Lighter? Stronger? Weaker? I can only assume that it is a stronger, stiffer material per unit volume, and therefore fewer "ribs" are needed along the fuselage.

I can make all the calculations in order to design the shape of the interlocking ribs and beams and bulkheads for the fuselage. But after that is where I'm stumped.

Given the size of the model I'm looking to make (i.e. 75" to 80" wingspan), what thickness carbon fiber would I need to use for the ribs and beams?

And, after I make the skeleton of the fuselage and wings, what do I cover it with to give it a smooth finish? Can I glue balsa wood to it like one can glue balsa wood to a plywood skeleton?

Or would I need to use something like fiberglass?

< Message edited by NebulaDDS -- 1/22/2008 2:24:46 AM >
       Post #: 1

RE: I want to build my own Extra 330L out of carbon fi... - 1/22/2008 3:59:10 AM   
BMatthews



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Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		The traditional use of carbon fiber is to replace fiberglass in a fully molded outer shell for the fuselage and in the case of the wings perhaps mold them in female molds or vacuum bag carbon skins to foam cores.

Truly to use carbon for a "wood" build like you're suggesting is not the optimum use of the material. Example. Ribs are a simple seeming item that we cut from balsa. For a model of this size we would normally cut them from 3/32 or 1/8 thick wood. If 3/32 then likely we'd use cap strips to stiffen the rib by giving it an I beam shape. The wood has a fair degree of compressive strength due to it's thickness and the stiffness imparted by the cap strips and even a lot just based on its thickness. Now you take that away and sub in some preresined carbon plate of the same weight and you'll likely have a rib that may only be .020 thick or perhaps .015. Some weight comparisons would soon find the right number. But now you have the carbon in a thin sheet trying to contain the same compressive forces and it wants to buckle into an S bend a lot easier than the balsa. Or you want to cap it but there's not enough contact area to properly glue the cap on without forming large and heavy fillets of glue.

The bottom line is that if you want to use a new material you need to use it in a way that promotes it's advantages and minimizes it's disadvantages. Just replacing wood with carbon isn't the way to do that. The typical structures of plywood and balsa were developed from years of previous practice to the form you see in models today. The use of carbon is making big inroads but the most successful uses of carbon have all been as skins over a light but supportive inner shaping structure. Namely carbon or other woven cloths like fiberglass or kevlar over foam and bonded on with a strong and fairly rigid binding agent that is typically epoxy resin or polyester resin in the case of skins done in female molds.

Carbon and balsa have been joined in a supportive way in a few areas of modeling these days. We first saw the use of carbon capping balsa in FAI free flight. That same methodology has migrated to some of the latest RC gliders. Another option has been to bond light skins of carbon matt to strongly supportive foam plastic cores to form composite sheets that can then be cut like wood and joined like wood. Such panels of material are not cheap though. Although with some care you could set up and make your own such panels.

Seriously though. At this point I think it's safe to say that you need a lot more answers than we can help you with. What you're asking would take a book to answer. Perhaps start with something a little less aggresive to test your carbon methods on.

EDIT- I reread your post and now I'm thinking that you're actually wanting to make this a scale stick for stick, tube for tube, etc for etc sort of thing. For that you're obviously looking at carbon tubing to replace the welded steel tubing of the original. That's were the trouble comes in. Unless you can find a way to bind and glue the carbon tubes together in such a way that the joint strength is both strong and rigid you won't be going anywere. I think if you look around this has been a problem with more than one such use of carbon tubing. You need some way to join them with more surface area than a simple T joint provides. That means a wrap of some form that transfers the loading from one tube to the other in a strong but resiliant manner so the tubes can still flex but not break the joint.

< Message edited by BMatthews -- 1/22/2008 4:11:10 AM >


_____________________________

Bruce-
Proudly wasting balsa since 1965.

Free Flighters go that extra mile........

(in reply to NebulaDDS)
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RE: I want to build my own Extra 330L out of carbon fi... - 1/22/2008 5:21:54 AM   
NebulaDDS


 

Posts: 213
Joined: 12/11/2002
From: Gainesville, FL, USA
Status: offline 		Thank you for your response. I do appreciate it.

Actually, I may have misspoken when I said "near perfect scale". I was referring only to the dimensions and shape of the plane. Not the build. Combining carbon fiber tubes to make the fuselage would be an absolute nightmare. Basically, I was thinking of--like you said--replacing the wood design with carbon fiber design....with some modifications that would "optimize" the benefits of carbon fiber over wood.

I think you might be mistaken in your statement that carbon fiber is unsuitable for making ribs and beams (i.e. vertical circular bulkheads and the long beams that run longitudinally between the bulkheads, respectively....just in case my terminology is incorrect.). You are correct about "optimizing" a material, though. And indeed, you are correct about carbon fiber having many special features. One of these special features is carbon fiber's ability to form strong monocoque shapes (i.e. like bicycle frames). But that's not carbon fiber's only desirable trait.

Ask yourself this question: why are most high-end R/C heli frames built with carbon fiber? The carbon fiber plates which make up the side frames (plates that are, I might add, riddled with servo-holes, lightening holes, engine spaces, battery bays, and screws/nuts/bolts galore) take much more abuse than any carbon fiber rib or beam on an airplane fuselage would. Miniature Aircraft would NEVER dream of using plywood plates to make their side frames. But carbon fiber does the job nicely. I think it's safe to say that carbon fiber is much stronger than wood.

With that in mind, I'm wondering is this: why not use 3mm-thick carbon fiber plates to make the ribs, and maybe 15 or 20 2mm-thick carbon fiber beams to go between the ribs to make up the fuselage? The pieces could be interlocking. If they are designed properly and machined accurately, there'd be minimal sanding to perform in order to achieve proper curvature without bumps. After that, something would have to be used to cover it all up.

Why would this not work?

< Message edited by NebulaDDS -- 1/22/2008 5:23:37 AM >

(in reply to BMatthews)
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RE: I want to build my own Extra 330L out of carbon fi... - 1/22/2008 5:41:32 AM   
haikt


 

Posts: 80
Joined: 7/12/2003
From: norman, OK, USA
Status: offline 		No one says it will not work...
Bit why complicate things, there are many many models of extra 300 that are made from balsa and plywood. the better ones use carbon fiber to laminate balsa in critical locations. Balsa/carbon nomex/carbon/fiberglass to mold the fuselage skins and use some formers in critical locations.
You most definitely can make it from carbon fiber to replace wood but it will be expansive and in the long run may not give much advantage.
If you want to use carbon fiber, then use it for molding the fuse from it you will like it lot more...

(in reply to NebulaDDS)
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RE: I want to build my own Extra 330L out of carbon fi... - 1/22/2008 6:42:46 AM   
BMatthews



Posts: 8949
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		I think it would work but I also think that much of what you are considering may be heavier than the wood equivalent. But without seeing the design and knowing the weights it's hard to say. For example, your 3 mm ribs would be lighter if they were done using a carbon laminate panel material of a lighter carbon matt or light cloth bonded to the two faces of a structural foam core. Same for your bulkheads. Again it's about using the materials to their best.

I still say that this is a rather intimidatingly huge project to do for a first go around with a new for you material. Some other simpler designs to learn the ins and outs may be a good idea. But I can see how it could end up being a great way to build if you can work out the bugs and optimize the materials and their uses.


_____________________________

Bruce-
Proudly wasting balsa since 1965.

Free Flighters go that extra mile........

(in reply to haikt)
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RE: I want to build my own Extra 330L out of carbon fi... - 1/22/2008 1:44:57 PM   
NebulaDDS


 

Posts: 213
Joined: 12/11/2002
From: Gainesville, FL, USA
Status: offline 		Well, you have to remember that carbon fiber plates themselves, i.e. the kind that you find used for R/C helicopters consist of three or four layers of carbon fiber sheets......if I'm not mistaken. So, laying two thin sheets of carbon fiber over a foam core would likely not be as strong as a carbon fiber plate, ESPECIALLY considering the fact that the carbon fiber would not bond as strongly to the foam as it would to another carbon fiber sheet and solidified with epoxy.

Those carbon fiber plates (2 or 3mm thick) are VERY strong. Hence, you could probaby get away with using fewer ribs and beams (or stringers, are they called?) in the fuselage.

(in reply to BMatthews)
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RE: I want to build my own Extra 330L out of carbon fi... - 1/23/2008 1:05:26 AM   
haikt


 

Posts: 80
Joined: 7/12/2003
From: norman, OK, USA
Status: offline 		Even real aircraft are laminated. they use nomax and two layers of carbon....
Also you dont need all that strenght that two 3 mm plates will give you, one 3 mm plate will be good..
also solid plate of carbon will be much more then a thin layer..
If you are interested look at some of the pattern planes for competition, such es AngelShadows and such they are top of the line, state of the art airplanes for competition... and even they use balsa ribs lol

(in reply to NebulaDDS)
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RE: I want to build my own Extra 330L out of carbon fi... - 1/23/2008 3:09:04 AM   
BMatthews



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Joined: 10/4/2002
From: Burnaby, BC, CANADA
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quote:

ORIGINAL: NebulaDDS
....So, laying two thin sheets of carbon fiber over a foam core would likely not be as strong as a carbon fiber plate, .......

Those carbon fiber plates (2 or 3mm thick) are VERY strong. Hence, you could probaby get away with using fewer ribs and beams (or stringers, are they called?) in the fuselage.



Yes they are stronger but obviously a lot heavier. The helicopter main plate needs that level of strength so it's an excellent application but most of the parts in a large airplace do not need that amount of concentrated strength in one spot so using such materials in many cases would be overkill and heavy. Case in point is your idea of using fewer but stronger elements to replace more voluminous but inherently more ridid assemblies. An eggshell is very light and uses a material that is not very strong. But by using it at the periphery of the egg and in a shape that transfers any outside loads into more of the surrounding shell like a 3 dimensional arch it achieves a high structural strength. Also you need to consider what defines "strength". A steel cable is very strong in tension. But if you try to compress the cable it easily buckles to one side unless you can contain it. Same with bending. But take those same steel wires that make up the cable and form them around a plastic tube in the right way and that tube and cable assembly suddenly can be very rigid and surprisingly strong for its weight. Same with your carbon. You're saying that a stringer of 2 mm x 4 mm carbon rod would be strong. And I agree. But that doesn't mean that it has sufficient rigidity and stiffness to avoid bucking to the side under compressive loads. But sub in a tube that uses the same amount of carbon that is 4 mm round and you have an element that is largely self supporting.

All of this is an attempt to help you see that there's a lot to learn about using carbon in the manner you want to and at the same time to use it to the best of its advantages. That helicopter main plate is a very special case and on a model like the Extra there's very little I can think of where an element of the model would see that level of loading outside of part of the firewall and the landing gear mount. For the rest of the model if you're wanting to use carbon to do a built up framework you're going to want to look at all the loadings and forces and how to best build in rigidity to the structure while using the least amount of material. Keeping in mind that small sections like rod (vs tubes) will not do well in compresion if the supportive frames are far apart.

To the best of my knowledge and from my few years here you're the first one to want to use carbon components to this extent. You've likely noticed that there aren't a lot of other replies from anyone that has done this sort of work. So you're really breaking new ground. As such a lot of homework will unfortunetly be required to get a feel for all this. And make no mistake about it. Much of model structural design comes from "feel" and "history". You won't have any history to work with in this manner.

If it helps I'd suggest you look into "honeycomb" composite materials information. The foam and skin deal I was suggesting was a substitute for honeycomb but on a model scale. And the foam I was suggesting isn't your basic hardware store crud. You were right, that sort won't hold up. I was trying to think of the name of the stuff but couldn't remember before so that's why I called it "structural foam". But I remembered while typing this that one type used by our european friends was called Rohacell. The glider guys that do the double skinned hollow molded glider wings used this Rohacell and it is intended for coring between such skins. The resins and such will bond much better to it and the foam itself has much higher yield strength than your typical white or blue insulating foam. The idea of coring like this is that you end up with panels that are inherently stiff (a good thing) without the weight of a carbon only solution where to be the same stiffness (not strength) you would have to use enough carbon and resin that it would easily be 4 or more times the weight of a skin-foam-skin panel. Remember that much of a model's structure does not need to hold a lot of forces at bay but it DOES need to be stiff to avoid the airframe flexing and allowing the surfaces to bend out of alignment.

I hope I haven't scared you away. It would be a very interesting project to see take shape and to see this sort of technology being used.


_____________________________

Bruce-
Proudly wasting balsa since 1965.

Free Flighters go that extra mile........

(in reply to NebulaDDS)
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RE: I want to build my own Extra 330L out of carbon fi... - 1/23/2008 6:40:05 AM   
NebulaDDS


 

Posts: 213
Joined: 12/11/2002
From: Gainesville, FL, USA
Status: offline 		Are you sure that a 2mm thick carbon-fiber rib would be heavier than a plywood rib? Honestly, I haven't compared the weights, but somehow I don't think the weight difference between them would be that significant. And even if it was, I think we can both agree that fewer carbon fiber ribs would be required than plywood ribs, and they would be more rigid and require less "bracing" (via longitudinal stringers).

As for your point about "buckling", no matter what an airplane is made of, each individual structural component of the fuselage AND wing (perhaps with the exception of the wing spar) would buckle if it experienced compressive forces on its own. What keeps the ribs and stringers from buckling is the "egg shell" property you mentioned...i.e. very even distrubution of forces over multiple structural components. With carbon fiber, given its added strength, fewer structural components are required.

Take a look at this drawing. It's just a very, very crude drawing I made in MS Paint, and it is VERY MUCH out of scale (i.e. the ribs would be much larger than the spine down the middle, and the wall of the spine are drawn very thick, etc. etc.). I might even ditch the idea of having a carbon-fiber spine down the middle. It may be overkill. What do you think?



< Message edited by NebulaDDS -- 1/23/2008 6:44:40 AM >

(in reply to BMatthews)
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RE: I want to build my own Extra 330L out of carbon fi... - 1/23/2008 6:55:01 PM   
BMatthews



Posts: 8949
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		Well... I for one don't make my ribs from plywood for starters. And the number of ribs is more a function of the need to support the covering material so it won't distort too much.

On your fuselage idea if you use the correct number of outer stringers and support them so they don't flex then the center boom actually does nothing for you.

Truthfully I understand what you're trying to say but I'm trying to show you that you can't just use less parts and get the same structural functionality. Carbon IS very strong but it still needs to hold all the other stuff where it needs to be and in a way that it supports the covering material. Strength is NOT the same thing as stiffness. You can have structures that are made from very strong material but the overall structure totally lacks sufficient rigidity.

This is why I'm suggesting that you start with something a little less ambitious to try out some of the techniques before jumping into a rather large and expensive build. There's a lot to learn about what you can and cannot get away with and to keep the weight under control.

< Message edited by BMatthews -- 1/23/2008 6:58:30 PM >


_____________________________

Bruce-
Proudly wasting balsa since 1965.

Free Flighters go that extra mile........

(in reply to NebulaDDS)
       Post #: 10

RE: I want to build my own Extra 330L out of carbon fi... - 1/24/2008 2:33:31 AM   
NebulaDDS


 

Posts: 213
Joined: 12/11/2002
From: Gainesville, FL, USA
Status: offline 		When you say "covering material", I'm talking about using a something thin, like fiberglass.....not bent balsa strips. Would fiberglass not support itself? Basically, what I'm thinking is that the covering material would serve no structural purpose. It would merely be decorative (i.e. to make the plane look like an Extra 330L) and for aerodynamics.

As for the central boom, I'm thinking that using a large carbon fiber tube with perhaps 1mm thick walls with a diameter of around 1.5" diameter would produce ample rigidity for the airframe, and it would weigh less than two-dozen stringers. In my "design", I have only three stringers present because I need them to handle two tasks; 1. to provide some support for the covering material between the ribs, and 2. to help keep the ribs from swiveling and/or tilting around the central boom. Neither of these put much force on the carbon fiber stringers.

True, strength is not the same as stiffness, but they are closely related. Stiffness (a.k.a. "modulus of elasticity) is the amount of pressure that can be applied to a substance without it undergoing plastic deformation. Strength (a.k.a. "yield" defines the point where a substance begins to deform plastically. Hence, a material that has a higher modulus of elasticity is also going to have a higher yield strength.

Now, you may be thinking of "absolute strength", which is the point where a substance actually breaks. In some substances (like glass), the absolute strength and the yield strength are the same. In others, like wood, the absolute strength is higher than the yield strength....meaning that wood can deform plastically before it actually breaks. But try to remember that where structures are concerned (like airframes), it is the material's yield strength that is important. Once an object deforms plastically, it is worthless to the structure...even if it hasn't broken apart. Where carbon fiber and wood are concerned, wood may (or may not) have a higher absolute strength, but carbon fiber's yield strength is higher than wood's. And again, it's the yield strength that's important.

As for the expense.....I can handle that. And I agree with what you're saying. However, by having this discussion with you, I am learning a lot.

(in reply to BMatthews)
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RE: I want to build my own Extra 330L out of carbon fi... - 1/24/2008 4:43:51 AM   
BMatthews



Posts: 8949
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		Fiberglass in a thin layer such as would be suitable for a light covering would not be self supporting covering any more than Monokote or any other covering material. Niether would the same very thin layer if it was carbon or anything else. Covering materials are more to seal out the air and give the model it's final shape like our skin over our skeleton and organs. The balsa you accuse me of confusing this with would have far more self supportive strength due to the thickness of the wood providing a monocoque aspect. It's because using the same materials to cover the model means that the covering requires the same spacing for support regardless of the size of the model. You see that in the old WW1 aircraft where the bombers and fighters all use the same linen. The rib spacing is set and when you build a bigger plane you just need more ribs. So unless you go with a stressed skin such as foam with a shell material or using something like balsa sheeting over the structure then the covering is just there to keep the air out and as a result it should be as light as practical and still do its job. A fiberglass skin is a very poor covering material in this case because it is heavy unless you go for the very like 1/2 or 3/4 oz/sq yard type. Or if you do then it'll be very messy since the lighter cloths of that sort will mess up the weave easily and you'd want to "dope" them with... er.. dope instead of resin both for a light weight and to help make the covering shrink to achieve a drum like tension in the skin. Anything heavier in grade would be much too heavy for a covering material. However at that point if you choose to switch to foam core with a stressed skin molded on then a 4 oz/sq yard material overall with a double layer in the high stress points becomes a viable stressed skin option choice. But ONLY because you're then avoiding all the heavy internal structure that we've been talking about.

It was me thinking that you had mistaken the ultimate strength for modulus of elasticity. However when you start using a material to produce a frame you want to forget about the moduli for the individual pieces and consider the modulus for the overall structure. In that light a center crutch is a very poor way to produce a stiff structure. Moving that longitudinal material out to the periphery of the structure will allow you to achieve far more stiff a structure with the same amount of material or alternately to achieve the same level of stiffness (same as the structural modulus of elasticity but with less words ) with far less material. Especially since, like it or not, if you want your Extra to LOOK like an Extra you need to match the number of stringers so the covering is supported in the same shape as the Extra. There's no avoiding that simple fact unless you go with self supporting shells such as the ones made in a fiberglass mold. And since you need that many stringers out on the outer surface anyhow you may as well make them pay their way and carry the loads as well.

I agree that if you only used 5 or 6 stringers at the periphery of the formers then the tube would carry a sizable portion of the load. Likely not as much as you would think if the stringers are not made so small that they bow under compression. But it sure wouldn't look at all like an extra unless you use a self supporting shell skin over the structure. And if you do that you may as well make the shell a little stronger and make it a fully self supporting shell and avoid all that pesky interior structure in the first place. But if you commit to a framework covered with a flexible covering then you commit to using the same number of stringers as the original or darn close to it. And at THAT point the center crutch tube is at or near the center of the forces where the amounts cancel out and it's just needless weight.

I could see a nice Extra being made from 8 to 10 bulkheads at key points with stringers of thinwalled tubing of perahps 3/16 other than the 4 main ones that should be from 5/16 thin wall tube. Diagonals between these 4