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Canard Spar

Ted Barstow

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Well, the current canard design fails above 14 g's( both with a good cover and a damaged cover) per testing done by Rutan and has recently recovered from going into flutter, without damage to canard or plane, why substancially increase cost, become a test pilot to try to improve that?


As a general rule, the components of the plane work as a system, if you stiffen one you are now putting more stress on other parts, a viscious circle.


As an example, a friend had a 34 foot boat, the bulkheads showed signs of falure and were stiffened by adding fiberglass layers. A few years later it had hull cracks, I asked him if they were in the area of the bulkheads that were reinforced, they were. The stiffened bulkheads were not bending as before, so the hull couldn't give in those areas and cracked.


If you stiffen one part, you may be transmitting stress to a diferent part and cause it to fail.


Carbon fiber is about 3 times as strong as e glass, if laid up perfectly straight, but if it is not laid up perfectly straight, then it has about the same strength of not perfectly laid up e glass.


This is not a problem with spar layups, as it is a simple matter to comb each layer straight, it is a problem with all other areas of the plane that I can think of.



maker wood dust and shavings - foam and fiberglass dust and one day a cozy will pop out, enjoying the build


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  • 7 months later...

I disagree to a certain extent. If you were to use the same amount of layers in a bulkhead layup then yes a carbon fiber bulkhead is going to be stronger than an e-glass bulkhead. If you make the carbon fiber bulkhead the same "strength" but with less layers you will derive a lighter part with the same integrity. If you were to incorporate this logic to all parts would you not end up with a much lighter plane doing the exact same job? ( I stress logic ):rolleyes:



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Using your logic. The notch you hot wire in would bee too deep after carboning. (Glassing) Then you would lose the weight gain with filler to fill the trough. Also if the carbon canard were to flex a lot the filler wouldent. craking filler, paint.:D


If the phone don't ring. It's me

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Using your lack of common sense wouldn't you make the notch the correct size? assuming you made the canard and did not buy it prefabricated. Chuck im not just speaking about a spar for the canard. Alot of the parts/mainly for skin and bulkhead applicatioins could be made the same strength but with a much lighter material such as carbon fiber. If it were the same strength it would'nt flex more , would it?

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Assuming cost where not an issue, are you saying that there is no way to plot a series of ribs for a canard? (compufoil software) Once again I'm not focusing on the spars or the wings. More specifically I am talking about parts that you would not have to modify the shape or size of. However, if you wanted to make a Carbon spar you could keep the airfoil the same and modify the notch to fit the spar that has been made. To imply that this is not possible is to say that you cant send a man to the moon even though we all know that it has infact been done. I did not want to start a pissing match here. I am not implying that you don't know what you are talking about or that you dont know how to build a canard or wing. Just simply that there is a way to design a carbon fiber part that won't be too rigid in place of the fiberglass piece that it will be replacing. Just because I design apartment buildings for a living does not mean that I know everthing about them. To say that your experince makes you knowlagable is true. But dont get so caught up in the plans that it retards your thinking outside the box. I know that I am not an expert, but do know that many people have made carbon fiber replacment parts for loads of applications. The one thing that they have stressed is that you do not need to build that part to the old specifications just because thats the way it was designed. If it is going to be stronger why not not make the part the same strength but modify the size so that it will retain the designs specifications but be even lighter? My bird is flying right @ the computer screen.:cool:

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Non-engineer speaking here, again. ;]


It seems to me that carbon fiber has different properties than glass. I know this seems like a ridiculously obvious statement, but the caveat here is that there's more to the difference between the two than just the overall strength-to-weight ratio. I'm not trying to rain on anyone's parade, certainly, but I wonder just how much solid unbiased research has been put into the idea of carbon fiber parts? Just how DOES carbon fiber affect the aircraft as a whole? I see a lot of folks who seem to be deciding, almost arbitrarily in some cases, to install or redesign parts with carbon.


My question is, if you build a part out of glass, and then build a similar part out of carbon using the same "recipe" (for lack of a better term), does the carbon part weigh the same but have greater strength? Or does it weigh less but have equal strength? These questions be come relevant below.


Second issue, drawing from the first, and referring to comments by TES111 and Mr. Slade;


Let's say you make a part out of carbon and alter the recipe, ending up with simlar strength but less weight compared to a similar fiberglass part. By the very nature of what you've done, you've changed the overall properties of the aircraft and the manner in which vibration or stress are transmitted to other parts of the structure.


Most significantly, two components that share similar physicals but different mass will vibrate in a different manner, and transmit those vibrations with different levels of efficiency. I tend to mentally compare it to the impedance mismatch beteween two conductors, or perhaps a bell made of glass as compared to one made of a polycarbonate. They're going to sound different; one will ring longer and louder, and the other will absorb the energy fairly quickly. One might be felt ringing through the handle, the other might transmit nothing.


Basically, what I'm getting at is that if you build a part with a different material that what's reccommended, just because the other material is physically stronger doesn't mean it's not weakening, or at least changing, something else equally critical.


I hear similar arguments against carbon or other "alien" (again, for lack of a better term) materials on a pretty regular basis. I obviously agree with them. However, since I don't have a lot of experience on the subject, I'm interested in finding out more. Where does one find data detailing the strength of carbon fiber parts in all the aspects that are relevant here? Torsional rigidity, compression, tension, etc? :(

Evan Kisbey

Cozy Mk IV plans # 1114

"There may not be any stupid questions, but I've seen LOTS of curious idiots..."

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Where does one find data detailing the strength of carbon fiber parts in all the aspects that are relevant here?


The guys who designed the Bercut (sp) must have done some analysis. I believe this plane was/is built with a lot of carbon fiber. It's a VERY fast plane, but I'm told that it feels like a Porche - no give and very hard on the bones.


The problem of transmitting stress may have been handled in the Bercut by making everything with carbon fiber. The result sounds like a light bird with no flex. One of the nice things about the Cozy, so I'm told, is that it rides the bumps very smoothly. The flex of the wings and canard allow it to handle turbulence without shaking the occupants up too much.


Unfortunately I think Bercut is now out of business.


Other than non structural parts like the cowl, removable seats, armrests etc. I think the best thing would be to stick to the plans materials, or do some very complex and costly analysis of the impact of the changes.

I can be reached on the "other" forum http://canardaviationforum.dmt.net

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I can appreciate your concerncs but once again I think that it is possible. Not that you are saying that it is not possible.;)


Just go to a few sail plane/Hang glider pages that utilze carbon. It is safe and there are a few that transition from glass to carbon in thier designs.


"does the carbon part weigh the same but have greater strength? Or does it weigh less but have equal strength? These questions be come relevant below."


My argument and fact finding is that you would want to make it the same strength but make the part weigh less. I think if the part has the same srength it would not change the properties of the aircraft just the weight. CG might be an issue but that can be worked out during the build/redesign process.


"I hear similar arguments against carbon or other "alien" (again, for lack of a better term) materials on a pretty regular basis. I obviously agree with them. However, since I don't have a lot of experience on the subject, I'm interested in finding out more. Where does one find data detailing the strength of carbon fiber parts in all the aspects that are relevant here? Torsional rigidity, compression, tension, etc? "


http://www.acp-composites.com/acp-ucfl.htm - Strengths & Pricing

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There is a book, written by andrew marshal that discusses this very subject. The main point he makes is that loads will always follow the path of greatest stiffness. On our canards, the load is taken by the spar in combination with the skin, thats why we use uni in the skin. Carbon fibre has a much greater stiffness than e-glass, therefore much more load will be taken by the spar.

Marshal also says that the strength of carbon fibre is much more dependant on lack of waviness in the fibres in the layup than e-glass. A major northwest kit manufacturer had to redesign their carbon spar because it failed at 60% of its designed load, according to him.

The problem with composites compared to structural materials like concrete and steel is that their mechanical properties are influenced to a very large extent by the manufacturer(you). Ten steel bars of the same dimensions may only vary by 1% in strength. Ten composite spars made by 10 different builders may vary by 10-15% in weight alone, let alone strength.(check cozy archives on builder reported center section spar weights). Thats why the canard is designed to fail at over 12 G's, to account for builder variations.

If you decide to substitute carbon fibre for e-glass, you are redesigning your plane just as surely as if you were substituting aliminum for e-glass. If thats what you want to do, go for it!

Just remember that unlike airplanes, buildings don't fly(except for that one second after the implosion):D


Richard Schubert

"We choose to do these things not because they are easy, but because they are hard."


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Back to the original statement that everyone agreed with:

<... If a material is substituted that has different structural attributes from the design material, loads that were shared would be concentrated in one component or the other ...>


Nobody argued with this. Then you proposed using less carbon to get same strength using less material and therefore end up with a lighter plane with the same "strength". It doesn't work quite that way. Trouble is, carbon fiber is stronger, but also stiffer. If you made a smaller carbon spar with the same "strength" as the S-glass spar, it would be the same "strength", but very different in "stiffness" (which is a different property altogether from "strength"). Your thinner carbon spar cap might have the same strength in compression and tension as the design spar cap, but would have different stiffness, and therefore would transfer either too much or not enough of its load to the rest of the wing, overloading one component or the other.


Visualize a 1/4" glass rod (not fiberglass but GLASS). It would be much "stiffer" than steel, but not nearly as "strong"; it's stiffness would make it much more brittle.


This sh*t involves VERY ELABORATE design and testing. Even if you stick to one material and not mix materials with vastly different structural attributes. The only thing most of us are qualified to do is oversimplify the problem and get ourselves into one hell of a lot of trouble.


I would trust Burt to mix and match carbon and S-glass. NO other names come to mind.


Just a theory .... Jim S.

...Destiny's Plaything...

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I dont have it all figured out. Never meant to imply that I did. I know that I alone could never figure out this design issue. Fortunatley there are smarter people than you or I out in the aviation inndustry. Fortunatley they have made this stuff work in the past. I've been talking with a guy that makes carbon fiber rods for spar caps. He says that it is quite possible. As long as I keep the spar as stiff or maybe alittle bit stiffer than the origional spar.




Heres a link to his site. This stuff is quite amazing.



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