Carbon design under FAR 23

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Markproa

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I've been corresponding with Serge Pennec, the French designer of the Gas'aile. I asked him if I could get the plane up to a MTOW of 600kg which is the max in Australia if I decide to register it as an ultralight. The plane is presently designed for a MTOW of 480 kg. The carbon spar caps are supplied professionally made to builders in Europe in one piece whereas I want to lay up my own using Graphlite carbon rod as it would be much cheaper to get it to Australia. I'm only asking because I know there are already a couple of Gas'ailes flying in Europe at 600kg.
In answer to my question he wrote: "it is ok for 500, but the certification FAR 23 say only 1/5 to the 2000 mpa, the carbone work only at 400 mpa for 6G !!"
Is that true? Does Far 23 restrict carbon use to 400Mpa or am I understanding him wrongly?

Mark


 

Swampyankee

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I didn't find any explicit stress values, and I believe there are none, but see http://www.risingup.com/fars/info/part23-613-FAR.shtml, especially part (a): "Material strength properties must be based on enough tests of material meeting specifications to establish design values on a statistical basis."

It may simply be that he can't demonstrate higher stress levels with the amount of testing he's willing to do. Rather cynically, he may be willing to accept the possibility of other people's death, but not if he may be sued for violating design standards.
 
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proppastie

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sounds to me he is telling you what the existing design might be good for/to.....perhaps he is not addressing your redesign question.
 

pictsidhe

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It's my understanding that composites should be designed with a 2x safety factor. So a 6G spar shouldn't break until loaded to 12G. The strength of the carbon is not the whole story though. Layups won't get the full strength of carbon, this is likely why the designer is a using a factor of 5. If you are going to use graphlite, you might want to check with Marske for the applicable safety factors, and also get the loadings to use from Mr Gazaille. I'm sure Billski will wade in with a better post soon enough.
 

BJC

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It's my understanding that composites should be designed with a 2x safety factor. So a 6G spar shouldn't break until loaded to 12G.
It takes ideal fabrication in the construction to make that an accurate statement.


BJC
 

wsimpso1

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I've been corresponding with Serge Pennec, the French designer of the Gas'aile. I asked him if I could get the plane up to a MTOW of 600kg which is the max in Australia if I decide to register it as an ultralight. The plane is presently designed for a MTOW of 480 kg. The carbon spar caps are supplied professionally made to builders in Europe in one piece whereas I want to lay up my own using Graphlite carbon rod as it would be much cheaper to get it to Australia. I'm only asking because I know there are already a couple of Gas'ailes flying in Europe at 600kg. In answer to my question he wrote:"it is ok for 500, but the certification FAR 23 say only 1/5 to the 2000 mpa, the carbone work only at 400 mpa for 6G !!"

Is that true? Does Far 23 restrict carbon use to 400Mpa or am I understanding him wrongly?
As far as I have been able to tell, Part 23 does not set stress limits, only tells you to use statistically determined values for materials and FOS of 2.0 for composites. As for 2000 MPa, that is about the published tensile limit for Graphlite, while the compressive limit is more like 1400 MPa. I do not get the 1/5 number at all, typical FOS for a composite structure is 2, not 5. Makes me wonder how it was designed and or beefed up during testing. I would need a lot more than one line from a chain of communication to even begin to understand what Pennec meant, maybe you should clarify with Pennec.

Let's remember that when you operate your bird at 25% more weight than is currently approved, you may well have put more than a 25% increase in the load that sits in at the wing mounts. Usually almost all of the increase is hung in the fuselage, which means considerably more than a 25% increase in strength required in the wing, and increased loads in the fuselage and tail as well. These are not just increases in the spars, these are increased loads on the skins, control surfaces, ribs, longerons, stiffeners, etc. Then your stall speeds, approach speeds, landing speeds, etc will all be 13% higher. And you will be hauling 25% more weight, are you planning to install 25% more engine? If not, the climb rate and climb gradients will be much lower. While many of these parts doubtless have enough margin to take a 25% increase in total load, which ones are closer to limits? The problem is you do not know which ones are OK and which ones need some beefing up... And it only takes one part to give up in flight and cause a nasty crash.

MARK! You keep running at designing a new airplane. It is a monster job. If you want to build a nice 600 kg airplane quickly and cleanly, get plans for a 600 kg airplane that otherwise does what you want. If you want to build this 480 kg airplane, try to come to peace with the fact that it is a 480 kg airplane...

Billski
 

Markproa

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I asked Serge Pennec to clarify: So are you saying that your spar designs are worked out at only 400Mpa despite the carbon being 2000mpa? "yes they do not like composite in aeronautical it is not a material stable , and so they have a big securty coef"

Maybe France has their own FAR 23, I'm not sure what Australia has, it usually follows U.S. specs.

Bilski, I do appreciate your philosophy about following the plans and fully agree with you however, Europe has a "rule" that says a UL plane can't be over MTOW of 470kg. I know that Pennec has redesigned a couple of planes that do not fly under that rule as they use a very heavy 100hp diesel engine taking their MTOW to 600kg. Those planes have hundreds of hours of flying, some of it in Nth Africa through very hot, windy and turbulent conditions. So it would seem sensible to not restrict myself to a 200kg useful load when there is a proven modification. Evidently the only difference is a 25% wider spar. And if he is designing at 400mpa then the spar must be bullet proof.

https://www.forum-ulm-ela-lsa.net/viewtopic.php?f=464&t=5667

Mark
 

proppastie

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I believe much of the strength quoted for CF is for professionally heat/pressure cured in autoclave assemblies. Unless you know what you are doing don't do it.
 

Markproa

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I believe much of the strength quoted for CF is for professionally heat/pressure cured in autoclave assemblies. Unless you know what you are doing don't do it.
Yes I get that. The spar caps used in the European built Gas'ailes are made by a large yacht spar builder using autoclave curing. I would use Graphlite. I make free-standing yacht masts for a living but they are hand laid and vacuumed then heat cured at room pressure. My composite engineer used 900Mpa for compression and tension with a SF of 2.0.

Mark
 

wsimpso1

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Big difference between designing new and using a proven design.

If the graphite wing spar design has been flying without failures in a bunch of planes at 600 kg, well that is tough to argue with. My issue is always about if the rest of the airplane still has an adequate FOS. If it was all originally designed and built and tested for 600 kg, but restricted to 480kg by local law, it might be fine. Has the wing and fuselage and tail all been tested to loads scaled for the 600kg version?

As to a proven design vs what you are proposing... There are spar caps and a shear web in the spar. Yeah, you may be duplicating the amount of pre-preg pressure cured graphite fiber in yours as in the other, but you also must have as good a shear web and connection between the spar caps and web. A new Graphlite rod design with its connection to the shear web is not the same as what is already proven.

Billski
 

wsimpso1

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I believe much of the strength quoted for CF is for professionally heat/pressure cured in autoclave assemblies. Unless you know what you are doing don't do it.
I advise folks to approach the 200-300 kpsi strength number for Graphite with a bunch of caution, and to not count on any of it with homebuilt methods and graphite roving. Some folks have reported strengths little better than with glass using homebuilt methods... which is what Markproa sounds like .

Graphlite is much better stuff though. Jim Marske sells his Graphlite rod as being better than 300 kpsi in tension, better than 200 kpsi in compression, and those numbers are in line with a lot of other published data on precured graphite-epoxy. I suspect that it can approach those numbers in reality IFF the web design/rod interlace actually prevents buckling/crippling in the web and in the rods AND the particular design is tested to show it works. It is possible to build a spar design that allows rod buckling and thus early failure. Better to know you are OK.

Billski
 

Swampyankee

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I asked Serge Pennec to clarify: So are you saying that your spar designs are worked out at only 400Mpa despite the carbon being 2000mpa? "yes they do not like composite in aeronautical it is not a material stable , and so they have a big securty coef"

Maybe France has their own FAR 23, I'm not sure what Australia has, it usually follows U.S. specs.

Bilski, I do appreciate your philosophy about following the plans and fully agree with you however, Europe has a "rule" that says a UL plane can't be over MTOW of 470kg. I know that Pennec has redesigned a couple of planes that do not fly under that rule as they use a very heavy 100hp diesel engine taking their MTOW to 600kg. Those planes have hundreds of hours of flying, some of it in Nth Africa through very hot, windy and turbulent conditions. So it would seem sensible to not restrict myself to a 200kg useful load when there is a proven modification. Evidently the only difference is a 25% wider spar. And if he is designing at 400mpa then the spar must be bullet proof.

https://www.forum-ulm-ela-lsa.net/viewtopic.php?f=464&t=5667

Mark
A 25% wider spar is a major difference; all other things being equal, it’s 25% stronger.
 

SpainCub

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Hi all and happy new year. What I’m about to say is here say, since I’ve not verified the information myself nor is there any data writen anywhere since I first heard of it, now this thread it’s the second time. This is coming from an attemp to build a certified trainer design in composites and having gone thr rough some of the loopeholes needed to stay with in CS-23, and failed at cost.

The way it was explained to me was as following, since Q&A of Composites is harder to maintain, any shop operating outside the Q&A control of the EASA stadards, all composites will have a QC factor in, essentially lowering the theoretical structural values substantially. For production, they are required a set of certified personnel, equipment, and data adquisition methodology that brings the cost of low production very high, and discureges to take advantage of the advanced weight savings and strength gains in composites. This way they can pull statistical parts from the production unit and test them. A single failure outside the expected range Winn mean a rethinking of the entire process and recertification to continue...

As an alternative, low production low costs, will incur this lower factor, and this avoids the administration any hassles since its the engineer that sings off on you your project held liable should you have a failure.

I was later explained to me that the issues with the CT tail and wings, something with one of the Pipistrel and Funk, all brought a high level of awareness since testing was not consistent part to part....

Again, this is comping form someone high trusted, but it’s still here-say.
 

wsimpso1

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OK, that is production airplane rules. What do the Aussie rules on homebuilts say? Markproa, that one is yours to comply with...
 

pictsidhe

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Homebuilt QC, a load test. It passes, use it. It fails, offer a prayer to the god of curses and make another. I believe that actual tests always trump theory? The designer will know what else needs beefing up. I believe load tests are done with the safety factor? So a 6g spar is tested to 12g.
 

proppastie

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I knew I had it somewhere....Design Allowable in Europe. Not sure exactly what that number means maybe 400Mpa ?

csvla allowables.jpg
 
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