Carbon fiber airplanes? - branch from titanic sub

bflynn

bflynn

Final Approach
Joined
Apr 24, 2012
Messages
9,603
Location
KTTA
Display Name
Display name:
Brian Flynn
branching from the titanic sub thread to talk more about carbon fiber as airplane structures

hindsight2020 said:
It's not just the obvious poor strength relative to competitor materials in compression, composites also have really dodgy fatigue curve characteristics. Treasonous would be my word choice.

Spent one too many years during my stay at Purdue working on these things, my master's concentration was on fatigue and crack propagation in aircraft structures. When it comes to predictive fatigue and propagation behavior, composite is the typical american wife hitting her 40s. She's gonna wreck your life, you just can't tell when.... and frankly neither does she. :D
Click to expand...

Interesting. how suitable is carbon fiber for airplane structures? I’m thinking of dark aero. Are they building an airplane that going to suddenly break apart one day at an unknown time?

https://www.darkaero.com/aircraft

are there other carbon fiber aircraft?
 
Non-issue as far as aircraft structures are concerned (with the exception of galvanic corrosion to any metal parts that come into direct contact since carbon fiber is electrically conductive). The forces and loads experienced by an aircraft have no relevance to those experienced by a deep sea submersible. Composites use in aerospace is very common, has a long track record, and its properties are well understood. Just because some asshat ignored sound engineering advice and industry practices doesn’t automatically condemn the material from use in proper application.
 
Last edited:
The weight to strength ratio of CF composite structures and the ability to form compound/complex curvatures for optimizing aerodynamics makes it an ideal material for aircraft construction. Why it was chosen in a subsea application where the above properties are of no value (hydrodynamic shape unnecessary and weight is needed) leaves me scratching my head.
 
robin ardoin said:
Why it was chosen in a subsea application where the above properties are of no value (hydrodynamic shape unnecessary and weight is needed) leaves me scratching my head.
Click to expand...
You want head scratching?
Where I used to work, they built a big, 3 story, black egg with stairs inside using carbon fiber. It's part of an effing building.
marburger_stem_center.jpg
 
I would assume that Mr. hindsight2020 has a deeper knowledge of this stuff than I do, but, my thoughts FWIW...

Steel has a fatigue limit - a value of stress that allows for a near infinite life as long as the stress remains below about 30 kilopounds per square inch (ballparkish). Aluminum, on the other hand does not behave that way - even small loads will eventually cause failure. So, who the heck would ever fly in an aluminum airplane?

S-N_curves.PNG


Carbon Fiber is not so simple. First, it is not a metal and the classic fatigue process - changes in the crystalline structure of a metal - does not apply. Plus, how a carbon fiber composite holds up depends a lot on the resin, the layup techniques, and the type of loading. If you were to look at the old part 23 the ultimate load requirements for a composite structure were higher than aluminum to account for the less well understood properties of composites in general.

And, given all that, composite aircraft have been out there for a long time without a lot of flaming death raining from the sky. So, while it's not stupid simple, and there is a potential to bite you in the ass, I wouldn't automatically say that carbon is bad. But, again, I ain't no expert.
 
Let'sgoflying! said:
Does an airplane ever get subjected to stresses similar to those at 12000’ below the surface, ie 6000psi?
Click to expand...
Its my understanding stress cycles on aircraft are different in that flexibility and yield play more into the equation than a single fixed stress like the 12,000ft depth. Regardless, where composites do under-perform is with impact damage. One impact dent in the wrong load path and you can structurally lose the entire panel. At least with aircraft. I'm sure once they recover this vessel they will determine the cause/failure path.
 
Let'sgoflying! said:
Does an airplane ever get subjected to stresses similar to those at 12000’ below the surface, ie 6000psi?
Click to expand...

Not remotely. Put a Dreamliner in deep space with the cabin at 1 atm, and the pressure differential is still maxed out at 14.7 psi.
 
Multiple Lancairs were offered 100% carbin fiber option.
 
donjohnston said:
IIRC, the only carbon fiber in the Cirrus is the wing spar. Everything else is fiberglass.
Click to expand...
Actually, fiberglass is a composite as is plywood and fiberboard, for example. I always think of the de Havilland Mosquito of WWII as the first composite aircraft since it was made primarily of plywood.
 
Let'sgoflying! said:
Does an airplane ever get subjected to stresses similar to those at 12000’ below the surface, ie 6000psi?
Click to expand...
Yes.
Sorta.
It's not that simple.

The issue is not the magnitude of the applied loads, but the loads in the internal structure (tension, compression, shear).
For example, my ride has a gross weight of 1300 pounds. If we assume a load of 3.8G that's 4940 pounds of lift required which works out to a bit less than 1/4 psi. Not a big deal, eh? That's how you get away with covering the wing with fabric.
However, if I calculate the internal stress in the wing strut, I come up with a bit over 15,000 psi in tension.
You can then compare the internal stress of 15,000 psi to a yield strength of about 38,000 psi for 6061 aluminum and conclude that if you were to go for a ride with me, you would be more worried about my piloting abilities than the strength of the strut.

To really compare to a submarine you would have know the shape and thickness of the pressure hull to calculate the internal stresses.
 
Last edited:
Let'sgoflying! said:
Does an airplane ever get subjected to stresses similar to those at 12000’ below the surface, ie 6000psi?
Click to expand...

Engineers are supposed to design to or beyond the planned loads and/or stresses. Airplanes aren't designed to go 12K' below the surface, so they are not designed for that environment. I suspect carbon fiber is a fine material for a deep submersible, but you still have to engineer the design correctly and properly build (and operate) the sub.
 
Last edited:
bflynn said:
branching from the titanic sub thread to talk more about carbon fiber as airplane structures



Interesting. how suitable is carbon fiber for airplane structures? I’m thinking of dark aero. Are they building an airplane that going to suddenly break apart one day at an unknown time?

https://www.darkaero.com/aircraft

are there other carbon fiber aircraft?
Click to expand...

Did I say they weren't?
 
Capt. Geoffrey Thorpe said:
I would assume that Mr. hindsight2020 has a deeper knowledge of this stuff than I do, but, my thoughts FWIW...

Steel has a fatigue limit - a value of stress that allows for a near infinite life as long as the stress remains below about 30 kilopounds per square inch (ballparkish). Aluminum, on the other hand does not behave that way - even small loads will eventually cause failure. So, who the heck would ever fly in an aluminum airplane?

S-N_curves.PNG


Carbon Fiber is not so simple. First, it is not a metal and the classic fatigue process - changes in the crystalline structure of a metal - does not apply. Plus, how a carbon fiber composite holds up depends a lot on the resin, the layup techniques, and the type of loading. If you were to look at the old part 23 the ultimate load requirements for a composite structure were higher than aluminum to account for the less well understood properties of composites in general.

And, given all that, composite aircraft have been out there for a long time without a lot of flaming death raining from the sky. So, while it's not stupid simple, and there is a potential to bite you in the ***, I wouldn't automatically say that carbon is bad. But, again, I ain't no expert.
Click to expand...

Bingo. You provided a nuanced response, and hit the nail right on the head regarding what I was referring to regarding composites in the fatigue realm vis a vis design tolerances. The rest of the cohort simply misconstrued my comment by applying that layman inclination of reaching a conclusion based on the presence of any negative opportunity cost. Reading comprehension fail basically.
 
I went to Europe last year in a 787. Very comfortable airplane. Very interesting to watch the wings. I enjoyed that ride.

When I first got my CF prop I mentioned to a big time aerobatics guy that I wasn’t confident about it. He told me it was way tougher than any aluminum prop I’d ever flown behind. It sure works well. I think as old men with old ideas go by the wayside, aluminum will go with them. CF and similar composites are the future.

Any F-1 fans here? Watch what those suspensions take. I got an up-close tour of a Williams F-1 car last year. Incredible.
 
robin ardoin said:
The weight to strength ratio of CF composite structures and the ability to form compound/complex curvatures for optimizing aerodynamics makes it an ideal material for aircraft construction. Why it was chosen in a subsea application where the above properties are of no value (hydrodynamic shape unnecessary and weight is needed) leaves me scratching my head.
Click to expand...
My understanding is that it was chosen because he wanted to fit more people inside and at certain size titanium only submersibles stop being practical because of weight limitations ( these things need to be picked up and handled by portable cranes etc )
 
The Airbus that lost its vert stab and came down in queens shortly after 9/11 I’m pretty sure was attributable to a composite failure.

the issue being it’s thought the structure took damage in a previous severe turbulence encounter that was not discovered using conventional NDT methods… so when it encountered the previous aircraft wake turbulence, it failed much sooner than it should have. Of course we (airline guys) were all subsequently schooled on not using the rudder too much…. Geesh.

So while I trust composite quite explicitly, I do believe there’s more to be learned to make it even better.
 
Tools said:
The Airbus that lost its vert stab and came down in queens shortly after 9/11 I’m pretty sure was attributable to a composite failure.

the issue being it’s thought the structure took damage in a previous severe turbulence encounter that was not discovered using conventional NDT methods… so when it encountered the previous aircraft wake turbulence, it failed much sooner than it should have. Of course we (airline guys) were all subsequently schooled on not using the rudder too much…. Geesh.

So while I trust composite quite explicitly, I do believe there’s more to be learned to make it even better.
Click to expand...

Thats incorrect.
 

Attachments

  • IMG_0891.jpeg
    IMG_0891.jpeg
    77.2 KB · Views: 38
You must log in or register to reply here.
Back
Top