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Enthusiast Joined: Nov 02, 2005 Posts: 72 Status: Offline

Living in dallas-ft-worth, im always running into F-35 engineers. Last week one told me. "were having major second thoughts on carbon fiber". I use carbon fiber to stiffen up the wing spars of many rc models I build so im aware of its properties... Question

But why would they have second thoughts?

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You know it's funny, I'm a Systems Test Engineer on F-35 and I was looking at one of the (Carbon fiber) LEF seals that was being fit checked the other night, this damn think is about 3/8 of an inch thick or more, I thought good God an F-16's is what .100 maybe. I picked it up and I swear it weighed next to nothing, I was shocked but we all know the deal with advanced composites.
But to answer your question, who know's, there's a lot of second guessing going on but A-1 is on track for first flight next August and has shed almost 3000 lbs.
Personally I think they got a little carried away with the carbon fiber, I can't wait to see what the engine heat does the the carbon fiber engine bay skin, there will be a really thin heat shield of some freaky material I assume is some kind of cosmic stuff NASA uses but I'm not sure. It's going to be interesting no doubt.
The fuel tanks got proof pressure tested last night and man they are tight, with the one piece upper wing skin it's a damn good thing.

Happy Gilmore, anychance of using Reinforced Carbom-Carbon for the heatshield?

Elite 1K Joined: Jul 26, 2005 Posts: 1455 Status: Offline

These advanced composites are also cosmically more expensive than aluminium.Their heat resistant and strength characteristics are usually better though depending on their application...
I suppose weight would be the major factor in it`s continued use as well as radar beating properties you can manufacture into them. It may be too late in the program to do a major u-turn?

Elite 1K Joined: Jul 26, 2005 Posts: 1455 Status: Offline

Johnghitman,

Carbon fibre is expensive, carbon-carbon composite is an order of magnitude more expensive because it takes so long to manufacture. That stuff is used for minuteman ICBM nozzles and space shuttle parts! It is also very brittle. If you want to re-inforce it, it takes even longer to make the stuff, making it even MORE expensive.

I was wondering if that was what they were using.

This is an interesting topic.

There is alot of talk regarding applications of composte materials in aircraft design. The main benefits often stated are weight saving and strenght. However comparing the Hornet and Eurofighter Typhoon their respective weights are 23500lbs & 22000lbs. The F-15C has an empty weight of roughly 28500lbs & the Raptor is apparently in the 35000lbs+ class, thats within range of a CFT equipped F-15E Strike Eagle. Granted the F-22 is a larger aircraft (wing area/control surfaces) and the integration of an internal weapons bay may have required additional airframe strengthing, however to the uninitiated (read as me) this seems excessive especially considering aircraft with similar G limitations. So is there an appreciable weight saving or have the designers opted for longer airframe life while maintaining similar maximum g-loads?.

Thanks
Obi

johnqhitman wrote:

Happy Gilmore, anychance of using Reinforced Carbom-Carbon for the heatshield?

Couldn't tell you one way or the other, I was told it's like a big sheet of aluminum foil, I was thinking more along the lines of what we saw on the Apollo program on the lunar lander.
You know the wing box and skin of the F-2 is all composite and is made here in F/W, apparently LM got a lot of technology of advanced composites from Japan as a trade off to build the wing.
If any of you have ever seen the composite layup department here it's pretty impressive watching them do their thing.

Isnt the F-2 having terrible structural issues too though due to the composites used in its wings? I heard even in their few short years of service the F-2s have been found to have stress fractures in their wings...

-Aaron

Quite late LM, with their 2n thoughts about carbon fibres. Boeing is constructing their B787 Dreamliner for approx 50% of composites, do they have2nd thougths too?. Carbon fibre has the same strength and stiffness as steel but is 5 times lighter. The problem with carbon fibres is that their good qualities only work when the fibres are orientated in the same direction. This is not easy to realize, maybe nature will provide the inspiration to do this. Nature has millions of years experience to optimise the strength of fibre materials, think about trees, corn, bamboo or even whales. Maybe LockheedMartin must consider composites with an aluminium/titanium/(stainless) steel matrix as applied in Formula 1 racing cars. Or laminates: carbonfibre/aramide (Kevlar/Twaron) combination, this is also used in F1 racing cars; Glare: a combi of 0.3 mm thick sheets Aluminium bonded with glassfibre sheets in an autoclave, Arall: Alu sheets bonded with Kevlar.

Yes, I have heard that too, that the F-2 has tons of problems with its wings, maybe because of changed wingbending moments as carbonfibre is stiffer than aluminium?

Elite 1K Joined: Jul 22, 2005 Posts: 1089 Status: Offline

Happy_Gilmore wrote:

johnqhitman wrote:

Happy Gilmore, anychance of using Reinforced Carbom-Carbon for the heatshield?

Couldn't tell you one way or the other, I was told it's like a big sheet of aluminum foil, I was thinking more along the lines of what we saw on the Apollo program on the lunar lander.

That would be gold. Not really cosmic but still damn expensive Shocked

The XB-70 used it for heat shield material back in the sixties. It's a good conductor of heat but apparently it's a good reflector of radiated heat and stays "shiny".

Enthusiast Joined: Nov 10, 2005 Posts: 39 Status: Offline

As it turns out, graphite composites do NOT provide any weight savings over metallic structures. In the real world, highly integrated composite structures cannot eliminate all stress concentrations due to fitting attachments for actuators and equipment mounting, routing wire harnesses, routing hydro/fuel/ECS tubing and maintenance access panels. Each of those stress concentrations requires heavy doublers to carry the load over the vehicle's fatigue life without failure. After beefing up the composite structure to handle those loads/fatigue, your composite structure is as heavy as metallic structure.

Graphite-epoxy composites do provide SIGNIFICANT assembly labor cost savings when advanced techniques such as VARTM are used, a 20-25% touch labor savings in fact. Boeing is using composites to lower 787 assembly costs.

The alternative is to design metallic structure and have it assembled in China or India for $1/hr.

Elite 1K Joined: Jul 26, 2005 Posts: 1455 Status: Offline

Hmm, got to disagree with you there DJCROSS. Carbon Composites offer huge savings in weight. Some small commercial aircraft have the entire fuselage made of Carbon fibre.
You can tailor composites to suit your needs. First you have a binder, or matrix, a polymer, basically plastics. Then you reinforce it with whatever you want, glass fibres, Kevlar, Boron fibres, Carbon, Ceramics, even metal fibres. You can use many types of polymers to get different characteristics in the material and any combination of reinforcement. Fasteners and high strength parts are simply reinforced composite sections that are stronger than steel and applied as inserts.

Making the stuff was done by hand, increasingly, machines are being used to form more basic shapes, like sheets and beams.
The biggest advantage of carbon composites has always been weight saving, strength and stiffness.They are less likely to crack, in metals, cracks spread, not in composites so much, the fibre structure tends to block this. They are corrosion and chemical resistant too.

The biggest downside is cost, the raw materials are very expensive.
All the major aircraft builders use composites, wings, tails, fuselage, rotor blades, engine casings, the list is long! Cool

This picture is of a Dassault front-end built by BAE systems entirely of composites. This was actually built by a machine.

Enthusiast Joined: Nov 10, 2005 Posts: 39 Status: Offline

Notice that the fuselage section in the picture is a shell that has none of the internal sturcture, nor attachments for equipment, nor cutouts for maintenance access. Once those are added and structure beefed up to handle point loads, you'll find the weight equals or exceeds a similar fuselage built in aluminum. This is one of the reasons why F-35 and F-22 do not have more composites - the weight trade studies did not show a payoff. This is also my personal experience from F-117 tails, X-33 tank structure, U-2 control surcfaces and other projects done in Palmdale. In fact, JAASM was willing to take the weight hit for use of composites in exchange for cheaper touch labor costs on a single-shot munition.

Aero-elastic tailoring?

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Late F-35 revisions - 2nd toughts on carbon fiber ?