Carbon Fiber Fuel Tanks: Possible?

Ok, this idea might be out there, but has anybody ever thought about building carbon fiber fuel tanks for their RV? Is this feasible? I got the idea from watching a video on building the hood of a car out of carbon fiber using moulds.

I thought that you could make a mould from a built tank. This would allow one to get the exact shape of the skin, then install the ribs, guts, then the back of tank?

I figure that since the tank is screwed in place that this could be possible. I imagine that it would be a stretch to do the outboard lead edge since it is riveted in place. Talk about a leak free tanks.

Would this be completely crazy? I know that there would/could be some W/B issues.

Definitely possible, but expensive, and a whole lot more work
I designed carbon fiber structures for McDonnell Douglas a while back (well before it became Boeing).
You'd need to put a layer of fiberglass along any surface that touches aluminum to prevent galvanic issues.
The skins would be more susceptible to impact damage than you might expect, and the exterior finish would be dependent on the quality of the mold. How much did you enjoy finishing your cowl and fairings?
Making the ribs and bonding them to the skins properly is not a trivial task, they are structural after all!
And finally, you'd have to use a resin that is impervious to fuel, and cure it properly, which may require an oven.
An interesting proposition though...

Too much corrosion

---- insert ---
[FONT=Arial, Geneva, Helvetica, sans-serif]For example, graphite fibers, which are used to reinforce some plastic structure, present a particularly challenging galvanic corrosion combination. The fibers are good electrical conductors and they produce a large galvanic potential with the aluminum alloys used in airplane structure.
--- end insert ---

The above Quote was taken from:
http://www.boeing.com/commercial/aeromagazine/aero_07/corrosn.html

Bottom line, Carbon Fiber and Aluminum in contact with each other will cause galvanic corrosion. Carbon Fiber should never come in contact with an Aluminum aircraft structure.


[/FONT]

It's possible, but a bit of research will probably show it to be frought with engineering challenges beyond the fabrication. It's also very tough to beat aluminum for weight in structures this small. So what would be the goal? To make leak free tanks? Plenty of those in aluminum flying around.

Many experimentals have composite tanks. My Longeze has fiberglass tanks. I would think that carbon fiber would work. But, other than the cool factor do not think it would be a major advantage over fiberglass. The problem is with the epoxy, fine for 100ll but motor fuel with ethanol will soften the structure. So if you ever think of using motor fuel, think ethanol fee

Gisnar said:

The problem is with the epoxy, fine for 100ll but motor fuel with ethanol will soften the structure. So if you ever think of using motor fuel, think ethanol fee

Click to expand...

While we're in the "no free lunch" mode, note that the standard for polysulfide sealants (AMS-S-8802B) does not require it to withstand any kind of gasoline....not even 100LL. The cert tests are done using JRF (Jet Reference Fuel).

JFR

No fuel expert here but a quick google search uncovered some interesting info.

JFR = Jet Reference Fluid. "To ensure reliable and reproducible results when determining the fuel resistance of materials, reference fluids of known composition, using worst case fuel compositions are used." see

http://standards.sae.org/air4275/

From what I can determine, the concept of JFR was developed in the 1950's to test fuel tank sealants for the their mysterious "chalking" phenomenon ("chalk" forming inside fuel tanks containing JP4 and possibly other similar fuels.) Not necessarily designed to test sealant for its ability to resist ALL chemicals (for leaks) found in all gasoline. However, since JP4 is a 50/50 mix of gasoline and kerosene, one would think that a sealant tested with JFR (and passed) would be suitable for our applications. BUT the recipe for 1950 gasoline (to be used for JP4) may not have contained the modern additives we have today in gasoline such as ethanol.

Bevan

Fuel contamination with fibers

I read NTSB reports of fuel starvation because fiberglass material clogged the fuel filter! Carbon fiber I'm sure would do the same if the building process was not adequate.

I did not say I was going to do this. I was wondering if it was possible.

If it's the "look" of carbon fibre you're after, why not wrap the tank with automotive vinyl wrap like this...

http://www.amazon.com/3M-Scotchprin...99&sr=8-3&keywords=carbon+fiber+wrap+3m+white

Comes in black too but I think we should avoid dark colors on fuel tanks.

Bevan

Gisnar said:

Many experimentals have composite tanks. My Longeze has fiberglass tanks.

Click to expand...

Excellent application for an EZ. In a bizarro world, would you replace them with aluminum? Probably not.
I don't have issue with the carbon or the epoxy itself. I think the challenge is in the interface to the existing RV aluminum structure since it is not designed for attachment of composite tanks. It is a significant load carrying joint that needs to stay "good" and not have corrosion issues.

rzbill said:

Excellent application for an EZ. In a bizarro world, would you replace them with aluminum? Probably not.
I don't have issue with the carbon or the epoxy itself. I think the challenge is in the interface to the existing RV aluminum structure since it is not designed for attachment of composite tanks. It is a significant load carrying joint that needs to stay "good" and not have corrosion issues.

Click to expand...

^^^^ Exactly this. I thought about composite fuel tanks for my -14 for about 5 minutes but dismissed the idea as impractical. I've done a lot of composite work; building the tanks would be easy, but the primary structure isn't a trivial task and would require a lot of engineering to cover structural and thermal interface aspects.

Anything is possible, but I wouldn't tackle it.

Actually, this idea does stem from the vinyl wrapping thread currently in the general discussion. I was poking around youtube to see how wrap was put on and happened upon the video on building the hood of a car.

I do like the carbon fiber look and I sort of like the idea of wrapping the plane instead of painting. I might try out wrapping parts of my car to see how challenging it is and to see how it holds up to the weather.

Certainly possible to build strong, leak proof, fuel resistant and long lasting fuel tanks from composite materials. As already indicated, there are pros and cons to be considered:
Weight – they can be built light but careful thought and testing required to come up with a suitable structure and manufacturing process that will at least equal what you would be replacing. Not something to be taken lightly.

Fuel resistance – Fire retardant vinyl ester would be my choice. Glasair used to use a Derakane product for their earlier designs. Not sure what they use these days. We built up an early Glasair1 kit for a customer back in the 1990’s and that aircraft is still flying today with no fuel tank issues that I am aware of. It used hand laid vinyl ester cured at room temp.

Strength- A well designed structure would be very strong. We make carbon fibre fertilizer tanks and hoppers (vinyl ester infused and cured at room temp.) that required testing for certification. Cannot recall the exact figures but one of the requirements was that the tank had to be able to keep over a ton of liquid or powder contained in the event of a survivable crash. They passed the various pressure and drop tests.

Bonding- With the tanks/hoppers above we use the same resin with a thickening agent and backed up with carbon cloth strips over the joins to bond the parts together. There are also various bonding agents available that will do the job just as well.

Corrosion- Some sort of barrier between carbon fibre and aluminium is a must.

Appearance- Done properly the tanks would be nice and smooth looking with no fuel leak issues. Developing the tooling to achieve this would be time consuming and expensive though.

As I see it the real deal breaker for this is mounting the tanks to the wing. Composite structures cannot be dimpled for screws etc. They must be countersunk which means the mounting flange would need to be thick enough to achieve this and be strong enough to support the loads. The resulting flanges would be quite a bit thicker than the aluminium skins. Not a good look, and I don’t know what this would do to the airflow over the wing in this area. Go with carbon wrap if you want the carbon look.