Carbon Fiber Panels - Touching Aluminum Frame

[D Egan]

Hello Group, 10-year EAA member - but brand new to the Forum Site here. (My return key doesn't appear to work, so apology for the run-on format) We are looking for Builders' wisdom on how to properly protect & separate carbon fiber panels from aluminum airframe structures. We understand that if carbon fiber panels come in direct contact with aluminum that more active "galvanic" material (the aluminum airframe) would suffer significant aggravated corrosion. We heard of possibly using tape or even layers of primer coat, etc. (1) Is this true? (2) Has anyone dealt with this? (3) What is the best method of separating the two materials, like for floorboards which will inevitably move and shift somewhat in flight and use? (4) Does this concern also apply to the stainless steel fasteners (bolts) that would pass through the carbon fiber into the aluminum? Do the fasteners need to be separated from the carbon fiber in some way also? - Dale, Milwaukee, Wisconsin

[Matt Gonitzke]

Put a fiberglass isolation ply on the side of the carbon fiber laminate that is contacting the aluminum. That's what's done with large aircraft.

[AcroGimp]

OEM's usually use a single/double prime on the AL part and a fiberglass layer over the CFRP, which is also sometimes primed.

AL to CFRP has the worst galvanic potential evaluated from a corrosion protection standpoint, it is important to deal with it.

Personally, I'd go single-prime on the AL part and a fiberglass layer over the carbon part - and make sure you check for corrosion as part of your conditional inspection.

For fasteners, we usually use passivated (e.g., cad plated, etc.) or corrosion resistant steels for removable fasteners (bolts/screws), and either passivated or wet install for rivets and the like.

'Gimp (Consultant Reliability, Maintainability & Safety Engineer)

[rleffler]

Also remember that most carbon fiber panels are encapsulated in epoxy, with additional finish coats, such as paint or just a simple clear coat.

[D Egan]

Thanks much or the input and detail. These panels could also be made out of simple aircraft-grade plywood; do you feel that would be wiser? (We are not concerned with the higher cost or effort with carbon fiber. Our primary goal with carbon fiber would be weight savings.) However, the potential for any protective material to 'rub away' causing the two materials to come in contact and corrode aggressively over time concerns us. What are your thoughts compared to plywood?

[rleffler]

Thanks much or the input and detail. These panels could also be made out of simple aircraft-grade plywood; do you feel that would be wiser? (We are not concerned with the higher cost or effort with carbon fiber. Our primary goal with carbon fiber would be weight savings.) However, the potential for any protective material to 'rub away' causing the two materials to come in contact and corrode aggressively over time concerns us. What are your thoughts compared to plywood?

Your wings could fall off in flight too, but the odds of that happening are almost nil. I would think the odds would be the same for any carbon fiber product installed properly.

Carbon fiber is not only used for weight savings, but strength too. I have a carbon fiber panel in my RV-10 and I don't believe that plywood would be strong enough to support my panel layout unless it was 1/2" thick or more. (too many holes) I would install an aluminum panel if carbon fiber wasn't an option.

In my particular case, the aluminum fuselage is primed AKZO epoxy primer and top coated. The carbon fiber is incapsulated in epoxy resin, then primed, and top coat applied. It would take quite a bit of rubbing to wear through all those layers with a panel that is rigidly installed to the fuselage (movement is severely restricted)

If this really keeps you up late at night, put it on your conditional or annual inspection checklist.

[D Egan]

Good points. Thanks for putting it in context. I guess as I read - the epoxy resin / fiberglass / primer is a thick barrier. Ok - thank you. We are going to build up one full section, add the resin and primer coats, and see what weight comparisons we find.

[Bob H]

The normal aerospace method for galvanic isolation is a layer of glass on the carbon to separate from alum. If faying/contact surfaces will be rubbing, any painted coating will eventually wear thru and allow electrical contact and eventual corrosion of the alum. Preferred fastener materials are clearance fit titanium or A-286 SS which won't react. These are expensive for homebuilders but give the best results.
You don't ever want to use any bucked rivets, no matter the material, as the radial swelling of rivet will preload fibers at hole surface and cause lower bearing/shear allowables.
Bob H

[RV8505]

Your wings could fall off in flight too, but the odds of that happening are almost nil. I would think the odds would be the same for any carbon fiber product installed properly.

Carbon fiber is not only used for weight savings, but strength too. I have a carbon fiber panel in my RV-10 and I don't believe that plywood would be strong enough to support my panel layout unless it was 1/2" thick or more. (too many holes) I would install an aluminum panel if carbon fiber wasn't an option.

In my particular case, the aluminum fuselage is primed AKZO epoxy primer and top coated. The carbon fiber is incapsulated in epoxy resin, then primed, and top coat applied. It would take quite a bit of rubbing to wear through all those layers with a panel that is rigidly installed to the fuselage (movement is severely restricted)

If this really keeps you up late at night, put it on your conditional or annual inspection checklist.


Did you think about the fasteners going thru the carbon to secure the panel and instruments. I am sure it is touching somewhere.

[c322348]

We understand that if carbon fiber panels come in direct contact with aluminum that more active "galvanic" material (the aluminum airframe) would suffer significant aggravated corrosion.

Check out a galvanic corrosion chart where the potential voltage indicates how likely corrosion is to occur:

http://www.thelen.us/1galv.php

If aluminum is -1.670 and carbon is +.810, the potential is 2.48. The potential between aluminum and cadmium is about 1/2 that, at 1.268.

Titanium pins and bolts are often used with aluminum in large aerospace structures. The potential there is 1.725, but 7000 series aluminum alloys are somewhat worse than 2000 series alloys. In most large aerospace structures fasteners are installet "wet" using sealant to keep electrolytes (moisture) out. BTW, monel "pop" rivets as used in some homebuilt aircraft have more galvanic potential with aluminum than titanium. Also, an "upgrade" for spam can owners is to install stainless steel screws to hold on fairings in aluminum aircraft. This is a "Bozo no no" in the aerospace industry because of the galvanic potential, yet it is commonly done in GA. A cadmium plated carbon steel screw, like came from the factory, would be much better.

The bottom line here is that aluminum corrodes with aplomb in seaplanes exposed to even fresh water without being in contact with dissimilar materials. It depends upon your environment.

The standard aerospace method is to add a fiberglass corrosion barrier ply to the carbon fiber panel and use a titanium screw. The aluminum would be coated with primer and topcoat.

In real life, a carbon fiber panel in a dry interior space in a homebuilt isn't at much risk, in my opinion. A external carbon fiber fairing on an aluminum stabilizer used in a seaplane would be much more worrisome. Airliners and cargo aircraft that see lots of cycles in inhospitable weather are where most of these measures are most needed.

[RV8505]

Check out a galvanic corrosion chart where the potential voltage indicates how likely corrosion is to occur:

http://www.thelen.us/1galv.php

If aluminum is -1.670 and carbon is +.810, the potential is 2.48. The potential between aluminum and cadmium is about 1/2 that, at 1.268.

Titanium pins and bolts are often used with aluminum in large aerospace structures. The potential there is 1.725, but 7000 series aluminum alloys are somewhat worse than 2000 series alloys. In most large aerospace structures fasteners are installet "wet" using sealant to keep electrolytes (moisture) out. BTW, monel "pop" rivets as used in some homebuilt aircraft have more galvanic potential with aluminum than titanium. Also, an "upgrade" for spam can owners is to install stainless steel screws to hold on fairings in aluminum aircraft. This is a "Bozo no no" in the aerospace industry because of the galvanic potential, yet it is commonly done in GA. A cadmium plated carbon steel screw, like came from the factory, would be much better.

The bottom line here is that aluminum corrodes with aplomb in seaplanes exposed to even fresh water without being in contact with dissimilar materials. It depends upon your environment.

The standard aerospace method is to add a fiberglass corrosion barrier ply to the carbon fiber panel and use a titanium screw. The aluminum would be coated with primer and topcoat.

In real life, a carbon fiber panel in a dry interior space in a homebuilt isn't at much risk, in my opinion. A external carbon fiber fairing on an aluminum stabilizer used in a seaplane would be much more worrisome. Airliners and cargo aircraft that see lots of cycles in inhospitable weather are where most of these measures are most needed.

Good post! I don't know why anyone would vary from the plans to add carbon to a aluminum aircraft as the benefits would be negligible and it would just introduce another pontenal problem down the road. I just had to retake a corrosion control class at work and aluminum and carbon are about as far apart on the chart as you can get.

[WLIU]

Part of the answer to your question is that there are a lot of compound curved components on aluminum airplanes that get done in composite these days. The list includes wing tips, cowlings, spinner assemblies, canopy structure, landing gear doors, wheel pants, etc. All are much easier to construct using composites vs traditional materiels. And since weight control is a primary focus of the design build process, many of these parts and others can in fact be built lighter using carbon.

An example is the Extra series of airplanes that are designed so that the entire top of the fuselage is composite and may be removed in two pieces for inspection and maintenance of the inside of the airplane. This would really not be practical in aluminum and even if done would be heavier than the composite used. And being able lean over the side of the airplane into a large open space to inspect and lube rather than crawl into the tail cone after removing the pilot seat obviously makes service easier which means better inspection and service. So looking at what you might call a holistic view of the aircraft design, working around the potential for corrosion that comes with carbon is an overall win.

Best of luck,

Wes
N78PS

[Bob H]

This galvanic potential doesn't cause an immediate pitting; it does take time to develop and it depends on the moisture present to act as an electrolyte. If you separate the alum and carbon with almost any kind of electrical barrier, it stops the reaction. You just want the barrier to be as permanent as possible in the operating situation. Glass on carbon is usually preferred for longevity but a tenacious paint would do the same as long as it doesn't rub thru by fretting.

The reason aluminum corrodes in plain moisture exposure (seaplane above) is because the heat treated aluminum alloys have microscopic precipitates of copper, zinc or magnesium which are dissimilar from the basic alum. So moisture makes a microscopic battery on the alum surface and it gradually corrodes. By applying a layer of pure aluminum on the alloy surface (cladding), the pure cladding has no dissimilar ingredients and no battery reactions.

[RV8505]

Part of the answer to your question is that there are a lot of compound curved components on aluminum airplanes that get done in composite these days. The list includes wing tips, cowlings, spinner assemblies, canopy structure, landing gear doors, wheel pants, etc. All are much easier to construct using composites vs traditional materiels. And since weight control is a primary focus of the design build process, many of these parts and others can in fact be built lighter using carbon.

An example is the Extra series of airplanes that are designed so that the entire top of the fuselage is composite and may be removed in two pieces for inspection and maintenance of the inside of the airplane. This would really not be practical in aluminum and even if done would be heavier than the composite used. And being able lean over the side of the airplane into a large open space to inspect and lube rather than crawl into the tail cone after removing the pilot seat obviously makes service easier which means better inspection and service. So looking at what you might call a holistic view of the aircraft design, working around the potential for corrosion that comes with carbon is an overall win.

Best of luck,

Wes
N78PS


I understand all of that but some builders want to use carbon for the cool factor for a straight sheet panel. I just don't uderstand why thy would want to deviate from the plans and add a future potential problem.. It is hard enough treating corrosion as it is. I worked for a major airline in the composite shop and now I treat and repair corrosion on navy aircraft and it is bad enough treating aluminum on aluminum. Once stared it is the gift that keeps on giving. There some very informed people posting on here with very good advice.

[WLIU]

I think that you are comparing apples and oranges. 99% of homebuilt aircraft will not see as much exposure to corrosive environments as one navy aircraft sees in one day at sea.

Carbon fiber components offer more advantages than disadvantages. Example, I built a seat that weighs half what the original seat weighs and is stronger. The manufacturing techniques mentioned in previous posts are easy to do and reduce the occurrence of corrosion to near zero in the environment my airplane lives in. In any case, fasteners like stainless steel machine screws are consumables that it is easy to discard at annual time and replace with new. The heads get torn up and they need to be replaced regularly anyway.

Should the material be used with some thought? Absolutely! But the advantages of the material are so great that the homebuilder community will learn how to use it. Over at the homebuiltairplanes.com forum there is a whole sub-forum for folks learning how to engineer their carbon airplanes. It is the new frontier for homebuilders who want to try working at the state of the art.

I wonder what the gallery told Burt Rutan when he showed them his first all composite Varieze? Likely something like what Wilbur and Orville heard.

Best of luck,

Wes
N78PS

[Bill Greenwood]

I can just see it at creation if there was an EAA homebuilder present.

The Lord is just finishing up Adam and Eve when the EAA guy comes up. "What ya making there? People, I call them human beings. Oh, looks interesting, what are ya using to cover them, Stits or cotton? No, it's a new thing called skin. It's pretty good, it's flexible, can hold in a lot of liquid if not punctured and can even repair itself when it suffers minor damage. And some if it is attractive, particularly the future models of Eve that I've got in mind. Well, I don't know, looks pretty simple to me, really just like an animal without the fur.
Couldn't you come up with something more high tech?
High tech, eh? Actually, I hear that that is something that Lucifer is working on. "

[rleffler]

There are many builders using carbon fiber products like Wes has elogantly described. For example.....

This is an Aerosport Products (http://aerosportproducts.com)carbon fiber overhead console. This photo is actuall my cabin cover for my RV-10. The first is the first fitting in the cabin cover and the second is after both are finished and installed in the aircraft. No aluminun touching except for the two inserts. I have no concerns about movement rubbing the paint and epoxy coatings off to cause corrosion.


3511

3513

This is an Aerosport Products Carbon Fiber Symetrical panel install in Rob Hickman's (Advanced Flight Systems) RV-10.


3512


This is a center arm rest in Geoff Comb's (Aerosport Products) RV-10.


3514


Here's a RV-7 carbon fiber panel that you may have seen in AFS's booth at Airventure.

3515

While this may come off as an advertisement for Aerosport Products, I wanted to use some products that I was familiar with since I have all of these installed in my RV-10 as well. My intent wasn't to talk about all the great carbon fiber products that are made by Aerosport Products, but to illustrate that there are very good applications of carbon fiber in the homebuilt market. I also doubt that any of their wings are about to fall off due to corrosion any day soon. :)

bob

[Bob H]

Going back 45 yrs or more when working at Douglas, the original applications of graphite/epoxy (carbon) were direct substitutes for aluminum called "black aluminum" which resulted in much more expensive structure for no real engineering benefit. As the manufacturing technology improved, we were able to build substructure with integral stiffeners and reduce the parts count significantly and reduce/eliminate the secondary labor required to join detail parts. Plus the elimination of all the paperwork and storage for each clip and detail part. That was the real advantage over aluminum. As designs matured and metal forming constraints disappeared, the use of composites allowed tailored stiffening and shapes not possible with metals. The goal was to build commercial transports at lower manufacturing cost over aluminum and provide a weight savings in the process. The 787 was the result of all that work, although it will take time to absorb the up-front tooling costs over traditional aluminum construction.
During these years of development, the problems of fastening, bonding, corrosion, UV exposure, fatigue, NDI, were gradually defined and solved. Now composites are being used for decorative purposes because the fiber/resin cost has been reduced and home builders can afford these parts on their planes. I guess we were finally successful.
Bob H