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Composites and surface cracking: what to look for.


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Here's another educational topic that I am passing along. I have an 08 CTLS that we use for flight training, and it's at 2100 hours airframe time now. She's had her fair share of bumps, scrapes, window damage, bent gear, etc. Because of the flight training environment, I am pretty scrupulous about watching the skin for cracks, and recognizing what needs to be fixed NOW, and what can wait a while so it's more convenient. I'm going to post some photos comparing various cracking, not all of which will be from airframes (a lot of them will be boats), but the concepts will still be the same.


Composite Primer:


First, our airframes are largely composed of Sandwich Material, using Carbon Fiber and Rohacell cores. It looks a lot like this, but the color of the core can vary, and we actually use thinner materials than pictured. But the basic idea is there. Carbon fiber has INCREDIBLE tensile strength, and decent compressive strength. We don't really want to compress fiber reinforced polymers because of issues with the layers separating, called delamination, so, we use sandwich construction to help spread out the forces. The sandwich core allows greater leverage and some flexibility to the fibers being placed in tension, while the fibers being compressed are relieved. We could also do this by building up a bunch of layers of fiber reinforced polymer, but this is very heavy, uneconomical, and completely overkill. Plus, repairing the material would be a couple order of magnitudes more difficult.


Spar construction is a little different, and I'm not going into it because I am not 100% sure which method flight design uses. I do not wish to speculate on the subject, and it's out of scope of this post anyways.


Identifying surface cracking and recommendations:


This is not an all inclusive list of types of damage! Just like my previous oil post, if I tried to explain everything, it would fill an encyclopedia. I am going over what to look for on the SURFACE, and overview of what causes it, and how critical it is to be repaired.


There is a golden rule to composites though that can be applied everywhere. Just like metal rust, damage to composites will only get worse, and the worse you let it become, the faster the damage accelerates.




Gelcoat Cracking:


We don't really use a "gelcoat" in our CT aircraft, but it still looks the same when it starts to crack. What we use is acrylic urethane paint; it polishes to a shine similarly to acrylic, but has some of the flexibility, and a LOT of the UV protection, of polyurethane paints. We also use a filler material underneath, like bondo, to smooth out the ripples caused by the natural surface of fiber polymers. That stuff is not very flexible at all, so many times you will have paint cracking as a result of the bondo cracking.


Here's an example from a boat of an isolated gelcoat crack:





It's a faint line, and it doesn't really spider or branch out. There's only one or two of them.


Here's another example from another forum user, CharlieTango, in his aircraft:




If I saw these kinds of cracks in my aircraft, I wouldn't really be worried about it. Paint needs to flex with our airframes, and sometimes it flexes just right to separate the paint. It's like a plastic bag: if you pull it apart, it gets really easy once it starts to tear, and paint will do the same thing. Just clean the surface and paint over with some touch up paint, and polish.





Stress cracking:


Stress cracking is a bit more serious. (Again a boat example)




Even though it all looks like gelcoat cracking, I would start to ask "Why does it look like that?". Stress cracking occurs because there is a LOT of flexing going on underneath. Now we need to start considering where it is happening at. If it's on a CT cowling, then it's not a problem, we have very flexible cowlings that can crack the paint when you take them off. But, if it's on the airframe body, then something is wrong with the sandwich underneath, and it needs to be examined. The most common cause is delamination, but it could also be caused by overstressing. The latter of which very rarely comes without other telltale signs, so don't freak out if you only see stress cracking.


I would consider the aircraft IMMEDIATELY unairworthy if this is found in a composite prop. A single small gelcoat crack isn't much to worry about in a composite prop, but keep an eye on it. If it starts developing more gelcoat cracks, especially if they seem to be centered around a particular area, or the gelcoat crack keeps getting bigger, then you need to send out the prop for inspection and overhaul.




Gelcoat cracking between stress points:


I don't know how to label these kinds of cracks, so I'll let it explain itself.





These kinds of cracks aren't all that uncommon, and they normally aren't a concern. They usually occur between areas that have bolts going through the frame in close proximity, like above. I had some cracking going on between the two bolts under my airplane's tail that is part of the elevator assembly. We sanded down the paint and it was just the bondo causing the problem, there was no airframe cracking. One thing of note though, just make sure the bolts are still tight. If they are loose or spongy when you check the torque, NOW that's a problem. Remove the bolt and look in the hole for deterioration. Loose bolts means that it's been jostling around in the hole and wearing on the material, and both loose/spongy could mean sandwich material breakdown. Just make sure that if it feels spongy, that there isn't a rubber mount or padding that the bolt goes through, or you are going to feel silly :P.






When layers separate their bond, we call this delamination. Delamination is serious and should not be ignored in critical areas. Unfortunately, delamination usually doesn't show itself until it gets bad. It's the cancer of composites, the sooner you catch it, the better.


This is what delamination on the surface looks like between overlapping layers:




This was CharlieTango's aircraft. If you look closely, you can see a little tuft of fiberglass poking up through the main crack in the middle. Fortunately though, the only fiberglass used near a window in CT aircraft is a 3-4 inch wide fiberglass overlapping part of the window and the skin, used to help smooth out the transition for a nice flowing paint surface.


Delamination can also be indicated by Stress Cracking, as explained earlier in this post.



Composite cracks:


These need to be addressed as soon as practical, and are an emergency if you see then in propellers and spars. Here's a photo from a boat:





This is the most serious type of surface damage. An open crack means that dirt, water, and solvents can reach the core. They also set up stress risers that will cause the crack to grow very quickly under stress. If you cannot address a composite crack right away, then take some epoxy and shoot it into the crack to seal it. Core breakdown isn't a joke. It will destroy a part if left unchecked.



Core Breakdown:


I get it, it's not a surface crack. It's still worth mentioning. Core breakdown is the worst damage you can have, short of catastrophic failure.





Floats here on this forum had this problem, and Flight Design was kind enough to step up and give him a new stabilator free of charge. Without a core, composite sandwich material loses almost all of it's structural strength.


If you see evidence of core breakdown on your airplane, get it to a shop NOW so they can drill out the affected areas. Core breakdown is usually the result of chemicals getting into the airframe core, and eating it away. Look for cracks that it might have gotten into.


Also, as paint ages, it becomes porous. This too can let chemicals seep past the paint and into the parts. Waxing your plane once in a while is very good practice, because it fills the pores and protects the paint, and by proxy, your airplane's skin.






If anyone was any suggestions or questions, as always, feel free to reply!

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I have been using the recommended ComposiClean Spray Wax and ComposiClean Polish which are expensive to order due to single source and shipping costs.


I also use microfiber cloths exclusively.


Are there other just as good or even better cleaners that are a bit more common and eaiser to get?

I think the main issue with choice of cleanser is to ensure you get one with the appropriate pH as anything too acid or too alkaline can cause problems with the composite materials.


The Flight Design literature recommends the Composiclean stuff so I've stuck with that, even though I had to get it shipped from the US. I took the view that having paid over £100,000 for the aeroplane, then potentially damaging it with the wrong cleaner for the sake of a few pounds would be a rather dumb choice!

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My composite Sky Arrow POH just calls for "water and a neutral automotive detergent".


The composite Cirrus SR22 POH just says: "The airplane should be washed with a mild soap and water".


I never had the impression that either was that finicky.


I just used any standard car wash that I had around on both, so far to no ill effect.


Don't know why the CT should be radically different if made with normal composites and paints.

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I have been using the recommended ComposiClean Spray Wax and ComposiClean Polish which are expensive to order due to single source and shipping costs.


I also use microfiber cloths exclusively.


Are there other just as good or even better cleaners that are a bit more common and eaiser to get?


I used to get these products from the CT dealer in Minden.  http://www.chemicalguys.com/category_s/129.htm


The clay bar was amazing especially on my warp drive prop.

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From the Maintenance & Inspection Procedures Manual


"The Rohacell foam, while highly resistant to fuel, is not resistant to strong Alkali cleaners or even water with very high alkali content. Therefore Flight Design requires that the cleaners used on the CTLS be pH neutral. Cleaners, such as Fantastic®, Formula 409®, Carbonex® and Castrol Super Clean®, which are otherwise good Alkali cleaning products, should not be used on the CTLS"

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Did I miss something?  I didn't see anything about the CT being "radically different"... 'sounded pretty generic to me.




I read between the lines that the CT was highly sensitive to the type of cleaner used, to the extent that Ian spent a lot just to get one specific cleaner recommended by FD shipped from the US. Owners of other composite planes don't generally worry very much about what kind of soap/detergent they wash their planes with.


If you're saying that the CT is like other composite planes, picking a soap/detergent should be pretty straightforward.


Unless it's really strong, you're washing it off in very short order anyway.

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A lot of cleaners for cars are not alkaline because clear coat is sensitive to it. If you were to use dawn dish detergent (which is highly alkaline) on a car, the clear coat would haze.


A good cleaner to use would be those that both wash and wax, that is as close to pH neutral as possible (composiclean's bucket wash also has wax). If you have to choose though, mildly basic or acidic is fine too. Remember that some compounds in gasoline are acidic, or form acids when mixed with water.


EDIT: As a tip, look up the MSDS for a cleaner. I was surprised to find out that meguiar's "ph neutral" cleaners are almost a pH of 8. It's still an extremely mild alkaline though, tap water is usually between 6.5 to 8.5 pH.

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I think the lesson to learn is that you have to give a little thought to the cleaner.  Is it overly acidic or alkaline?  Does it have ingredients that could harm the paint?  A degreasing cleaner may have other things in them that are harmful.  I know plenty of people that use vinegar as their go-to household cleaner.


For the record, I've tried to contact Composiclean and it's CEO Ken Godin a dozen times, offering them our forum as a free place to discuss their products, and discuss the care and feeding of composite aircraft.  I'm not sure if his lack of response has anything to do with his exit at FD, and subsequent stint at Remos.





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I have not found Composiclean to be unreasonably expensive, and it works great. I buy it through Aircraft Spruce.

I agree, and I got mine from Aircraft Spruce, though they don't sell directly in the UK, they have an arrangement with a UK company who makes their catalogue available then amalgamates shipments to keep the price down.

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