![]() Stress cracking occurs because there is a LOT of flexing going on underneath. ![]() (Again a boat example)Įven though it all looks like gelcoat cracking, I would start to ask "Why does it look like that?". Just clean the surface and paint over with some touch up paint, and polish. 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. Paint needs to flex with our airframes, and sometimes it flexes just right to separate the paint. If I saw these kinds of cracks in my aircraft, I wouldn't really be worried about it. Here's another example from another forum user, CharlieTango, in his aircraft: It's a faint line, and it doesn't really spider or branch out. Here's an example from a boat of an isolated gelcoat crack: That stuff is not very flexible at all, so many times you will have paint cracking as a result of the bondo cracking. We also use a filler material underneath, like bondo, to smooth out the ripples caused by the natural surface of fiber polymers. 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 don't really use a "gelcoat" in our CT aircraft, but it still looks the same when it starts to crack. Just like metal rust, damage to composites will only get worse, and the worse you let it become, the faster the damage accelerates. There is a golden rule to composites though that can be applied everywhere. 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. 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. Identifying surface cracking and recommendations: I do not wish to speculate on the subject, and it's out of scope of this post anyways. Spar construction is a little different, and I'm not going into it because I am not 100% sure which method flight design uses. Plus, repairing the material would be a couple order of magnitudes more difficult. 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. The sandwich core allows greater leverage and some flexibility to the fibers being placed in tension, while the fibers being compressed are relieved. 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. ![]() Carbon fiber has INCREDIBLE tensile strength, and decent compressive strength. It looks a lot like this, but the color of the core can vary, and we actually use thinner materials than pictured. 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.įirst, our airframes are largely composed of Sandwich Material, using Carbon Fiber and Rohacell cores. 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. She's had her fair share of bumps, scrapes, window damage, bent gear, etc. I have an 08 CTLS that we use for flight training, and it's at 2100 hours airframe time now. Here's another educational topic that I am passing along.
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