Cotronics Leading Edges

[JimJarvis50]

I like to coat the leading edges of my fins with Contronics 4525 epoxy. If provides a very hard protective layer that has some heat resistance. I have used this on rockets at speeds up to Mach 3 with no apparent deterioration of the coating.

One of the problems with the epoxy, though, is that it is difficult to apply to get a smooth coating. If the material is brushed on, the resulting layer can be very uneven, and once the epoxy cures, it is impossible to sand.

In my most recent application of the epoxy, I used a foam roller to "brush out" the epoxy. By working the epoxy for a minute or so with the roller, I was able to get a smooth layer and a reasonable final finish.

Pic 1 shows the starting epoxy with some milled glass added to the epoxy. This is 1/16" material. I don't think I'd use anything longer than that. Pic 2 shows a small amount of epoxy applied to the leading edge. As shown in Pic 3, the layer is initially very gloppy from the milled glass. However, if you work on it for a minute or so with the roller, it can be rolled smooth. This would not be possible using a brush. The surface after using the roller is in Pic 4.

(Continued)

 

[JimJarvis50]

(Continued)

The roller leaves a bit of a rough finish. I usually use a foam brush to lightly smooth the surface of the epoxy shortly after applying it. The resulting finish is in Pic 1.

The finish of the epoxy can be further smoothed after the epoxy partially cures. I believe the optimum time for this is after 36 hours at room temperature. At that point, the finish is hard enough that it can be sanded, but soft enough that it can still be sanded. After this sanding (Pic 2), additional layers can be applied. I typically start the first layer at about have of the final width and make each layer just a little wider. This is so there isn't much of a ledge at the transition from the Cotronics to the fin surface. For this rocket, I applied five layers.

After the final layer is sanded, the fin should be heat treated. Ideally, the epoxy calls for an hour at 250F and an hour at 350F. I don't have the capability to do that, so I cure at about 200F for several hours. After that, the surface can be wiped with a thin layer of laminating epoxy to shine it up a little (Pic 3). The final results for the two rockets I treated are in Pic 4.

Jim

 

[mpitfield]

Hi Jim,

Remind me again on your process/jig that you use to get the leading edges prepared for the epoxy. That leading edge of CF is, visually, perfectly lined up and even with the leading edge of the airfoil.

 

[JimJarvis50]

Hi Jim,

Remind me again on your process/jig that you use to get the leading edges prepared for the epoxy. That leading edge of CF is, visually, perfectly lined up and even with the leading edge of the airfoil.

I think it's mainly just by eye, but I do have this little jig that I use to initially sand the leading edge to a consistent width. It has a few miles on it.

Jim

 

[mpitfield]

I think it's mainly just by eye, but I do have this little jig that I use to initially sand the leading edge to a consistent width. It has a few miles on it.

Jim

Well I know your vision is not going...good eye. I will try to make my own, thanks.

That work surface looks like it has seen a bit of activity as well.

 

[JimJarvis50]

That work surface looks like it has seen a bit of activity as well.

But I could tell you what every gouge, hole and mark is and how it got there.

Jim

 

[watheyak]

Hi Jim, I know this thread is a few years old, but it's still a valuable resource.

I am about to give Cotronics 4525 leading edges a try for the first time. My question is about the milled glass fibers. What are their purpose? I have some from Fiberglast that I think are 1/32" that I'll mix in, but I'd like to be able to explain the why's and what for's.

Thanks.

 

[ksaves2]

Man oh man, you've done a really great job. I've found over the past 15 years 4525(b) has changed a bit as to how it cures up. Works fine though no matter what. Some of the earlier batches I bought sanded up ok and the later ones took a bit more work. One has to remember if they invest in it, it technically has a limited shelf life but I've used "expired" 4525(b) on modrocs and it worked fine. For casual readers this epoxy costs a fair amount of money to buy and really isn't needed for sport flying.

I would have thought it would take a "wider" coating on the leading edge to provide protection. Have you found any evidence of erosion of the coating with high speed flight? I ask out of curiosity as I haven't gone there. Thanks for posting a great question. Kurt

 

[watheyak]

Absent any input from Jim I'm going to do a couple experiments based on a a collection of comments here on TRF and other places.

I'll report back!

 

[JimJarvis50]

Hi Jim, I know this thread is a few years old, but it's still a valuable resource.

I am about to give Cotronics 4525 leading edges a try for the first time. My question is about the milled glass fibers. What are their purpose? I have some from Fiberglast that I think are 1/32" that I'll mix in, but I'd like to be able to explain the why's and what for's.

Thanks.

I don't really know if the glass fibers do any good or not. I haven't done any testing in particular. The cotronics tends to be a little brittle, so maybe the fibers keep things together? Just experiment and see what you like. If you use the fibers, be sure to brush out the clumps with a roller. I have flown up to around Mach 3 with this approach and I have never observed any damage or change in appearance of the coating. But I wouldn't recommend going above that, at least if ground-launched.

Jim