pmccoy

pmccoy

Well Known Member
I've done it, bummer. The canopy is now officially cracked. I was using the countersink bit on the top spine when I noticed a problem. I now have three or four holes that look like the one below. Very small hairline cracks, moving about 1/16 to 1/8" out from the hole.



I called Vans. They said to use Weld-On to wick into the cracks and glue them shut. I also called Jeff from Airplane Plastics, since Vans gave me his number, and he said it would be better to drill out the holes large enough to remove the crack. There should be plenty of holes left to secure the canopy, and that would remove the risk of further cracking from the Weld-On joint.

So now I have conflicting advice. What do you think? Glue with Weld-On, or drill them out bigger???
 
Peter-
Sorry to see that. It's hard to know if those cracks will ever propogate, but I guess it's better to do something about them now, rather than wait.

I don't know anything about Van's suggested fix. Might work. Drilling the hole out will, from the looks of the picture--I think, require a much bigger hole than you'll have a good rivet for, although you could use some vinyl or surgical tubing in the oversized hole as some have done trying to forestall canopy cracks. See archives for this canopy construction method.

My thought would be to drill that hole as large as necessary to eliminate the cracks. If you haven't already drilled the aluminum strip, I'd then abandon that hole and drill a couple extra holes forward and aft of that location. Then, before putting the aluminum strip on, fill your problem canopy hole with some type of sealant (proseal or lexel would work great) and then lay the aluminum strip over it. Aside from the irregular rivet pattern on top of the canopy (which no one would see once painted), I think you'd be good to go.

If you have already drilled the aluminum strip, just order a new strip and match drill all the holes (except for your problem hole) from the old strip to the new one.

Just my initial thought. Good luck with whatever you do. I was pretty stressed with all that drilling and cutting too. I'm just glad (knock on wood) that it's over!
 
Crack

I can't really tell from the picture. Is that a crack? Or a scratch? Is it all the way through?
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Yes, it is cracked. At the top of the hole, two small cracks extend out from 11 & 1 on the clock. Very small, but still significant. I looked at all the remaining holes with a magnifying glass, and found a total of seven holes that have these small cracks. All of them are on the top spine of a slider. If I drill them out to a larger size, the cover strip (c-653) will hide the holes.
 
What'd you use to countersink?

What did you use to make those countersinks? Do you think that is what caused the cracks?

I'm going to try those special bits from Avery, since I've already paid for a new canopy, but that's another story...

Thanks,
Greg
 
A thought

May I suggest that if you do drill out enough to clear the cracks, you do so extremely cautiously.

I can just see the drill catching/chattering in the current crack-----wont take much of a snag----and causing an even worse crack.:eek:

I would think about stabilizing the existing small cracks with glue, then drilling.

If you have any scrap, might try a practice run or two.
 
Pete, I'm guessing you used a standard countersink bit, the kind with three or four straight cutting edges. They chatter on plexiglass. Take a close look with a 10x magnifier. Instead of a conical cut, I think you'll see "flats" with a gouge between each, and the cracks radiating from the gouges.

Switch to a single-hole countersink (aka "zero-flute") and just barely tweak the edge of the hole. Better yet, whenever possible deburr by grinding with a small conical stone.

I like Mike's idea about wicking some Weld-On into those cracks before doing anything else.
 
How about stop drilling each crack with a small bit, say about 1/16". Another suggestion, carefully grind out each crack individually with a small grinding bit ( I am thinking no bigger than about 1/16") in a high speed Dremel. With either method the original drill hole would be left as is so you would not risk more cracking by using a bigger drill. You may consider filling in where the cracks were ground out/stop drilled with a filler/glue that sticks to plastic.

Fin
9A
 
So now I have conflicting advice. What do you think? Glue with Weld-On, or drill them out bigger???
__________________
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FWIW I would drill them out.

A Unibit (step drill) will make very clean holes in acrylic without chattering, and will easily enlarge your existing holes. Go slowly and use very little pressure.

You want all the holes to be significantly larger than the fasteners, to allow the canopy to expand and contract without being trapped between two rigid points.

You don't need to countersink the plexi holes at all. you can make them a little bigger than the outside of a dimple, dimple the "targa" strip, and avoid any contact between the canopy and the fastener
 
Burn a hole

Isn't there a way to burn, or better yet, melt through the plastic materials? Seems to me if you got a pointed rod red hot it wouls just go right through.
 
Don't do that

Robert M said:
Isn't there a way to burn, or better yet, melt through the plastic materials? Seems to me if you got a pointed rod red hot it wouls just go right through.
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Not a good idea. The melted and refrozen plastic is more susceptible to cracking. Changes the temper, or sumptin.
 
Thanks for all the advice.

- I used a standard countersink bit. Looking back I would say that was a big mistake.

- Drilling out seems to be the opinion shared by most. I stepped up the bad holes with Avery Plastic bits. All but two removed all cracks by a 1/4". The last two I used a unibit to go up to 5/16". I did this very slowly, hoping for no additional cracking. It worked.

- Now I have no cracks, but several holes that are larger then normal.

- I need to decide if the larger holes should be left as is, or filled with something? Since they are on the spine and will be covered with the C653 cover strip, I am thinking about leaving them be. Rivets through the cover strip will hold the canopy down. I would think there are plenty of good rivets left to secure everything in place. Especially since the canopy is designed to have all the rivets sit loosely in the holes anyway.
 
Here are a couple of photos of the bigger holes. This one shows a hole drilled out to 1/4" and sanded smooth. All the cracks are gone!



With these holes at 1/4" the dimpled skin on the C653 cover strip will fit just inside the holes. Check out this photo. I think I can leave these without filling them with anything?

 
pmccoy said:
With these holes at 1/4" the dimpled skin on the C653 cover strip will fit just inside the holes. Check out this photo. I think I can leave these without filling them with anything?
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Just make sure no water can get under the strip. Seems like filling the holes, even if just with proseal or lexel would be good insurance. Good luck, though, whatever you do.
 
Water Leakage

Just make sure no water can get under the strip. Seems like filling the holes, even if just with proseal or lexel would be good insurance. Good luck, though, whatever you do.
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Since all the canopy rivets have holes drilled out to 5/32" for a 1/8" rivet (#4), it seems as if they would all leak water. I have to admit I didn't think of that. Are others having trouble with the canopy leaking? Filling the holes with proseal would be easy enough, and I would think still allow room for the canopy to expand and contract with temps differences. Maybe a bead of silicone caulk under the C653 cover strip would help as well?
 
pmccoy said:
Since all the canopy rivets have holes drilled out to 5/32" for a 1/8" rivet (#4), it seems as if they would all leak water. I have to admit I didn't think of that. Are others having trouble with the canopy leaking? Filling the holes with proseal would be easy enough, and I would think still allow room for the canopy to expand and contract with temps differences. Maybe a bead of silicone caulk under the C653 cover strip would help as well?
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Put a bead of Lexel under the edge of the cover strip when you rivet it on. No need for it to adhere to the acrylic as it will act as a compression seal. No chance of water getting into the holes then.

Lexel has a good track record in this application (RVs). I wouldn't use proseal because it is grey/black and you will see it from inside the cockpit (will look just terrible). Lexel is clear and will be invisible. I used it on ALL skirts and it cannot be seen.

Builders who have been flying for up to 6 years report no problems with Lexel.
 
pmccoy said:
Since all the canopy rivets have holes drilled out to 5/32" for a 1/8" rivet (#4), it seems as if they would all leak water.
Click to expand...
I agree Peter, they all have the potential to leak, but at least with the riveted holes water wouldn't have a free pass in. Whatever leaks there were would be much smaller. That said, as mentioned above, if you are sure you've got the alum. strip completely sealed, sealing the holes is probably irrelevant, but it still wouldn't hurt to cover your bases now while everything is accessible. Again, good luck.
 
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