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Crack in 7A Windshield

CD.

Active Member
So I rolled my 7A out this afternoon to take her for a flight and found this crack in my windshield. :mad: I don't know when it happened. The last flight was 2 weeks ago with my wife and was about 2.5 hours. No extreme temps or turbulence. And I didn't notice it then, so I don't think it was there. The temps did get down to the low 20's last night from low 70's yesterday afternoon. But if I look at the crack with just the right light and angle, I may can see some indication of an impact at the end of the crack. Maybe. But maybe not. I don't remember anything hitting my windshield. So I rolled her back into the hangar while I figure out what to do. I assume I can stop drill it, but I wanted to ask for advice. So I am all ears.

What should I do?

Thanks,
CD
 

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Start with a stop drill

Do what you can right away to relieve any residual stress concentrating at the end of that crack. In other words, stop that crack progress first.

Lots of techniques: My choice was a very fine step, stepdrill. I have the kit to replace the lost acrylic but haven't used it yet. My crack stopped at the stop drill.

Know that temperature cycles are enough to keep that crack progressing.
 
It is hard to tell from the pictures, but it looks like it could be connected to the cracking in the windscreen skirt? Maybe an impact on the skirt or fastener under the skirt?

As for fixing, it is in an area not real noticeable from the pilot so stop drilling and then trying to glue it back together like Scott Chastian did would work. Good luck.
 
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My first guess would be from compressive strength from thermal contraction. definately stop drill the end and doubt it will progress further.

The fact that the front base of the windshield moved, causing the paint cracks, points to the canopy shrinking more than the securing method up front could deal with. You would need a serious impact to crack that far into the base and a witness mark from that impact should be visible somewhere. The star crack in the paint is just where a rivet is buried, causing more paint damage in that area as the epoxy or filler moved.

Larry
 
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Special bit required?

Thanks for the responses. I do appreciate it. I did not build this plane, so I am not familiar with the process for stop drilling the crack. I will be soliciting the assistance of my A&P to do this. Is there a special bit that I need to use/purchase to do stop drill it?
 
'Special Bit'

There are specific twist drill bits for plastic. But I like to use a very fine step 'step drill'
 

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What diameter bit/hole is recommended? I assume on the step drill bit you only used the first step?
 
What diameter bit/hole is recommended? I assume on the step drill bit you only used the first step?

I would just use a 1/16" standard drill bit and go very slowly. The goal is to keep the sharps edges of the bit from grabbing the plexi at that end of the hole. That is what usually causes the cracking. By going slow, the friction from the bit heats up the plexi and that minimizes cracking potential. On my 6, I used a #40 bit to start every hole in my canopy and used the step bit to enlarge the holes. I don't think a hole larger than 1/16 is required, but upsize to an 1/8 if it makes you feel better. I would lightly hit the hole with a 1/8 bit to put a chamfer on the hole to avoid cracking. Just kiss the 1/8 bit to the plexi.

Larry
 
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Thanks for that info. I watched plexi-glass drilling videos where water is used to keep the temp down. At aircraft Spruce, the smallest acrylic bit that I can find is 1/8"... Same with the Plas-Drill bit at Wicks.. 1/8". So I guess 1/8" is ok?
 
Windshield

If there is no known history of unusual impact the next most likely cause is failure to properly smooth the edge of the acrylic. That can be done with progressively finer sanding blocks working length wise along the edge.
I see no reason to stop drill larger than 1/8" diameter. Use a drill bit that is specific for acrylic.
For a longer term repair the crack can be ground in a v shape, mask the inside and inject with acrylic cement. Using an acrylic repair kit the repair can then be finished flush with the original acrylic. Not much can be done finishing the inside in this case.
The B25 people in Chino CA have an acrylic shop that repairs very old acrylic windshields and windows. You might offer them payment for some advice on how to deal with this.
Any big city plastics shop will have the drills and the cement.
 
Crack

Given the small crack along the line where the fiberglass joins the aluminum there was likely some kind of impact.
 
How big a hole?

This is a best guess/gamble. The point is to spread the stress over a larger radius than the knife edge of the current crack propagation.

The more stress present the larger the radius to keep it below ultimate strength of the material.

We often see in highly stressed skin areas like trailing edges of thinly skinned wings, a crack that got stop drilled, then cracked some more, and stop drilled again, etc.

It is also very difficult to know where the end of the crack really is since it is literally microscopic.

Since my crack wasn't in my eye line I went with a 1/4" final stop drill hole. It will be filled with the acrylic repair which will stop the whistling but will be plainly visible.

I wonder about grinding the crack itself and filling the area with the repair acrylic. If the stress is still present and it has a solid path of acrylic then one might expect the crack to return and the repair might act as a conduit to propagate further into the canopy.

So go with the smallest hole you want to drill that will give you the comfort of not having it crack some more. Sorry, but this is half art half science.
 
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Drill just past where you can see evidence of the crack. Start the hole drilling clockwise, then as soon as the end of the bit penetrates the surface reverse the rotation for the rest of the hole. Use light pressure, take your time.
 
I don’t think you need to replace your windshield, and the advise you’ve been given about stop drilling is what you should do. You could then call Jeff Rogers at Airplane Plastics - 937-669-2677- (they make Vans canopies) and ask him how to fix this permanently. There are ways, and he is an expert. Many cracked canopies can be saved.
 
Stop drill

I recommend also a 1/16” bit slowly. Then use a debur tool to soften the edges both sides. 1/8” is very large and not needed.
 
This is a different situation/place, but for my slider I opted for a straightforward and durable - yet butt-ugly solution. I liken it to having an ultra-hot girlfriend who wears braces...
And for cyclists, that's an XL roadbike in the back, just before heading to Colorado for the Triple Bypass!
home
 
Just dealt with my own crack

I just dealt with a crack on the rear pilot side of the canopy on my -7. It originated at one of the fastener locations. I used the method outlined in AC43.13-1b to repair it.

Stop drilled it with a 1/8" drill. I used the special acrylic drill because I had one but I've also used the regular drills without issue by going slowly (as others have suggested). I wicked a little of the thin Weldon 4 into the crack to try to bond it together. Then I used Weldon #40 2-part adhesive to fill the hole with 1/8" acrylic rod, followed by trimming, sanding and polishing. Obviously you can still see the impacted area but it's not too bad. I drilled two holes to "stitch" at both ends and prevent any further movement or stress but that's probably unnecessary. The repair seems fine so far but this was literally only a couple weeks ago and I've only flown once since.

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My 1/8" acrylic drill bit from Spruce came in today and I stop drilled the crack. It appears to have turned out pretty good. I went extremely slow. Now I guess I need to fill/epoxy... Do I need to not fly until that is completed?
 

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Chad, good to see you got the immediate issue solved. Beware that the crack extends forward through the transition to the forward skin. There is a perimeter crack also in your OP post. You should prepare yourself for a front windscreen replacement and repair of that transition area. I don't think the cracks alone make it unairworthy, but they should be closely monitored to see if they progress. The video below shows that area should be a very strong fiberglass layup. Cracks there are not a normal occurrence, you can see why.

Here is a video starring Scott McDaniel (Vans) demonstrating the process on an RV14. It is an excellent demonstration.
 
My 1/8" acrylic drill bit from Spruce came in today and I stop drilled the crack. It appears to have turned out pretty good. I went extremely slow. Now I guess I need to fill/epoxy... Do I need to not fly until that is completed?


NOT regular epoxy, it isn't generally UV stable. It'll turn brown and crumble over time. Use an Acrylic repair.
 
I've ordered some Weld-On #4 and some 1/8" acrylic rod to put in the hole. Will have wait another week or so.

Thanks for all the info. I watched the videos. Thanks!
 
I've ordered some Weld-On #4 and some 1/8" acrylic rod to put in the hole. Will have wait another week or so.

Thanks for all the info. I watched the videos. Thanks!
I believe Weld-on #4 would be too thin for the hole. I would suggest Weld-on #16 for the hole itself and #3 for the crack. If you have TAP plastic near you, you can just get it from the store. I have had great success with #3 for the crack
 
What I researched was #3 and #4 were almost identical as to wicking into a crack. #4 does have a longer cure time but wouldn't 'blush' or turn white as bad as #3 due to higher humidity. I was planning on plugging the hole with the 1/8th rod and using the #4 to bond the plug in the hole and wick into the crack.

Unfortunately, the nearest TAP Plastics store that I saw is almost 2000 miles away in CA..:eek: Since I live in the sticks in AL... If Wal-Mart, Tractor Supply, or Advanced Auto doesn't have it, I typically have to drive a ways! :)
 
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What I researched was #3 and #4 were almost identical as to wicking into a crack. #4 does have a longer cure time but wouldn't 'blush' or turn white as bad as #3 due to higher humidity. I was planning on plugging the hole with the 1/8th rod and using the #4 to bond the plug in the hole and wick into the crack.

Unfortunately, the nearest TAP Plastics store that I saw is almost 2000 miles away in CA..:eek: Since I live in the sticks in AL... If Wal-Mart, Tractor Supply, or Advanced Auto doesn't have it, I typically have to drive a ways! :)

Try PORT plastics, they might be out your way...
 
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