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Cracked Windscreen; help requested!

G

gerrychuck

Well Known Member
Posted on the "How cold is too cold" thread, but thought I'd post it separately to solicit advice:

I think I jinxed myself; after bragging about my New Years Day cold weather flight, I went back to the airport yesterday to fly (in much milder temps) and discovered a 2" crack in the windscreen, probably from the cold temps last time out. Bummer. The crack is at the bottom of the screen on the passenger side at the "corner" where the screen curves from the frontal plane to longitudinal. It can't be seen from inside the cockpit at all, and there is zero separation; can barely feel it with a fingernail.

I know I should stop drill it; but would appreciate advice re: techniques, bit sizes, etc for doing so. With no separation, I don't know if it needs to be glued as well or if this isn't needed. Again, any advice appreciated.

I apologize for the lack of pictures; for some reason I left my phone behind at the house when I left for the airport yesterday.

From what I have been able to learn from searching this forum and a more general google search, it appears that stop drilling is a given, but the questions I have are mainly related to drill bit type and size: Some people are adamant that you need to use a plastic bit, which I don't have, and others say that for stop drilling it isn't important since the plastic is already cracked. Also, many suggest a very small bit size, such as 1/16, and the plastic bit sets I've looked at online seem to start at 1/8.

After drilling, it appears that gluing/sealing with Weld-On #3 is in order, according to what I've read. Any hints on how best to do that would be appreciated.

Fortunately, the repair does not have to be perfectly transparent or even look particularly good cosmetically; as noted above it is not visible from the cockpit at all, so my concerns are purely to prevent further propagation of the crack.

Thanks in advance!
 
Regular drill bits have a flute which will tend to grab the plastic and may very well cause a crack. You can dress a regular bit to remove the flutes so it won't do this, or you can play it safe and order a plastic bit.

When I put holes in my wingtip lenses I used a soldering iron to melt
The hole. You might consider using a 1/16 or 3/32 bit in vice grips to heat and do the same. It would be great if you had a piece of scrap to try it on first (whether hot method or modified bit).

Good luck.
Don
 
If using a very small bit, like a 1/16 or 3/32, would the grabbing be much of an issue? That seems to be Derek's experience. Again; going to Spruce through Derek's link, they don't sell acrylic bits smaller than 1/8 at all. I can certainly appreciate how the grabbing would be a factor with larger bits. I do wish I had some scrap lying around to experiment with, but I don't think I do.
 
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I don't know, Bob; if you were looking down on the windscreen from above, with the screen covering the clock from 9:00 to 3:00, the crack would be at about 2:00.
 
one data point

Gerry, sorry to hear that.
I found a small crack starting in my canopy ( slider) and just held a block of wood tightly on the inside, and drilled with a new, sharp 1/16" bit just at the end of the visible crack. I was surprised that I almost missed getting it centred on the tip of the crack, I'd advise making a small dot or crosshairs with a waterbased marker to make it more accurate!...or use a 1/8" bit.
I did touch a drop of methylene chloride (weld-on) to help cement the crack together, but hard to say if that really helps, ( without pressure on the joint.)
been ok for 2 years, even with the flex of the canopy.
I'd venture that the windshield is more rigidly supported, but also gets prop and aerodynamic loads, so my experience is that cracks will continue.
 
Thanks Perry; I went out today and did pretty much exactly what you did. I spent lot of time making sure I was lined up with the end of the crack, and then drilled through with a 1/16 bit. With the location over the glare shield I ended up not using any backing, but drilled very carefully: very light pressure, fairly low speed, frequent stops to check progress and make sure there wasn't any significant heat buildup. I tried to avoid any chipping from punching through the back, and was pretty successful; I could feel a bit of an edge with my finger after drilling, so I smoothed it off with some 1000 grit paper to hopefully avoid any stress risers. I'll put some pics up after I have a chance to load them into the computer.

I don't have any Weld-On lying around, so that will have to wait, but hopefully I have stopped the crack from propagating until I can get some. Quick question; is the 1/16 hole okay, or should it be enlarged a bit to get a bit more radius? Would a slightly larger hole be more effective, or am I okay as is?

Film at 11...
 
one more caution....

looks good to me Gerry, but I'm not sure it's an exact science.
Just be sure to resist the urge to stick some sealer in there.....unless it's just putty, or paintable latex caulk or something, sealers tend to have a lot of things in them that can attack the raw core of the acrylic that you've now exposed, and 'presto', a bunch of cracks radiating from the new hole you drilled.
The sanding was a good idea, if you can get a cone-shaped stone, or a tool in there with some fine sandpaper and deburr the edges, just like an aluminum fitting, that would be a good idea. ( obviously tough on the inside)
 
1/16 should be fine

Any size hole is better than the crack tip, which is a nearly infinately small radius. The stresses at a sharp crack tip in a brittle material are very high, which is why it can propogate so easily. The hole blunts the crack. I don't remeber all the fracture mechanics i studied years ago, but there should be only a marginal reduction in stress going from 1/16 to say 1/4 hole. However, you want to be sure that you cut out all of the crack tip with the small hole. Use a magnefying glass to be sure. If not, open up the hole size until you get it all.
 
Drill Bits for plexi

For what it's worth, I was at a local Tap Plastics store over the weekend to pick up some polishing/cleaning products. While I was there, I noticed that they carry drill bits for plexiglass. I didn't drill any holes in my canopy or window, but it might help someone to know that there's a local source without mail order time and shipping costs.

Sorry to hear about the crack in your canopy. It looks like you got it stop drilled successfully. :)
 
Crack

Gerry,

I found the same crack in my canopy at the same spot after test running my engine. I must have left a bit of a hard point at the screw hole. I was not happy about it, but I stopped drilled the crack just like you did. After 125 hours on the Hobbs, the crack hasn't grown, and I rarely notice it.

John
 
Great information; I think checking the crack and hole more carefully with a magnifying glass is a good idea - thanks Brice. Lighting in the hangar wasn't perfect; I was going to put a work light on it but got lazy. A closer look to make sure I got the whole end of the crack blocked with the hole seems like a pretty good idea. If I didn't quite get it, enlarging to 5/32 would be easy enough.

John; that is very reassuring! Did you use solvent/cement or just drill?
 
Gerry, I don't think you would accomplish anything by trying to glue the crack with solvents. I would leave it alone.
 
Crack

Gerry, sorry to hear of the crack in your windscreen. The slider on my 6A cracked on a cold day, I ended up repairing it rather than spend $$ replacing it, link to what I did here; http://www.vansairforce.com/community/showthread.php?t=68505

It has held up fine, no issues other than it is a visible. Not sure if I would do this if it were in my field of vision but as it is above my head not a problem.
 
Thanks for that link, Skid; I had found and read your post earlier, and very much appreciate that information. Fortunately, the crack in my screen is not in the field of view; can't see it from the left seat at all, which makes it all about preventing further spread of the crack and makes the cosmetics of the repair, including optical acuity, irrelevant. Thank heavens.

John; thanks for that input. I'm inclined to think you're right, and likely won't go out of my way to do any further repair with solvent/cement. If I happened to have some lying around I would probably use it, though. I actually was talking to a patient who is a maintenance tech with the Snowbirds today, and asked if they have any just, you know, lying around, hint, hint. As I expected, even with their 50 year old Tutors, they don't repair, they just replace! Oh, well; had to ask;)
 
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