Looked at a glider today that has a 3 inch cracked running forward from the vent window opening. Crack caused by someone reaching in to pull release and catching a coat sleeve on the edge of the opening. Owner drilled a 1/8" hole at the leading edge a number of years ago, and the crack has not spread further, but the glider has not been flown much since then. The area is in compression, so there is no gap. I don't think it has been glued.

How stable is this repair? Is there anything else to be done that would make a better repair, and can the edge of the vent hole be reinforced (on the inside) to make the hole less prone to crack.

On Thursday, June 17, 2021 at 7:53:58 PM UTC-4, son_of_flubber wrote:
> Looked at a glider today that has a 3 inch cracked running forward from the vent window opening. Crack caused by someone reaching in to pull release and catching a coat sleeve on the edge of the opening. Owner drilled a 1/8" hole at the leading edge a number of years ago, and the crack has not spread further, but the glider has not been flown much since then. The area is in compression, so there is no gap. I don't think it has been glued.
>
> How stable is this repair? Is there anything else to be done that would make a better repair, and can the edge of the vent hole be reinforced (on the inside) to make the hole less prone to crack.

Son - I have done several repairs of cracked canopies. The hole to stop the crack from propagating is a good start.
Here is how I did it but there are probably many different opinions on this:

1.) Use clear packaging tape and apply it centered along the crack on the outside and the inside of the canopy
2.) Get a Dremel-motor with an end-mill type bit (one that cuts on the cylindrical and the front end). A cone-shaped bit may work but keep in mind that the peripheral speed at the tip is near zero. I prefer the end-mill type.
3.) Holding the tool at a 45* angle, carve out the crack down to half of the thickness of the material. You will slip once in a while but the clear tape prevents major tool marks where you don't want them.
4.) Remove tape and clean thoroughly with denatured alcohol.
5.) Position Canopy with the crack as level as possible and fill the V-groove liberally with Acryfix cement. You must over-fill the V-grove since the cement shrinks considerably as it cures and you don't want to fill it again..
6.) Let it cure under a UV-light or in sun-light.
7.) Do the same on the opposite side making sure you carve into the first V-groove you just filled. That way, you eliminate all surfaces, which are perpendicular to the crack and are amazingly reflective.
8.) After the second side has cured, apply again clear packaging tape but this time use two strips, leaving the repair seam, which should be proud of the rest of the canopy, exposed.
9.) Start with a file to remove the excess material, then gradually move to less coarse tools, as you approach the original surface level.
10.) Finish it up with Micro-Mesh or similar and buff it to perfection.
11.) Repeat these steps on the opposite side.

The repair will last for ever and will leave only a very small optical distortion. It's a lot of work but the result will be rewarding.
Best of luck,

Uli
'AS'

On Thursday, June 17, 2021 at 9:01:20 PM UTC-4, AS wrote:
> On Thursday, June 17, 2021 at 7:53:58 PM UTC-4, son_of_flubber wrote:
> > Looked at a glider today that has a 3 inch cracked running forward from the vent window opening. Crack caused by someone reaching in to pull release and catching a coat sleeve on the edge of the opening. Owner drilled a 1/8" hole at the leading edge a number of years ago, and the crack has not spread further, but the glider has not been flown much since then. The area is in compression, so there is no gap. I don't think it has been glued.
> >
> > How stable is this repair? Is there anything else to be done that would make a better repair, and can the edge of the vent hole be reinforced (on the inside) to make the hole less prone to crack.
> Son - I have done several repairs of cracked canopies. The hole to stop the crack from propagating is a good start.
> Here is how I did it but there are probably many different opinions on this:
>
> 1.) Use clear packaging tape and apply it centered along the crack on the outside and the inside of the canopy
> 2.) Get a Dremel-motor with an end-mill type bit (one that cuts on the cylindrical and the front end). A cone-shaped bit may work but keep in mind that the peripheral speed at the tip is near zero. I prefer the end-mill type.
> 3.) Holding the tool at a 45* angle, carve out the crack down to half of the thickness of the material. You will slip once in a while but the clear tape prevents major tool marks where you don't want them.
> 4.) Remove tape and clean thoroughly with denatured alcohol.
> 5.) Position Canopy with the crack as level as possible and fill the V-groove liberally with Acryfix cement. You must over-fill the V-grove since the cement shrinks considerably as it cures and you don't want to fill it again.
> 6.) Let it cure under a UV-light or in sun-light.
> 7.) Do the same on the opposite side making sure you carve into the first V-groove you just filled. That way, you eliminate all surfaces, which are perpendicular to the crack and are amazingly reflective.
> 8.) After the second side has cured, apply again clear packaging tape but this time use two strips, leaving the repair seam, which should be proud of the rest of the canopy, exposed.
> 9.) Start with a file to remove the excess material, then gradually move to less coarse tools, as you approach the original surface level.
> 10.) Finish it up with Micro-Mesh or similar and buff it to perfection.
> 11.) Repeat these steps on the opposite side.
>
> The repair will last for ever and will leave only a very small optical distortion. It's a lot of work but the result will be rewarding.
> Best of luck,
>
> Uli
> 'AS'

P.S.:
Forgot to mention: when you get a tube of Acryfix, store is with the tip up for several days. That way, you allow any air-bubbles to rise to the top. An air-bubble in the repair is hard to get out.

Uli
'AS'

Ditto.
Or, just leave it.
The hole or "drill stop" should be stable for the rest of your life.

On Thursday, June 17, 2021 at 9:47:16 PM UTC-4, Guy Acheson wrote:
> Ditto.
> Or, just leave it.
> The hole or "drill stop" should be stable for the rest of your life.

What Uli said, except I use several thin applications which are easier to work with. Specifically, it is Acrifix 192, which I got at Knauff and Grove. You should keep it frozen or refrigerated since a tube goes a long way, and also keeps those air bubbles out.
For what Guy said, probably ok unless you intend to do any wave soaring, where the large temperature changes add a bunch of stress and are more likely to bypass the drill stop crack further; also, it doesn't look very nice.

I greatly appreciate the technical advice offered here. Thanks.

On Friday, June 18, 2021 at 8:25:37 AM UTC-6, son_of_flubber wrote:
> I greatly appreciate the technical advice offered here. Thanks.
Two part mixes that are primarily methylene chloride mixed with an acrylic monomer work well, but takes some practice and doesn't require UV to cure. Once mixed, still requires some time for micro bubbles to release for best results. Too fast of a cure can result in fine stress cracks. Done properly and with lots of polishing, cracks can be made to vanish. Acrifix 192 also works well and the UV curing is not a huge issue, just be prepared to roll from shade to sun.

Frank Whiteley

Decades ago I repaired a crack in my LS-6a canopy with a small stop
drilled hole and then applied a very tiny amount of (IIRC) acetone to
the crack with a hypodermic needle. I forgot where I got the advice but
it worked.

Dan
5J

On 6/18/21 4:09 AM, Dan Daly wrote:
> On Thursday, June 17, 2021 at 9:47:16 PM UTC-4, Guy Acheson wrote:
>> Ditto.
>> Or, just leave it.
>> The hole or "drill stop" should be stable for the rest of your life.
> What Uli said, except I use several thin applications which are easier to work with. Specifically, it is Acrifix 192, which I got at Knauff and Grove. You should keep it frozen or refrigerated since a tube goes a long way, and also keeps those air bubbles out.
> For what Guy said, probably ok unless you intend to do any wave soaring, where the large temperature changes add a bunch of stress and are more likely to bypass the drill stop crack further; also, it doesn't look very nice.