canopy crack help needed

I just went to the garage to start taping the canopy for the fiberglass fairing and was absolutely disgusted to find this crack. After all the trouble I have had with the canopy part of me was going to be surprised if there was no crack when I was done. Well now what to do? I obviously have to stopdrill it and have read through a bunch of posts on others cracks. I have to get a plexi bit, but where do I get it and what size? I used sika so I haven't drilled any holes in the canopy. Any help or tips is mch appreciated.
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Drilling plexiglass

You want to drill it soon so it doesn't grow longer.

A small finishing nail or brad will work, or use a number 40 bit held against concrete to dull it.Try it first on a scrap.

Sorry!

I started all holes in my canopy with a #40 drill bit and never had a problem. A 1/16" bit would work even better for stop drilling. You can then put a bit of epoxy or silicone in the drilled hole. It doesn't look that bad, though it will be a constant reminder of your struggle.

For others reading this, It is important that spacers be used so that the plexi is not forced too hard to fit bends. The problem with the sika approach is the use of clamps. It is critical that the clamps are not used to force the plexi to make too aggressive of a bend without using heat or spacers to help it conform and not create too much stress. The sika is very strong and will easily hold the plexi in that stressed position, if held there with clamps as it cures.

Larry

lr172 said:

I started all holes in my canopy with a #40 drill bit and never had a problem. A 1/16" bit would work even better for stop drilling. You can then put a bit of epoxy or silicone in the drilled hole. It doesn't look that bad, though it will be a constant reminder of your struggle.

For others reading this, It is important that spacers be used so that the plexi is not forced too hard to fit bends. The problem with the sika approach is the use of clamps. It is critical that the clamps are not used to force the plexi to make too aggressive of a bend without using heat or spacers to help it conform and not create too much stress. The sika is very strong and will easily hold the plexi in that stressed position, if held there with clamps as it cures.

Larry

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Thanks! I don't know how I could have used spacers on the sides of the canopy because I had to pull the canopy in a fairly decent amount to get it to line up with the fuselage sides. After all is said and done the passenger side lines up almost perfectly with the fuselage but the pilot side sticks out about an eighth of an inch. I have a friend building a 14 and I can't wait to see his canopy go together.

On the 14 a some of us had to cut relief points, kind of like a keyhole shape, to relieve stress points. You can’t tell after the layup.

Sorry to see that crack!

I stop drilled it with a 1/16" regular bit. Hopefully that stops the crack. I may order a 1/8" plexi bit just in case. I saw many posts saying to use Weldon #3 in the crack. Is that still the preferred way to seal the crack?

dwranda said:

Thanks! I don't know how I could have used spacers on the sides of the canopy because I had to pull the canopy in a fairly decent amount to get it to line up with the fuselage sides. After all is said and done the passenger side lines up almost perfectly with the fuselage but the pilot side sticks out about an eighth of an inch. I have a friend building a 14 and I can't wait to see his canopy go together.

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When the plexi wants to bend in one place, you use spacers further out from the bend to make the bending force more gradual and less concentrated in one area. Heat can also be used to relieve the stress, but it takes a bit of experience to keep from getting it too hot and creating visual distortions.

My 10 windshield had a bulge in an area similar to yours and I used a heat gun to get it back in without using force.

Larry

Heat

Careful on application of heat via heat gun or heat lights. A friend just melted his finished 8 canopy with 'just a little heat'.

A hot summer day is perfect heat!

Sorry to hear it, tough luck.

I am thinking, would this stop at this point or is it going to grow eventually even with stop drilling a hole. If it needs to be replaced, this is the best time before the fiberglass lay up and paint has been applied.
I guess it will be tough call to make but it is worth spending a bit of time and research.

Bavafa said:

Sorry to hear it, tough luck.

I am thinking, would this stop at this point or is it going to grow eventually even with stop drilling a hole. If it needs to be replaced, this is the best time before the fiberglass lay up and paint has been applied.
I guess it will be tough call to make but it is worth spending a bit of time and research.

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Oh man don't say that! I am praying to the canopy gods that it will stop there. If I had to replace the canopy at this point I would be tempted to push the plane into the back of my garage and take a couple years off! The thought of starting a new canopy takes ALL the wind out of my sails!

The thought of starting a new {INSERT MISTAKE HERE} takes ALL the wind out of my sails!

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Been there, done that, ordered the parts, happier for it

... 5 trim tabs
... 2 rudders
... 3 nosewheel pants
... 2 emp fairings
... countless others I've mentally blocked

Still worth it

Certainly did not mean to bring discouraging thoughts and I feel your pain.

I assume you intend to cover the entire crack in the layup in which case you do not need to worry about how it may look. Weldon #3 is a great adhesive that will flow into the crack and creates a rather strong bond. If you did not know about it, give it a try.

lr172 said:

I started all holes in my canopy with a #40 drill bit and never had a problem. A 1/16" bit would work even better for stop drilling.

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The #40 bit is recommended by Vans to pilot the holes in the canopy and frame and to allow 3/32" clecos to be used. These holes are eventually enlarged to 5/32" for the 1/8" rivets or for the screws.

AC43.13-1B section 3-24 "At the first sign of crack development, drill a small hole with a #30 or a 1/8" drill at the extreme ends of the cracks as shown in figure 3-21. This serves to localize the cracks and to prevent further splitting by distributing the strain over a large area."

The large area they are referring to is the drill hole, and it is large compared to the area at the end of the existing crack which is very small. As the tension strain (tendency to elongate due to external loading) gets distributed over a large area, the unit stress becomes lower. Using too small a drill will allow the crack to keep propagating through the stop drill hole until it relieves the tension strain.

Sikaflex and canopy cracks

It is interesting that a significant number of RV builders who used the Sikaflex technique on their canopies have now reported cracking of the acrylic. After all, one presumes they used Sikaflex to avoid cracking. One theory for this suggests that microscopic discontinuities in the canopy edge treatment might be susceptible to chemical attack by the very aggressive solvents in the Sikaflex primer causing the microscopic flaws to turn into microscopic cracks which then propagate into larger cracks by the ?work of fracture? principle.

dwranda said:

Oh man don't say that! I am praying to the canopy gods that it will stop there. If I had to replace the canopy at this point I would be tempted to push the plane into the back of my garage and take a couple years off! The thought of starting a new canopy takes ALL the wind out of my sails!

Click to expand...

The crack is caused by energy stored from bending it into the shape you did. That energy wants to dissipate. Heat, even from the sun, can relieve it and the crack does the same thing. Once the crack starts, the plexi can EASILY take the crack further if it has more stored energy. The fact that is stops indicates that all of the stored energy is gone. Initiating the crack requires substantially more energy than continuing it. It usually cracks enough to relieve the stress/energy introduced and once that is relieved, there is no more stored energy to create further cracking. The stop drilling removes the ease of continuing, so the small energy created from thermal cycling and vibration can't do more damage. Just be sure to use a countersink to bevel the edges of the stop drill hole.

Larry

Captain Avgas said:

It is interesting that a significant number of RV builders who used the Sikaflex technique on their canopies have now reported cracking of the acrylic. After all, one presumes they used Sikaflex to avoid cracking. One theory for this suggests that microscopic discontinuities in the canopy edge treatment might be susceptible to chemical attack by the very aggressive solvents in the Sikaflex primer causing the microscopic flaws to turn into microscopic cracks which then propagate into larger cracks by the “work of fracture” principle.

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Read the label. No agressive compounds in the sika or it's primers. Sika 295 is THE gold standard for insalling plexi windows in boats. Literally millions of plexi windows installed with Sika 295. If your theory were correct, I am sure Sika would have figured that out by now and specified edge treatment requirements to prevent it. The primer is basically an adhesion promoter and it's chemical makeup looks similar to the epoxy primer that everyone spray on the border ends of their plexi.

The linkage to the cracking I outlined above. Or at least my theory. But I am quite confident it is not from a chemical attack from the Sika.

Larry

Seems to me there are good numbers of cracks with sika or with screws. Maybe it's more of a case that the canopies are just slightly sized wrong and have to be coaxed into position resulting in stress that then results in cracks. I did have to push the sides in a fair amount to get them to line up with the canopy frame that was even with the fuselage sides. I believe that's what caused the crack.

lr172 said:

Read the label. No agressive compounds in the sika or it's primers. Sika 295 is THE gold standard for insalling plexi windows in boats. Literally millions of plexi windows installed with Sika 295. If your theory were correct, I am sure Sika would have figured that out by now and specified edge treatment requirements to prevent it. The primer is basically an adhesion promoter and it's chemical makeup looks similar to the epoxy primer that everyone spray on the border ends of their plexi.

The linkage to the cracking I outlined above. Or at least my theory. But I am quite confident it is not from a chemical attack from the Sika.

Larry

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I'll deal with your comments sequentially. Your comments in color black and my answers in blue:

Read the label. No aggressive compounds in the Sika or its primers. Totally incorrect. The most common primer/adhesion promoter for Sikaflex 295 is Sika 209D which contains 25% MEK and 20% Ethyl Acetate. Both of these solvents are known to break down the cross links of the long chain molecules in PMMA (Acrylic). Sika 209D is a solvent based adhesion promoter that works by "etching" the acrylic to form a mechanical bond.

Sika 295 is THE gold standard for insalling plexi windows in boats. Yes it is used in boats but the windows are generally small compared to an RV canopy. And Vans canopies are thermally formed to double curvature which induces locked in stresses. It's an entirely different game.

If your theory were correct, I am sure Sika would have figured that out by now and specified edge treatment requirements to prevent it. Sika recommends that 209D is a product "suitable for experienced professional users only". That hardly applies to your typical RV builder. Sika themselves do not recommend the use of this product to retain an aircraft canopy. Additionally they say: "Always seek the plastic manufacturer's advice before using 209D primer on thermoplastics that are prone to stress cracking such as acrylic". And who is the manufacturer of the canopy....its Vans Aircraft. And what do Vans have to say on this matter...they also recommend against using it.

The primer is basically an adhesion promoter and it's chemical makeup looks similar to the epoxy primer that everyone spray on the border ends of their plexiglass. I don't know what you're talking about here. I didn't spray any "epoxy primer" anywhere on my acrylic canopy.

Hopefully it won't feel so depressing after a few nights sleep.

There is a tendency to say good enough, or "I'll live with it" when building. However, things feel different later, flying and on the flightline. Plus there's your financial investment to consider.

Take a deep breath, cut it off the airplane, and replace it. The high probability reward is that you'll do it better the second time. We all do.

feel/felt your pain . . .

dwranda said:

If I had to replace the canopy at this point I would be tempted to push the plane into the back of my garage and take a couple years off! The thought of starting a new canopy takes ALL the wind out of my sails!

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I hear ya - been to the edge a couple of times, had to go home and do something else for a while.

9 months and 5 failed plenum covers was a test of endurance.

I had a layer of epoxy on fairing in the front of my tipper did not set up, I had to peel it off. WEST said use a little heat. The protection was still on the plexiglass but somehow I must have heated a little too much as I found distortion in the same region as your anomaly. Minor, but definitely visible when knowing it is there. Schit happens.

My canopy did not fit right. Off sets and gaps no matter what I did. I stopped removed the front skin, tested for new shims to make all correct and made a new front skin. It fits properly not - quite happy. This is one of about 5 issues that a reboot and 40 -50 hours of work each to fix.

Hang in there, get her flying and then dial in all the issues you can, then replace it. It will still fly like an RV!!

EDIT: Dan is probably right, do it now and you won't have the pain of a plane that should be flying. That seems worse than a delay.

My experience

I'm a new builder - RV-12 so I have very little experience building airplanes - but - I have a lot of experience building or repairing other things - houses, cars, machinery, etc. AI learned early that if I had to ask myself if my work was ok or not, then it it wasn't good enough for me and I didn't care that a few others said that my work was 'good enough', carry on.

This mid set carried over into my RV build - if I had thoughts that my work was only marginal. then I replaced the part with a new and better one. That way, I never had any doubts that my finished product (RV) was good or better. That way, I avoid seeing poor quality work that I did during pre-flight, etc. I'd replace it now with a new canopy. Do it better and then you will never again have to look at your cracked canopy. Just my two cents worth.

I really appreciate everyones input. My choices are to fix the crack and hope it stays where it is now that the stress is reduced or spend most likely over $2000 and replace the canopy. This is my dilemma.
I am not building a show plane or a GA commercial aircraft. I read one internet page that said stop drilling is not an acceptable fix for certified aircraft. Yes I will see that visible 1" crack every time I fly my plane. This is my first build and is not perfect. Right now I can't fathom replacing it. The thing is whos to say I won't have the next one crack on me as well? It's all a gamble and right now I am going to continue on with this canopy. If it lasts me 20 years I will know I made the right decision. If it cracks more when I get airborne then I didn't. I have ordered the weldon supplies to try to repair the crack and hole. Wish me luck!

Canopy

I am not an RV builder and have never done a two place canopy. I have done a quite large single place canopy and a lot of Plexiglas work.
The two biggest enemies are improperly finished edges and shop temperatures that are not hot enough.
I finish all edges by sanding with a sanding block. Then I use a large flat file held at 90 degrees to the edge and "draw file" the edge. The corners are very lightly chamfered with a very small, fine tooth flat file. I then go back to the sanding block with 500 wet or dry paper. Sand along the edge, not across the edge. A few light passes on the chamfered edges.
I absolutely believe that great attention to detail on these two items will eliminate almost all future cracks.
Drill bits: if you can't find a special purpose Plexiglas drill bit, take a regular drill bit and dull the cutting edges on a bench grinder. Then use very light pressure when drilling including a backup block, preferably hardwood, so that the drill bit exits the hole smoothly.
Regarding repairing the crack, the B25 people at Chino CA have a Plexiglas shop. If you offered to pay one of the experts for an hour of shop time they MIGHT be willing to talk you thru a process for repairing the crack so perfectly that it will virtually disappear. The airshow is this weekend so best to wait a week or two until they recover from that. If you were in that area I would say just take them the canopy and ask them to fix it.
I know very little about this process but I think it involves the special Plexiglas glue which is available from any large plastic shop and possibly some ground up acrylic to mix in as filler. If a filler is used it should be the same tint as the canopy.
Then many hours of sanding and polishing with one of the scratch remover kits.
I have built two colored wingtip light covers for my airplane that have a glued joint. After many hours of work the glued joint is almost invisible.
To summarize a perfect repair is not that complicated but VERY time consuming.

Canopy

DWRanda, Check your pm

Canopy

The company in Chino is Aero Trader 909-597-4020. They allegedly have an in house Plexiglas shop.

Replace it. I damaged my canopy thirty years ago and didn't replace it. Spent the next thirty years answering questions on how I damaged it!

Oh man David I feel your pain. I say, fix the crack, leave the canopy and go on to other things. Come back in a few weeks and see how it looks and how you feel about it. But I suggest you make your decision before you fiberglass else you’ll be replacing the front deck too.

Years ago when you could purchase this sort of stuff I used chloroform to weld plexiglass. I could glue two 1/4” thick sheets at 90 deg to one another and could NOT break the bond. The way you would use it is to clamp he two pieces together and squirt the chloroform along the seam and it would wick into the joint. Remarkably strong. Don’t know if you can still buy it, but it you can I bet good money it would be a permanent fix.

Just found this. See if they’ll sell it to you and experiment on some scraps. If you don’t have any scraps left, call me and I’ll send you some.

https://www.laballey.com/products/c...MI1d7o8tuC4gIVyISzCh3csg0ZEAQYASABEgLdKfD_BwE

2johns said:

Replace it. I damaged my canopy thirty years ago and didn't replace it. Spent the next thirty years answering questions on how I damaged it!

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If my canopy lasts 30 years then I know I made the right decision by not replacing it!

Av8torTom said:

Oh man David I feel your pain. I say, fix the crack, leave the canopy and go on to other things. Come back in a few weeks and see how it looks and how you feel about it. But I suggest you make your decision before you fiberglass else you?ll be replacing the front deck too.

Years ago when you could purchase this sort of stuff I used chloroform to weld plexiglass. I could glue two 1/4? thick sheets at 90 deg to one another and could NOT break the bond. The way you would use it is to clamp he two pieces together and squirt the chloroform along the seam and it would wick into the joint. Remarkably strong. Don?t know if you can still buy it, but it you can I bet good money it would be a permanent fix.

Just found this. See if they?ll sell it to you and experiment on some scraps. If you don?t have any scraps left, call me and I?ll send you some.

https://www.laballey.com/products/c...MI1d7o8tuC4gIVyISzCh3csg0ZEAQYASABEgLdKfD_BwE

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Thanks! I've got some extra on the rear window that I will be cutting off. I am going to experiment with that. Never knew chloroform could do that.