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Cracked Canopy

Skot7ac

Member
After reading several posts, I see I’m not alone in the cold night at the hanger canopy crack club. It looks like the consensus is plastifix after stop drilling. Does anyone have any other thoughts or guidance on the repair? Crack is just behind the back seat, left side of the canopy, a little over 14” long.
 

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After reading several posts, I see I’m not alone in the cold night at the hanger canopy crack club. It looks like the consensus is plastifix after stop drilling. Does anyone have any other thoughts or guidance on the repair? Crack is just behind the back seat, left side of the canopy, a little over 14” long.
That's a heck of a crack. I have no suggestions but will be following this thread in case it happens to mine.
 
How cold did it get & was there any movement that you are aware of that caused this? In other words, did you walk out to the plane in the morning and discover this happened overnight?
 
No movement, I had been in training for a little over a month and came back to find the crack waiting for me. I did have a friend working on an airplane in the hangar with bullet heaters running quite a bit, I think the heat cycles may have contributed. That being said, there were quite a few threads where people had the canopy’s crack in nearly the same spot.
 
After reading several posts, I see I’m not alone in the cold night at the hanger canopy crack club. It looks like the consensus is plastifix after stop drilling. Does anyone have any other thoughts or guidance on the repair? Crack is just behind the back seat, left side of the canopy, a little over 14” long.

Sorry to see you have joined the cracked canopy club.

If you are going to use plastifix be careful not to use too much which would require additional sanding. The additional sanding can create a lens effect.
You may see tiny cracks radiate from the ends of the crack after filling with platifix. Not sure what causes this other than the heat from the chemical reaction.

I am still flying with the canopy I repaired years ago, I may opt to finally replace it later this year, as I am planning on repainting the airplane.

Good luck!
 
OUCH! That's gotta hurt! -( I have a small crack about 3.5cm long from the same spot on my 8, near cried when I first saw it -)Stop drilled it and filled with clear silicon, not grown in over 2 years. My machine came from a warm climate all its lofe (18 years) I bought it back to a cold climate and the canopy wasn't happy! -(
 
A cracked canopy would give me a headache.

I am surprised having seen posts for cracks in the same area for 10 yrs that no self-respecting analyst has done an FEA to determine precisely why it happens and generated an alternate design as a fix.

Paul Dye posted his floating sleeve fix, but have not seen "field test" results since then for a definitive confirmation.
 
Crack Cause

Okay...Ill jump in here with my 1 1/2 cent thought...

It seems that everyone blames the canopy crack on the temps...or the drilling...or something else. I'm wondering if it's something that hasn't even been discussed yet.

When I was rigging the canopy latch to the frame/bow, I noticed that if the latch was pulling too much (a good tight fit!)...then the whole canopy frame would twist under the strain of the pulling latch. Could this be the cause of some of these cracking shells? Pulling and twisting the frame (and not very much) would certainly put a significant strain on the canopy when latched....then expose her to all the environmental stresses and that might be too much.

The latch doesn't have to be tight to the point of pulling the frame closed, it just has to be tight enough to latch.

Just my thoughts...
 
A cracked canopy would give me a headache.

I am surprised having seen posts for cracks in the same area for 10 yrs that no self-respecting analyst has done an FEA to determine precisely why it happens and generated an alternate design as a fix.

Paul Dye posted his floating sleeve fix, but have not seen "field test" results since then for a definitive confirmation.
There used to be speculation that this was caused by drilling holes in the canopy, but not that long ago someone had a crack in a canopy glued on with sika, which was pretty surprising. Perhaps that was a very special case, but not a lot of data. I'm only aware of that one case of a cracked canopy which was glued on.
 
I feel a little lucky with the canopy on my 10 year old RV-9A slider. It was built in Atlanta and lived there for 5 years, then 5 years in St. Louis. This is by far the farthest north it's ever been (central Minnesota). My hangar is unheated and it was -30F more than a few days and nights here this winter, and the canopy tolerated my intermittent bullet-heater use for my little avionics project well, as well as the in-and-out of the heated hangar I borrowedfor part of it. This time of year is often even worse in terms of temp swings. This morning it was 12F outside...this weekend it will be 70F.
 
There used to be speculation that this was caused by drilling holes in the canopy, but not that long ago someone had a crack in a canopy glued on with sika, which was pretty surprising. Perhaps that was a very special case, but not a lot of data. I'm only aware of that one case of a cracked canopy which was glued on.

I have kept a mental tally of crack reports on -8’s for a lot of years, and can think of several that were glued on that cracked. It does not seem to be a panacea....

Paul
 
I have kept a mental tally of crack reports on -8’s for a lot of years, and can think of several that were glued on that cracked. It does not seem to be a panacea....

Paul

I agree.

There have been glued side by side canopies that have cracked as well.
 
Relieving stresses?

If, and it’s a big if, there are significant residual stresses from mounting and latching, is there a way to bring the canopy up to temp for a while to stress relieve it?
 
There used to be speculation that this was caused by drilling holes in the canopy, but not that long ago someone had a crack in a canopy glued on with sika, which was pretty surprising. Perhaps that was a very special case, but not a lot of data. I'm only aware of that one case of a cracked canopy which was glued on.

Sorry to disturb your slumber Mickey but there have been many reports of cracking of Sikaflex canopies. The primer is virtually 50% volatile solvents. It actually works by attacking the acrylic. Any microscopic discontinuities around the prepared edge of the canopy and microscopic cracking can occur. By the work of fracture principle these microscopic cracks can progress into something much bigger during heating and cooling cycles (read expansion and contraction cycles).
There might be some advantages to using Sikaflex (although I'm not sure what they are) but avoiding cracking is not one of them. For those who wish to persist with Sikaflex I highly recommend they treat their canopy edges with the highest respect. That means that all edges should be continuously rounded and sanded with ever increasing grit to achieve a glass finish....not the slightest imperfection. Mind you, I would recommend that for any canopy...however it is attached. And if you drill holes then apply the same principle to the holes...no microscopic imperfections, particularly where the drill breaks through. A no-flute permagrit stone countersink (or similar) to clean up both sides of holes is highly recommended.

The problem with the cracking of canopies, whether attached with fasteners through holes, or via Sikaflex, is a matter of insufficient time being spent to prepare the canopy. There's a lot of builders who are very impatient, who want to see rapid progress...unfortunately the canopy is one area that does not lend itself to sloppy workmanship. It's an area that requires meticulous care and a fair bit of knowledge.
 
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My RV-8 had a 2-inch crack in about the same place when I bought it. It had been stop-drilled and covered with a decal. A couple of years later it cracked another 2 inches while sitting in a near-zero F hanger. I stop-drilled it again. I considered using repair materials, but then noticed that the crack seemed to widen and narrow very slightly with temperature, so I assumed the stress has been relieved and left it as is. Now it has two decals covering it. When I close the slider canopy, I push both sides forward just before latching it. I'm told that the process for manufacturing the canopies can produce stress points in the material. I recall seeing a photo years ago of a crack that started and ended well away from any edge.
 
Okay...Ill jump in here with my 1 1/2 cent thought...

It seems that everyone blames the canopy crack on the temps...or the drilling...or something else. I'm wondering if it's something that hasn't even been discussed yet.

When I was rigging the canopy latch to the frame/bow, I noticed that if the latch was pulling too much (a good tight fit!)...then the whole canopy frame would twist under the strain of the pulling latch. Could this be the cause of some of these cracking shells? Pulling and twisting the frame (and not very much) would certainly put a significant strain on the canopy when latched....then expose her to all the environmental stresses and that might be too much.

The latch doesn't have to be tight to the point of pulling the frame closed, it just has to be tight enough to latch.

Just my thoughts...

Mine cracked over night with an unlatched canopy (I never close it in the hangar all the way). Same place as all the others. 4 inches long or so. Fixed it using this approach:

https://www.polyvance.com/pfxcanopy.php-1

Followed by a sticker. 500 hours later doing lots of acro still holding up just fine... .. .

Oliver
 
CTE

I think a large contributor to many of these crack incidents is the vastly different coefficient of thermal expansion (CTE) between The Acrylic canopy and the steel frame. Acrylic has a CTE of at least 6X that of steel. Since we’re all taught to work on our canopies in a warm environment, we typically drill or bond to the frame in a warm shop - usually around 80 degrees or more. So if we attach the canopy in such a warm environment, then in cold weather we can expect the canopy to shrink 6 times more to than the steel frame. In the case of the -8 with the long canopy and frame and the fact that steel is much stronger than acrylic, I think we are seeing the canopy becoming over stressed in very cold weather. This coupled with the fact that acrylic becomes brittle at cold temperatures means that at some point the canopy is getting subject to tensile stresses that it can’t tolerate, hence the crack. Perhaps we would all be better served to perform the final match drilling to the frame or bonding in cooler temperatures to reduce the risk of cracking at low temperatures.

Skylor
 
Update on the canopy, to my untrained eye it looks like it’s attached with sikaflex as I can’t see any attaching hardware on the canopy frame.

Enlisted the help of some friends and followed the excellent thread on using the Polyvance method to repair the crack(s). All went well and I should start sanding/polishing the repaired area this weekend.
 

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Update on the canopy, to my untrained eye it looks like it’s attached with sikaflex as I can’t see any attaching hardware on the canopy frame.

Enlisted the help of some friends and followed the excellent thread on using the Polyvance method to repair the crack(s). All went well and I should start sanding/polishing the repaired area this weekend.
Very cool - please keep us posted. Those of us with sikaflex-bonded canopies who have recently been slapped out of our slumber can go back to sleep knowing that there is a fix in case it happens. :)
 
-8A Canopy Cracks

I was an early Sikaflex user (nearly ten years ago). Even though there is not a single rivet in the canopy assembly I still ended up with a long crack that occurred about two years ago in non-turbulent air cruising at 6500 feet. Crack is about 6 inches forward of the cross brace.

Hindsight: Meticulous edge dressing wasn't emphasized when I did the canopy (or I missed that part of the briefing). I believe I failed to adequately finish the canopy edges - coupled with introduced stress points when I clamped the canopy to the frame. The idea of stress introduced by the latching lever is a new twist that could be the cause.

I drilled a 1/8" hole about midway down the crack, used a screw, two washers and nut to clamp the edges together, then ran a strip of black electrical tape from side to side over the crack and non-cracked side. Looks like a two-part canopy. No issues in two years. I plan to replace the canopy someday when I overcome my current case of laziness.

The plane is a fler - I've flown between CA (IYK) and PA (FWQ/AFJ) 26 times since first flight, including eight times since the canopy cracked.
 
I wonder if there's a way to mount the canopy which allows for some small flex?

In woodworking, you have to allow for the fact that the wood will shrink and expand with changing humidity when the piece is large - like a table top.

So you elongate the fastener holes slightly in the direction of movement. The screw head keeps the piece tight to whatever you are screwing it to.

Or you use things called "buttons" which hold the table top down snugly but allow movement because of the elongated slits they fit into (see thumbnail).

I wonder if there isn't some analogous riveting method that will hold the canopy snugly to the frame yet allow some movement in a helpful direction.
 

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I was an early Sikaflex user (nearly ten years ago). Even though there is not a single rivet in the canopy assembly I still ended up with a long crack that occurred about two years ago in non-turbulent air cruising at 6500 feet. Crack is about 6 inches forward of the cross brace.

Hindsight: Meticulous edge dressing wasn't emphasized when I did the canopy (or I missed that part of the briefing). I believe I failed to adequately finish the canopy edges - coupled with introduced stress points when I clamped the canopy to the frame. The idea of stress introduced by the latching lever is a new twist that could be the cause.


The RV Sikaflex phenomenon was driven purely by a fear of canopy cracks. Neither Vans Aircraft nor the Sika company ever approved of the application (and both actually advised against it) but nervous builders, encouraged by other nervous builders, disappeared down the rabbit hole. It was also a case of the blind leading the blind and there were no established protocols and as a result probably no two canopies using the Sikaflex method were ever fabricated exactly the same. And as we now know many of them cracked.

The Sikaflex sealant would not adhere to the acrylic without the prior application of the Sika 209 Primer. But the 209 primer was 20% Ethyl Acetate, 25% MEK, and 0.5% Isocyanate Monomer. All of these solvents are "N" class which means that on acrylic sheet "immediate damage may occur such as severe crazing, cracking, or permeation losses".

But builders did not know this and as they saw an increase in builders on VansAirforce adopting the technique they felt there was safety in numbers. And they intuitively felt that any primer produced by Sika to be used on acrylic must be safe. But in reality Sikaflex was aimed at the yachting/boating market for relatively small acrylic windows, mostly unformed (or with very little heat forming) and so with relatively low internal stresses.

The Vans canopies were another game altogether....very large polymeric areas with extreme curvature and high internal stresses. And with often crudely prepared edges exhibiting microscopic discontinuities and aggressive solvents in the primer it was a bad combination.
 
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I wonder if there's a way to mount the canopy which allows for some small flex?

In woodworking, you have to allow for the fact that the wood will shrink and expand with changing humidity when the piece is large - like a table top.

So you elongate the fastener holes slightly in the direction of movement. The screw head keeps the piece tight to whatever you are screwing it to.

Or you use things called "buttons" which hold the table top down snugly but allow movement because of the elongated slits they fit into (see thumbnail).

I wonder if there isn't some analogous riveting method that will hold the canopy snugly to the frame yet allow some movement in a helpful direction.


Actually yes! You drill the holes in the plex oversize to 1/4” and get a length of surgical rubber tubing with an interior diameter of 1/8” (the size of the rivets), and an O.D. Of 1/4”. You cut very short lengths of the tubing, the thickness of the canopy, to make soft “washers”. Pop a washer in the hole in the plex and the rivet goes through that - you now have given the plex some room to move if it grows or shrinks. I got the idea from someone else, and it seems to have had no adverse affects on the canopies I have installed. I know lots of others that have done it as well. Does it do any good? I haven’t done a statistical analysis.....or even gathered the data...
 
Actually yes! You drill the holes in the plex oversize to 1/4” and get a length of surgical rubber tubing with an interior diameter of 1/8” (the size of the rivets), and an O.D. Of 1/4”. You cut very short lengths of the tubing, the thickness of the canopy, to make soft “washers”. Pop a washer in the hole in the plex and the rivet goes through that - you now have given the plex some room to move if it grows or shrinks. I got the idea from someone else, and it seems to have had no adverse affects on the canopies I have installed. I know lots of others that have done it as well. Does it do any good? I haven’t done a statistical analysis.....or even gathered the data...

I plan to do the same Paul but with #6 Rivnuts and C/S Screws. I've been told the same method with the rivets work well but can wear loose at the skirt after several hundred hours..
 
Does anyone know if any of the Todd's Canopies have cracked? Just wondering if there is a significant enough difference between Todd's and Van's that it might provide a clue for a more permanent solution. I realize that there aren't very many of Todd's out there.

Personally I would prefer a thicker canopy than what Van's provides although I have no data to suggest that would solve the problem.

Chris
 
I plan to do the same Paul but with #6 Rivnuts and C/S Screws. I've been told the same method with the rivets work well but can wear loose at the skirt after several hundred hours..

That’s exactly how we’re installing the canopy on our Xenos....screws instead of rivets.
 
Todd’s canopy crack

Chris, I have a tinted Todd’s, screwed on. Every screw hole was buffed and polished... On a -20 night I was in my hanger and heard the break. Stop drilled it and it hasn’t migrated in 5 years. The crack is 5” long, in the usual spot. I fixed the crack using a method I picked up here and aside from the “scar” it has held up well. I have a Todd’s canopy in a box to replace the cracked one if I ever have the time to take that on 😁
 
Canopy repair is completed and flight tested with good results. Some final thoughts now that it’s repaired. I took a closer look at the canopy frame and it’s riveted not bonded. The canopy had one small crack further back when I bought the airplane and then this large one. Both cracks originated at rivet holes. If I had to start over, I think I would have made the V cut a little larger and sprayed water on the Dremel bit as we were going. We were very careful about temperature when cutting the V but it would have been cheap insurance. Overall, there is a definite scar that’s very visible but not in the line of sight of either passenger or pilot. Glad to have her back flying and can move on to other improvements and putting more time on the overhauled engine.
 
Some pics of the final repair, still working on how to rotate the image. Sanding/polishing used the following grits 220/400/600/1000/1200/1500/2000/2500. Polyvance was out of headlight polishing kits so Novus 2 and 3 were used for final polishing. We also used 2/3 on the windshield and it came out great. Not quite like new but close.
 

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Ordering my new canopy tomorrow - going to try the tubing flex method. Not happy about how many cracks the 8's have seen. All my close friends with 8s all have cracks on them. Recently my original one doubled in size and a new one propagated. Definitely there seems to be a need for a change of some sorts in the design or materials used as this seems to be too common of an issue.
 
For those of you using screws, can you provide more info on how to fasten to the canopy frame, types of screws, etc? I'm doing a canopy crack replacement next week and leaning towards silkaflex.
 
I have installed quite a few canopies on RVs over the years. At least a dozen or more. So far none have cracked.
I have never used sikaflex but have always drilled the canopy and either used rivets or screws to fasten them.
When using screws, for example on the RV8, I initially drill the canopy to the frame and cleco it on. There are some high stress areas, noteably behind the cross brace where the tubing size changes. I typically do not put anything in that area, for maybe 8 inches or so, depending on how the canopy is fitting, but leave it to float. The canopy does require some pressure to make it conform but I try to reduce that as much as possible. The way the canopy rests on the frame is critical. It is important to initially get it to rest as snugly as possible all around the frame before you drill any holes. Once you drill the first hole you are committed.
I drill the frame holes to #35 and tap threads into the frame for #6 screws. Just chuck a tap into a drill and use some Boelube. Debur them before putting the canopy back on.
Then I drill out the canopy holes to 1/4" using a plexiglass drill bit. If any of the holes look rough, I use a pencil soldering iron to touch the inside edges of the hole and seal any tiny cracks or burrs. Usually, I just do them all, just in case.
The countersinking for the screw gets done on the skirt.
Also, sand and smooth all the edges of the canopy.
When installing the canopy, I use the vinyl hose washer Paul mentioned on a few of the holes, just to line things up. The rest I leave open for maximum expansion and contraction.
If I am riveting the canopy on, I drill to at least 3/16" on the canopy and only use a hand blind rivet puller, since the pneumatic pullers can pull with too much sudden pressure, in my opinion. I always prefer a 1/4" hole in the plexiglass but if the skirt is trimmed a little close, the 1/4" hole may show a little, so I go to 3/16" and so far it hasn't been an issue.
In spots where there is a space between the canopy and the frame, or the skirt and the frame, use nylon washers as standoffs to keep the pressure off the canopy/skirt. You may have to stack them in some places to get the fit just right. It will make everything look nicer and relieve stress areas.
Fitting the canopy can be stressful.
Hope this helps and good luck.
 
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Definitely there seems to be a need for a change of some sorts in the design or materials used as this seems to be too common of an issue.

When Todd was still with us I had him make a custom canopy for my F1. He special ordered heavier material to make it and it took five tries to get it right. I have been very happy with it, and its glued on with Sikaflex. It has never creaked or popped in the hangar like my RV6 canopy did. Perhaps you can reach out to Jeff and Becky at Airplane Plastics, who are Van's supplier of canopies, for a custom heavy canopy. My Comanche has 3/8" windows all the way around including the windshield. I've had them out and don't find them objectionably heavy.
 
Find Paul Dyes post on this subject.

For those of you using screws, can you provide more info on how to fasten to the canopy frame, types of screws, etc? I'm doing a canopy crack replacement next week and leaning towards silkaflex.

You will have to do some searching of VAF, but Paul Dye has an excellent series of posts showing how he put an elastic sleeve around the fasteners to allow movement of the canopy from thermal expansion conflict with the steel frame. One other noted that he used screws rather than rivets and it worked well too.

Since it is your canopy, I'll allow you to do the sleuthing to find the posts. ;)
 
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