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The Unhappy Cracky Club

Nicely done repair! Canopy issues are my biggest worry. I really don't want to go through all that again. More so the windshield....
 
It's surely interesting that so many canopies bonded on with Sikaflex have developed serious cracks.
 
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oversize the holes and screw it on so it floats and has room to expand may be one method to eliminate cracks.
 
Compae the coefficient of expansion of the plexi with the aluminum (IIRC it's a factor greater than 3) It gets significant as the edge of the canopy grows in length.
 
That's a nice repair. Too bad there isn't a similar product that is transparent. Or is there? There is a guy near Montreal here with a 4 who has had to replace his canopy several times. Big temperature variations up here between winter and summer and his hangar is not heated.
 
Compae the coefficient of expansion of the plexi with the aluminum (IIRC it's a factor greater than 3) It gets significant as the edge of the canopy grows in length.

yes, by analysis, if you connect two dissimilar pieces and then apply a temperature delta, the max stress is in the middle. exactly where this canopy cracked.
 
Sport Aviation Plexi Repair Article

Recently, I've been editing articles from old copies of Kitplanes and Sport Aviation. The November 2007 issue of Sport Aviation has a great article regarding a major Plexi repair. It was on the top turret for a B 25. Repair came out very well, with good optics. The article author is Scott Bixler. Article starts on page 100. Check it out.

Charlie
 
Too bad someone couldn't come up with a canopy material that was crack proof!
That would be great until you are upside down and trying to get out. It is a pain to change the canopy, but not that bad once you just dive in and do it.
 
Timing

Yes, but surely the migration to Sikaflex was prompted almost exclusively by the initial belief that bonded canopies would not crack.

Would that be at the time of construction (i.e. drilling a hole and hearing a "crack"), or sometime later, once the plane is flying? It is a bit of a dilemma to decide what fixing method to use; some hard data would be helpful.
 
Yes, but surely the migration to Sikaflex was prompted almost exclusively by the initial belief that bonded canopies would not crack.
I thought the benefit was that canopies wouldn't have stress concentration points (like screw holes). So the chance of cracking would be *reduced*. I don't think anyone could guarantee no cracks ever, when moving between temperature extremes.

I'm without numeric data to support this, but based on posts here on VAF there still seems to be a higher percentage of cracks in screwed canopies than Sikaflex'd canopies.
 
-8A Canopy Cracking

I was an early Sikaflex user - not a single rivet in the canopy. However, after six years I developed cracks on both sides forward of the cross bar.

I believe they were caused by inadvertent stress risers induced during the clamping steps of attaching the canopy to the rails. And perhaps not paying excruciating attention to smoothing the canopy edge.
 
Anybody out there have an UNCUT RV-8 plexi that could do a simple experiment?
Measure the overall length of the canopy at 55F (or colder) - than again after you bring it in & warm it up to about 85F, to determine how much that big hunk of plastic actually expands lengthwise. This will illustrate how much tension this assembly has to deal with on a repetitive basis.
Unfortunately, it would be much harder to measure sideways & diagonal expansion rates.

Another experiment - anybody out there have a bit of Sikaflex that they could rig a test fixture to see how flexable a 4 ft glued flat plate to a 1/2 tube will move laterally to simulate plexi expanding along a RV8 canopy frame?
I know it is called SikaFLEX, but is it in this situation?
 
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The expansion coefficient is part of the manufacturers data and specification.

Here's something I just copied

Product

Inches/Foot/100 ?F
Plexiglas? sheet Aluminum Lexan

.062 .016 3.75 10^-5 in/in/Deg F
 
I woke up to a big crack in my canopy last time I was home and also used PlastiFix to repair it. That stuff is legit! I haven't had time or warm enough temps to polish it out yet, but I got a MicroMesh kit to do the work and hopefully it turns out as nice as yours did!

Sadly I didn't take any good photos of the crack or PlastiFix work (I know, I know. I've already reprimanded myself), but my crack is on the right side just forward of the crossbar behind the pax seat and is about 11" long. I flew a few times after the repair with no issues...and it was COLD!

My canopy was drilled and riveted, with oversized holes, but the crack did originate from one of the holes. My thoughts are that the builder didn't spend enough time deburring and polishing the holes before installation...they look kinda rough.

I'll be sure to take pix of the fix and polishing this summer when I get round to do it...

PlastiFix - https://www.aircraftspruce.com/catalog/cmpages/01-00542-45.php?clickkey=8215

Micro-Mesh - https://www.aircraftspruce.com/catalog/cspages/micromeshstd.php
 
I wonder if some of this could be exacerbated by assembling the canopy in a relatively cold shop, vs doing it in a warm shop? If it were assembled and allowed to cure at the upper end of the temperature range, then if it were cooled the Plexi would only be placed in compression, not tension. Does anyone know if Plexi has better compressive strength than tensile?

Scott, it might be a good idea to add this writeup to the RV Wiki on the Matronics page. The writeup I did that compiled all of the knowledge available at the time for installing canopies with Sika (back when I was building and planned to use that method, but before I got that far and eventually bought a flying RV instead) is still there, with photos, 13 years later.

http://www.matronics.com/wiki/index.php/Gluing_Your_Canopy
 
The coefficient of thermal expansion of plexiglass sheet is .062" per foot per 100 F. That's a lot. Also, from ePlastics.com:
To ensure good performance in environments where temperature varies widely, Plexiglas® sheet should be installed in a channel frame that permits the sheet to expand and contract freely. The channel frame should be deep enough for the sheet to contract fully and still stay within the frame.

Avoid inflexible fasteners such as bolts that do not permit expansion and contraction. Tapes and sealants that adhere to both the acrylic sheet and the frame should be sufficiently extensible to accommodate thermal expansion of both.
 
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Seems to me the optimal temp for gluing is the mean between the highest expected temp, and the lowest expected temp that the unit will be exposed to.
The last Sika job I did, I modified the frame to under size enough to provide a 1/2 inch buffer/ gap between the frame and the canopy. Further, I put 'lightening' holes (gaps) in the Sika to enable more flexion in the Sika joint. Plastic soda straws placed on 1/2 inch spacing, to create the holes in the Sika.
 
However, I wanted to address the repair option when posting this thread, not the original build process with either rivets, screws, or SikaFlex. For those whose canopies suffer cracks---even severe ones like mine---it might be good to know that repair is not only possible but also acceptable. I'll leave the debate over why the cracks occur to another thread.
Understood! I was suggesting that the article go on the Wiki, but not on the Sikaflexing page... that was just an example. A different page could be added for Repairing Canopy Cracks.

It was an excellent writeup, and it would be great if it could withstand the test of time and not be subject to image hosting companies disappearing.
 
Missing posts?

Be ever hearing, but never understanding,
Be ever seeing, but never perceiving.
---Isa 6:9

I don't understand. The first post I see in this thread is this one but members are commenting on the repair. Did some posts or pages get deleted?
 
I don't understand. The first post I see in this thread is this one but members are commenting on the repair. Did some posts or pages get deleted?

It would appear that Scott has edited all of his posts, removed the aviation content, and replaced it with scripture.
 
It would appear that Scott has edited all of his posts, removed the aviation content, and replaced it with scripture.

That is unfortunate. I have a crack in my -4 canopy plexiglass and was optimistic this thread would hold the answers. IIUC, it has been referred to many times.

I will keep looking or start a new thread.

Thanks Kyle
 
Posting Rules:
"Posts should not contain a single molecule of anything even remotely resembling politics."
 
Original Thread

Since the original thread was so valuable, I saved it as a PDF way back when. Unfortunately it's more than 70 megs, too busy to post here. If someone has a place where I can upload it without establishing a new account anywhere, I'll do that. Ideally, then you'll post that link here for everyone to access.

Send me a PM.

Dave
 
Generous offer Dave, thanks.
One way would be to use a Dropbox, of which you could publish the link here for anyone interested to download.

Now I would think the mods could delete this very thread, then reinstate and close the original?
It was indeed a reference, pity to see this kind of information mine go...
 
Since the original thread was so valuable, I saved it as a PDF way back when. Unfortunately it's more than 70 megs, too busy to post here. If someone has a place where I can upload it without establishing a new account anywhere, I'll do that. Ideally, then you'll post that link here for everyone to access.

Sadly, at just under 80 total hours on the airframe, I have joined the Unhappy Cracky Club. RV-7A, Tip-up, bonded with Sikaflex. Small 1cm crack forming on the rear window near to the center channel that runs aft of the rollbar.

David generously sent me the large PDF of the original thread with pictures, and I've offered to host it here, unless and until the bandwidth required exceeds my budget, so please go easy on my web site.
 
My "team" just finished doing a Sika attach of the aft part of an -8 canopy to the frame. It's a lot of work, and tests your patience!

I noticed in this amazing write-up from Draker regarding this repair, that during the widening of the groove, the crack was pulling apart and "flattening out".

"... A piece of wet-dry sandpaper was inserted at the base of the crack as the V-groove channel widened.
For the final 5 or 6 inches near the apex, the crackline suddenly began to flatten out and carry the fissure into the interior of the bubble" page 9

You can also see later in the finished repair, via the distortion in the surface, not the repair itself, how the canopy is flexed to a relaxed state causing straight lines seen through the canopy to distort. Take a look at the runway edge paint and horizon in two photos.

Don't get me wrong, this was an amazing job of doing a nice repair. The point is that despite the Sika process and no holes through the canopy what-so-ever, residual stress captured during the original fitting is driving a load towards an edge of the canopy somewhere. We all know that these canopies don't just lay down on the frame without any pressure. If they did, you would likely never see a crack. But the reality is that there will be some stress built in, it's unavoidable. So, if that edge isn't polished, the load transferred to the defect at the edge may eventually turn into a crack.

Lesson learned: Do whatever it takes to allow the canopy to fit to the frame (notice the word "allow"). Avoid holes. Leave a gap between the frame and canopy to allow the Sika to flex. Polish all edges until clear and notch free.

We did some things different that may get posted in the future, especially changes that helped working in cold temps. Stay tuned.

Build on ...
 
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