Cracking the cracking canopy code?

[AN23]

For years now, I've read about canopy cracks. For a while, we were all convinced that it was an installation problem (screws versus rivets/deburr better/ polish all the edges/sikaflex was the answer) but then, years later, we're still seeing cracks and lots of them. We also blame temperature changes but they still crack even in fairly mild temps. I just read where a Todds canopy mysteriously cracked while sitting on the ground. A buddy of mine recently taxiied his RV-8 in the grass (fairly rough) and came back two days later and there are multiple cracks. In my opinion, something else, perhaps in combination with all of the above, is at work. One thing I've observed is that many of us jack our airplanes one side at a time for tire changes, brake or bearing work. . Is it possible that if we're doing that with the canopy closed, we are applying a slight twist to the canopy that could cause micro-cracks that could, I repeat could, later develop into big cracks when presented with the above build and environmental conditions? The cracks on my friend's 8 are very similar to the most recently reported cracks and are on opposite sides of the canopy. They all occurred simultaneously, while sitting on the ground. I can honestly say that I don't recall ever moving my canopy while its on the jack so I can't say for sure if there was any additional resistance indicating it was in a bind. My canopy is usually open during maintenance.

 

[rv8ch]

I too am very interested in understanding the reason a canopy cracks.

One thing I think that recently came to light is that the edges of the canopy need to be very smooth, sanded, polished, etc.

When someone has a crack, it's rare to get a detailed analysis of what might have caused it, since usually people repair the crack in-situ, rather than taking it all apart. Some things I think we might hear, if we had a detailed forensic review of the failure, would be "The edge of the canopy was not polished well enough" or "The canopy was cleaned with an inappropriate chemical" or "The crack started in one of the rivet/screw holes" or "There was not enough thermal expansion thickness between the canopy and the frame"

Keeping these in mind when building the canopy might reduce the chance of cracking, but no idea if it will completely eliminate it.

 

[wirejock]

Testing

Save those scraps and perform some tests.
I don't claim to be an expert and certainly don't have a degree in an applicable area. I did try some tests on scrap before bonding mine.
I found an unpolished edge was pretty easy to break. A polished edge took a lot more force. I concluded some cracks start at stress risers on edges or holes and propogate due to tension. I should have examined the break edges. To that end, I used the fewest clamps possible. The canopy was bonded in as natural a curve as possible. Basically just the sides were clamped with a straight edge. The bow forward and aft are almost natural curves. I put one clamp forward and one aft. I still cross my fingers a lot.
I may be way off base.
My $.02

 

[Marc Bourget]

All plastic will crack but I think builders are observing the "manifestation" of "hidden"* internal stresses introduced at the time of forming the canopy and expressing themselves as cracks.

Poorer the forming technique/process, greater the potential for cracking.

I have a blown canopy formed in 1970 that started showing cracks about 5 years ago. I've known guys trimming their brand new canopy to have cracks form "spontaneously".

* hidden ignores the fact that the material is "see thru"

 

[Roadjunkie1]

Cracking in canopies.....

For years now, I've read about canopy cracks....The cracks on my friend's 8 are very similar to the most recently reported cracks and are on opposite sides of the canopy.

Maybe this has been discussed before, but I am wondering what model is most prone to canopy cracks? Seems like most are in the -8 and in the same places.

I don't want to even say anything about my -4 canopy. I do look every time I go out and fly or work on her. Perhaps the expansion of the aluminum frame is such they are not as prone to cracking. And, of course, installation techniques. I had experience with plexiglass before replacing glass on production aircraft including windshields and applied what I knew to my installation. I have been in heat that did not allow me to close my canopy after I had fueled up and had to "borrow" the shade of a hangar to get her to cool down enough to close. (I worked hard to get a tight-fitting canopy! It paid off!) This has happened once. I have also had her in sub-freezing temperatures and am VERY cautious as to how I open and close the canopy. So: there are some extremes of temperature that, luckily, have not bothered the canopy.... (insert crossed fingers and touching wood here).

Do we need to do a survey, or has that been done?

 

[DanH]

For years now, I've read about canopy cracks. For a while, we were all convinced that it was an installation problem (screws versus rivets/deburr better/ polish all the edges/sikaflex was the answer) but then, years later, we're still seeing cracks and lots of them.

Sure we are. Nothing much changed. Some builders are very detail oriented, some not so much.

Early Kitfox models had wrap windshields, not molded. They required a U-shaped cutout for the front spar carry-through. The plastic was bent 45 degrees at the cutout, and the lower side got an additional bend aft to meet the doorframe. See below.

As you might imagine, all this bending in a small area created quite a lot of residual stress in the flat sheet. The common result was cracking radiating inboard from the "U", even when the edges were sanded smooth with wet 600 grit. First windshield for this project was showing crazing and cracking in less than a week.

Sanding with 600 wet, then two passes with methylene chloride on a Q-tip to melt the edge smooth and clear, i.e. no microscopic gouges? No cracks when I last saw it about five years ago, after flying 40 or 50 hours. I'll send a note and ask if it ever formed any.

Dan

 

[graywoodworking]

Stress cracking and crazing

Plexiglass is highly susceptible to stress cracking and crazing from exposure to various chemicals. Do not use Windex or anything with ammonia to clean the canopy. Same goes for denatured alcohol and acetone, etc. Also, do not use Loctite on any fastener that is used to attach the canopy.

 

[SabreFlyr]

Do not use Windex or anything with ammonia to clean the canopy. Same goes for denatured alcohol and acetone, etc.

Oops! No denatured alcohol? How about isopropyl? Not sure about the difference between the two (I believe that denatured is stronger). I've used a little iso to clean off some Sharpie marks.

 

[RV8JD]

From Van's: "ACRYLIC CANOPY AND LEXAN WINDOW HINTS"

https://www.vansaircraft.com/faq/acrylic-canopy-and-lexan-window-hints/

"Cleaning And Use Of Liquids

For general cleaning use Dawn dishwashing liquid or equivalent and water followed by a clear water rinse. To prevent water spots, blow dry with compressed air or wipe dry with soft cotton flannel. Plexus, Sprayaway #848 Industrial Plastic Cleaner, or All Clear can also be used for day to day cleaning. Grease, oil, tape residue, etc. may best be removed with Mineral spirits, refined kerosene, white gasoline, naphtha, or isopropyl alcohol. Wash approved solvents off of canopy with Dawn dishwashing liquid and water. It is best to avoid using products that are not specifically formulated for acrylics on your canopy such as Rain-X or Lemon Pledge.

CAUTION: DO NOT use Loctite, aromatic solvents, acetone, benzene, ethyl acetate, carbon tetrachloride, lighter fluid, lacquer thinners, gasoline, toluene, window sprays, concentrated alcohols, keytones, scouring compounds, ammonia, or 409 cleaner on or around acrylic or Lexan canopy materials.

CAUTION: DO NOT allow electrical tape or auto fuel to come into contact with Lexan as it will also cause crazing. This is a particular issue to beware of when refueling the RV-12."​

 

[Draker]

Huh--I never gave much thought to what to use to clean the canopy. I've been using Sprayway 443331 Ammonia Free Glass Cleaner, but even if it doesn't have ammonia, it might not be appropriate. I'll give that plastic cleaner a try.

 

[rv8ch]

Every time I go to an airshow and see a booth where they sell windshields and door windows for aircraft I ask them what I should use to clean, they always say the same thing - lots of water and a microfiber cloth. They don't recommend any "product".

 

[PaulvS]

The above advice from Van's says to avoid electrical tape on the canopy because it can cause crazing. Yet the instructions for fairing with fiberglass on the canopy include a double layer of electrical tape around the edge of the fiberglass, so I question whether it is safe or not? Maybe there is a time limit for how long to leave electrical tape on the canopy.

 

[mburch]

The above advice from Van's says to avoid electrical tape on the canopy because it can cause crazing. Yet the instructions for fairing with fiberglass on the canopy include a double layer of electrical tape around the edge of the fiberglass, so I question whether it is safe or not?

The prohibition on electrical tape is specific to Lexan only, not fiberglass.

 

[BillL]

Fantastic!!

Sure we are. Nothing much changed. Some builders are very detail oriented, some not so much.

Sanding with 600 wet, then two passes with methylene chloride on a Q-tip to melt the edge smooth and clear, i.e. no microscopic gouges?
Dan

Methylene chloride? That is the main ingredient in the classic useful paint remover. I thought acetone, but heard it was really not good.

How did you come on this Dan, a secret polymer chemist friend?

The 8 seems have a statistical zone for cracks, not sure others do. And, the surface finish of a hole (loading point AND stress concentrator) would seem to be a significant factor. If we could treat the hole interior to get a rounded and fine surface finish that would help greatly. This, as an experiment in coupon strength, could make a great KP article, if done well.

To the OP - - - But, a full scan of the RV8 plexiglass and model of the frame all for FEA analysis would seem to be the first step. i.e. find the load. This 8 failure has been occurring for 10+ yrs. A hangar colleague, said he landed and rolled the plane in the hangar was and was walking away when the snap occurred. Cool, but not frigid day.

BTW Here is a solid mechanics view of a stress concentrator, it would show (at least) a 2X stress increase even for an unloaded fastener hole (not reality). So, since we know that the Sikka canopies fail in the same "critical" zone, it would seem that a different solution should be pursued in addition to polishing the edges/holes. Until the pressure loading , stress, and differential expansion are quantified a solution pursuit is shooting bats in the dark (experimentally).

 

[DanH]

Methylene chloride? That is the main ingredient in the classic useful paint remover. I thought acetone, but heard it was really not good.

How did you come on this Dan, a secret polymer chemist friend?

Naa. Methylene chloride (dichloromethane) is the base ingredient in solvent adhesives for plexiglass. Example: https://www.eplastics.com/pdf/IPS3SDS.pdf

Let me stick a warning in here. Methylene chloride vapor can kill you dead as canned tuna. Work in a well ventilated place.

To the OP - - - But, a full scan of the RV8 plexiglass and model of the frame all for FEA analysis would seem to be the first step. i.e. find the load.

Yep.

 

[Webb]

It seems like a lot of the 8 cracks occur in the same spot around the crossover bar. It’s just a guess but I think the size/length of the 8’s or canopy is one of the main reasons it cracks in the spot.

When I looked at John‘s first crack, I thought to myself, that perhaps a preemptive strike of cutting the canopy above the crossover bar would reduce the potential for a crack and reduce the stress focal point.

If the canopy was split by a purposeful cut and an aluminum band replaced on top I think it would look in as though it belong there.

Other thoughts about it?

 

[rocketbob]

Seems like one could come up with a tube within a tube expansion joint to allow for expansion. Like a bridge...

 

[PaulvS]

Methylene chloride

Here is a test pic of methylene chloride applied in some holes on a bit of scrap canopy, these are the holes with red arrow labels. The chemical has kind of etched into and roughened the surface of the plexi. The MC was applied inside the holes with a pipe cleaner but there is no noticeable improvement in the hole smoothness. Clearly the process is not idiot proof!
The cleanest holes that I've been able to produce is by using a new drill bit that was sharpened with zero rake per the article that was in Kitplanes.

 

[rv8ch]

Failure bias?

We are hearing a lot about cracked canopies, but not much from people that have not (yet?) had a cracked canopy. Is this an example of failure bias where we focus on the things that failed, not on those that succeeded?

When I go to Oshkosh and look at the 100s of RV-8 canopies, I certainly remember those that had a crack. My recollection is that it's way down in the single digit percentages, but I could be mis-remembering, and perhaps those with a crack chose not to display their aircraft at the big show. ¯\_(ツ)_/¯

 

[Webb]

I noticed in the picture that the hangar door edge kinda gives the appearance of a split canopy like I suggested. The split could be oriented to the builder’s preference.

Also, on my original thought of splitting the canopy, have there been cracks in Show Planes’ canopies which are shorter than the stock Van’s?

 

[AN23]

We are hearing a lot about cracked canopies, but not much from people that have not (yet?) had a cracked canopy. Is this an example of failure bias where we focus on the things that failed, not on those that succeeded?

When I go to Oshkosh and look at the 100s of RV-8 canopies, I certainly remember those that had a crack. My recollection is that it's way down in the single digit percentages, but I could be mis-remembering, and perhaps those with a crack chose not to display their aircraft at the big show. ¯\_(ツ)_/¯

A recent poll on cracked canopies. This certainly shows the problem is much greater than a single digit issue!

 

[Tag]

Rocket Bob: This idea, I’ve been wondering about for a while now. The canopy frame has a welded joint where the tube size drops at the aft end. I’m curious about the possibility of leaving said “joint” unwelded allowing the separate tubes to slide in/out during the temperature changes? This would account for much of the differing rates of expansion (plexiglass vs. metal tube)

 

[AN23]

Thats a great idea!

 

[BillL]

Methylene Chloride is hard to find.

SR-97 is lowest cost, (not $26/oz) and pretty pure.

 

[DanH]

And doesn't last forever in an opened pint can. I just shook mine and it's dry.

Ordered more, just for fun, about $40 shipped. www.dawnscientific.com

The posted Kitfox example was lexan, and yes, I know lexan is polycarbonate, not acrylic, but I don't recall any difference with the dichloromethane. We'll see.

There is also flame polishing...

 

[Shark]

Old and Brittle

I replaced my slider canopy this past winter and I really believe it simply gets old and brittle with time, you could break the piece's with you bare hands, not the case with a fresh new plex. Mine was from 02-03ish and as much as we hate to admit it that was 20 years ago. Also, I Sikaflexed the top and drilled the sides, if I could do it again I would have Sika'd the hole thing

 

[PaulvS]

The prohibition on electrical tape is specific to Lexan only, not fiberglass.

The electrical tape gets applied to the plexiglass as a boundary for the fiberglass. I don't expect the tape will affect the epoxy glass, but Section 5 of the build manual states that some electrical tape can damage plexiglass (acrylic) and Lexan (polycarbonate) and to avoid leaving it on for more than 2 days. That's not much time to get the fairing done!

 

[Dr_Dave_MT]

Sure we are. Nothing much changed. Some builders are very detail oriented, some not so much.

Early Kitfox models had wrap windshields, not molded. They required a U-shaped cutout for the front spar carry-through. The plastic was bent 45 degrees at the cutout, and the lower side got an additional bend aft to meet the doorframe. See below.

As you might imagine, all this bending in a small area created quite a lot of residual stress in the flat sheet. The common result was cracking radiating inboard from the "U", even when the edges were sanded smooth with wet 600 grit. First windshield for this project was showing crazing and cracking in less than a week.

Sanding with 600 wet, then two passes with methylene chloride on a Q-tip to melt the edge smooth and clear, i.e. no microscopic gouges? No cracks when I last saw it about five years ago, after flying 40 or 50 hours. I'll send a note and ask if it ever formed any.

Dan

Methylene chloride! That is Unicorn saliva now. I have 1/2 can left of that magic chemical.

 

[DanH]

Methylene chloride! That is Unicorn saliva now.

IIUC, bathtub refinishers were using it as a stripper in enclosed spaces, resulting in fatalities. Don't do that.

Got a fresh bottle, and I'd like to run some experiments. Digging around in the shop, I seem to have lexan, but no plexiglass. I started to pick up some generic acrylic plastic at the big box store, but I know little about variation in plastic formulations. Is big box plexiglass sheet the same as that used for a blown canopy? I dunno.

So, a question; does anyone out there have canopy cutoffs they might care to send?

 

[lr172]

S

Sanding with 600 wet, then two passes with methylene chloride on a Q-tip to melt the edge smooth and clear, i.e. no microscopic gouges?

Dan

what a great idea!

 

[wirejock]

Plexi

IIUC, bathtub refinishers were using it as a stripper in enclosed spaces, resulting in fatalities. Don't do that.

Got a fresh bottle, and I'd like to run some experiments. Digging around in the shop, I seem to have lexan, but no plexiglass. I started to pick up some generic acrylic plastic at the big box store, but I know little about variation in plastic formulations. Is big box plexiglass sheet the same as that used for a blown canopy? I dunno.

So, a question; does anyone out there have canopy cutoffs they might care to send?

I have a piece of one aft side cut. How much do you need? I have a strip about 2" wide x 12".

 

[DanH]

I have a piece of one aft side cut. How much do you need? I have a strip about 2" wide x 12".

Perfect. Ck PMs