What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Canopy Crack - What Would Cause This?

pvalovich

Well Known Member
-8A with 130 hours since April and not a single rivet in the canopy. Sikaflex all the way.

Friday morning on preflight I discovered a 3+ inch vertical crack in the canopy starting from the left side rear canopy frame cross brace. After going through my vocabulary several times, and stop-drilling, I'm still trying to figure out "why?".

Any other Sikaflex users have mystery canopy cracks? Anyone have a theory besides random canopy gremlins roaming the country creating havoc?
 
Stress, When Acrylic is heated by cutting, forming, etc. It is a crack waiting to happen. It can be anealed BUT it takes about 8 hrs in a controlled enviroment. time will also take some stress out. You can test using a scrap of left over plexi, wipe some Trichlorethylene (weldon) on the edge and see what happens. The edges of our canopies have to be sanded smooth allaround. Or use a cutting method that generates no heat
 
Canope crack

I have exactly the same crack in my canopy. It was caused by the canopy latch pulling too hard on the left side of the canopy frame, resulting in distortion of the frame and acrylic canopy. There should be virtually no tension on the canopy hook when the canopy is latched. It needs no tension, as the air loads in flight firmly close the canopy. If you are not satisfied by the lack of overcenter latching pressure on the canopy handle, install a "ball screw" from McMaster-carr. I would be glad to send a photo of this system installed in my RV8 if you would like.

I have repaired this crack using catylized acrylic monomer, but the repair was unsuccessful. It cracked again, whereupon I stop drilled it. The final solution was to put an American flag sticker over the crack to hide it.....

Hope this helps!
 
Total sikaflex

I have two cracks. One, aft right side - about two inches. Happened on a very cold winter morning.
The second is right dead center (top) aft about two inches.

Heaven knows how many cracks I would have had I riveted the thing together.
 
Ouch!

.....Sikaflex all the way.....I discovered a 3+ inch vertical crack in the canopy....
I have exactly the same crack in my canopy.....
I have two cracks. One, aft right side - about two inches....
Telling, very telling. With so few views on this thread so far, these initial comments suggest to me that Sikaflex attachment is not necessarily the silver bullet fix for canopy cracking many builders have anticipated. :confused:
 
Amazing. Years ago I took the larger pieces of my canopy scraps and tried to crack them. I froze them, bent them, beat on them, hit the edges, did everything I could to induce a crack and could not. I don't get it.
 
It seems to me that the -8s are more susceptible to canopy cracks than the side-by-sides. The long canopy on the long frame + different expanding properties = cracks. I've had several -8 friends who cracked their canopies in-flight -- usually when climbing to altitude.

I've always wondered this but never seen it addressed here on VAF: When the canopies are riveted you oversize the holes to allow for expansion and contraction. With a glued canopy there is no room for this expansion. Plexi and aluminum obviously have different rates of expansion and contraction. So I guess my point is that you can't win and there is no silver bullet.

I would say with Sika you are much less likely to crack the canopy during construction -- but anecdotal evidence suggests that Sikaflexing a canopy does not reduce changes of cracks while in service.

Of course there are many Sika'd canopies that are not cracked, but I would just like to add that riveting your canopy on does not result in instant crackiness. I've got 400 hours with an all-riveted/screwed on canopy and not a single crack. Ok...one crack...but it was completely my fault. :rolleyes:

There are other advantages of using Sika though. If you Sika your canopy you get an automatic seal. In rain I can get a little water on my rear baggage wall where rain comes in between the skin and plexi in the 'rear window' of my tipper. If I had to do it again, I would probably use Sika (at least in that area) for that reason alone.
 
This isn't the first time I've heard of sika'ed canopies cracking. I think the most important thing is to be very careful with how edges are smoothed to prevent stress risers. I rounded the edges of my canopy on the rocket for this reason with fine grit sandpaper. The bubble canopies expand and contract a surprisingly large amount since they are much longer. In cold temperatures my rocket canopy will pull up the skirts by a fair amount leaving gaps, but in the sun everything fits nicely.
 
Sikaflex Bead Depth

One possible contributor to cracks could be inadequate space between plexiglass and aluminum, especially on long runs (RV-8s). I suspect that many canopies have been built with less than Sika's recommended bead thickness. Sikaflex 295UV has very good ability to stretch, but I'm pretty sure it becomes much more rigid as temperatures drop. Since plexiglass shrinks much faster than aluminum, and properly applied Sikaflex has a tenacious bond, the thing most likely to give is--the plexi.

This would be just one more factor in a complex problem.
 
SIKA thicknes?

Hello,

as Charly mentioned, you should have at least a 1/4" bead thickness, so that there is some "flex" area.

But for sure also, a canopy as long as a RV-8 one will streach quite an amount under heat/cold.

Interestingly how many builder build there canopys in warm condition. This is ok for trimming and drilling it. But for the final position, when it is screwing or riveting time, it maybe better to do this when it is not so cold, lets say in the lower half of the temp-range of usage. So the stress should also be eaqual for cold/warm wether.

Regards, Dominik
 
In the great Sikaflex debate of 2008 on VansAirforce I stated the following:

In actual fact polymethyl methacrylate (PMMA) or "acrylic" as it is commonly known is notoriously difficult to obtain a good bond to. Sikaflex itself will not bond to PMMA. An interlayer (or adhesion promoter) needs to be used. In the case of Sikaflex its 209 primer contains 25% MEK and 20% Ethyl Acetate. Both of these are solvents known to break down the cross links of the long chain molecules in PMMA. In the longer term that might very well lead to edge cracking (one case already reported).... the very thing builders were anxious to avoid.

I am providing a link to the thread if anyone is interested. The comments above were in Post #6

http://www.vansairforce.com/community/showthread.php?t=29804
 
Chloroform

Is anyone using chloroform to "weld" these cracks? I used to use it all the time to assemble plexiglass parts. Created a joint that was stronger than the material itself. I don't know how readily available it is, but it worked amazingly well. Besides its performance as an "adhesive" is has such low surface tension that it wicks into the crack by capillary action so there is no need to open the crack to get the glue in there.
 
Last edited:
I have tested Weld-on 3 on Plexi with great success. It does create a great bond and wicks into very tinny cracks. Fortunately, I have not had to repair any canopy and l will keep my fingers cross that I won't ever have to.
 
Sikaflex vs. Rivets, Alternative vs. Lycoming. It's amazing how similar the arguments look. :)
 
Peer Pressure

Sikaflex vs. Rivets, Alternative vs. Lycoming. It's amazing how similar the arguments look. :)
Amazing similarities, that's for sure. Technically, we truly are "experimenters" and we enjoy great flexibility with the liberty to go down the wrong road if we so choose. But speaking directly to the sometimes hotly debated arguments you mentioned in your post, I would add that thanks in part to the ever-growing body of information contained within the VAF archives, it would appear the passage of time and operational experience does have a way of sometimes exposing persuasive or cleverly crafted opinion to the cold hard realities and glaring light of day. If anything, The Great Sikaflex Debate clearly demonstrates the intellectual minefield we sometimes deal with and an objective review of that lively discussion can be a fascinating and instructive exercise in Monday morning quarterbacking complimentary to your favorite cup of joe:

http://www.vansairforce.com/community/showthread.php?t=29804

Kudos to Captain Avgas:
.....it is my observation that those who desperately WANT to believe in the merits of the Sikaflex solution are often quite hostile to any poster who puts forward any information or data that suggests there might be intrinsic problems....
 
Last edited:
I'd be willing to wager that there are a lot more canopies that have cracked that have been installed by conventional means. The noises my canopy made on my -6 when warming up the hangar made me nervous about getting a crack.
 
Last edited:
I have two cracks. One, aft right side - about two inches. Happened on a very cold winter morning.
The second is right dead center (top) aft about two inches.

Heaven knows how many cracks I would have had I riveted the thing together.
I've never seen anything to suggest that Sikaflex actually reduces cracks occurring? Lots seem to think it will though...
 
Part of the issue with Sikaflex (or, more generically, adhesives) is that the material, installation requirements and limitations need to be understood in order to use it properly. Which I suspect doesn't always happen. One hazard of using an adhesive is that it can possibly fasten the acrylic too well, which could make for some big localized stresses, depending on part geometry.

Confession: I used Sika products to bond my canopy (RV-7 slider). I was attracted to the process because I have some background with bonding and casting with urethanes (Sikaflex 295UV is a single part, moisture-curing urethane) to a variety of substrates. Most of that experience was in a marine environment, with equipment that had fairly severe environmental exposure. One thing that I learned was that urethanes worked very well for various chosen applications if used correctly. If not, it could be pretty frustrating. Sometimes the process was pretty fussy.

Because of a dumb, careless mistake, I had the pleasure of doing two Sika canopy installations: I dropped pliers on the first one a couple of months after completing it, making a nice star-shaped crack above the pilot's head. Duh. There was one advantage to the Sika process: I got the broken canopy off the frame with only a razor blade, in about 10 minutes. Well, not completely off. I left it attached to part of the forward bow, then tried tearing it off. Might as well see how strong the stuff is. Obviously a purely qualitative test. Still, I was pleased. I wasn't strong enough to pull it free.

Bob Barrow pointed out that the 209D primer contains substances known to attack acrylic. That could lead to some interesting stresses at the primer line edges. I didn't see any evidence of failure along the primer line on my first canopy when I was tugging on it. Sika does represent the system as being suited for bonding acrylic windows into boat hulls, but the jury is still out.

Please don't take the above as an argument in favor of using Sika. I was interested in using due to my aforementioned previous experience. So far I'm happy, but I'm not flying yet. I too have noted that the canopy shape is temperature dependent. The skirt on mine hugs the fuselage a lot more tightly when it's cold.

There's one other thing I thought of, maybe it's been mentioned in canopy threads, but I haven't seen it. The side-by-side canopies are formed in a mold- I believe they are vacuum-formed. The tandem canopies are blown. Would the difference in the two processes lead to different surface conditions on the plastic, possibly making one or the other more susceptible to cracking?
 
Post #3

I think Craig (post #3) is spot on.

I used Sika for my -8 canopy after the usual careful cut and fit. There is very little risidual stress if the canopy has been cut and fit with care. It more or less lays exactly in place.

I noticed after it was all cured that the entire frame/canopy assembly would deform significantly when engaging the canopy latch by pulling the left side further forward than the right side. This resuts in a noticible torsion applied to the entire canopy against many compound curves - not good.

Filing the latch to the point where it just engages the lock pin prevents any deformation and in my opinion will significantly reduce the chance for cracking.
 
...After going through my vocabulary several times, and stop-drilling, I'm still trying to figure out "why?"...

If you remove the canopy, that is if you don't just do a spot crack repair, can you please get some closeup photos of the edge at the crack initiation site? I would be very interesting to a lot of people to see whether there are any notches or other imperfections that would constitute stress risers.

Thanks, Bob K.
 
Canopy crack

Well I'm on my third canopy on my RV-4 and I used Sikaflex for the last one with only 4 screws loosly installed as a precaution and guess what...After painting the skirt back and removing all the taping, there was a very small crack just above the screw...dang...:mad:

Whenever I have to do it again, it will be Sika all the way and no sticking screw for this boy...
-------------
Ken
'' canopy assembly would deform significantly when engaging the canopy latch by pulling the left side further forward than the right side. This resuts in a noticible torsion applied to the entire canopy against many compound curves - not good.

Have you thought about installing a latch on both side like they do on Rocket so you can close the canopy evenly..?


Bruno
 
Well I'm on my third canopy on my RV-4 and I used Sikaflex for the last one with only 4 screws loosly installed as a precaution and guess what...After painting the skirt back and removing all the taping, there was a very small crack just above the screw...dang...:mad:

Whenever I have to do it again, it will be Sika all the way and no sticking screw for this boy...
-------------
Ken
'' canopy assembly would deform significantly when engaging the canopy latch by pulling the left side further forward than the right side. This resuts in a noticible torsion applied to the entire canopy against many compound curves - not good.

Have you thought about installing a latch on both side like they do on Rocket so you can close the canopy evenly..?


Bruno


No need. Careful trim of the latch assembly keeps the canopy closed against the seal without any noticable deformation.
 
Part of the issue with Sikaflex (or, more generically, adhesives) is that the material, installation requirements and limitations need to be understood in order to use it properly. Which I suspect doesn't always happen. One hazard of using an adhesive is that it can possibly fasten the acrylic too well, which could make for some big localized stresses, depending on part geometry.

In the great Sikaflex debate of 2008 on VansAirforce I stated the following:

The problem with this discussion is that many people are totally confused about the difference between an adhesive and a sealant.

For everybody's information.....a sealant is designed to allow movement at a joint....whereas an adhesive is designed to restrict movement at a joint. And it is important to know which is which because the optimum geometry of a joint bead will differ depending on whether it is acting primarily as a sealant or primarily as an adhesive.

Sikaflex is an elastomeric SEALANT formulated with the specific intent of allowing movement at a joint. In the case of an RV canopy the Sikaflex acts as a "structural" sealant in that it is also expected to transmit loads. That the Sikaflex "adheres" to the surfaces does not make it primarily an adhesive. Virtually all sealants are "adhesive" by nature but they are not PRIMARILY adhesives. Epoxy is an adhesive (and it may act also as a sealant...but it is not PRIMARILY a sealant).

It is relevant that in his post #78 Jim McChesney claims he asked Sika "how to apply the ADHESIVE between 2 perpendicular surfaces" and was advised to use a full depth fillet. And that would be correct advice for an ADHESIVE....but incorrect advice for a SEALANT.

I have done a few quick (and rough) drawings to indicate a cross section of the canopy to side bow joint on the RV7 slider with 3 possible joint geometries utilising Sikaflex. You will need to left click on the thumbnail to bring it up to a reasonable viewing size.

Drawing A shows the detail that I proposed in my post #82 using a polyethylene bead. I recommended this because it's probably the simplest solution to implement.

Drawing B shows an alternative joint geometry using a polyethylene bead (between the bow and the acrylic) which would work just as well but would require moving the acrylic away from the bow by up to 1/4" which might prove difficult without serious mods to Vans canopy design.

Drawing C shows a Sikaflex joint geometry that is destined to fail. It's a good geometry for an adhesive but a very poor one for a sealant. Infinitely large stresses will arise at the root of the fillet as the side bow and the acrylic move differentially.



The fact that I make these points should not be construed by anyone as an indication that I recommend using Sikaflex to bond RV canopies without an additional system of mechanical attachment.


I am providing a link to the thread if anyone is interested. The comments above were in Post #86.

http://www.vansairforce.com/community/showthread.php?t=29804&page=9
 
Last edited:
In the great Sikaflex debate of 2008 on VansAirforce I stated the following:

The problem with this discussion is that many people are totally confused about the difference between an adhesive and a sealant.

For everybody's information.....a sealant is designed to allow movement at a joint....whereas an adhesive is designed to restrict movement at a joint. And it is important to know which is which because the optimum geometry of a joint bead will differ depending on whether it is acting primarily as a sealant or primarily as an adhesive.

Sikaflex is an elastomeric SEALANT formulated with the specific intent of allowing movement at a joint. In the case of an RV canopy the Sikaflex acts as a "structural" sealant in that it is also expected to transmit loads. That the Sikaflex "adheres" to the surfaces does not make it primarily an adhesive. Virtually all sealants are "adhesive" by nature but they are not PRIMARILY adhesives. Epoxy is an adhesive (and it may act also as a sealant...but it is not PRIMARILY a sealant).

It is relevant that in his post #78 Jim McChesney claims he asked Sika "how to apply the ADHESIVE between 2 perpendicular surfaces" and was advised to use a full depth fillet. And that would be correct advice for an ADHESIVE....but incorrect advice for a SEALANT.

I have done a few quick (and rough) drawings to indicate a cross section of the canopy to side bow joint on the RV7 slider with 3 possible joint geometries utilising Sikaflex. You will need to left click on the thumbnail to bring it up to a reasonable viewing size.

Drawing A shows the detail that I proposed in my post #82 using a polyethylene bead. I recommended this because it's probably the simplest solution to implement.

Drawing B shows an alternative joint geometry using a polyethylene bead (between the bow and the acrylic) which would work just as well but would require moving the acrylic away from the bow by up to 1/4" which might prove difficult without serious mods to Vans canopy design.

Drawing C shows a Sikaflex joint geometry that is destined to fail. It's a good geometry for an adhesive but a very poor one for a sealant. Infinitely large stresses will arise at the root of the fillet as the side bow and the acrylic move differentially.



The fact that I make these points should not be construed by anyone as an indication that I recommend using Sikaflex to bond RV canopies without an additional system of mechanical attachment.


I am providing a link to the thread if anyone is interested. The comments above were in Post #86.

http://www.vansairforce.com/community/showthread.php?t=29804&page=9

Thanks for the thoughtful reply. Just to be clear though, Sikaflex 295UV is described as an adhesive by the manufacturer. Ref: http://usa.sika.com/en/solutions_products/01/01a006/01a006sa02/01a006sa02100/01a006sa02105.html

That said, I agree with you regarding joint design as regards polyurethanes. Unlike epoxies, for example, they require a gap in the joint. In the case of bonding a material with a high coefficient of linear thermal expansion (let's call that one CLTE) to one with a low CLTE, that's a good thing, because there is more compliance in the joint. In consideration of your description, you could say that urethanes have properties of both adhesives (strong joint) and sealants (joint flexibility).

Sika lists the elongation of 295UV at failure as 500%. In the case of a joint in shear, say with a 1/8" gap (thin, but probably common) that means it should shear 5/8" before it lets go. I've done some shear tests on urethanes bonding metal in a university lab years ago; it's crazy to watch how far the stuff goes before it rips.

Oh yeah... my canopy incorporates some mechanical attachment.
 
I had a thought... (Please ignore the snarky comment that my son will leave as soon as he reads this...)

My -7 canopy is bonded using the Sikaflex method. There is only one hole in the canopy - the hole for the canopy latch. I was very careful while building and bonding the canopy to polish all of the edges with polishing compound. I worked in a heated shop, which I had heated to about 80 degrees while cutting/polishing/gluing the canopy. I haven't experienced any problems with my canopy.

So - back to that random thought that I had.

We all agree that canopies crack when there is stress on the acrylic, and a stress riser or focal point. Stress can occur due to thermal expansion or contraction, or it can occur for other reasons, such as a dropped canopy or some other form of stress. I've read about lots of thermal expansion issues on the -8 with the long canopy that could conceivably expand 1/8" or more in temperature extremes. But that doesn't explain the cracked canopies that have sikaflex and sufficient expansion built in to avoid stress.

***drumroll*** So here's the thought that I had. ***drumroll fades***

What if the expansion wasn't along the surface of the canopy, but through it's thickness???

In other words, the canopy isn't stressed because it's expanding 1/8 from front to rear. What if it's cracking because the inside of the cockpit is being heated with cabin heat (around 65 degrees), and the outside of the cockpit is cold - like 20 degrees. This would cause the inside skin of the canopy to expand, while the outer skin is contracting - causing all kinds of stress in a cross-section of only 1/8".

Well, right after I had that thought, I saw a shiny airplane and promptly ran into the wall while looking up.
 
Back
Top