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  #1  
Old 11-28-2011, 12:08 PM
pvalovich pvalovich is offline
 
Join Date: Dec 2005
Location: Ridgecrest, CA
Posts: 436
Default Canopy Crack - What Would Cause This?

-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?
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  #2  
Old 11-28-2011, 01:04 PM
FrankK90989 FrankK90989 is offline
 
Join Date: Feb 2005
Location: N Mn
Posts: 299
Default

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
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  #3  
Old 11-28-2011, 08:54 PM
gear1 gear1 is offline
 
Join Date: Sep 2008
Location: Pagosa Springs CO
Posts: 249
Default 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!
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  #4  
Old 11-29-2011, 06:40 AM
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N282S N282S is offline
 
Join Date: Aug 2007
Location: Topeka
Posts: 328
Default 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.
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  #5  
Old 11-29-2011, 07:42 AM
RV8R999 RV8R999 is offline
 
Join Date: Dec 2008
Location: na
Posts: 1,457
Default

You guys have the stock Van's canopy?
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  #6  
Old 11-30-2011, 08:19 AM
Andy Hill's Avatar
Andy Hill Andy Hill is offline
 
Join Date: Dec 2005
Posts: 976
Default

Quote:
Originally Posted by N282S View Post
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...
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  #7  
Old 11-30-2011, 11:59 AM
Lars Lars is offline
 
Join Date: Nov 2005
Location: Davis, CA
Posts: 1,176
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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?
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  #8  
Old 11-29-2011, 07:46 AM
Rick6a's Avatar
Rick6a Rick6a is offline
 
Join Date: Jan 2005
Location: Lake St. Louis, MO.
Posts: 2,346
Default Ouch!

Quote:
Originally Posted by pvalovich View Post
.....Sikaflex all the way.....I discovered a 3+ inch vertical crack in the canopy....
Quote:
Originally Posted by gear1 View Post
I have exactly the same crack in my canopy.....
Quote:
Originally Posted by N282S View Post
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.
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  #9  
Old 11-29-2011, 07:55 AM
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JonJay JonJay is offline
 
Join Date: Jan 2007
Location: Battleground
Posts: 4,348
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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.
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  #10  
Old 11-29-2011, 08:06 AM
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Jamie Jamie is offline
 
Join Date: Jan 2005
Location: Atlanta, GA
Posts: 2,295
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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.

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.
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