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-   -   Canopy Crack - What Would Cause This? (https://vansairforce.net/community/showthread.php?t=79420)

pvalovich 11-28-2011 12:08 PM

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?

FrankK90989 11-28-2011 01:04 PM

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

gear1 11-28-2011 08:54 PM

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!

N282S 11-29-2011 06:40 AM

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.

RV8R999 11-29-2011 07:42 AM

You guys have the stock Van's canopy?

Rick6a 11-29-2011 07:46 AM

Ouch!
 
Quote:

Originally Posted by pvalovich (Post 600606)
.....Sikaflex all the way.....I discovered a 3+ inch vertical crack in the canopy....

Quote:

Originally Posted by gear1 (Post 600834)
I have exactly the same crack in my canopy.....

Quote:

Originally Posted by N282S (Post 600930)
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:

JonJay 11-29-2011 07:55 AM

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.

Jamie 11-29-2011 08:06 AM

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.

rocketbob 11-29-2011 09:07 AM

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.

chunt0 11-29-2011 11:59 AM

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.


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