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  #21  
Old 11-30-2011, 02:14 PM
Bob Kuykendall's Avatar
Bob Kuykendall Bob Kuykendall is offline
 
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Location: Douglas Flat, CA
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Quote:
Originally Posted by pvalovich View Post
...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.
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  #22  
Old 11-30-2011, 02:39 PM
RV-4 RV-4 is offline
 
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Location: St-Jerome,Quebec,Canada
Posts: 1,148
Default 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...

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
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  #23  
Old 11-30-2011, 02:50 PM
RV8R999 RV8R999 is offline
 
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Quote:
Originally Posted by RV-4 View Post
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...

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.
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  #24  
Old 11-30-2011, 07:18 PM
Captain Avgas Captain Avgas is offline
 
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Location: Melbourne, Australia
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Originally Posted by Lars View Post
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/communit...t=29804&page=9
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Last edited by Captain Avgas : 11-30-2011 at 07:27 PM.
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  #25  
Old 11-30-2011, 09:18 PM
Lars Lars is offline
 
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Location: Davis, CA
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Quote:
Originally Posted by Captain Avgas View Post
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/communit...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_pro...06sa02105.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.
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  #26  
Old 12-05-2011, 10:15 PM
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DakotaHawk DakotaHawk is offline
 
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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.
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  #27  
Old 12-05-2011, 10:32 PM
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Casey Stewart Casey Stewart is offline
 
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Quote:
Originally Posted by DakotaHawk View Post
I had a thought... (Please ignore the snarky comment that my son will leave as soon as he reads this...)
I dont even need a "snarky comment when you end your post with this:
Quote:
Originally Posted by DakotaHawk
Well, right after I had that thought, I saw a shiny airplane and promptly ran into the wall while looking up.
Just sayin
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  #28  
Old 12-06-2011, 09:06 AM
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Mark12A Mark12A is offline
 
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Quote:
Originally Posted by Casey Stewart View Post
I dont even need a "snarky comment when you end your post with this:

Just sayin
I'm feeling the love here. I guess all grown sons are pretty much alike.
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