Weldon 16 is cracking my canopy. Why???

[rvbuilder2002]

Originally Posted by jcarne

As far as I know the Sika method is 100% as well if it was properly done but I haven't been around long enough to know this for sure.

That is the primary reason that Van?s has never endorsed the installation of canopies to flexible frames, and probably never will.
It is a pretty simple process to confirm that a rivet or screw is properly doing what it is supposed to do. An adhesive structural bond... not so much.

I agree that the canopy is the most challenging part of an RV build and everyone at Van?s will typically tell prospective customers that (with the cowling being a close second).
BTW, this is not something limited to building an RV. Most anyone that has built a kit plane design that has a large movable canopy will tell you that was the most challenging part.

[jcarne]

Originally Posted by rvbuilder2002

That is the primary reason that Van’s has never endorsed the installation of canopies to flexible frames, and probably never will.
It is a pretty simple process to confirm that a rivet or screw is properly doing what it is supposed to do. An adhesive structural bond... not so much.

I agree that the canopy is the most challenging part of an RV build and everyone at Van’s will typically tell prospective customers that (with the cowling being a close second).
BTW, this is not something limited to building an RV. Most anyone that has built a kit plane design that has a large movable canopy will tell you that was the most challenging part.

Thanks for the expert comment Scott. I certainly understand Van's stance on this one but not fully. There are quite a few things while building the plane that must be done correctly for safe operation. After attaching my canopy to the frame with Sika I don't see how one could possibly screw it up royally unless a person skipped a major step. Just out of curiosity as I know you guys did some testing on this, did you find that if you skip certain steps such as scuffing, priming, cleaning, etc. that it resulted in sub-par strength? After following Sikas directions (which were pretty simple) it sure is amazing how strong this stuff is. Totally agree with you though that riveting/screws would be more "fool proof" for sure.

[dwranda]

Well I just did some testing with a scrap piece. I bent it a little bit with some clamps to put some stress on it. Under stress even putting one drop of weld-on on the surface makes it developed cracks. Might be considered crazing? Putting a drop of number 3 on a piece that is not under stress does not create any cracks. I obviously still have some stress in that corner where the crack developed. Who knows how far up from the edge the stress is strong enough to continue to develop cracks. I'm experimenting with the scrap piece to figure out what I can get away with before I do anymore on the actual canopy.

[rvbuilder2002]

Originally Posted by jcarne

Thanks for the expert comment Scott. I certainly understand Van's stance on this one but not fully. There are quite a few things while building the plane that must be done correctly for safe operation. After attaching my canopy to the frame with Sika I don't see how one could possibly screw it up royally unless a person skipped a major step. Just out of curiosity as I know you guys did some testing on this, did you find that if you skip certain steps such as scuffing, priming, cleaning, etc. that it resulted in sub-par strength? After following Sikas directions (which were pretty simple) it sure is amazing how strong this stuff is. Totally agree with you though that riveting/screws would be more "fool proof" for sure.

We have never done any testing of the Sika process on any of the two seat canopy installations.

The only testing done was with the bonding installation process recommended for the RV-10 which is a bit different since it is bonding the acrylic to a composite surface and the adhesive process used is a single part / step.

I don't disagree, that there are many critical construction steps in building an RV but I think that most all of them are very easily inspectable post process, to confirm that they were done correctly. Like my example of fastening something with a rivet vs Sika adhesive..... The rivet joint can easily be inspected by someone else, or even by the builder over time with doing condition inspections. A Sika bonded joint can be inspected to confirm the two surfaces are still joined together but that is about it.

The one exception made in the RV kits to the general line of thinking I mentioned in my other post is the bonding of the RV-10 windows.
I admit it is also process sensitive just like the Sika process, but to a lesser degree in my opinion. Fewer steps in the process mean fewer instances where a critical mistake can be made. Even then, there has been a few bond failures on RV-10's, but if we consider that 933+ RV-10's have been completed and flown, the failure rate appears to be extremely low so I believe that shows that the process is reliable.

If we had more long term service data on canopies installed with the Sika process, we may be able to say the same for it as well. But since my engineer mentality has to always look to data, right now there is not anywhere close to enough data to indicate that.

[jcarne]

Originally Posted by rvbuilder2002

We have never done any testing of the Sika process on any of the two seat canopy installations.

The only testing done was with the bonding installation process recommended for the RV-10 which is a bit different since it is bonding the acrylic to a composite surface and the adhesive process used is a single part / step.

I don't disagree, that there are many critical construction steps in building an RV but I think that most all of them are very easily inspectable post process, to confirm that they were done correctly. Like my example of fastening something with a rivet vs Sika adhesive..... The rivet joint can easily be inspected by someone else, or even by the builder over time with doing condition inspections. A Sika bonded joint can be inspected to confirm the two surfaces are still joined together but that is about it.

The one exception made in the RV kits to the general line of thinking I mentioned in my other post is the bonding of the RV-10 windows.
I admit it is also process sensitive just like the Sika process, but to a lesser degree in my opinion. Fewer steps in the process mean fewer instances where a critical mistake can be made. Even then, there has been a few bond failures on RV-10's, but if we consider that 933+ RV-10's have been completed and flown, the failure rate appears to be extremely low so I believe that shows that the process is reliable.

If we had more long term service data on canopies installed with the Sika process, we may be able to say the same for it as well. But since my engineer mentality has to always look to data, right now there is not anywhere close to enough data to indicate that.

Thanks for clearing that up and giving us a bit of history Scott, many people including myself have wondered about this. Also, I was under the false notion that you guys tested Sika at one point so thanks for setting the record straight. Too bad no one has tracked Sika installations since they came about. If I am remembering my research correctly I want to say it started around 06'-08'. I based my decision in part on how long the Sika method has been around and it seems to be pretty common now with no structural failures to my knowledge. However, I agree that on a company standpoint it is a different ballgame.

Sorry for the thread drift dwranda but hopefully it was worth it to some degree.

[dwranda]

No problem at all for the drift. In my novice opinion any canopy statistics have to be taken with a grain of salt. We all know these canopies are far from perfect. There is a lot of play in almost every dimension. Too many builder variations could come into play. Walking around Oshkosh I found it very hard to find a canopy that is perfect. There is usually an almost 1/8" gap or overhang somewhere. This is just something we agree to live with to have an awesome airplane in nearly every other aspect. I have a friend building a 14 and can't wait to see his canopy go together. I thought my canopy fit great on my frame when I glued it on but I guess not.

[Captain Avgas]

Originally Posted by dwranda

Putting a drop of number 3 on a piece that is not under stress does not create any cracks.

It probably is cracking, but at a microscopic level. I did some testing with methylated spirits on formed acrylic some years back. The acrylic looked OK with the naked eye but when I viewed the sample under my Olympus stereoscopic microscope at 30X magnification you could clearly see fine cracks emanating from the affected area. Then, flexing the sample caused the microscopic cracks to propagate by the “work of fracture” principle so that they became larger and visible to the naked eye. The “work of fracture” principle postulates that stresses at the tip of a crack can be extremely high.
In reality acrylic (poly methyl methacrylate) is prone to breakdown at the molecular level by a really wide range of chemical substances, including virtually all solvents.
My best guess is that your particular problem arose because of inadequate treatment of the canopy edge after cutting. The edge was probably not sanded sufficiently to remove all discontinuities at the microscopic level. Most RV builders do not understand the extent of sanding required to remove microscopic discontinuities at edges and penetrations of the canopy. In your case the solvents in the Sikaflex primer probably attacked the acrylic causing a crack to emanate from one of those edge discontinuities.

[Aviaman]

Here are some thoughts, admittedly purely speculative.

Internal stresses in materials are often relieved with “annealing”, which is the application of heat, allowing the molecular structure to “flow”. As the temperature rises, eventually the acrylic will melt. Before that temperature is reached, there may be a point where a slow stress relief occurs, without large scale distortion. If this is true, you would think it would be part of the manufacturing process. But even if so, the NEW stresses caused by installation of the canopy will not be neutralized by the previous annealing. So I’m wondering if an application of heat might anneal the new stresses caused by the installation process. I’m not recommending any particular procedure. Experimentation with scrap materials might shed some light on this. Even if positive results are obtained with experiments, scaling it to a whole installed canopy introduces the problems of temperature control, distribution of air temperature, duration of heating, etc. Perhaps relatively high ambient temperatures should exist during drilling, and installation.

[rv9builder]

Originally Posted by Captain Avgas

My best guess is that your particular problem arose because of inadequate treatment of the canopy edge after cutting. The edge was probably not sanded sufficiently to remove all discontinuities at the microscopic level.

Bob,

What grade of sandpaper do you recommend for the final sanding? 400 wet? Or maybe something else?

Thanks,

[dwranda]

Played around with some samples. Bent it with clamps so it was under stress. Put 2 drops of weld on 3 on the surface. Within a minute the surface cracks showed up under the drops. After that dried I heated the piece with a heat gun. I moved the gun back and forth about an inch from the sample for about 45 seconds to a minute. The surface was quite hot to the touch. After it cooled I placed 2 more drops on the piece. NO cracking. I must have heated it enough to relieve the stress and there is no distortion visible in the piece.
I will do 1 more sample then if that is also successful I will heat the corner of the canopy which should relieve any stress in that area. Then I should be able to fix the holes with weld on with no further cracking.
Has anyone else taken a heat gun to their canopy?