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  #21  
Old 01-12-2018, 01:07 PM
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mfleming mfleming is offline
 
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What a great write up. I've saved this post as a webarchive for the future.

This should be a sticky!
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  #22  
Old 01-12-2018, 03:20 PM
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goatflieg goatflieg is offline
 
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As I come closer to building up my own RV-8 canopy, this thread answered one important question I had: would bonding with only Sikaflex (no rivets) eliminate the chance of developing a crack later? The answer here is, no. Although you flew for many hours, it eventually happened... and since I live in Michigan and face some daunting temperature extremes, I can't help wondering if a canopy crack is likely (or perhaps unavoidable) in my future, however distant. I will build with the mind of making sure there is no built-in stress during fabrication, and the question of whether or not to use the suspenders-and-belt method of blind rivets and Sikaflex remains unresolved. I'm very glad you shared the details of your experience... I also agree that this post should be saved as a sticky in the RV-8 archives. It certainly is helpful information.
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  #23  
Old 01-12-2018, 04:56 PM
scsmith scsmith is offline
 
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Thanks Scott, great informative write-up.

As I read along, I kept hoping for the big surprise that the filler material would in fact become transparent. I wonder if there is a product that would be more clear?

One question I have on your Sikaflex installation. One variable in various people's use is how thick a layer of Sikaflex they create between the frame and the canopy acrylic. I used garden-hose spacers that created a gap of almost 3/16" that got filled with Sikaflex. The hope was/is that this would provide enough expansion flexibility to allow the canopy to grow or shrink without (or with less) stress.

So-how thick is your Sikaflex layer between the frame and canopy?
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  #24  
Old 01-12-2018, 06:38 PM
Captain Avgas Captain Avgas is offline
 
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Quote:
Originally Posted by scsmith View Post

One question I have on your Sikaflex installation. One variable in various people's use is how thick a layer of Sikaflex they create between the frame and the canopy acrylic.
The thickness of the sealant will indeed be crucial. But there will be other important factors as well. In a post on VansAirforce dated 5/8/2008 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.

My opinion is that the ongoing reports of cracks in Sikaflex canopies result from the following factors:

a) Vans canopies are either vacuum or pressure formed and will contain locked-in internal stresses.
b) Builders do not properly treat cut edges to fully remove microscopic imperfections/discontinuities.
c) The aggressive solvents in Sikaflex 209 primer will attack the acrylic, particularly at edges where there are discontinuities.
d) Builders are not using the recommended thickness of Sikaflex sealant.

Once small cracks form at the edges of the canopy they will continue to run due to an engineering phenomenon known as "work of fracture".

However despite all of the above, the main reason I would be very cautious about using the Sikaflex approach is because it may be bonded to Vans powder coating which itself has no known quality control and therefore provides no guarantee of adhesion.
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  #25  
Old 01-13-2018, 01:11 AM
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Scott Chastain Scott Chastain is offline
 
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Default PPG DX330 Acryli-Clean

Quote:
Originally Posted by Captain Avgas View Post
Scott, a fascinating and instructive post with great pix. However a couple of questions for you.

Can you tell me why you used the PPG DX330 Acryli-Clean to prep the canopy. My understanding from reading the product tech info is that the DX330 is not suitable for acrylic (despite the name). The PPG SX103 might have been the more appropriate product. Is it possible that the occlusions you developed arose through using the wrong cleaner.

I am surprised that when you were using a Polyvance adhesive (ie Plastifix Clear) you did not use the Polyvance cleaner (1000-A Super Prep Plastic Cleaner) to maintain compatibility.

And finally, the fact that the Plastifix Clear was not in reality clear at all (but translucent) must have been disappointing. Is it possible that a Weldon Product might have been more transparent.
Hi, Bob!

That's a great question about using the DX330 Acryli-Clean. I actually thought the same thing when the occlusions became visible.

From the PGG DX-Series Data Sheet: "PPG DX Cleaners are specifically designed to remove waxes, grease, silicones, road tar, engine oil, overspray, adhesives and other contaminants. They can be used on bare metal, primers, automotive finishes or to clean plastic parts <emphasis mine>. DX Cleaners are packaged ready-to-use."

Throughout the painting process some 11 years ago, and especially when it came time to paint the areas around the windscreen and the canopy skirts, the DX330 cleaner proved the most effective cleaner for the job, and it does not attack acrylic like other cleaners.

The Polyvance website instructs users to apply their Super Prep 1000-A, the contents of which are:

Petroleum gases 13-30%
Naphtha 60-70%
Xylene 13-30%

The contents of the DX330 Acryli-Clean are:

Methylcyclohexane <10%
Toluene <5%
N-Heptane <10%
Naphtha 70-80%

Both cleaners use mostly Naptha as the contaminant removal solvent. The funny thing is, when I opened up the package of Plasti-Fix, the instructions directed me to use denatured alcohol! To that end, I used the Acryli-Clean because it has proven a great contaminant removal solvent so many times before. I didn't have any denatured alcohol around, so I used it. I found no literature anywhere, at the PPG website or otherwise, stating that it should not be used on acrylic. Please direct me to where you are finding that information.

Are the occlusions caused by the cleaner? I doubt it, but the thought has crossed my mind just as it has yours.

With the exception of the occlusions, I was not disappointed with the result. I knew from others far more experienced that the final result would be opaque, not nearly as transparent as the acrylic. As noted in the original post, I tried a Weld-On product and was disappointed. The crack not only remained visible, but it continued to migrate after application.

Thanks for your insight, Bob. If the San Joaquin Valley fog manages to break open this weekend, I'll let you know how flight testing goes. I may end up using denatured alcohol or the Super Prep 1000-A after all!

Smooth Air!
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Last edited by Scott Chastain : 01-13-2018 at 01:13 AM.
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  #26  
Old 01-13-2018, 01:26 AM
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Scott Chastain Scott Chastain is offline
 
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Default Sikaflex Crack

Quote:
Originally Posted by Captain Avgas View Post
My opinion is that the ongoing reports of cracks in Sikaflex canopies result from the following factors:

a) Vans canopies are either vacuum or pressure formed and will contain locked-in internal stresses.
b) Builders do not properly treat cut edges to fully remove microscopic imperfections/discontinuities.
c) The aggressive solvents in Sikaflex 209 primer will attack the acrylic, particularly at edges where there are discontinuities.
d) Builders are not using the recommended thickness of Sikaflex sealant
Hi, Bob!

It certainly is an imperfect world, and I think most of us try our very best to build our airplanes with whatever information, guidance, advice, and certainly friendships we have at the time. All I know is that my canopy cracked, and I did my best to fix it. One of these days, if I get the opportunity to replace the canopy, I will again do the very best job that I can, knowing all the while that there are no guarantees. Especially in aviation.

Smooth air, my friend!
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  #27  
Old 01-13-2018, 01:49 AM
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Scott Chastain Scott Chastain is offline
 
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Default Sika Thickness

Quote:
Originally Posted by scsmith View Post
So-how thick is your Sikaflex layer between the frame and canopy?
Hi, Steve!

I followed the Sikaflex instructions to the letter. On the windscreen, I used 1/8" spacers. For the bubble, I used 3/8" spacers at the windscreen mate-up, 3-16" around the frame. There was a lot of preload at the rear of the canopy when I fitted it, and I remember how anguished I was about having to install it that way.

I wish there were the big surprise some people hoped for at the end of the process, but no---the reality of this kind of plastic surgery means that scars will remain no matter how small or large such wounds might be. Such is life.

Smooth air, my friend!
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  #28  
Old 01-13-2018, 09:18 AM
Captain Avgas Captain Avgas is offline
 
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Quote:
Originally Posted by Scott Chastain View Post
From the PGG DX-Series Data Sheet: "PPG DX Cleaners are specifically designed to remove waxes, grease, silicones, road tar, engine oil, overspray, adhesives and other contaminants. They can be used on bare metal, primers, automotive finishes or to clean plastic parts <emphasis mine>.
Hi Scott, PPG DX330 cleaner is a general purpose auto refinish wax and grease remover. It may be safe to use on some auto plastics such as thermoplastic olefins etc as used in bumper bars and other trims but I wouldn't use it on an acrylic aircraft canopy. I suspect that the fact that your gas inclusions are occurring mainly along the interface of the acrylic and the Plasti-Fix adhesive points to a problem with the cleaner.

But I'm also amazed that the Plasti-Fix instructions called for the use of denatured alcohol as a cleaner. To me, denatured alcohol is methylated spirits which generally contains products that are certain to cause stress cracking.
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Last edited by Captain Avgas : 01-13-2018 at 09:25 AM.
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  #29  
Old 01-13-2018, 03:57 PM
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snopercod snopercod is offline
 
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Awesome job, Scott, and a very helpful writeup. It gave me chills looking at you gouging out the crack with a Dremel tool. I could just see the tool skittering across the canopy I don't know if this information will be of any benefit to the RV Community but on my Lancair, I used Hysol structural adhesive to bond the acrylic canopy to the fiberglass frame. IIRC, I cleaned both pieces with automotive wax and silicone remover, and lightly sanded the canopy in the joint area. On the inside, I reinforced the joint with BID tapes. No cracks in 220 hrs. (knock on acrylic). I have nightmares about cracks in my canopy...
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  #30  
Old 01-14-2018, 09:01 PM
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Scott Chastain Scott Chastain is offline
 
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Default Flight Testing

The San Joaquin Valley winter fog finally broke on the afternoon of Sunday, January 14. I raced from Church to the airport and put some preheat on the engine while I prepped the Dove for her first flight since the canopy repair. After taking my dog, Gracie, for a little adventure walk around the airport, I pushed the plane out of the hangar for the first time since Christmas Day. It had been 3 weeks since the canopy was removed.



I noted the occlusions and took a few pictures of the repair. Then I strapped in, cranked over and taxied out. The first hour of flight testing would be simple but comprehensive.



I took off and performed a simple 270-degree, 1.5-G turnout departure over the field in the direction of Mariposa, CA (MPI). After reaching 4,500 MSL, I leveled off and prepared the engine and propeller for a set of aerobatic maneuvers with 180-mph entry speeds.



I began with a set of clearing turns and pulled a sustained 2-Gs in the process. A few lazy-8s were performed, followed by three loops with sustained 3-G entries and exits at the bottom, followed by some rolls. The repair held.

With daylight fast disappearing, I then climbed toward Yosemite National Park until I reached 14,500-feet MSL, watching the OAT on the AF-2500 engine monitor. It was a toasty 47-degrees Fahrenheit at 4,500. Ten thousand feet later over Yosemite, the OAT gauge was showing -3. I guessed that some serious re-calibration of the gauge was in order. It was not that cold outside, but it was certainly colder than when the Tehachapi fissure occurred.







The acrylic bond appeared to be unaffected by the cold, and in the occlusions no apparent changes were seen.



I made an aggressive descent back into the haze to attempt cracking the repair with a rapid temperature increase and near-VNE airflow over the canopy, but the repair was stable and remained unfazed.





Back in the hangar, I took another close-up shot of the worrisome occlusions that became apparent during the polishing process. Compared to photos taken before startup, no changes were noted during the 1.03-hour flight.



I was pleased that initial flight testing had proven the PlastiFix repair process airworthy. Further testing at 17,500 MSL, experimentation with 4-G split-S maneuvers, and even more rapid temperature and airspeed fluctuations would come next. It would be interesting to see how the repair would handle those stresses.

It was good to get airborne again.
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Last edited by Scott Chastain : 01-14-2018 at 09:04 PM.
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