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RV-12 Aft window crazing

B

BlueColorKat

Member
We just completed our first engine starts and taxi test. The next day I noticed a few areas of small cracks or crazing lines on the aft window. The crazing lines are on both side of the aft window in the same location (where the lexan has the most radius bend). I will try to post some photos here.
They appear to be from the stress of the bend of the lexan. They do not appear to start at the screw holes or the aft edge of the lexan glass. If you run your finger nail across the outside, you can feel it line, but nothing on the inside.
I know there was some comments about not using black electrical tape for masking, which we did not. We used painters masking tape when it was painted. We have not spilled or used any gas or solvents on the glass. Anyone else have any problems here??? Vans said fly it and keep an eye on it, they dont know what is causing it.

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Jim Olson
RV-12 N126BJ
SN: 120141
Blairsville, Georgia
email: [email protected]
 
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Just curious

How tight are the screws ? If too tight, they could still cause stress in those areas. I can turn all mine with my fingers, or VERY little torque from a screw driver.

John Bender
 
I'm just curious, Jim- did you guys remember to open up the screw holes in the Lexan, to #27? The screws need to be a fairly loose fit in the holes- no binding. Steve
 
I was very carefull with the screws so not to over tighten, and I can also turn them by hand. The crazing lines do not appear to be coming from the screw holes but more along the bend radius. There are a few lines, then it skips for a few inches and another small crazing line shows up lined up with the few back by the turtle skin. I also enlarged the screw holes as per the plans with a plexiglass bit and hand drill, and sanded the edges.

The lexan has been installed for a few months, but the lines did not show up until after the vibration of engine start and taxi.

Jim Olson
[email protected]
 
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Still curious

Did you have the rear glass in when you painted it ? The screws seem nicely painted like they were in place when painted. Just wondering if thinners may have worked their way in and had some affect.

John Bender
 
...have seen that once before

....but it was on our windshield, at the area of max curvature and stress, but only after several YEARS in the sun etc. ( I think it was plexi GP though, not lexan!!!???)
Lexan has some moisture present, so we figure as it 'dried out' it lost its flexability. Of course those that talk of solvents are making the point that these can accelerate degradation of plexi and lexan.
Is it possible that the crazing's been there since installation ( ie bending and stressing) and you just noticed it since the lighting or viewing angle has changed?
I'll bet if you took away the stress, the lines would disappear. The bad news is; after we replaced the windshield, the old one broke easily along these lines.
Instead of 'keep an eye on it' .......I'd ask for a replacement. could be the material supplied is not up to spec. ( lots of confusion with grades and types of 'plastic')
 
Some of the screws in my rear window originally fit very tightly. I had to thread the screws in and knew that was NOT right. I called Van's and was instructed to enlarge the holes for clearance.

I enlarged all of the holes in the rear window and all of the holes in the canopy exclusively using the Uni-bit. As the step sizes on the Uni-bit are in 1/32", I enlarged the rear window holes (originally specified #27) to 5/32". This is equivalent to a #22 to #23 hole.

The screws then fit well. As John mentioned the screws were then only loosely tightened. You should be able to spin them with your finger or very lightly with a screwdriver.

IMHO, Van's should change the plans to allow the rear window and the canopy to float a bit more by changing the plans and enlarging the holes.
 
Mel said:
I'm pretty sure that the back window in the RV-12 is NOT Lexan. It should be plexiglass. It would not be smart to use Lexan that close to the fuel filler. Lexan and fuel do NOT get along at all.
Scott, Can you confirm this one way or the other?
Click to expand...

The RV-12 rear window is Lexan. It is a special version that has a surface coating on it that is commonly used by NASCAR.
Even with the surface coating there is evidence that it is not impervious to everything (builders have had crazing just from leaving tape stuck to it for an extended period while it was under tension as installed. It is also very possible that solvents from paint can cause a problem. I would suggest that builders do their painting with the window removed to avoid any possible problems with tape or paint.
 
rvbuilder2002 said:
The RV-12 rear window is Lexan. It is a special version that has a surface coating on it that is commonly used by NASCAR.
Even with the surface coating there is evidence that it is not impervious to everything (builders have had crazing just from leaving tape stuck to it for an extended period while it was under tension as installed. It is also very possible that solvents from paint can cause a problem. I would suggest that builders do their painting with the window removed to avoid any possible problems with tape or paint.
Click to expand...
Then I withdraw my comment. Sorry about that. First misteak today!
 
Not a Bad Suggestion

I would suggest that builders do their painting with the window removed to avoid any possible problems with tape or paint.
Scott:
Hard to tell what the problem is here but mechanically, sounds like everything was done correctly. With many builders doing their own paint work or taking the planes to non-aircraft painters, there's always a chance of using solvents that may not be friendly to the plastics. Like Mel, I assumed the rear "glass" was acrylic (plexi). In any event, many polymers are susceptible to a wide variety of common household chemicals, let alone metal cleaners and solvents. When it comes to the canopy, I agree - if there's any uncertainty, remove it if you can. OUCH!
Terry, CFI
RV-9A N323TP
 
I have used lexan on three different aircraft and Lexan can be rolled cold. However, in two of the aircraft (Durand Mark V and Thatcher CX 4) craze lines showed up the full length of the glass. The craze lines were not felt on the glass from the outside or the inside but seemed to be internal. This was just regular Lexan, no special coating. So far our RV 12 rear window does not seem to have that crazing. The crazing was only visible when the sun was a a vey low angle relative to the angle of the glass.
John Foy
 
Another maybe

As it appears - the painting was done with the rear glass in place. If any paint got under the tape, and on the 'glass', using whatever for a cleaner could have been an issue also.

John Bender
 
Just a shot in the dark. Do you have all of your "tunnel door" screw holes filled in. If you left the ones out behind the fuel tank it could mean structure/stress issues that work their way up to the turtle deck. And if that was the case, nothing would happen till the engine ran and the airplane flew. Like I said, grasping for straws.
 
Yes, had all panels and screws in place when we did our first start up and taxi. These crazing lines are hard to see unless the light and angle is just right.

Jim Olson
BlueColorKat
Blairsville, GA
 
Like the factory said I would just watch it and fly it and see how it reacted. If it cracks through replace it.
 
Jim,

Any chance you might have used a citrus-based cleaner? To see what happens to polycarbonate when it comes into contact with that type cleaner, get a piece of scrap 10-12" long and about an inch wide. Drill a hole in each end, bend it, then string it like a bow with a piece of safety wire. Spray with a citrus-based cleaner and wait a couple of days.

Tony
 
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