Stress/crazing/heat forming relationship
 

In my ignorance, I'll ask any noted authority on everything: Does the crazing we see happen only when the polycarbonate is stressed? Or will spilling Mogas on an unstressed sheet also cause crazing? Could we heat-form the polycarbonate, like one does with acrylic, to relieve the stress and thereby minimize the crazing??? As I understand it, polycarbonate can be heat formed, but requires a bit higher forming temperature.

OK - here's a data point in the FWIW Dept:

Just spent 3 days flying the RV-12 demo airplane (N412RV) down in Oregon with Mike Seager (43 landings and not one call to the insurance company.)

The airplane rear windows looked brand new (maybe they WERE brand new! Ha!)

Asked Mike about this fuel surge back situation. Never happened to him he said. We stopped for gas twice in Scappoose. I asked the gas boy - who fuels the airplane almost daily. He claimed no knowledge and no problems. I noted the gas pump nozzle was BIG - almost completely filling the filler neck (and allowing minimal space for expansion air to exit the tank.)

He let me listen to the gurgle as it came up the filler neck. He completely "topped off" each time and seemed to be running the nozzle "wide open."

Like I said - FWIW.

Bob Bogash
N737G

BUT...Has there been a change in the filler location?
 

I remember Scott saying also in an earlier thread that there was never a splash back problem with the demo aircraft.

BUT, the original demo plane had the gas filler in a different (lower) location than the kits actually shipped.
This is a picture of the original plane when Karen and I rode in it at Van's in '09. Bob, you just saw it, is the gas filler now higher or in the same lower location??

This is an apples and oranges comparison if the filler is still in the lower position.

If it is still in the old position and it works so well in the old position why was it moved?

Tony

Position different ...
 

If you look at the picture of the fuel filler on my -12 (as well as everyone else's) on page 6 of this thread, you'll see our tank filler is higher up and considerably closer to the rear window than on Van's demo RV-12. Perhaps the longer neck builds more back pressure when fuel is pumped in.

Oh well ... It is what it is ... :mad:

The fuel filler on 412RV is not still in the location shown in the photo. It was moved because it works better in the higher position.

I didn't take any pictures of that side of the airplane, but I agree with RV-Builder, it was in our current position - right next to the window edge - complete with both decals above and below the filler port.

Bob Bogash
N737G

Note to new buiders. We all hope to hear about the tests as mentioned earlier in this thread. In advance of those results, If I was building now, I would do the following. It might help and I can't see how it would hurt - maybe someone can point out where I am wrong!

1. Seal the bottom edge of the lexan with metal tape up to the edge line of the fuselage skin overlapping the holes. (this would be hidden under the fuselage skin). This would be a "u-shaped" piece of tape. That would be about 1/4 inch "above" the holes on both sides of the window. Then cut out the holes.
2. Somehow seal the holes with silicone as I was putting in the attachment screws. Probably using masking tape "above" the screw holes on the inside as I was doing it to avoid a mess.
These two things should be of no concern to any DAR.
3. After certification I would strongly consider adding an external auto window tint film. That might cause a DAR comment if done before and it also might not stand up to being bent if applied when flat before window installation. That's one of the test items.

FWIW -just what I'd do and I am no expert!
I am being really careful while I wait for the tint film results. That might address the surface crazing remote from the edge that the pics show that I have not yet experienced.

A possible 'simple' solution!
 

Hey folks,
I have not forgotten about the tests we all discussed a few weeks back, the data points sent to me, and nor have I forgotten about the generosity of the college teacher and his aviation students who volunteered to conduct the tests.

However, I recently found out that there may be a really easy solution to the problem at hand. While a l do a huge amount of in-house manufacturing (vacuum-forming, assembly, etc), all of my acrylic and polycarbonate work is done by another facility. Think of the included rear -12 window that Van's provides: that is precision cut-out, on a VERY expensive multi-axis CNC router. While I haven't seen the Van's rear windows being made, it is the exact same final result that I have produced for my windshields that we manufacture. Anyway, I don't wish to dole out hundreds of thousands for the CNC when I can have it done by someone already owning the machines. Let me give you a quick background on where I am heading with all this nonsense, that so far has nothing to do with fuel on an RV-12 window:

I recently was courted by the largest acrylic and polycarbonate producer in the world. They were formerly known as GE Plastics (if that tells you anything about their size) until they bought the company from GE and changed the name. I was anticipating this meeting with the new company for almost 2-weeks, and planned to ask about the fueling issues on the polycarbonate as soon as we hashed out all the details of my business. I was speaking to the GM of the facility, surrounded by engineers, and when I told them the RV-12 issue, and he immediately said "That's a no-brainer. That's a simple fix!" He proceeded to rattle off a part number of some product that seemed like it was 20-digits long. Of course I[had no idea what product he was referring to. But as he elaborated, he said to think of any gas pump in America. All the signage on the pumps, let's say where a pump may say 'TEXACO'....that's polycarbonate material. And it is final-covered in the material I was told about. Think of an actual gas pump being subjected to fuel for years, yet you never see any signage on the pumps that is crazed or distorted.

The solution he mentioned is exactly what some folks proposed early on: it's a clear specialty film that can be applied to the polycabonate sheet, in the area that would possibly be splashed by fuel. It wouldn't need to cover the entire rear window. I was told it is crystal clear, and while it would be easier to apply it with the back window still flat, it would be possible to apply the material once the window is bent and installed. I was told that the film is a highly specialized substrate, with a specialty adhesive backing. I asked if it was similar to a window tint, and was told yes, but that fuel would melt a typical window tint film in a heartbeat.

So here's what I'm now proposing we do: they ordered me a sample sheet of this specialty material, and I should have it within a few weeks. They didn't stock it in their Dallas facility. I will send this new material up to the college (testing) folks, along with the samples of scrap polycarbonate. We will find out VERY quickly if it performs as stated, but I have tremendous faith in the guys that told me it was THE solution to the problem.

If it works, I would think that Van's could possibly look at providing a small die-cut piece of this material with the kit, and the problem of the window crazing talk will quickly diminish? We will see. I will notify Van's of our findings. If it performs flawlessly, I will see what I can do about being able to provide the material to folks who already have their pink slips. For those (like me) still building, we'd need to ask Mel or one of the other DAR's if we can apply it before installing the window. It's what I would consider purely 'cosmetic', but I don't know the specific rule that the Feds require the DAR's to follow. I'll also gladly provide the magic part number / manufacturer of this material, once I get it. Then while the tests are being done, perhaps we can do some online research of the product in the interim.

Really sorry for the loooong post, guys. But I wanted to fully explain where we are at, and let you know the solution sounds extremely promising. :)

Take care,

"works better" means? holds more? better venting? angle of filler neck too low? larger fuel tank? curious the thinking behind the move...sounds to be a favorable one.

Great news Gary!! But remember even with a new Magic film, I bet if the fuel touches an edge or seeps around the screw into the inside of a hole (as mine did in one second) that film would not stop that edge-based crazing. I bet we will still need a method for edge and screw-hole sealing but this does sound like a very possible solution to surface crazing.