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  #11  
Old 04-10-2016, 09:39 PM
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DonFromTX DonFromTX is offline
 
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As I had misquoted Scott earlier, to be fair here is a direct copy of exactly what he said back then:
"The window does have structural significance to the aft fuselage.

Use caution if you choose to install something in its place. It would be foolish to make the assumption that aluminum would be stronger.
It might be... but depending on the thickness chosen... it might not (lexan is extremely tough material)
I personally would not want to give up the side visibility to the aft rear just for something totally opaque to bloc the sun. (and don't say I don't know the heat issues... I flew in Phoenix for 20 years)."

A better choice in my opinion would be to add some very dark tint film to the top areas of the canopy and rear window
Quote:
Originally Posted by C120driver View Post
I was going back and looking more carefully at what had been said in previous posts and this one quote from DonFromTX caught my attention. Windows in airplanes are not counted on for structural load paths. They are there to take wind loads and if pressurized the internal pressure loads. The structure for strength purposes is analyzed as an open hole and all airframe loads have to flow around that open hole. The structure around windows in jet aircraft are structurally sized with the window as a hole in the fuselage. If windows were assumed to be structural, light airplanes that have a bird impact on the windscreen should experience a catastrophic structural failure of the airplane, but that does not happen because the flight loads are flowing around the windscreen. You should be able to experience a structural failure of either one of the RV-12 rear windows and not experience an airframe failure.

I think there must have been a misunderstanding at one time as to what Vans Aircraft engineering said.
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  #12  
Old 04-12-2016, 07:56 PM
RFSchaller RFSchaller is offline
 
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My 12 has 4 years and about 440 hours since licensing. I have light crazing (I.e. several crazes about 1/2" long) along about 3 inches of the rear edge on the filler side and a few crazes about 1 "long 5 inches from the rear edge.

My crazing has been stable over the last two years since appearing, so I am questioning the value of replacing the rear window which I had originally planned for next year along with a preemptive replacement of the flaperon bell crank angle, but so far the bracket has no issues either. Given the hassle involved in each I am beginning to think I'll wait until one requires rework for cause and then do both.

An interesting note: I flush my fuel tester tube with denatured alcohol each use to keep the auto gas smell out of my cockpit (BTW it works great) and so far two have cracked!
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  #13  
Old 04-12-2016, 10:31 PM
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flyboy1963 flyboy1963 is offline
 
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Default one more idea

....from an earlier thread, but worth mentioning....old lexan has pretty much 'dried out', so it should be safe to apply a window tint film to the exterior....not too hard, since it's a flat wrap. ( new lexan has a fair moisture content, if I recall, so can cause bubbles under film as it tries to 'out-gas' ).

this tint film will hold the thing together a bit, should the stress risers become structurally significant, and it will act as a partial shield to splashed fuel, although it will still run down to the bottom edge, which ain't good,as we all know!
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  #14  
Old 04-13-2016, 07:38 PM
D&M Dan D&M Dan is offline
 
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I am at the point of installing the rear window. Was wondering why I couldn't buy a piece of plexi locally the same thickness as the lexan window I have (what ever tint I choose) and cut my own new window. I would use the old lexan one as a template.
Also would it be necessary to do the fuel tank sealant around the plexi window?
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  #15  
Old 04-13-2016, 08:34 PM
Mich48041 Mich48041 is offline
 
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Acrylic will break if bent cold, unlike polycarbonate.
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  #16  
Old 02-28-2017, 10:48 AM
AirHound AirHound is offline
 
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Quote:
Originally Posted by flyboy1963 View Post
Not to beat up on anyone, but polycarbonate is simply not a good fit for this application. I'm sure the manufacturer, and engineers at Van's, were ok with this cold formed wrap at such a large radius, and it was within 'spec' for the material.
The problem is; this imparts stress, and sooner than later, these cracks, risers, and crazing results. On a rear window, this is not critical, but I hardly see how it can be 'stronger' than plexi, when it eventually busts in half!
The F-22 has a huge lexan canopy, but you can be sure that it is surface hardened, tinted, and molded at something well above room temperature!!!

Lexan was first spec'd as a security glazing, and as such, needed to perform vertically, and relatively flat. Laying it down and bending it is starting to depart from its' best characteristics; such as impact resistance.
Yes, in the 1970's, we were lured by the appeal of Makrolon, Lexan etc. and made several windshields flat wrapped, siliconed, and drilled, 6" from a fuel filler cap, before coming to our senses.
I just hate to see others go thru the same trial and error process .......45 years later!!!!!
Hello,
On and RV12 is it easier to use the old rear window as a mold/shape with which to shape some storebought lexan rather than order a VAN's plexiglass replacement? I'm thinking RV12 rear windows will not last forever and simple/ease of replacement is better. Cheers! Doug in IL
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  #17  
Old 02-28-2017, 11:55 AM
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rzbill rzbill is offline
 
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I am unfamiliar with the details of RV12 windows. The following is simply about the two materials in general.

As has been alluded to in the previous posts, here are some points about the two materials.

Acrylic (Plexiglas) has a wide range of strength ratings but in most common aircraft use it is stronger (tensile strength) and stiffer (modulus) than Polycarbonate. Polycarbonate in comparison is more ductile.

It would be worth digging into the chemical resistance charts of the two materials. They are different and if memory serves, polycarb may have some weaknesses that are surprising with common chemicals. Sorry, I don't have time to review those at the moment.

Similar to metals, strength and ductility are generally a zero sum game. One trades one for the other.

So, in practical use, Plexiglas will be stronger, harder (more scratch resistant) but unfortunately more brittle (with related low notch strength) so an installation that includes stress and imperfect holes/edge finishing will promote cracking.
On the other hand, Polycarbonate is slightly less strong, more flexible and LESS scratch resistant, but with its increased ductility it will accommodate some installed stress plus edge/hole imperfection.

Reminiscing........
I do remember the videos of the Polycarbonate F16 canopy tests from Engineering school 35 years ago. The chicken cannon shot at the canopy. The canopy flexed (A LOT) and survived. The pilot did not. The flex was so far that it "killed" the pilot. They increased the thickness to get the flex down to where the pilot just got a headache.
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Last edited by rzbill : 02-28-2017 at 12:02 PM.
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  #18  
Old 02-28-2017, 01:35 PM
Gandalf Gandalf is offline
 
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"I do remember the videos of the Polycarbonate F16 canopy tests from Engineering school 35 years ago. The chicken cannon shot at the canopy."

Quote from engineering after chicken shot out of cannon penetrated canopy.

"Defrost the chicken"

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  #19  
Old 02-28-2017, 06:22 PM
E. D. Eliot E. D. Eliot is online now
 
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Default We went with the plexi

We used the lexan rear canopy part as a pattern. The plexiglass part that came with my finish kit did not have the holes for the screws in it. Worked carefully and we are happy with the result so far.

Crazing of the lexan due to fuel spillage just isn't attractive to me. I am very well acquainted with the guy who will do most/all of the fuel tank filling and know that he won't go for long before he would spill fuel on the lexan. By the way, the lexan was about twice as thick and didn't want to bend to form at all. Just hope that the plexi will hold up - I trust Van's on that one!
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  #20  
Old 03-04-2017, 07:04 AM
AirHound AirHound is offline
 
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Quote:
Originally Posted by E. D. Eliot View Post
We used the lexan rear canopy part as a pattern. The plexiglass part that came with my finish kit did not have the holes for the screws in it. Worked carefully and we are happy with the result so far.

Crazing of the lexan due to fuel spillage just isn't attractive to me. I am very well acquainted with the guy who will do most/all of the fuel tank filling and know that he won't go for long before he would spill fuel on the lexan. By the way, the lexan was about twice as thick and didn't want to bend to form at all. Just hope that the plexi will hold up - I trust Van's on that one!
Good-day Elliot,

Generally, how did you arrange or trace or however pattern it?
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