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Rear window plexi vs. Lexan

EastCoastFlyer

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I'm just about to tackle the rear window. Does anybody know what the pros and cons are of the Plexiglass vs. the Lexan rear window? I have the old Lexan window from my fuselage kit, but now Van's ships the Plexiglass rear window with the finish kit. I'm about to order my finish kit, so I could get and install the Plexiglass window if it is worth it. Any thoughts?
 
I have one of each, the plexi is tinted. I have not yet decided which one to use, but probably will use the plexiglas one. Way back when this first came up, the fact that the plexi would not craze and could be tinted was a plus, but Vans claimed it was a structural part of the plane, and the plexi was not strong enough. With my wing tanks with the fillers on each wingtip, I would have to get pretty sloppy to get fuel on the window now. I am down in the lower Rio Grande Valley and I think the tinted will serve me better in the hot sun.
 
but Vans claimed it was a structural part of the plane, and the plexi was not strong enough.

I am pretty sure all that was said was that the rear window was structural so you are correct on that part, but it was never said that acrylic wasn't strong enough (or it wouldn't be offered in the kit now).

I think what was said was that if you substitute some other material for the lexan, make sure it is at least as strong (the comment was made in a thread where someone was saying they would look into providing an alternative window).
 
From what I recently witnessed with my rear window that decision would be a no brainer. I installed the window about 6 months ago and only this week removed the protection paper from the window. I found a small crack near the edge, does not go to the edge, nor radiates from a mounting hole. It is just there! So for now, a small stress relieving hole,then in the future, a generation II window will be installed after window tint is applied.
 
You were correct Scott, I went back and checked. You urged caution in making a change to a structural part, and someone else chimed in that Plexi was far less in strength than Lexan.
I had worried about that ever since, but now that it is solved, I think my tinted plexi is going in, not quite Phoenix, but we get a lot of sun here in the lower Rio Grande Valley area..

I am pretty sure all that was said was that the rear window was structural so you are correct on that part, but it was never said that acrylic wasn't strong enough (or it wouldn't be offered in the kit now).

I think what was said was that if you substitute some other material for the lexan, make sure it is at least as strong (the comment was made in a thread where someone was saying they would look into providing an alternative window).
 
An Update: I looked at 12-15 RV-12's at Sun-N-Fun and almost all with Lexan rear window had cracks/crazing. Everyone I spoke to said to definitely wait and install the Plexiglas rear window.
 
An Update: I looked at 12-15 RV-12's at Sun-N-Fun and almost all with Lexan rear window had cracks/crazing. Everyone I spoke to said to definitely wait and install the Plexiglas rear window.

Where were the cracks/crazing? I slopped some fuel about the second time I filled up, and ended up with some fine cracks starting from 2 or 3 screw holes near the fuel cap. No real surprise there. However, the plane now has 85 hours on it, and I've just returned from a 4 week trip, covering about 4,000 miles. To my surprise, there are now very fine cracks starting from a number of screw holes on both the front and rear edges for the Lexan on both sides of the fuselage. The window was installed as per the plans (ie. no oversize holes, bushings etc), so I'm wondering if these are expansion/contraction cracks. Has anyone else come across this?

Incidentally, the plane performed very well on the trip. Landed on some very rough outback strips with no problems. The autopilot was a very useful addition, well worth the money. To my mind, the only drawback of the RV-12 as a cross country flyer is the relatively small fuel tank. Travelling across the remote parts of Australia you often have very few options for alternates if you can't make it into your destination. I like to land with a decent amount of fuel left in the tank, and another 5-7 gal would make all the difference. Not a complaint, just an observation, as I know all aircraft designs are compromises, and I'm aware of solutions others have put forward.
 
I had very bad crazing in my rear window in the center of the window on both sides so I know it wasn't fuel related. This was in an SLSA with less than 100hrs. I've replaced it once and am seeing a few early signs that its returning, Plexiglass next time for sure! I also agree with your assessment regarding the fuel limitations.
 
. . . but Vans claimed it was a structural part of the plane, and the plexi was not strong enough. . . .

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.
 
not 'the right stuff'?

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!!!!!:eek:
 
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
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.
 
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!
 
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!
 
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?
 
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!!!!!:eek:

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
 
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. :rolleyes:
 
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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"

:)
 
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!
 
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?
 
Pattern

Doug - hope that this helps. I don't claim to be an expert - this how we did it.

Please measure and make sure that this will work for you before you drill any holes into your plexiglass. We checked by installing our lexan piece onto our 12 sheet metal. It fit great and the holes matched the pre-drilled holes of the aluminum pieces of the 12. We did not have to trim the plexiglass sheet prior to drilling - only a very small amount of filing the two corners and edges as we final fit the rear canopy. That allowed us to go directly to marking and drilling the holes into the plexiglass.

Place the lexan on top of the plexiglass flat on your work bench and align the straight edges of the two sheets exactly - the plexiglass underneath the lexan 'pattern'. Carefully mark thru the Van's factory holes in the lexan 'pattern' onto the plexiglass using a fine felt tipped pen. These marks are where you will drill your holes into the plexiglass. Be as accurate as you can.

Remove the lexan 'pattern' and begin the drilling process. Take your time, be as accurate as possible in drilling exactly where you marked the plexiglass. Use Van's method of drilling plexiglass or whatever method suits you.

If you haven't drilled plexiglass before, I recommend that you research how to do it including Van's instructions at the first few pages of the plans set. Again, take your time while drilling. Best of skill to you.
 
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