windshield fabrication

Discussion in 'Workshop Tips and Secrets / Tools' started by aeromike49, Dec 16, 2012.

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  1. Dec 16, 2012 #1

    aeromike49

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    I am rebuilding an antique experimental plane and the original windshield is missing so I need to fabricate a new one. I tried Lexan and used a heat gun to bend the material and now the vision through it is distorted. Any ideas as to what material to use and how to bend it without distorting the vision through the clear material?
     
  2. Dec 16, 2012 #2

    SVSUSteve

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    Depending on what you have access to, an oven might be a better option to assure even heating. The distortion is likely the result of uneven heating or poor shaping on a mold both of which are very easy to do.

    Personally, I would stick with Lexan or a similar polycarbonate. In fact, I am planning to use a bonded double layer windscreen of polycarbonate to provide bird strike protection, structural integrity (since mine is a pressurized aircraft) and sound attenuation. The material will be heated in an oven then draped over a positive mold for the inner pane and a negative mold for the outer. The bonding will most likely be accomplished with the same urethane material used in commercial windshields for the same purpose. The hardest part of the whole thing is going to be figuring out how to include a defrosting/anti-ice option.
     
  3. Dec 16, 2012 #3

    4trade

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    Last edited: Dec 16, 2012
  4. Dec 16, 2012 #4

    StarJar

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    You can make your own small oven made out of foam board sandwiched with reflective sheeting bought at Home Depot/Lowes. Then find some old electric ovens and take the heating elements out and put them in your new oven. (Make the oven just big enough to hold your parts.)
    Then put some sheet metal on two wood formers, that duplicates the shape of your present windshield.

    Test it first with some cut scraps, to get the temp. right. Then lay your lexan on the sheet metal, and fire up the oven. You can make a small peep hole to peek in with a flashlight.

    I did this for a small bubble canopy and it worked. I didn't use the sheet meatal former, but I think it would work.

    I'm not saying this is the 'right' way or best way, but it worked fine for me under my circumstances.

    I not a professional at this, so if someone else has further advice, please, give it.
     
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  5. Dec 16, 2012 #5

    thump

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    this was done using a propain heater and an old trailor,You can build a wooden box and line it with 5/8 sheet rock for fire res. 20121216-134913.JPG 20121216-134918.JPG 20121216-134920.JPG 20121216-134933.JPG 20121216-134935.JPG 20121216-134938.JPG 20121216-134943.JPG
     
  6. Dec 16, 2012 #6

    Dan Thomas

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    Is this a bubble-type canopy or a flat wrap? If it's flat wrap, just bend the Lexan cold. It won't break. Heating Lexan is a really touchy thing; it absorbs a small amount of water, and that water vaporizes in the plastic and fogs and ruins the clarity. Any polycarbonate will do that, and polycarbonates also give off poisonous gases at certain temperatures, so I would avoid heating it for forming and use acrylic (Plexiglas) instead.

    Port Plastics > Architecture

    Material Safety Data Sheet for Makrolon® AR Polycarbonate Sheet

    Dan
     
  7. Dec 17, 2012 #7

    aeromike49

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    A simple flat wrap type windshield with minor bends. Low speed aircraft - less than 110 knots max speed - I will try some fairly thin Lexan - without heat - on a warm Arizona day.
     
  8. Dec 17, 2012 #8

    SVSUSteve

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    The major problem is that thin Lexan or acrylic tends to vibrate like the skin on a drum in the wake of the prop which adds a major component to cockpit noise. It's a huge annoyance (at least in my book) that detracts from flying. Also it tends to shatter if impacted by anything bigger than a robin. One of the other forums a while back posted pictures of what happens when a flock of quail hit the thin (I believe it was 1/8") windshield of a light aircraft.

    I didn't think about that. You may have just made my build a bit easier.
     
  9. Dec 17, 2012 #9

    Toobuilder

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    Lexan is a bad choice for windscreens. It scratches very easily and very shortly loses its impact resistance due to damage from UV. So what you end up with is a very scratched, weak windscreen.

    Anyway, I did my own windscreen for the RV here.
     
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  10. Dec 17, 2012 #10

    SVSUSteve

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    So what do you recommend then? Is there another form of polycarbonate that is a better choice?
     
  11. Dec 17, 2012 #11

    Toobuilder

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    Good high quality plexiglass is plenty tough stuff. Stay away from the extruded sheet from the big box stores - its junk. There will be a plastics distributor in most large cities where you can get "real" Plexiglass.

    The idea behind Lexan is a good one - and was my first choice too. But after talking to my engineers here at work, they told me just how tough it is to get Lexan to survive. First off, it will be a laminate with a tougher material (like Plexi) bonded to the outside for scratch protection, then the whole mess has to have a UV coating applied so the Lexan can retain the impact properties. While it is worth the effort for F-35 canopies, this is well outside the scope for us.
     
  12. Dec 17, 2012 #12

    henryk

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  13. Dec 17, 2012 #13

    PTAirco

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    You cannot possibly be talking about Lexan - it does not shatter under impact. You could throw 10lbs rocks at it all day and you will not break 1/8 polycarbonate. It's used for bulletproof riot shields for a very good reason. I work in the optical industry and polycarbonate lenses have pretty much taken over the standard plastic. The only way to destroy them is with certain solvents. I have used a hammer on ploycarbonate lenses 3mm thick and it just bounces off.

    Yes, it will scratch a little easier, but careless cleaning of plexiglass will ruin it almost as fast. I personally won't ever use plexiglass in any project again, unless there is some compelling reason.

    As for UV damage, I think all cockpits and windshields should have covers, unless you have a hangar.
     
  14. Dec 17, 2012 #14

    autoreply

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    Just to illustrate your point:
    [video=youtube;ibJ3aXrvaCs]http://www.youtube.com/watch?v=ibJ3aXrvaCs[/video]
    I understood in recent years much better formulations have come up that are reasonably resistant to UV (Lexan). Is that correct?
     
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  15. Dec 17, 2012 #15

    Hot Wings

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    This is the same material from which soda bottles are made. I've decided to try a flat wrap of this on my part 103 project because it should be more resistant to penetration/shattering than standard acrylic, even though it costs more. Scratching may be more of a problem and if it is I'm prepared to try a "tear off" laminate like used on helmet shields.
     
  16. Dec 17, 2012 #16

    Toobuilder

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    Dramatic, for sure... But that gold plated (literally), Polycarbonate/Acrylic laminate on the F-16 is light years different than blowing a canopy from some Lexan sheet sourced from Home Depot.
     
  17. Dec 17, 2012 #17

    SVSUSteve

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    Yes, it does. Any material has a yielding point. A ten pound rock thrown by a person has a lot less energy than a 4 lb bird at cruising speed of even a slow aircraft like a Cessna or a Piper. If 1/8" lexan were sufficient, we would not have multiple pane windows on higher end aircraft for bird strike resistance.

    It's also used in multiple layered applications often in excess of an inch thick in exactly those applications. Saying that it is practically unbreakable is akin to saying that a single ply of Kevlar is satisfactory because they use it in ballistic vests (ignoring the fact that it is also used in multiple plies to achieve that performance). Lexan is wonderful stuff but it has its limits which must be respected. We can't give ourselves over to magical thinking or reliance on examples that don't have direct applicability (see below). There is nothing wrong with the material itself. We simply need to work to maximize the good properties of it while accounting for its weaknesses.

    Just to illustrate the invalidity of your illustration....there is quite a big difference in thickness and construction between a single pane of 1/8" Lexan and the material used to make the canopy on an F-16.

    One of my best friends is an egress and life support systems mechanic for an F-16 squadron so I asked him about this once when I was looking for insight on how to design a scratch resistant windshield for my aircraft. The 550 kt impact tolerant canopy on the F-16 (which was developed after a fatal birdstrike with an earlier canopy) is made up of three different plies and is over an inch thick. The outer layer is polyurethane for scratch resistance and is about 0.5" thick. The middle layer is polycarbonate is 0.75" thick and the inner layer is 1/8" of acrylic and designated as a "spall prevention layer" to prevent fragments of the polycarbonate from penetrating into the cockpit.

    So....slight bit of difference there from a non-coated 1/8" thick sheet of Lexan. Trying to compare the two is like trying to compare being a Dutch glider pilot and being an American powered aircraft pilot. The basic "materials" might be the same but there are some distinct differences in how they are used.

    On this we agree 100%.

    You are correct. There is at least one formulation that is UV resistant because it is used in some greenhouses for the better tolerance to impacts from small hailstones that would shatter traditional glass panes. The high school I attended from had its greenhouse refitted with it after a severe storm a few years back.
     
  18. Dec 17, 2012 #18

    StarJar

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    Having read all that......Isn't Lexan tougher than plexiglass? Somewhere from a long time ago, I always had that impression.
     
  19. Dec 18, 2012 #19

    cavelamb

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    Not if any solvents touch it...

    The uncoated side will craze to the point of open holes rather rapidly if bent.
     
  20. Dec 18, 2012 #20

    Dan Thomas

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    For the average homebuilt, Lexan is fine. I got 16 years out of the Lexam windshield on my Jodel, and recently replaced with a new piece that took me a couple of hours to make. Bent cold. 1/8". The airplane is hangared. The Lexan failed because I splashed a little fuel on it while refuelling, in cold weather, and the thermal shock cracked it. I had never seen that before. The crack started at a screw hole. I think there was microscopic cracking occuring there anyway, from age and stress. The windshield straightened right out when I took it off.

    You can take a piece of Lexan, lay it on the concrete, and pound the daylights out of it with a hammer and it won't break. If you place a piece of it over the open end of a short length of 2" pipe and hit the center with a hammer, hard, it MIGHT punch out a piece. I have had a smaller bird bounce off my windshield, leaving no marks or damage. A larger bird of the four-pound variety will come through a Plexiglas windshield much more easily than through Lexan. The typical low speed Cessna/Piper windshield is 1/8" Plexiglass, and the Lexan is far stronger. Sure, Lexan scratches more easily, but with care that is minimized, and the homebuilder with a flat-wrap affair can replace it easily anyway. Plexiglass of that thickness requires heat-forming, which takes far more work and time and expense. Lexan can be treated as if it was steel: drill it, cut it with a bandsaw or shears, bend it sharply in a brake. Just try any of that with Plexiglass---even the drilling is risky.

    Four-pound birds are a threat to more than a windshield. They can cause structural damage.

    Never use Windex or just about any other solvent on Lexan. Brake cleaner ruins it right away. To be fair, Plexiglas is intolerant of many solvents, too. I have used Lemon Pledge furniture polish on my Lexan, to clean it up and hide the fine scratches it picks up with time.


    Dan
     

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