Windshield cracks

[runwayrex]

I typically get 2-3 years out of a lexan windshield on my Model 4. This last one needs replacement in only eleven months. I used a heat gun to warm the plastic a bit by the wing root on one side when I installed it, but that side is just as bad as the non heated side. Has anybody found a method to prevent the crazing at the sharp bend by the wing root?
Rex Phelps / Michigan

[DanB]

Hi Rex,
If you do a search on this site for cracked windshield you will get several ideas. Best Wishes,

[runwayrex]

Dan,
I researched windshields and found some info regarding preformed and different materials. Is there something regarding preventing the crazing at the wing root when using lexan?

Changing every 2-3 years was okay, but less than one year has me searching for a new solution.

Thanks,
Rex

[DanB]

Rex,
To the best of my knowledge (limited I'm sure), the Lexan usually does craze at the wing root as the complex bend surpasses the limits of the material. Many of the guys that have this happen either live with it, replace them often or use the acrylic pre-formed from LP. I am personally a few weeks out from trying my hand at installing the LP screen that came with my kit. I am not going to hold my breath because I don't expect much from a screen that has gone through several winters in a Idaho hanger only to follow up with a few summers in a Phoenix storage unit.
I have some of the best Kitfox experience available to man here in the Valley and I plan on rallying the troops for this one. If we get it on without a hitch, I will be a happy camper. If not...Lexan it is.

Has anyone out there not had the Lexan craze? If so, we would like to hear how you did it.

[HighWing]

Rex,

It is a cosmetic issue - crazing - or are you seeing cracks that are propagating down the windsheild.

I replaced once in ten years, but the crazing was always there. I didn't use the heat gun, but always installed in the heat of the summer. The mimimum bend radius recommended by the manufacturer is 100 times the thickness. so if you have the .093" the minumum bend radius shouild be 9.3 inches. The two inche radius we use will most certainly result in crazing. I just measured my original 1993 windshield - I just can't seem to throw anything away - and it measures .060". I suppose it is the 1/16" I see offered online. If yours is much thicker, it will more readily craze. I used the .060 for 900 hours and it did cave in a bit if I pointed the nose to the ground and approached redline, but except for the movement in fast air, never had any issues. The reason I finally raplaced it was surface clarity and to finally get rid of the scar I placed in it while I was building.

Lowell

[runwayrex]

Lowell,

I don't mind the crazing until it turns into cracks that have sharp creases extending 5-6 inches toward my cowling. Mine is the .093 thickness. Maybe I'll go back to the .060 and see how that works. My VNE on this Model 3 is officially 100 mph. Hopefully the .060 can handle that. I do have the Model 4 cowling on the plane which I believe is a bit easier on the bend. I appreciate your ideas.

[dholly]

I just replaced my Model III windshield/skylight (.060). The Lexan polycarbonate manufacturer says controlled and uniform heating of the entire polycarbonate sheet is a critical factor for hot bending, as slow heating rates will balance out hot spots and allow the sheet edges to reach the required forming temperature. After attaching to the butt ribs, on a suggestion from a fellow Avid builder, I hooked a 1000w halogen work light to the headrack inside as a heat lamp and my twin halogen stand positioned outside so as to shine not only on the bend area but to warm the entire sheet for a period before getting started. I think this has merit and, like super smooth edges, may help reduce future crazing. Time will tell but I figure it can't hurt. I used a heat gun on low setting for extra stress relief while forming the acute, compound curve in the pre-warmed sheet. Blasting the acute curves only with high heat certainly makes the bending process easier but, unless you heat the entire sheet (or as much as possible around the target work area), you have not done everything you can do to help reduce crazing. Good info about hot and cold bending, tips working with poly etc. can be found HERE (http://www.theplasticshop.co.uk/plastic_technical_data_sheets/lexan_polycarbonate_sheet_processing_guide.pdf).

[Av8r3400]

If crazing is an issue for you, the ONLY way you will avoid it is by using a formed acrylic windshield. Lexan will craze, period.



I have a formed windscreen and bubble doors on my model IV flyer. They are well worth the money, in my opinion.