I gotta ask... how often have you seen frozen chickens flying by when you are airborn?
Terry,
I was referring to the testing the Air Force once did on canopys. They were trying to test for breakage should a midair occur with a duck or goose, problem is they were not thawing the chickens they were blasting at the canopies! I thought everyone had heard this story.
Or was that McDonald Douglas.
Terry,
I was referring to the testing the Air Force once did on canopys. They were trying to test for breakage should a midair occur with a duck or goose, problem is they were not thawing the chickens they were blasting at the canopies! I thought everyone had heard this story.
Or was that McDonald Douglas.
I did hear the story, but the mental picture of frozen chickens flying around was perversely funny.
I did hear the story, but the mental picture of frozen chickens flying around was perversely funny.
The first images that came to mind for me were those of the Mythbusters' "chicken cannon" where they fired frozen chickens at an old Piper's (I think) windshield. Made pretty short work of it, IIRC.
Now that's quality television!
__________________
Cheers, Dave "Finish." AIR #4188 1994 34' Limited / 2002 Chevy 3500 CC 4x4 D/A Equal-i-zer Hitch / Jordan Ultima 2020
Just to clear up a couple of error in this thread. The original transparent rock guards were make from Acrylic sheet. Acrylic is the short generic name for PMPA polymer. Rohman Haas's trrade name fo it was Plexiglas. Dupont's trade name for the same material was Lucite. Cyro Industries and several others also made this polymer under various trade names. In error, many people just call it all Plexi.
Polycarbonate is the generic name for a higher impact polymer that was invented much later. GE named it Lexan. Bayer and Monsanto made the same kind of material under their own tradenames. GE recently sold their plastic business to Sabric so they now own the Lexan tradename. PC as the plastic people call it is a little softer than PMPA but much more impact resistant. PC and PMPA are sensitive to a variety of chemicals and can craze or crack when exposed to them when the material has a stress or an internal stress (like poorly drilled holes) applied.
Just to clear up a couple of error in this thread. The original transparent rock guards were make from Acrylic sheet. Acrylic is the short generic name for PMPA polymer. Rohman Haas's trrade name fo it was Plexiglas. Dupont's trade name for the same material was Lucite. Cyro Industries and several others also made this polymer under various trade names. In error, many people just call it all Plexi.
This is the same issue that gets all cola-flavored sodas called "coke", all snowmobiles called "skidoos", and all aluminum trailers called "airstreams"...
There are two kinds of acrylic. The one they used was a thermal plastic and could be heat formed at about 250-290 F. It does not need to be dried before you heat it. It can be bent with a heat shrink gun or a paint peeler type hot air gun or radiant heat source. Polycarbonate should be dried before you try and heat it too high. It absorbs water in normal conditions and will bubble up if you try to heat it too fast. If you get it dry, you can bend it successfully when it reaches about 350 F. You can evaluate the amount of stress in either PC or acrylic by using a polarized light source on one side and an other Polaroid filter to look through from the other side. Rainbows indicate the severity of the stress. Slump heating, in which the material losses enough strength to simple relax and fall to contact a wood mold give the least stress. A little gentle encouragment usually is not too bad. A cold bend with result in high stress in PC and will crack acrylic. By the way, some sheets Acrylic also comes as thermal set grade material and will not lose strength when heated and so can not be formed.
I've got a '49 Spartan Mansion I'll need to do curved window replacement on. It still has the original (I assume) acrylic in place but is badly star marked and crazed. Most of the Spartanizers seem intent on Polycarb but unless they use enough thickness (requiring heat bending) the windows tend to blow/pop out on them. I would probably use Polycarb too if I could but in my research I too found that drying is required to stress relieve/ bubble prevention. I'm familiar with resin drying (I work with PET injection molding and stretch blowmolding at the bottling plant where I work), but wonder if you could give me some info on Sheet material drying. Is this something that could be "homebrewed"? I work in maintenance and have the tech skills to build apparatus or a heating oven but I don't know anything about reqd. temp/ dewpoint, time to dry etc. Is this feasible and or worthwhile, or should I stick with Acrylic/ Plexi that doesn't require drying to heat bend? ...I guess 59 years is pretty good for the original windows. Also, what should I look for in fade/ UV protection. Any books on the subject you know of?
When I was restoring my old Fighters I had a canopy maker in LA make several plexiglass/acrylic canopys for me. He made a plaster form and heated the material over the form in a large oven affair. There are several places where you can still have that done and I am sure your windows would not be a problem.
I've never heat formed anything this large before but interestingly enuf I too was thinking of a plaster form cast from the inside surface of the original. Did your source use any coating/covering over the plaster or just on the bare surface? I seem to remember reading once of laying felt or flannel over the forming surface to protect the plastic from damage.
I have access to heating elements from several machines where I work so the oven shouldn't be too much trouble. I also have a buddy who has done Plexy/ acrylic forming as a resource but he does'nt have any background in Lexan/Polycarb drying if I were to go that route.
The more I mull this over the more I'm thinking Plexy may be the cheapest, easiest route to take.
GE owned and produced Lexan until recently. They had some very good Tech literature which might be available off the web. Sarbic recently bought the name, business, and people from GE. I have not done business with them yet. Drying proceedures and vac forming used to be in their literature. You need to be about 220 F to get effective drying. You need to be about 100 F higher temps to do the vac forming. From memory, I think it was about 350F. Acrylic would be easier to do and half the money but can be fairly easily broken by flying rocks. Spartans were never intended to be really road machines like the Airstreams and so did not need to be as resistant to abuse.
As the story goes the USAF exported their canopy chicken cannon to Canada or Britain so they could check their latest NATO design and completely destroyed several of their best designs - As you can imagine it took some long-distance trouble shooting over velocities and mounting etc. and it was by deduction from a description of how messy a raw chicken can be that it dawned on the other end of the line "Oh gawd - you mean we should thaw the bloody birds!...
Mar-guard scratch resistant lexan is great stuff and comes in some 20% tints, but the lexans and polycarbonates etc. with scratch resistant coatings will not heat form without stripling the coatings into zebra stripes.
Rain-X does a great job reducing hazing - crazing - sand blasting - UV damage on polycarbonate. Be generous so it can fill the pores and dry slowly in the pits and scratches, use a couple of coats.
As to drying Lexan/ polycarb....... Is the drying done with dehumidified (over a desicant bed)air or just by heated air only. The dryers we use for resin drying are hoppers which have dehumidified and heated air blowing thru them. I didn't know whether the desicant was required for lexan or not. Anyone know???
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