This page collects "hands-on" information about the materials that are used to build a BD-4. It is not an attempt to present the complete specifications. The emphasis here is more on what other builders found out when working with that stuff.
Right now the statements are all from RAH (rec.aviation.homebuilt) without references to the authors. For the future we should try to always mention the source.
Plexiglas:
Source: RAH:
Acrylic can be broken with a hammer in the event of a crash, whereas polycarbonate
is tougher and difficult to break. Polycarbonate offers better bird strike
protection than acrylic, but also scratches easier than acrylic. Acrylic is
easier to form compound curves, but is harder to drill. Polycarbonate will
craze if gas gets on it.
For some technical data on polymethylmethacrilate (Plexiglas, Perspex) go
to http://www.psrc.usm.edu/macrog/pmma.htm
and the Roehm web at http://www.roehm.de/en/plexiglas.html
and for polycarbonate (Lexan) see http://www.psrc.usm.edu/macrog/pc.htm
and the General Electric web site http://www.ge.com
(where you will have to register in order to find anything of value).
Neither material is particularly resistant to abrasion (without anti-scratch
coatings) but polycarbonate is somewhat better. The antiscratch coat on polycarbonate
CDs is so good that it is almost impossible to polish out scratches. Neither
is particularly resistant to organic solvents. Density is about the same so
weight is not a consideration for aircraft windscreens. Polycarbonate is better
for impact resistance so that would make it superior for keeping birds out
of your face, but could make it more difficult to exit a damaged aircraft.
Both are available in sheets larger than 4 X 8.
Acrylic, by the nature of how it is made, will give you better optics as well.
Acrylic doesn't typically have the "wave" you sometimes have to look through
in a poly canopy.
Polycarbonate yellows with exposure to UV much faster than acrylic. there
are coatings to prevent this so if the polycarb is your preference, be sure
to get such a coating.
....
I've bent [Lexan] in a brake, works fine,,, just scary the first time... Lexan can only be heat molded if essentially cured out in a dry hot oven. It takes about 1 hour per 1/16 inch of thickness in my limited experience.... At about 180 to 200 degrees F. If you cold bend it and put 1 drop of gasoline on the bend it will craze and break/shear. You can vacume form it some when hot (I think it was at 250 deg F)with a buc and a good large pump. I cut it with a good carbide blade in a skillsaw. I was building the small windshields/wind deflectors for a Formula Car.
Aluminum Alloys:
For resistance to general corrosion and stress corrosion cracking 6061 is
superior to everything else (except the low strength 6063) and can be used
bare. The 2XXX alloys are inferior (except 2219 and 2011 which are not particularly
susceptible to stress corrosion cracking). Corrosion resistance of 7075-T73
is similar to 2219 and 2011 and is superior to 7075-T6 (which is comparable
to the other 2XXX alloys). However, with some sort of protective coating any
aluminum alloy will be suitable, but unless the part is highly stressed play
it safe and stick with 6061. The wing spars and other structural elements
of commercial aircraft from Boeing, Douglas and Lockheed are made from 7075
or other 7XXX alloys but all need a protective coating.
The real problem with the 7075 series is its notch sensitivity. Round all
your corners and polish it well. :-)
All high strength aluminum alloys are susceptible to stress corrosion. In
a massive part I wouldn't worry about it very much. Paint all non-mating surfaces
with a good quality two part epoxy primer.