The UV Index and why your canopy is your friend

Earlier, Eric Greenwell wrote:

All the ones I've looked seem to be about 0.60"...

Now wait a minute, Eric, I think you might have slipped a decimal
point.

A thickness of 0.60" is about 5/8" or about 15mm. I have seen aircraft
transparencies that thick, but only on transport and jet fighter
windshields. In polycarbonate, thicknesses like that might be
considered "Bullet Resistant" (I don't like the term "bulletproof").

The canopies for the early HPs are usually made from 1/16" thick
material; that's about 0.063" or just over 1.5mm. That's pretty typical
for small transparencies such as you'd find in older sailplanes with
two-piece canopies. It makes for a transparency that is perfectly
adequate for most sailplane flight loads, but with less than inspiring
stiffness; especially for limber plastics like polycarbonate. For a
while I had a 1/16" Lexan forward canopy on my HP-11, and I remember
once when I was pressing back to Truckee through hail that the whole
forward canopy shimmered like a soap bubble with each hailstone strike
- and they were not much bigger than peas.

More typical sailplane canopies, such as the HP-24 transparencies I've
been ordering, are made from 1/8" material; that's 0.125" or just over
3mm. Most of the broken European sailplane canopies I've seen seem to
be about 3mm thick.

Here's the thing, though: The minimum thickness of a canopy is often
substantially less than that of the original sheet of material. Since
most canopies are either free-blown or stretch-formed from flat
material, the finished canopy has more surface area than the original
pre-formed material. That extra area doesn't come from nowhere; it
comes from stretching the acrylic while it is hot and rubbery. Since
(for our purposes at least) acrylic is incomressible, it has to get
thinner where it is stretched, and gets thinnest where it is stretched
the most.

It's hard to say how much thinning is typical, but I have seen pieces
of a broken canopy made from 1/8" material where the minimum thickness
at the crown was about 0.090".

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com