Willy VINKEN wrote:
ar/canopy - related charring happen in flight?

I completely agree, focusing solar light can only occur by reflection,
by transmission, as the canopy has parallel inner and outer surfaces, it
cannot change the direction of light rays, only slightly offset them.
Hey, this is just why we can see through it as if it were not there !


I don't agree.
I've seen it happen at EBSH on the rear headrest of a Ka21 with open
canopy. The sunrays incidence was almost tangential to the uprised
dome and definitly crossed the plexiglass from the outside.
Even if outer and inner surfaces are parallel (which has not been
proved), near-tangential rays have to cross more material where they
hit the vertex of the curved dome, which acts as a biconvex lens,
with in this case, a bright focal point on top of the headrest.
I don't believe in reflection-related damage. The sun would have to
be too low on the horizon for this to be possible with open canopies
and due to the see-through qualities of plexiglass, at best only a
very small part of the energy would be reflected.

Believe it! Numerous people, including myself, have carried out the
experiment. Schleicher gliders, in particular, raise the canopy high
enough that it can easily happen during the soaring hours. I always
check the sun position before raising the canopy on my ASH 26 E (as I
also did on my ASW 20) to avoid any more burns on the black cloth I have
on the glare shield. Even the manual warns about it.

The transmission-type damage I witnessed could not happen in flight
either: the sun would also have to be too low -thus weak-, even the
slightest movement of the glider would prevent the same spot to be
focused continuously, and above all, the lens-effect of closed
canopies would focus well above anything in the cockpit.

I am not aware of any transmission-related damage, and have never seen
any through-the-canopy focusing with the gliders I've had.

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Eric Greenwell
Washington State
USA