On Sunday, November 12, 2017 at 3:25:23 PM UTC-5, john firth wrote:
A comment on the report of the ASK 21 disintegration .
( see Starer Nov 8th)

lightning strike protection: a fallacy

At the end of a report describing a strike on an ASK 21, causing disintegration, we find
" the BGA speculated on the improved lightning protection which
non-conducting composite material control push rods might provide."

this is a fallacy;
imagine the glider flying in a strong horizontal electric field ( which may extend over kms).
Remove the glider and leave the aileron pushrods. Locally the rods "short circuit" the local field but create a high field region at the ends of the rods. If breakdown occurs here it may lead to creation of a channel for
a discharge, which is what probably happened in the case of the unfortunate ASK21.

Now insert an insulating spacer in the rods; at it's ends there will be a similar high field region, which will simply break down if a strike is initiated; an arc will jump the gap.
Hence the fallacy.

Protecting a carbon spar would seem to require an alternative low impedance path, ideally encasing the spar in a good conductor; this would add weight and pose skin bonding problems. It might be sufficient to provide an adjacent conductor (aluminum has the best conductivity/density value) wingtip to wingtip, which will not approach melting temperature in severe discharges. The laminate could still be damaged by heat.

Rod deformation; a rough calculation based on a current of 100 KA, suggests that Al tube 15 mm dia, 1mm wall, will reach 600 C ( close to melting temp)
in 60 micro secs, the same order as observed lightning strikes. Magnetic compression will then easily deform the tube.

John Firth

If the whole wing is an insulator, you are better off; However, as the electric field near a T storm at altitude will likely be in the to-from direction,
and the fuselage has rudder cables, do not fly to or from a storm cell.
Simple enough?

JMF