lightning strike protection

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