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

What'd he say?
JJ

On Monday, November 13, 2017 at 4:37:48 PM UTC-5, wrote:
What'd he say?

Try not to get hit by lightning.

Let's ask KM what he thinks.

Chip Bearden

On Mon, 13 Nov 2017 13:37:46 -0800, johnsinclair210 wrote:

What'd he say?
JJ

I thought it boiled down to "don't put short bits of insulator in metal
push-rods".

I read the BGA advice as "use stiff non-conducting tubes for aileron
pushrods and restrict the use of metal to the end fittings on them"
because this would stop the push-rods from being used as conductors which
generate enough ohmic heating the blow the wing skins off.

However, I don't have the background to understand whether this would
prevent the aileron-aileron linkage (via the in-fuselage control
connections) from acting as a lightning short-circuit without somehow
causing the main spars to take over as the prime short circuiting element.

Over to you, Mr. Firth


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

What if the whole wing is non-conductive (including spar and control rods)? Would that help?

There is still the fuselage to worry about. Cables and control rods and electrical wires (including radio antenna and its cable) and the pilot.

Ah, the joys of flying aluminum...

Boggs

To paraphrase:Â* If you see a bolt of lightning coming your way,
immediately go to negative flaps and dive away to outrun the stroke.Â* If
you're flying an unflapped ship, you're screwed.

On 11/13/2017 2:37 PM, wrote:
What'd he say?
JJ

--
Dan, 5J

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