Lightning and Electrostatic Charge Effects And Protection Design Approaches For Large Transport Airships

Airships are necessarily exposed to the atmospheric electrical environment, which includes electric fields, electrostatic charges, and lightning. Protection from hazards presented by these environments is necessary. One of the main issues is protection of the envelope, which is usually filled with helium. Helium poses special problems in this regard, even though it is an inert gas, because electrical breakdown of helium occurs at about 1/3 the electric field required for air. This means that it is possible to have thunderstorm induced electrical discharges in the helium volume before discharges occur in the surrounding air. Therefore, it is possible to have initial lightning attachment to structures inside the envelope resulting in envelope puncture. It is therefore necessary to include the presence of helium in the lightning protection design. This paper offers lightning and static electricity protection concepts for a large semi-rigid airship based on simulated lightning tests of simple 1/25 scale model envelope enclosures combined with laboratory testing and numerical simulations of thunderstorm electric field interaction with the airship geometry and materials. The relevant properties of helium compared to air are provided based on laboratory testing. A description of how the examples of the scale model laboratory tests of simple helium filled envelope shapes, combined with numerical electric field analyses approach and test results can be used to verify adequacy of protection of a full size airship will be presented, as well as correlated numerical simulations. Additional concerns related to potential lightning and electrostatic effects on airship propulsion, flight control and electrical and avionic systems will be described, together with approaches with addressing them. Proposed new transport airship certification requirements, adapted from transport aircraft certification requirements, will also be reviewed. Airships and balloons in tethered or free flight have been struck by lightning and have experienced effects of static charge accumulations. Often the details of these encounters have not been fully understood or investigated, and there has been uncertainty as to whether the event was associated with static charge accumulations, or with an actual lightning strike to the envelope. Lightning strikes to tethered airships (i.e. aerostats) seem to have been reported more frequently than have strikes to free flying airships, but this may be only coincidental, or it may be because the free flying airships usually have the option of avoiding thunderstorm (and other electrified cloud regions), whereas a tethered craft may be exposed to whatever conditions pass by. Also, the electric field environments and charge distributions about tethered and nontethered balloons (for example) may differ, and this may render the tethered variety somewhat more susceptible to lightning strike attachment. More importantly, envelope lightning protection methods that have been successful for (comparatively) small tethered balloons may not be as effective or applicable at all, when employed on larger, free flying airships. Moreover, future airships intended for commercial and/or industrial roles may be more susceptible to lightning and static charge encounters than have present airships utilized for advertising or recreational functions. AIRSHIP LIGHTNING EXPERIENCE Previous large airships, such as the Zeppelins (Graf, Hindenburg, Los Angeles) contained the lifting gas within balloons within rigid metal frames. When lightning strikes occurred the lightning current entered the metal framework and did little damage to envelope materials. The Hindenburg is a possible exception, where it is believed that an electrical discharge of some sort, either a strong streamer (100’s of A) or a lightning strike (1000’s of A) flowed within or across the partially conductive fabric and set it afire. A conductive coating had been provided to protect this fabric from degradation due to sunlight. The more ubiquitous advertising blimps seen today usually avoid known lightning conditions and