Physics-Based Zoning of Unconventional Aircraft: The Swept Stroke Phase

Lightning zoning is a critical exercise for the design and certification of the lightning protection measures of aircraft. As the aviation industry powers towards a low-emission future, novel zoning methodologies, compatible with non-standard aircraft configurations, need to be devised. Current practices for evaluating the swept stroke region (zone 2) rely on empirical models, which are trusted for ‘tube-and-wing’ concepts, but their validity remains unproven for less conventional, next-generation aircraft. Physics-based models, which are increasingly feasible with modern computing capabilities, have the advantage that they are agnostic to the particular aircraft being considered and theoretically do not require prior experience with that vehicle. Therefore, the use of physics-based models for lightning zoning might be the only path to certify aircraft for which no in-service experience is available. They will also align industry standards for zoning with the state-of-the-art in engineering simulations.

In this contribution, we present a physics-based model for the swept stroke zone, building on previous work which identified initial attachment points (zone 1). The swept stroke is simulated using an idealized lightning arc, which is linearly advected in a flowfield, and realistic flowfields for 3D aircraft models are obtained using computational fluid dynamics (CFD). The proposed tool uses physical models for reattachment and reconnection of the arc and also tracks the delivery of current to the aircraft surface. Any number of arcs can be simulated in parallel, randomly positioned in the first lightning zone. The distributions of attachment probability and dwell time across the aircraft surface can be mapped to identify lightning zones 2A (swept stroke) and 2B (swept stroke with long hang-on).

This work presents results of the swept stroke model for various aircraft configurations. First, data from the NASA Storm Hazards Program is compared to simulated results on the Convair F-106B to validate the behavior of individual lightning arcs and to understand the effect of simulated flight conditions. The model is found to be most sensitive to aircraft attitude, while altitude does not significantly impact results. The Storm Hazards Program data also validates the model’s ability to simulate the distribution of attachment probability over the aircraft surface. Second, a generic commercial aircraft is ‘zoned’ and compared to published zoning diagrams to validate the output of simulations with thousands of arcs, focusing on mapping probability and dwell time data into a zoning diagram. Finally, the model is applied to different unconventional aircraft configurations to demonstrate a zoning result which could not have been achieved following the current Aerospace Recommended Practices.