Lightning strike simulation helps designers protect composite structures

Lightning strike tests of a helicopter at Eurocopter's German headquarters in Donauwörth have verified the accuracy of finite element analysis techniques for characterising the electromagnetic behaviour of modern aircraft structures made using advanced composite materials.

The exercise was performed using the Opera electromagnetic design software from Cobham Technical Services, as a final element of the company's work for the ILDAS (In-flight Lightning Strike Damage Assessment System) project.

Simulation of the ILDAS tests highlighted how finite element techniques can generate accurate models of complex assembled airframes, and simulate the effects of lightning strikes rapidly - in around a day on a standard office PC. The work will help developers evaluate and optimise lightning protection measures during the design cycle.

Commercial passenger aircraft are struck by lightning once a year on average. Powerful strikes can result in costly delays for inspection and repair. The industry's current certification against lightning is based on threat levels derived from measurements of cloud-to-ground strikes. While this approach has served well for traditional airframes with good metallic conduction, modern aircraft are incorporating increasing amounts of lightweight composite materials. This makes them more susceptible to direct damage at lightning entry and exit points, and potentially to indirect energy coupling effects into the electrical systems as current flows through the aircraft. As a result, it becomes increasingly important to understand the exact nature of the threat by accruing data on actual in-flight strikes. Modelling the current flow patterns within complete assembled airframes with validated software can also reduce costly testing procedures.

The ILDAS project was conceived to develop an in-flight embedded system for measuring actual lightning strikes. Coordinated by The Netherlands' National Aerospace Laboratory, the partners are Airbus, Air France Industries, EADS Innovation Works, Cobham Technical Services (Lightning Testing & Consultancy), Cobham Technical Services (Vector Fields Software), Eurocopter Deutschland, Groupe Socius, LA Composite, Lufthansa Technik, ONERA, and Technische Universiteit Eindhoven.

"Airframe structures making extensive use of composite materials have less natural protection against lightning," explains John Hardwick of Cobham Technical Services (Lightning Testing & Consultancy). "As lightning protection measures such as conductive coatings or strips add weight it's important to optimise the design. Simulation provides an effective means of achieving this."

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