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Lightning strike simulation heralds simplified design

Lightning strike simulation heralds simplified design
Tests verify accuracy of finite-element current flow simulation in complete aircraft structures including client-specific cabling.

Lightning strike tests of a helicopter at Eurocopter's Donauwörth facility have verified the accuracy of finite element analysis techniques for characterising the electromagnetic behaviour of complete modern aircraft structures constructed 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 easily generate accurate models of complex assembled airframes, and simulate the effects of lightning strikes rapidly - in around a day on a standard office PC - to 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. 

The project was conceived to develop an in-flight embedded system for measuring actual lightning strikes. This will help in understanding the threat, aid the design of protection measures, and streamline post-strike inspections and maintenance by capturing actual data on occurrences, intensity, and strike points. Opera software was employed to predict lightning strike current flow patterns on structures with carbon fibre composite materials. This knowledge helped ILDAS partners to select the best locations for sensors, and then to compare current flow predictions against actual measurements. 

Electromagnetic design
To achieve this goal, Cobham Technical Services generated an electromagnetic design model of a specific airframe configuration for an EC135 helicopter using CAD files from Eurocopter. This part of the exercise mainly involved simplifying non-critical parts of the original design data in order to minimize simulation times, while maintaining good representations of critical elements such as metal space frames and surface panels, carbon composite panels, electrical bonding and cable harnesses - including those for client-specific equipment. With this supporting work in place, subsequent models of variations on the basic helicopter airframe would be much quicker to create, providing a simple means of evaluating aircraft construction programs. 

Once the model was ready, the simulation itself took a little over a day to run on a good-specification office PC. Real-life tests at Eurocopter's Munich facility then showed that the theoretical predictions of energy diffusion effects agreed very well with simulation predictions.

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