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This paper reports the results of swept frequency transfer function and pulse testing performed on a small graphite test article with an aluminum tube attached to it and driven in parallel. The data is being used to support the validation of analytical models and tools for lightning indirect effects. In addition, swept frequency and pulse testing techniques are compared.

Lightning testing performed on fuel tank components or structure utilizes photographic techniques or an explosion test cell to determine tendencies of the hardware to produce sparks. The photographic technique utilizes the no-light-on-film pass/fail criteria with a Polaroid camera at an f-stop of 4.7 and a film speed of 3000 ISO, or a 35 mm camera equivalent (reference 1 and 2). There is speculation that although, per the test specifications, the requirements of the test are met if these film and camera settings are as specified, the various film types would not produce equivalent results. It is common for faster speed films to be grainier which could affect the ability of the film to detect small sparks. There are also color films which, depending on their manufacturer, can have different sensitivities to various light frequencies. The processing of the film can also affect the ability to discern small sparks.

The electromagnetic coupling between a conductor and a composite structure by wire-mesh techniques using method of moments is investigated. A three-bladed composite panel with an aluminum tube located above the panel is considered in this analysis. Computations are made of the current on the tube for various frequencies with height above the panel as a parameter. The numerical results compared reasonably well with the measurements. The technique proves to be useful in modeling the composite structures such as wings of an aircraft.

The Finite Difference Time Domain (FDTD) method (as implemented in the commercial software package EMA3D) is used to model indirect lightning effects in a composite three bladed panel and a composite wing-box. The analysis is compared with low level continuous wave (LLCW) tests performed in the Boeing Lightning Effects Laboratory. Measured data include transfer functions for currents induced on metal tubing interior to the three-bladed-panel and wing-box. The thin wire and thin plate formalisms provided in EMA3D are used to model the composite surfaces and metallic conductors such as pipes in the wing-box. The wing-box is simulated both with and without apertures. The modeling results showed excellent agreement with measurements over a broad frequency range demonstrating the usefulness of FDTD as a predictive tool for lightning frequencies.

A comparison was done of the prediction capabilities for lightning indirect effects of two two-dimensional (2-D) computer codes using two graphite structural test fixtures. The two codes evaluated were an internal Boeing Method-of-Moments code and a commercially available Boundary Element method code. The codes were compared against each other and against test data. The purpose was to evaluate the prediction capabilities of both codes for use in predicting lightning indirect effects on internal components of graphite structure. Since 2-D codes are much easier to use than 3-D codes, they could be widely used in trade studies and design evaluations for lightning indirect effects protection of composite aircraft. The first code, REDIST, is a Method-of-Moments code developed in the 1980’s for use on the B-2. The REDIST code has short run times and is somewhat easier to use than the second code that was investigated.

A comparison of various numerical tools and techniques was performed for calculating the lightning indirect effects to composite structures and internal systems. This paper is a summary of the initial comparison results. Detailed results of each technique considered are given in additional separate papers presented during this conference. The modeling considered current distributions over and within composite surfaces and the coupling of current and voltages to internal systems such as wire bundle cables and hydraulic and fuel tubes. The models were compared to each other and to measured data from low level swept continuous wave (LLCW) tests performed on two test fixtures. Other features of the codes such as run time, ease of use, computer requirements, availability of documentation and technical support, etc. are compared as well.