Practical Electric Field Simulation to Evaluate Aircraft Initial Lightning Attachment Locations

Lightning zoning is the process of identifying areas on an aircraft that are most likely to be struck by lightning. It is a necessary step to gaining aircraft certification. Determining zoning and mitigating potential problems keeps the aircraft working as intended.

Lightning Zones

SAE ARP5414 provides guidance on how to establish aircraft lightning strike zones. Table 1 describes the zones. Lightning is most likely to attach to Zone 1 areas.

Table 1. Lightning Strike Zones.

There are three primary techniques to determine initial lightning attachment zones: similarity to aircraft with service history, testing, or analysis. Figure 1 outlines the lightning zone location process. This article will focus on analysis and the process of determining initial leader attachment locations with a minimum set of practical steps.

Fig. 1. ARP5414 Zone Location Process

Electric Field Modeling

Analysis is a beneficial approach for new aircraft designs since there are no previous zonings to reference. Experts suggest using electric field modeling (EFM) or a rolling sphere assessment. Simulation is the best option because of its efficiency and ability to explore many possible electric field scenarios and aircraft configurations. However, no standard guideance currently exists on how to utilize EFM to identify initial attachment locations.

This simulation approach is not new, but it is different in that it stops at the electric field enhancement study. The primary reason for this simplified approach is because of its similarity to the accepted rolling sphere technique. Both methods rely on identifying the maximum electric field stressors around an aircraft to determine the most likely initial attachment locations.

This approach is practical because once you have an aircraft outer mold line (OML) model, designers can identify the maximum electric field enhancement locations within one day, generating substantial supporting data. Utilizing simulation for zoning adds the benefit of easily adapting the model for lightning indirect effects, fuel system current distributions, and HIRF analysis.

Initial Attachment Simulation Approach

We performed the aircraft moel simulations in this article using Ansys EMC Plus. EMC Plus is a full wave finite-difference time-domain (FDTD) code with an integrated multi-conductor transmission line (TL) algorithm. The aircraft model used in this study, Figure 2, is a simplified F16 aircraft. The first step in the process is to import the OML model geometry into the software.

Figure 2. Model F16 aircraft OML used for analysis.

Assign Material Parameters and Mesh

Here, we use a full-vehicle computational electromagnetic modeling (CEM) model development process. The goal here is to capture the pertinent electromagnetic parameters that contribute to field enhancement.

The software assigns a general aluminum material with a conductivity of 2e7 S/m to the aircraft and a dielectric polycarbonate material with a conductivity of 1 S/m to the cockpit canopy. Users can easily change the component assignments in EMC Plus, thanks to the built-in property values, which include carbon fiber-reinforced polymer (CFRP) and engineered coated fabrics (ECF). This gives simulation an advantage over rolling sphere or scale model testing approaches.

This case uses a surface mesh presentation of the aircraft OML with a 5-cm cubic cell size for the baseline analysis. It is important to note that EMC Plus does feature a variable grid mesh allowing smaller mesh sizes near tiny features and larger mesh sizes in areas without such features for faster computation.

Electrostatic Field Simulations in X, Y, Z

In this example, we probe 10 locations on the OML. Figure 3 shows the locations including the nose, left wingtip, left ventral fin, left horizontal stabilizer, and vertical stabilizer. We chose these regions based on previous analysis and engineering judgement.

We do not probe the right-hand wingtip, horizontal stabilizer, and ventral fin for this analysis due to the aircraft’s symmetry combined with the static field environment. Instead, the simulations focus on central and left-hand extremities.

Fig. 3 Probed field regions of the F16 model, highlighted in blue

The illumination source for the EFM simulation environment is an electromagnetic plane wave with a sine-squared ramp signal, Figure 4. Analyzing this problem using the FDTD approach allows the field to equilibrate within the problem space in a relatively short amount of time. After roughly 500 ns, a static electromagnetic field encompasses the entire FDTD domain, and two spherical polarization parameters control the electric field polarization.

Figure 4. Initial 1-µs of Sine-Squared Ramp waveform.

You only three electric field orientation simulations to capture the full enhancement profile: the X-, Y-, and Z-orientated background electric fields. By combining the vector fields from these results, you can understand the field enhancements for any polar or azimuthal orientation of the electric field.

We completed all simulations in this example using graphical processing (GPU) acceleration on Windows desktop machines. This is a standard feature of EMC Plus. An Intel® i9-14900f CPU and an NVIDIA GeForce RTX 4090 GPU powered the Windows desktop machine. The problem space for a 5-cm mesh step size was roughly 300 x 200 x 100 cells in the X, Y, and Z orientations respectively. We simulated the problem out to 5 µs (114,000 time steps), completing the talk in approximately four minutes.

Lightning Simulation Results

Figure 5 shows that field levels reach asymptotic stability within 1 µs for this aircraft model and enhancements measurements are made at 5 µs, which is equivalent to an electrostatic background environment.

Figure 5. Simulation probe electric field plot at vertical stabilizer.

EMC Plus simulation generates images that display the normal electric field behavior on the entire aircraft. Figure 6 is a snapshot of the aircraft with an X-polarized static field. The areas in orange and red indicate larger normal electric fields. As anticipated, the largest field enhancement areas are the sharp extremities such as the nose, wingtips, and stabilizers. An additional 3D probe looking at a slice of the electric field enhancement profile is provided for a central X-Z plane, Figure 7, and central X-Y plane, Figure 8.

If any hotspots are identified, electric field probes can be added to the model and re-simulated. Combining actual field enhancement values and visual inspection of the normal electric fields through animation probes provides high confidence that you will identify all the likely attachment points using this simulation approach.

Fig. 6. Animation probe snapshot of normal electric field on the aircraft OML.

Fig. 7. 3D probe snapshot of electric field with central slice in the X-Z plane for X-oriented field.

Fig. 8. 3D probe snapshot of electric field with central slice in the X-Y plane for X-oriented field.

Observations and Results Interpretations

The electric field magnitude at each location is analyzed from the resultant field vector output. Three baseline cases with electric field polarization along the X, Y, and Z cartesian axes were simulated. Field enhancement for any arbitrary polarization angle can be calculated with a vector composition of simulation cases. This methodology is shown in Table 2 for 2.5-cm cases. The results are shown with 30-degree increments for azimuth and polar angle sweeping. The magnitude of the resultant field vectors is used to extract the minimum field enhancement for each region in that polarization. It is suggested to consider the top three enhancement locations for each polarization angle when considering attachment risk. In this case, significant field enhancement is observed on the nose tip, wingtip, and horizontal stabilizer, suggesting a high likelihood of attachment to one of these areas.

Table 2. Maximum electric field enhancement values for a range of field orientations with a 2.5-cm mesh.

Six of the 10 probed locations are present in the top three field enhancement values across the polarization sweep data as shown in Figure 3. The vertical stabilizer is the most frequent and largest field enhancement location, appearing in the top three enhancements 91% of the time. The nose at 68%, and wingtip at 45%, are the other two locations most likely to receive an initial attachment based on field enhancement evaluation.

Table 3. Number of occurrences in the top three field enhancement locations for each probe out of 22 polarization cases.

We identify the nose, ventral fin, wingtip, horizontal stabilizer, vertical stabilizer, and rudder base as initial attachment locations using this approach.

Further Reading

We performed an additional study to evaluate the sensitivity of the results to the mesh size. You can find the results in the full paper by clicking here.

This paper was first published by EMA at ICOLSE 2024.

EMA developed and updates EMC Plus with new features twice a year. You can learn more about what the software is capable of by clicking here. If you need support with your lightning zoning or other aircraft certification projects, we are here to help. You can contact us by clicking here.