The Complexities of Modeling Lightning Strikes on Aircraft
The biggest threat to an aircraft is lightning. Every second of the day there are anywhere between 40 and 100 lightning strikes happening across the world.
Getting hit by a lightning strike can lead to the total loss of an aircraft if it is not properly mitigated. The effects of lightning strikes are divided into two categories: direct and indirect. Direct effects impact the physical structures, while indirect effects affect electronic devices.
Lightning Attachment
Lightning attachment can happen anywhere from on the ground up to 50,000 feet in altitude. This means that all aircraft, ground structures, and air platforms must consider the effects of lightning. It is so important that a significant portion of the cost for development, certification, and modification of air platforms is devoted to analysis, design, and testing of ways to protect against lightning.
One of the most important design tasks is to determine where lightning will attach on an aircraft or another object. In more than 90 percent of aircraft lightning strike cases, it is the aircraft itself that causes the lightning strike. During the attachment process, leaders originate from the aircraft and spread through the air until they intercept the approaching lightning leader, or the region of opposite polarity charge in the clouds. These leaders from the aircraft can travel several meters. Since aircraft moves faster than the lifespan of a lightning event, there tend to be multiple strike points because the lightning sweeps back as the plane moves forward.
During the design process the outer surface area of the aircraft is broken into categories based on the likelihood of lightning attachment. The zones are lightning attachment, lightning sweep, and lightning hang-on. Each zone is determined by the SAE International standard ARP 5414A: Aircraft Zoning.
Three legacy methods are used to determine attachment locations:
• Scale-modeling testing: A small model version of the aircraft is taken to a high voltage lab. The model is adjusted to see where lightning arcs are likely to attach.
• Rolling sphere analysis: A sphere whose radius is related to the possible peak lightning current is rolled over the aircraft outer mold line (OML) to see where an attachment is likely to occur.
• Electric field modeling: Using 3D electromagnetic (EM) simulation, a leader is brought near an aircraft model to see where the electric field (E-field) is most likely to attach.
Modeling Lightning Attachment
All three methods are accepted by standard authorities, but there are advantages and disadvantages to each. By using scale-modeling, the dimensions of the aircraft are scaled down. By scaling the dimensions, the radius of curvature is adjusted artificially, skewing the results. The rolling sphere method has a long historical heritage for terrestrial installations, but there has yet to be a first-principles derivation for its effects from the basic physics involved in lightning attachment. E-field modeling has the greatest technical basis.
E-field modeling previously used linear methods for calculations. This is because of the lack of simulation tools that were able to accurately model the non-linear and multiphysics effects of corona in air. Nonlinear methods are preferred because they are the most accurate and quantitative method of determining lightning attachment location.
It is no longer the case that designers need to use linear methods. That is because of the development of simulation software like Ansys Charge Plus. EMA developed Charge Plus for the analysis of charging and discharging phenomena, including coupled multiphysics solvers to model the full dynamics of lightning attachment. Charge Plus is able to solve the problems that nonlinear E-field modeling could not. It solves the dynamics of corona effects in air by solving nonlinear air conductivity effects in the time domain. It also calculates the densities of positive ions, negative ions, and electrons as a function of space and time.
Included in the software is the physical processes for air ionization and recombination such as electron avalanching, electron attachment to neutral molecules to form negative ions, electron-positive ion recombination, and negative-positive ion recombination.
Fluid dynamics are modeled by solving the fluid equations for momentum, energy, and particle density conservation. These equations include the magnetic and electrical forces of charged particles. The fluid conservation equations are solved simultaneously and on the same grid as a solution of Ansys Maxwell, a low frequency EM field simulation, and the air ionization and recombination equations.
Charge Plus enables engineers to predict where air breakdown and corona will form. Further simulation will also predict the exact location on aircraft and structures where lightning will attach.
Using Charge Plus to Determine Lightning Attachment
This example will include finding lightning attachment on an electric vertical take-off and landing (eVTOL) vehicle. Figure 1 shows the approaching lightning leader that is charged with a current source. The eVTOL rotorcraft is connected to the computational boundary.
Fig. 1. eVTOL modeled in Ansys Charge Plus with an approaching lightning leader.
Charge Plus’s nonlinear air chemistry model captures the corona dynamics from the approaching leader and determines where the lightning will attach. In this case, it is the rotor itself. This is seen in Figure 2. This result may not have been guaranteed prior to running the simulation.
Fig 2. Simulation showing the lightning attachment point on the roto of the eVTOL.
The simulation is also dynamic, meaning it can also determine the waveform for the discharge, shown in Figure 3. The recombination dynamics also resolve a reduction in carriers between discharge events.
Fig. 3. Charge Plus plots the waveform for the lightning discharge.
Validation
The solver behind Charge Plus’s nonlinear air chemistry module has been validated in a government test. An F-16 aircraft was instrumented with sensors and intentionally flown into thunderstorms with the purpose of getting struck by lightning. More than 700 strikes were recorded and the Charge Plus solver interpreted and reproduced the data. This effort led to new lightning waveforms being discovered and is the basis for many aspects of modern lightning development programs.
Charge Plus is a validated, multiphysics solver that can determine the location of lightning attachment based on lightning event details. It is a must have tool for engineers who design, certify, and test aircraft and structures for lightning.
Charge Plus is maintained by EMA, with new features released regularly. It is sold exclusively through Ansys. EMA has consultants on staff that can perform lightning measurements including indirect effects aircraft testing and fuel ignition testing. To learn more, click here.
