Direct Effects of Lightning (DEL)
Table of contents:
1. Indirect effects of Lightning
2. Direct Effects of Lightning
2.1 Zoning Assignment
2.2 Set Current Distribution in Structures2.3 Evaluate the Performance of Structures to Lightning Arc Attachment and Conducted Current
2.4 Design Guidance for DEL
2.5 Component and Representative Coupon Testing at High Current/High Voltage
2.6 Corrective Measures
2.7 Team Experience in DEL
3. Lightning Fuel Systems
EMA can determine the likely lightning attachment locations for the aircraft during all applicable mission configurations and control surface positions. The primary attachment locations are mainly based on the outer mold line (OML) and the material properties of exterior items.
The initial attachment locations will be determined by electric field modeling approaches, as prescribed in SAE ARP 5414A. EMA can initially use engineering judgment to determine the attachments. This determination will be verified and modified by E-field modeling using low frequency methods in a quasi-static condition considering all OML surfaces. The initial attachment work should consider the proper mission configurations, including the most extreme control surface extensions.
Next, the initial attachments are used as the basis for the zone assignment for all OML areas of the aircraft. WMA will use SAE ARP 5414A in order to accomplish this task. Various mission parameters, such as the velocity profile at various altitudes are needed for this task.
An example zoning is shown in the figure below. We can use an acceptable method to determine the initial attachment locations in the present effort.
EMA’s simulation can be used to estimate the lightning current distribution in all critical aircraft structures. In certain assemblies and components, further direct effects effort can be eliminated by showing the current level is below the threshold to risk damage. For other items, the current may be used as a basis for the testing at high current labs.
It will be important to understand what will be required to protect structures to the direct effects of lightning. EMA has significant experience in determining methods to protect composite materials from the damaging effects of lightning. EMA’s team has experience setting the mesh thickness as well as the appropriate application of the composite interface protection schemes. EMA’s team knows the conditions under which novel material solutions, such as micron-sized silver dispersed in co-polymer can be used to protect composites, and these in which a more robust bonding surface must be provided.
Further, our team’s multi-physics simulation capabilities allow for prediction of the damage level from lightning to composites to further optimize the design to meet the performance and mass requirements.
EMA can apply the Zoning and Analysis levels to each structure, sub-system, and system and consider their susceptibility to the prescribed levels. In areas in which there is a concern, we can consider the criticality based on the hazard assessment. In cases in which the criticality is catastrophic or hazardous, mitigation should be applied. In cases in which the criticality is severe or lower, mitigations will be considered in light of mass and performance requirements, using analysis methods to assess the benefit and cost of each mitigation.
EMA can consider structures in the vehicle that may carry lightning current, assess each structure’s possible failure modes and determine if that failure might result in a catastrophic, hazardous, or severe consequence to continued safe flight, per the hazard assignment. The main areas of Direct Effects concern that will be analyzed in this manner are the integrity of carbon fiber reinforced polymer composite (CFRP) surfaces, sparking across structural junctions and joints, control surfaces and actuators, fuel ignition, and ordnance ignition.
EMA can design mitigations to the Direct Effects of Lightning based on the Lightning Zoning requirements. The Lightning Zone requirements along with the Hazard Analysis of the aircraft are the inputs to this process, following the steps in SAE ARP 5f77. The effort has an associated test and analysis program. Corrective measures will be specified in the event of failures from test and analysis.
2.4.1: Integrity of Carbon Fiber Reinforced Polymer Composites
Attachments to carbon fiber areas will be given special attention. Carbon Fiber Reinforced Polymer Composites are characterized by a much smaller conductivity (104 S/m) than that of Aluminum (3.5 x 107 S/m).
The Zoning and analysis maps the required current levels to every composite structure. Several factors can affect the susceptibility of the CFRP material:
- Thickness and type of resin used for each layer
- Number of and type of reinforcing carbonaceous fibers (such as weave, unidirectional)
- Surface roughness
- Nature of and connection to fasteners
- Addition of metallic foils and expanded foils
- Connectivity to neighboring and underlying structures
EMA’s team can use engineering judgment, analysis, and write sample coupon test plans to determine the anticipated effect on the CFRP material in each required location and current level. Next, the estimated damage will be assessed for the hazard imposed on the system. For example, if CFRP layers delaminate to expose the underlying items to the outside aerodynamic environment, what affect will this have on the continued safe flight?
Once this analysis is complete, EMA can develop mitigations for each component that presents an unacceptable hazard in the lightning environment. The possible mitigations include:
- Addition of Expanded Copper Foil (ECF) co-cured as the outermost layer of the composite material
- Adjustment of the thickness of the composite part
- Adjustment or metallization of dielectric coatings or paints
2.4.2: Structural Junctions and Joints
The simulation analysis will determine if any critical seams or junctions are susceptible to sparking, with for example standard limits of 500 V on exterior surfaces. The CFRP-fastener interface in many cases will be a critical interface to assess.
Analysis that considers an evaluation of the joint seams will reveal the voltage in all cases. These will be used to set the junction impedance levels for all major interfaces. Follow-on analysis and coupon testing of representative joints will be used to verify the mitigation of these concerns.
2.4.3: Control Surfaces and Actuators
Actuators and control surfaces will be carefully considered so that excessive current does not damage moving components and joints. The analysis and Zoning levels will be used with the material properties to determine the potential for heating. If it exceeds the estimated susceptibility of the parts, mitigation will be designed. The mitigation will include additional bond paths or changes in the gauge or thickness of parts. In some cases, testing may be used to determine the susceptibility of parts.
1.4.4: Determine Physical Damage
First, EMA can estimate the CFRP and other material effects from the lightning levels. For example, we can estimate the number of CFRP layers that will be delaminated in the worst case configuration and attachment scenario. Additionally, it will determine how much coating material may be stripped away by the attachment. These estimates will be based on experienced engineering judgment for similar levels in similar material. EMA can write example test plans for any coupon testing may be required for exotic or critical material/level combinations.
All critical structures that are deemed to have appreciable lightning current from the analysis as well as potential for arc attachment will need testing per the provisions of SAE ARP 5416.
The high voltage testing determines risk of attachment and punctures of transparencies. The high current testing determines the risk of physical damage to the aircraft that would affect continued safe flight.
EMA has significant experience in performing such testing, writing test plans, and witnessing testing.
EMA has significant experience in providing design changes to meet IEL requirements in the event of test failure. Our multi-physics tools that co-simulate the electrical and thermal equations allow for us to reproduce the failure and design the appropriate action to prevent the problem.
Our team has developed protection schemes for bonding and panel protection that allow us to prevent damage and harmful effects from lightning.
Past Performance Success Demonstration: EMA has provided design guidance directly for many programs. Past customers include:
- Bell Helicopter Relentless 525
- Orbital Sciences Missile Defense Agency GMD Vehicle
- Lockheed Martin AC-130
- Stratolaunch Roc
- XAC MA 700
- Sierra Nevada Dream Chaser
- Mitsubishi Regional Jet
- Bombardier CSeries
- Lockheed Martin AC-130
- NASA, Lockheed Martin Orion MPCV
- Sierra Nevada Dream Chaser