1. Introduction
Lightning is a natural discharge process with a high voltage and a large current at the boundary between high-density positive and negative charge concentrations. Its current rise speeds can reach 10–20 kA/μs, and its discharge energy can be up to hundreds of megajoules [
1,
2,
3]. Lightning occurs at high frequency in nature, especially in the troposphere and stratosphere, which are the main areas in which aircraft fly. Statistics show that commercial aircraft are struck by lightning once a year [
4]. When an aircraft is struck by lightning, the lightning’s transient pulses will electromagnetically couple to the internal cables of the fuselage and generate induced currents. Once the induced current is higher than the equipment interference threshold, it will cause damage to the airborne equipment or interfere with the internal systems [
5,
6]. Therefore, the electromagnetic compatibility (EMC) prevention of cables is the main measure for preventing electromagnetic interference caused by the indirect effects of lightning on aircraft [
7]. Compared with traditional metal aircraft, Carbon Fiber-Reinforced Plastic (CFRP) aircraft are not able to channel and release lightning currents on the fuselage in a timely manner because of the low electrical conductivity of the material. And, the lightning electromagnetic pulse will penetrate the composite structure, further reducing the lightning protection capability of the fuselage of airborne equipment.
The main method for analyzing the indirect effects of composite aircraft is numerical simulation calculations [
8,
9]. To verify the shielding performance of the indirect effects of cables inside composite aircraft, anisotropic composite material models and electromagnetic coupling simulations of door joints and structural fasteners have been carried out. However, the mechanism behind the impact of lightning on aircraft has not been analyzed in detail [
10]. Numerical simulation technology is used to estimate the coupling of the indirect effects of lightning on cables [
11]. The shielding performance of internal aircraft cables in response to the indirect effect of lightning is not only affected by the fuselage material and skin gap but also by the layout of the cables in the fuselage. Aguilera et al. conducted a high-current pulse injection test analysis on various aircraft, including passenger aircraft, military aircraft, helicopters, and unmanned aerial vehicles. The distribution of surface currents and electromagnetic fields during different lightning current paths on the aircraft was analyzed. In addition, the effect of the layout of the cabin cables on the induced currents of the cables during lightning strikes has been discussed, but the effect on the protection of composite aircraft against lightning has not been further analyzed [
12,
13,
14,
15,
16]. Piche analyzed the EMC performance of metal track structures inside composite aircraft by studying the mutual interference of cables in different slots inside the track [
17]. Jaehyeon Jo et al. established an aircraft electromagnetic simulation model by analyzing the coupling effect of a lightning electromagnetic pulse on cables and further analyzed the factors that affect the electromagnetic coupling of indirect lightning effects. The design of protection measures against these indirect effects on cables was carried out, and a reference for the principles of cable laying was given [
18].
In summary, many scholars have analyzed the indirect effect of lightning on aircraft and its protective measures, but further research is needed on the coupling mechanism between lightning and aircraft and the factors affecting cable shielding performance. Therefore, this paper will address this need in several parts, as follows:
Part 2 introduces the theory of induced currents generated by an aircraft after being struck by lightning, which includes factors such as the impedance of the aircraft fuselage, the impedance of the internal cable shield, and the aircraft grounding network, and provides a direction for further in-depth research on the indirect effects on aircraft.
Part 3 constructs an aircraft simulation model in CST Studio Suite 2020 software, observes the changes in the electromagnetic fields inside and outside the aircraft by injecting lightning current into the model, and analyzes the key changing parts of an electromagnetic field on the fuselage of the aircraft.
Part 4 uses different types of cables to observe the induced currents when the aircraft is struck by lightning. At the same time, based on the different cable shield grounding methods, a more in-depth investigation of the effects of induced currents on the aircraft model after lightning injection is carried out.
After the induced currents of the cables mentioned above are observed, in part 5, based on the obvious areas of electromagnetic field changes shown in part 2, the different types of cables in part 4 are placed in several key areas for experiments to obtain and analyze the induced currents in different areas. Based on the induced currents in the cables, the effect of metal track grooves on the generation of induced currents in the cables is investigated.
Finally, all the above experiments and results are analyzed to provide important references and protection suggestions for future all-electric/electronic aircraft against EMI.
5. Principles of Laying Cables
5.1. Cable Layout
During the selection of the cable shielding layer inside the fuselage, one must consider the importance of the equipment, the type of transmission signal, the cable installation location, and working environment. The layout of the cables in the composite fuselage was analyzed to study its influence on shielding performance. Fourteen single wires were laid at different positions inside the composite aircraft fuselage, marked as P1-P14. The cables were parallel to the aircraft floor, with a longitudinal length of 9 m along the fuselage. The y-z section layout is shown in
Figure 14.
For the comparison of the coupling of cables at different locations inside the cabin layout after the aircraft was struck by lightning, 14 cables were divided into the following four groups: cables P1-P5 formed group A, cables P5-P8 formed group B, cables P8-P11 formed group C, and cables P10 and P12-P14 formed group D.
The calculation results of the induced current of the four groups of cables are shown in
Figure 15, including P1 induced current of group A, P2-P4 induced current of group A, and the induced current of cables of groups B, C, and D. The grouping descriptions of the four groups of cables are shown in
Table 6.
The four groups of cable induction currents were calculated, as shown in
Figure 15.
In group A, the current of cable P1 is much larger than the cable induction current inside the aircraft because it is located on the aircraft surface. In addition, the closer the position of the internal cable of the composite aircraft is to the skin, the smaller its cable induced current, i.e., the higher the shielding performance of the lightning indirect effect. In group B, because of the proximity to the left side of the hatch, the closer the cable is to the skin, the higher the induction current, and the weaker the shielding performance. As the cable moves further form the aircraft skin, the induced current of the cable decreases gradually. This is because the electromagnetic leakage effect of the doors and windows on the cable is less than the shielding ability of the skin on the cable. In group C, as the cable moves from the hatch to the window, the induction current increases, and its shielding performance is weakened. However, as the cable close to the aircraft skin is moved further, the cable induction current gradually reduces. In group D, as the cable is moved away from the windows and doors, the cable induction current drops significantly, but compared with group B, the induction current of cables P12–P13 is still greater than cables P5–P8.
In summary, the cables laid on the surface of the aircraft skin should be reduced as much as possible, and attention should be paid to the connection between the cables on the skin surface and the interior. In addition, cables should not be arranged near the doors or windows of the fuselage. When cables must be arranged on the fuselage, the closer the cables are to the skin, the better. The shielding layer with good shielding performance must be selected.
5.2. Cable Track Groove
Cables should be arranged far away from the gap and close to the ground or structure to reduce the loop magnetic flux formed by cables and structures. In practice, cables are often laid in metal track grooves. Both the Current Return Network of B787 and the Electrical Structure Network and Metal Bonding Network of A350XWB contain a metal frame structure. Therefore, the cable track was set inside the composite aircraft fuselage, and the track model is shown in
Figure 16.
The influence of the track on a single wire was analyzed, and the cable induced current results are shown in
Figure 17.
Based on the calculation, when the cable is laid inside the track, its induced current is much smaller than when there is no track, which verifies the shielding effect of metal track on the cable. To assist in analyzing the principle of laying cables inside the fuselage, the laying positions of the cables relative to the structural components are given in
Figure 18, where cables 1–3 transmit current in their axial direction.
The shielding performance of the cables in the figure is from weak to strong, namely, cable 1, cable 2, and cable 3. Simulation verification was carried out using
Figure 18d as an example to obtain the induced currents of the cables laid at different locations, as shown in
Figure 19.
According to the “Boeing 787 Electrical System System & Component Description/Operation, and Maintenance Training Course”, the 28 VDC bus voltage fluctuation range is within 6 V. And according to ANSI/NEMA WC 27500 [
25] and SAE AS22759 [
26], the simulation cable impedance is 0.02016 Ω. The maximum peak current of cable 1 in
Figure 19 is 6.41 A, and the interference voltage caused by the induced current is 0.12923 V. Therefore, it meets the corresponding airworthiness requirements. Meanwhile, the graph suggests that cable 3 has the best shielding performance and cable 1 has the worst shielding performance. This is because the magnetic field is concentrated at structures of greater curvature and dispersed at structures of lesser curvature. Based on this property, the magnetic flux through the loop of cable 1 is greater than the magnetic flux through cable 2. As cable 3 is shielded by a structural member or track recess, the magnetic flux through cable 3 is much less than the magnetic flux between cable 1 and cable 2.
6. Conclusions
This work investigates the indirect effect shielding performance of composite aircraft cable shielding measures by establishing an analytical model of aircraft cable shielding during lightning strikes. The various factors affecting shielding performance are considered, and the following conclusions are obtained:
(1) Based on the spatial electromagnetic field distribution after lightning injection into the aircraft model, the aircraft is prone to generating large electromagnetic fields at the nose, position, and fuselage. It is necessary to insulate the aircraft fuselage, such as at the porthole glass and the hatch, to reduce the electromagnetic interference of the backdoor coupling to the on-board equipment. At the same time, laying cables near doors, window openings, and protruding structures should be avoided as much as possible.
(2) In comparison with four different types of aircraft cables and different cable shield grounding methods, based on the analysis of the induced current after a lightning strike, it can be concluded that if the shielding performance of the aircraft internal cable needs to be enhanced, the corresponding shielding layer can be increased to meet the requirement, and the selection of the twisted-pair cable indirect effect is better than the other cables. The shielding performance can be significantly improved when the shielding layer and shielding layer double-ended have a balanced grounding. For the future development of electric aircraft, this can be selected for use in the aircraft electromagnetic exposure area or sensitive equipment with a shielding layer of a twisted pair or with a shielding layer suitable for cables. Based on the electric aircraft electrical structure network, the appropriate choice of shielding layer grounding to meet the ARP 1870 required bonding impedance is less than 2.5 mΩ to achieve a safer flight of electric aircraft.
(3) When a fuselage gap cannot be avoided, or needs to be close to the ground, the cable can be laid in a closed metal groove to reduce electromagnetic interference. This is an excellent way to reduce induced current and standardize cable routes for electric aircraft that require a large number of electrical cables to be connected and operated, greatly increasing the electrical safety of electric aircraft.