Lightning is a common natural phenomenon, with an average of about 8 million lightning strikes occurring every day in the Earth's atmosphere. Among them, the amplitude is as high as 200 kA, and the current rise rate can reach up to about 1000 kA per second. UAVs flying at high altitudes are very vulnerable to lightning strikes. Without adequate lightning protection measures, UAVs will become very vulnerable in thunderstorm environments, unable to perform normal work, or even crash out of control, greatly reducing combat indicators. Therefore, it is of great significance to study the lightning protection and anti-lightning electromagnetic pulse interference of UAVs.
1 The harm of lightning to drones
1.1 Direct Effects
The direct effect mainly refers to the physical damage to the structure caused by lightning directly hitting the UAV. The degree of damage is mainly related to factors such as the size of the lightning current, duration, action integral, and charge transfer amount, which mainly include the following aspects.
(1) Mechanical effects. UAVs make extensive use of composite materials, such as radomes, fuselage structures, wings, etc. The voltage of lightning is as high as thousands of kilovolts, and the action time is only a few milliseconds. Therefore, the impact force generated is very large, which may cause breakdown, opening and tearing of structures such as radomes and fuselage skins.
(2) Thermal effect. The electrical conductivity of the composite material is very low. When the drone is struck by lightning, the composite material cannot effectively conduct the lightning current, and the electric charge is accumulated in a large amount near the lightning strike point and converted into heat energy, which may cause the composite material to burn, melt and structural deformity. Wait. Thermal effect is a typical damage form of lightning, which seriously threatens the flight safety of UAVs.
(3) Spark effect. Cremation effects include thermal sparks and electrical sparks. Heat refers to the burning of materials when debris is ejected from the hot spot, and spark refers to the breakdown of the medium due to the action of voltage. Since the lightning instantaneously releases the energy, the peak value of the lightning current is very high, and if the impedance of a certain point is large, it is easy to produce a spark effect. Spark effects pose the greatest threat to the fuel system and can cause fuel vapors to ignite, causing the drone to explode.
(4) Electromagnetic force effect. When current flows through conductors in the same or different directions, forces that attract or repel each other are created. The lightning current is very large, so the electromagnetic force generated is very strong, which may cause damage such as cable breakage, skin tearing, bending, etc., affecting the flight safety of the drone.
1.2 Indirect effects
Indirect effects mainly refer to the interference or damage of lightning to the electronic and electrical equipment of UAVs, resulting in indirect hazards that may endanger the normal operation of UAVs.
(1) Electromagnetic effect. After the lightning channel is formed, the impedance of the entire channel is very low. The strong current brought by the lightning can generate a strong electromagnetic pulse. After the electromagnetic pulse enters the drone through various coupling channels, induced voltage and induced current will be generated on the device. Due to space constraints, many electronic devices are densely distributed inside the drone, and the electromagnetic pulses generated by lightning will cause serious interference to these devices. When the UAV is subjected to electromagnetic interference, it can cause the data link to be blocked, the communication is not smooth, the mission instructions are difficult to execute, and even run out of control and crash.
(2) Electrostatic induction effect. When the lightning strikes, the surface of the drone can induce an induced charge with the opposite polarity to that of the descending leader of the lightning. When the lightning disappears, the induced charge on the shell will be redistributed, which may generate a higher voltage, which may cause damage to the internal equipment. interference.
2 Lightning protection recommendations for drones
The UAV has a complex structure, high system integration, and different lightning protection requirements between different systems, so the protection methods are also quite different. This paper suggests that the design unit should consider the following suggestions when conducting UAV lightning protection:
(1) For the work of lightning protection equipment, the difference between large drones and manned drones is small, and the design experience of manned drones can be directly referred to; small drones are significantly different from manned drones in terms of structure and size, and are affected by lightning effects. The difference is also very large, so this paper suggests that the basic research work on lightning protection should be specially carried out for small UAVs to ensure that various protection measures are reasonable and reliable.
(2) UAVs use a lot of composite materials, composite materials have low conductivity and poor shielding performance, and the internal electronic equipment is more susceptible to the interference of lightning induction. Therefore, when designing UAV lightning protection, attention should be paid to the protection of indirect effects of lightning. work and increase the level of protection.
(3) UAVs have stricter weight requirements. For the consideration of weight reduction, it is recommended to design lightning protection and other functions, such as heating and deicing, electromagnetic compatibility (EMC), high-intensity radiation field (HIRF), Electromagnetic pulse (EMP) should be considered at the same time to improve the comprehensive technical level of UAV.
UAVs will be more and more widely used in the military and civilian fields in the future. With the increasing integration of the system, the impact of lightning electromagnetic pulses on UAVs is also increasing, so it is necessary to take appropriate lightning protection measures. At the same time, the lightning protection of UAVs is a complex project, which requires the overall department to make an overall plan for the lightning protection of the entire aircraft at the project approval stage, and then decompose the tasks step by step, formulate protection plans, and conduct corresponding tests.
