Sonlu elemanlar yöntemi ile yıldırımın uçaklara olan etkilerinin incelenmesi
The analysis of lightning strike effects on aircrafts by finite element method
Yazar: SEMEN MEMİŞ
Danışman: PROF. DR. ÖZCAN KALENDERLİ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Elektrik Mühendisliği Ana Bilim Dalı / Elektrik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering ; Mühendislik Bilimleri = Engineering Sciences

Özet:
Yıldırım bulut ile yeryüzü arasında meydana gelen gerilimi 100 MV'lara, akımı 200 kA'lere varan hızlı bir boşalma olayıdır. Yıldırım, çarptıkları nesnelerde tehlikeli olaylara neden olmaktadır.
Bu çalışmada, yıldırım darbesinin uçaklara olan etkileri araştırılmıştır. Teknolojinin gelişmesi ile beraber, uçaklarda ağırlığı azaltmak için iletken malzemeler ile birlikte yalıtkan malzemeler de kullanılmaya başlamıştır. Kullanılan yalıtkan özellikli fiber-karbon kompozit malzemeler ve elektronik kontrol sistemleri, uçaklardaki doğrudan ve dolaylı yıldırım etkilerini artırmaktadır. Artan yıldırım etkilerinden dolayı, uçakların yıldırım koruma seviyeleri artırılmalıdır. Doğrudan etkilerin çoğu, uçak yüzeyinde, içyapıda ve metal bağlantılarda fiziksel hasara yol açar. Yıldırım çarpmalarının meydana geldiği bölgelerde, yanma erime hasarı olur. Dolaylı etkiler, genellikle uçağın uçuş kontrol ve haberleşme sistemlerini içeren aviyonik sistemlerin hasar görmesine neden olmaktadır. Dikkatli ekranlama, topraklama ve aşırı gerilim bastırma cihazlarının kullanılması, yıldırım etkilerini azaltabilir ya da önleyebilir.
Yıldırımın uçak üzerinde meydana getirdiği elektriksel alan dağılımını belirlemek için Comsol'da elektrostatik bir model sunulmuştur. Yıldırım bulutu, öncü boşalma, toprak ve uçak modellenmiştir. Yıldırım bulutu, toprak ve öncü boşalma bakır malzeme ile modellenirken, uçak kompozit, alüminyum ve titanyum malzemeleri ile modellenmiştir. Comsol'da kurulan modele sınır koşulları uygulanmış ve model sonlu elemanlar ağına bölünmüştür.
Uçaklarda meydana gelen elektriksel alan dağılımının bulut potansiyeli ile olan değişimi elde edilmiştir. Yıldırım bulutunun gerilimi 10 MV'dan 100 MV'a kadar değiştirilerek uçakta oluşan elektrik alan şiddeti hesaplanmıştır. Yıldırım bulutunun potansiyelinin artması ile birlikte uçak üzerinde meydana gelen maksimum elektrik alan şiddetinin lineer olarak arttığı gözlemlenmiştir. Kanatlar, kuyruk ve radom gibi eğrilik yarıçapının küçük olduğu uçak bölgelerinde elektrik alan şiddetlerinin büyük değerlere ulaştığı, uçağın uçuş konumuna göre elektriksel alan değerlerinin değiştiği gözlemlenmiştir. Uçakların yunuslama ve sapma açısı değiştirilerek uçak üzerinde meydana gelen maksimum elektrik alan şiddetleri elde edilmiştir. Uçakların iniş, kalkış ya da yuvarlanma hareketleri sırasında yıldırımın uçağa çarpma olasılığının normal uçuş konumuna göre daha yüksek olduğu elde edilmiştir.
Öncü boşalma uçağın kuyruğuna 50 m sabit adımlarla yaklaştırılmıştır. Öncü boşalmanın uçağa yaklaşması ile birlikte uçak kuyruğunda ve öncü liderde meydana gelen elektrik alan dağılımının arttığı gözlemlenmiştir. Öncü boşalma ile yakalayıcı boşalma uçağın kuyruğunda birleştirilmiştir ve kuyruk, kanat, radom ve motor bölgelerinde meydana gelen maksimum elektrik alan şiddetleri elde edilmiştir.  Uçaklarda yıldırım bölgeleri belirlenerek, yıldırım için hangi bölgelerde daha fazla önlem almak gerektiğine karar verilebilir. Elektrik alan dağılımının yüksek çıktığı bölgelerde yıldırım koruması güçlendirilirse ve donanım yerleşimleri bu bölgeler göz önüne alınarak yapılırsa yıldırımın uçaklara verdiği zarar azaltılabilir.

Abstract:
Lightning is a fast discharge that occurs between the atmosphere and the Earth's surface with voltage up to 100 MVs and current up to 200 kAs. Lightning causes dangerous events in the objects it hits. In order to analyze the lightning impulse, parameters such as front time, tail time, peak value of current and voltage are used.
In this study, the effects of lightning strike on aircrafts are investigated. Aircraft structure is made of aluminum material with high electrical conductivity. With the development of technology, insulating materials have been used along with conductive materials to reduce weight in aircraft. The use of non-conductive fiber-carbon composite materials increases the direct and indirect lightning effects on aircraft. Due to the increased lightning effects, the lightning protection levels of aircraft should be increased. The continuity of the Faraday cage shall be ensured by using woven or non-woven copper or aluminium mesh, or an expanded foil. Most direct effects cause physical damage to aircraft surface, internal structure and metal connections. Burning and melting damage occurs in areas where lightning strikes occur. Indirect effects often cause damage to avionics systems, including flight control and communication systems. Careful screening, grounding and surge suppression devices can reduce or prevent problems from lightning strike.
All areas of the aircraft are not exposed to the same level of current or energy. The zones may vary in each aircraft depending on the geometry of the aircraft and the materials used (conductive or non-conductive) and the design. Radom is the region where lightning strikes occur most. In the radom region, there is aircraft radar and behind the radome there are critical electrical-electronic systems for the aircraft and the radome is made of dielectric material. Adding lightning diverters to the radome during the design phase significantly reduces lightning damage. Control surfaces in aircraft are located in 1B or 2B lightning zones. To protect control surfaces from lightning strike, conductive material should be using on all control surfaces. To eliminate the potential difference between the two metal surfaces, the application of bonding is an effective protection method. The grounding process will eliminate static electricity through a conductor and protect the aircraft systems. One of the most effective methods of protecting aircraft from lightning is to add electrical static discharger to the aircraft. 
Finite element method is used in the analysis of Comsol. Basic process steps of finite element method have been applied. These steps include determination of the problem geometry, limiting the geometry with a closed region, writing neumann and drichlet boundary conditions, separating the solution region into finite elements (discretization), writing the basic equations for each finite element, combining the finite elements in the solution region, combining the finite elements in the solution region, the solution of linear algebraic equations resulting from the combining elements and obtaining the results.
An electrostatic model is presented in Comsol to determine the electrical field distribution caused by lightning on the aircraft. The model includes thundercloud, lightning leader, ground and aircraft. Lightning cloud, ground and stepped leader are modeled with copper material, aircraft is modeled with composite, aluminum and titanium materials. Since the solution is made in the electrostatic field in Comsol, the relative dielectric constants of the conductive materials are taken as 105. Boundary conditions are applied to the model established in Comsol and the model is divided into finite elements. Using the scale command, the mesh density of each object is selected differently. The scale factor was 0.6 for the aircraft, 2 for the electrodes and 3 for the outer cylinder. The smaller the scale factor, the more frequent the mesh density will be. In the model considered, lightning clouds are about 5 km in diameter and 4 km above the ground. The modeled electrical structure is a uniform field, disc-shaped plane-plane electrode system in which the cloud is a plane electrode (5 km in diameter) and the ground is the opposite plane electrode (5 km in diameter). The external cylinder used in the simulation to make the solution with Finite Element Methods. The cylinder material has no effect on the solution. The cloud (upper plane electrode) potential is defined as 108 V (100 MV) and the ground (lower plane electrode) potential as 0 V. The length of the aircraft used in the model is 72 m, the height is 23.3 m and the wingspan is 79.9 m. 
The variation of the electrical field distribution in the aircraft with the lightning cloud potential is obtained by using Comsol software. The voltage of the lightning cloud is changed from 10 MV to 100 MV and the electric field density is calculated. The cloud potential is taken as 100 MV. The potential on the aircraft is calculated as 90 MV. When the cloud potential is 100 MV, the maximum electric field density of the aircraft tail is 970.2 kV/m, the maximum electric field density of its engines is 235 kV/m, the maximum electric field density of its wings is 130 kV/m and the maximum electric field density is 93.2 kV/m. It is found that the maximum electric field density on the aircraft increases linearly with the increase of the potential of the lightning cloud. It has been observed that electric field density has reached great values in aircraft regions where the radius of curvature is small such as wings, tail, radome and motors.
Also, the electrical field density values have changed according to the flight position of the aircraft. Pitch angles of the aircraft are selected as 15°, 20°, 25° and 30° towards the ground and maximum electric field densities on the aircraft are obtained. The variations in the electric field density in the aircraft zones due to the pitch angles are not the same for each aircraft zone. When the pitch angle is set to 30 degrees towards to ground, the maximum electric field density in the aircraft tail is 3.20 x 106 V/m, the maximum electric field density in aircraft engines is 1.5 x 106 V/m, the maximum electric field density in aircraft wings is 4.49 x 105 V/m and the maximum electric field density in aircraft radome is 2.58 x 105 V/m. The angles of rolling are then changed to 45°, 60° and 75° respectively. When the rolling angle is set to 75 degrees, the maximum electric field density in the aircraft tail is 7.8 x 105 V/m, the maximum electric field density in aircraft engines is 1.2 x 106 V/m, the maximum electric field density in aircraft wings is 1.18 x 106 V/m. When the pitch angle of the aircraft is 30°, the value of the maximum electric field density on the aircraft is the highest among all positions and occurs at the tail. It has been observed that the roll and pitch movements of the aircraft increase the probability of lightning strikes. The electric field density values measured in the engine and wing regions during the rolling movements of the aircraft increased significantly compared to the normal flight position. 
Lightning leader is modelled with a fixed stepped leader length (50 m) that approaches to an aircraft tail vertically. Stepped leader is modeled in Comsol as conductive copper and rod electrode with a voltage of 100 MV. Stepped leader progress by branching and return streamer occur from the aircraft to the stepped leader. The branching of the stepped leader is not considered in the model. The stepped leader is approached to the aircraft in 7 steps. It is observed that the electric field density on the aircraft increased as the stepped leader approached the aircraft's tail. The distance between the aircraft and the stepped leader should be at least 50 m. As the stepped leader approaches the aircraft, it is seen that the electric field lines formed in the tail are close to each other. When the stepped leader's length is 350 m, the maximum electric field strength in the aircraft tail is 5.61 x 106 V/m, and the maximum electric field strength on the stepped leader is 3.74 x 105 V/m.
When the stepped leader approaches the aircraft, return streamer occurs from the aircraft to the stepped leader. The total length of the stepped leader and return streamer is 400 m. When the return stroke occurred at the tail of the aircraft, it is seen that the electric field lines formed on the aircraft close to each other in the regions where the radius of curvature is small. The leading attached to the tail and maximum electric field values in tail, wing, radome and motor regions are obtained. When the stepped leader and return streamer are merged in the tail region of the aircraft, the maximum electric field density in the aircraft tail is 7.17 x 107 V/m, the maximum electric field density in aircraft engines is 2.6 x 107 V/m, the maximum electric field density in the aircraft wings is 3.85 x 106 V/m and the maximum electric field density in aircraft radome is 4.17 x 106 V/m. It is confirmed by analysis that the electric field lines on the aircraft become more frequent at sharp points. The results of the simulations can be used in risk assessment studies. If lightning protection is strengthened in areas with high electric field density and system installation shall be made considering lightning zones, the damage caused by lightning strike to aircraft may be reduced.