The real scale simulator is described which was designed by the Centre d'Etudes de Gramat (CEG) to study the coupling of fast rise time Lightning Electromagnetic pulse in a fighter aircraft. The system capability of generating the right electromagnetic environment was studied using a Finite Difference Time Domain (FDTD) computer program. First, data of inside stresses are shown. Then, a time domain and a frequency domain approach is exposed and compared

Gauthier, D.

Serafin, D.

English

NASA Technical Reports Server

A REAL SCALE SIMULATOR FOR HIGH FREQUENCY 
LEMP 
D. GAUTHIER - D. SERAFIN 
CENTRE! D’ETUDES DE GRAhlL4T 
46500 GRAMAT 
ABSTRACT : The paper describes the real scale simulator designed by the Centre d’Etudes 
de Gramat (CEG) to study the coupling of fast rise time Lightning Electromagnetic pulse 
with a fighter aircraft. The system capability of generating the right electromagnetic 
environment has been studied using a FDTD computer program. 
First data of inside stresses are then shown. A time domain and a frequency domain approach 
are exposed and compared. 
1. INTRODUCTION 
Nowadays aircrafts become more and more sensitive to the high frequency electromagnetic 
pulses generated during a lightning strike event. This is due to the extensive use of both com- 
posite material in the fuselage and digital electronic whose susceptibility towards High Fre- 
quencies (HF) is important. Recent in flight lightning measurement campaign [l], [2] have 
revealed significant single current pulses whose rise time can be much faster than the micro 
second range. Such pulses seem to originate from the re-ignition phase when the lightning 
channel is already ionized. Their spectrum can show a significant frequency content up to few 
tens of Megahertz. 
It is not yet well established whether this new kind of threat has to be normalized or not and 
taken into account during qualification process. In order to study the coupling of such high 
frequencies Lightning Electromagnetic Pulse (LEMP) with inner aircraft cables a simulator 
has been designed by the Centre d’Etudes de Gramat (CEG). 
The fast rise times associated with high current amplitude suggest to use a coaxial return path 
technique to be able to simulate faithfully the electromagnetic environment created by light- 
ning. 
2. OVERVIEW OF THE SIMULATOR DESIGN 
The simulator is based upon the coaxial return path technique. This technique has previously 
been used with Transall and Mirage Scale Model Studies [3]. The aircraft under test is consi- 
dered as the central conductor of a coaxial line whose outer conductor is made by a set of 
wires surrounding the aircraft (see fig 1). Under the aircraft the return current is driven 
through a copper ground plane. The distance between the central core and the return path is 
adjusted to avoid any air breakdown during the high voltage tests. Two other parameters 
which are of great importance for the set up of the return wires are the electric field at the 
fuselage surface and the characteristic impedance of the line. We tried to maintain the second 
around the fixed value of 50% while the first one is compared with 3D computer calculations 
(see next section). 
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https://ntrs.nasa.gov/search.jsp?R=19910023386 2020-03-17T15:02:48+00:00Z
The aircraft stands on metallic adjustable posts. Those three posts are situated inside the line. 
It has been checked that the change in the characteristic impedance due to this configuration 
does not affect strongly the rise time of the injected current. The current is injected at the 
nose of the aircraft, the end of the line can be terminated by various charge impedance or 
short circuited. 
The following experiments have been performed using a low level generator (IC = 500 A). 
Future works will be proceeded with a variable (5 - 15 kA) high level generator whose rise 
and decay times will as well be adjustable. 
3. OUTER COUPLING 
As a first step it seems of the greatest interest to measure the electromagnetic environment 
created by the simulator using Finite Difference Time Domain (FDTD) computer program. 
The code called GORFF-VE has previously been used to interpret the data from in flight 
lightning measurement campaign [4]. The good agreement between real data and numerical 
calculations has allowed to get a good confidence in the ability of the code to reproduce in 
flight lightning electromagnetic environment. Hence numerical calculations are now used to 
validate the simulation tool. 
Accurate description of the GORFF-VE 3D code can be found in numerous publications [5], 
161. 
The current shapes injected at the nose of the aircraft experimentally and numerically are 
presented figure 2. 
Electric and magnetic fields have been measured for various points on the half of the total 
fuselage by reason of symetry. Measurements have been performed using active sensors and 
fiber optic link. Mappings of the peak values of the electric and magnetic fields are reported 
figure 3 and 4. 
For a better understanding temporal shapes of the recorded signals are compared with nume- 
rical ones. This is only possible using a transfer function technique because of the difference 
in the experimental and numerical injected currents (see fig 2). 
To be consistent and to eliminate the mismatch problems at the aircraft nose, value of the 
major magnetic field component at the nose (Hyl) is taken as a reference. 
Only the time domain derivative is compared for the electric field because the numerical 
decay time is strongly dependent upon the aircraft capacitance. 
Both equations are used : 
73-2 
Fourier transform analysis are performed in the lo4 - lo8& range. After inverse Fourier 
transform (1) and (2) are compared with the experimental signals (fig 5, 6). A study of all the 
experimental points allow to conclude that two zones appear on the aircraft : 
Booster and back fuselage zone where experimental recordings are much higher than 
what is expected from the calculations, 
Everywhere else experimental signals fit quite well the theoretical predictions. 
Hence, comparison between the electromagnetic environment created by the simuiator and 
calculated from the 3D FDTD GORFF-VE code indicates that the coaxial return path simu- 
lation is appropriate to reproduce the fast rise time lightning electromagnetic stress. Due to 
the good agreement on the outer electromagnetic peak values and time domain shapes, inner 
coupling results should be representative of the in-flight situation. 
4. INNER COUPLING 
Ten different points have been instrumented inside the Mirage aircraft. Six cable currents 
have been measured and for four points inner fields have been recorded. The levels recorded 
are significant, six typical recordings are presented fig. 7. As well as time domain measu- 
rements, frequency domain measurements have been performed using a vectorial network 
analyser. From those experiments transfer functions up to 200 MHz, normalized to Hyl value 
are deduced (fig. 8). 
Using these functions and the Fourier transform of Hyl at the nose of the aircraft, we have 
been able to deduce stresses inside the aircraft. Theses stresses are compared with the measu- 
red one for four different cases (two currents, two H fields). 
As the frequency spectrum of Hyl is well defined only up to 20 MHi a good agreement can 
be obtained as long as the inner signal does not contain too high frequencies (HYIPPB, 
ITPBC). If this is not the case discrepancies are observed (HXIS, ITSER) because the high 
frequency exitation is not the same in the two cases (fig 9). As the frequency spectrum of Hyl 
is limited to 20 MHz (due to the 80 dB dynamic measurement range), only noise is then 
recorded after this limit and the frequencies higher than 20 MHz are then altered. Limiting 
the analysis up to 20 MHz then cancel out those frequencies. 
5. CONCLUSION 
We have presented a real scale simulation tool able to reproduce quite faithfully the electro- 
magnetic environment generated by fast rise time lightning pulses. 
73-3 
The first series of measurements inside the aircraft have been presented. They show the limi- 
tations of the frequency domain analysis. Still more work has to be done on a more realistic 
aircraft and at real threat to define Thevenin equivalent generators for the s 
cable bundles and to compare those values with advisory circular recommended waveforms 
for aircraft certification. 
A link between Thevenin 
future qualification tests. 
This word was supported 
generators and cables bundle currents would also be helpful for 
bY 
French Ministry of Defence. 
AKNOWEDGEMENTS 
DRET (Direction des Recherches, Etudes et Techniques) of 
REFERENCES 
J.P. MOREAU - J.Y. JOUAN - F. ISSAC 
Transall 88 lightning characterisation program 
1989 ICOLSE - Bath. 
V. MAZUR 
Lightning initiation on aircraft in thunderstorms 
Aerospace and ground conference on lightning and static electricity - Oklahoma city 
1988. 
D. GAUTHIER - D. S E W I N  
Simulation des effets impulsionnels de la foudre par ligne coaxiale adaptke 
Colloque CEM - Evian 1989 
Exploitation de mesures acquises lors de la campagne 1988 de caractbrisation de la 
foudre sur un abronef en vol. 
F. FLOURENS - D. GAUTHIER - D. SERAFIN 
N.T. 90-73 - CEG - 1990 
F. FLOURENS - D. GAUTHIER - D. SERAFIN 
Lightning strike simulation using coaxial line technique and 3.0 linear injection current 
analysis 
1989 ICOLSE Bath. 
F. FLOURENS - D. S E W I N  
Remarques sur la simulation de l’espace libre au sein du programme GORFF et sur la 
modklisation des extrkmitks de firs electriquement adaptkes. 
NE 88.04 - CEG - 1988. 
73-4 
FIGURE 1 : Picture of an overall view of PARSIFAL simulator 
%+ GORFF ** 
E3 R 
16 
14 
12 
10 
0 
6 
4 \ 
9% MRNIP ** 
E3 A 
0.6 
0.5 
0.4 
0.3 
0.2 
0.1 1 
E-6 E-6 Sec. 
FIGURE 2 : Time domain behavior of the experinlental 
and the numerical currents injected on the Mirage 
73-5 
FIGURE 3 : Peak values of Hy fields on the fuselage. Numbers in brackets are from the 
calculations. Units are in A/m 
FIGURE 4 : Same representation as figure 3 for the E peak values. 
Units are in E9 V/m/s 
73-6 
E0 R/n 
0 .5 1 1.5 2 
E-6 SEC 
-2 
0 .5 1 1.5 2 
E-6 SEC 
** HY-15 ** 
EB w n  
18 
16 
14 
12 
18 
8 
6 
4 
2 
0 
-2 
B .5 1 1.5 2 
E-6 SEC 
0 .5 1 1.5 2 
E-6 SEC 
** HY-iE +* 
EB wn 
0 .4 .E 1.2 1.6 
E-6 SEC 
I 
I 
I 
I 
I 
I 
30 
20 I 
10 
0 
0 .5 1 1.5 2 
E-6 SEC 
FIGURE 5 : Time domain comparison for 6 Hy points of the fuselage 
73-7 
E12 
0.6 
E12  :::I ; 
0.3 
0.2 
0 .1  
0 . 0  
-0. I -0.2 r 3 I I I t ( (  ' I  I I a I 
0 . 5  1 1.5 2 0 .5  I I . 5  2 
E-6 SEC E-6 SEC 
** dE/dt-15 ** * *  dE/dt- l6 ** 
E3 E12  
250 r 
0 . 5  1 1 .5  2 0 . 5  1 1.5 2 
E-6 SEC E-6 SEC 
E9 
E 
150 
E3 
300 
250 1 
200 
e 
FlGURE 6 : Same comparison as figure 5 for the E time dcmain values 
73-8 
** ITPBC ** ** ITPFF X *  
E-3 R 
25 € 
2 0 i  I 
** ITPRC ** 
E-3 R 
90 E 
0 2 3 4 
E-6 Sec. 
I* HXIS tit 
0 . 5  1 1 . 5  2 
E-6 Sec. 
*it HYIPPB ** *tt HYIS ** 
E0 R/M 
4.0 r 
3.0 
2.5 5 
Ee R/M 
0.25 r 
t 
FIGURE 7 : Samples of currents and fields measured inside the aircraft 
73-9 
E0 dB 
51e7 510' 
Hertz 
Hertz 
E0 dB 
510" 510f 510' 
Hertz 
EIGURE 8 : Examples of transfer function measured with a network analyser : 
HYIPPB : Hy field in the cockpit, near the UHF command box 
HXIS : Hx field in the equipment compartment 
ITSER : current on the cable bundle of the emitter 
ITPBC : current on the cable bundle of the UHF emission box 
73-10 
** HYIPPB ++ ** HXIS ++ 
Ea 
5.8 
0 1 2 3 
E-6 SfC 
-O.s LJ ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' * ' ' ' ' ' a ' ' I ' ' ' I ' '.A 
0 1 2 3 
E-6 SEC 
*+ ITSER +* 
E-3 E-3 
200 f L 
150 
100 
5% 
0 . . . . . .  
-50 1 
120 
40 
20 
-20 
** ITPBC ** 
. . . . . . . 
-40 
0 1 2 3 4 
-100 L
0 .5 1 1.5 2 
E-6 SEC E-6 SEC 
FIGURE 9 : Experimental and frequency domain calculated inner currents and fields. 
The points presented are the same as for figure 8 
73-11 


A real scale simulator for high frequency LEMP

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