Practical Approach for High Current Radome Testing

Practical Approach for High Current Radome Testing Christian Karch Airbus Defence and Space GmbH, 85077 Manching, Germany [email protected]

Fridolin Heidler University of the Federal Armed Forces Munich, EIT7 85577 Neubiberg, Germany [email protected]

Wolfgang Zischank University of the Federal Armed Forces Munich, EIT7 85577 Neubiberg, Germany [email protected]

Mircea Calomfirescu Airbus Defence and Space GmbH 85077 Manching, Germany [email protected] Abstract—EM-transparent radomes constructed from dielectric material offer no EM screening of radomes to the external EM fields associated with lightning strikes. The antenna structure presents a major electrical stress raiser and is a likely target of a lightning strike. Therefore, a lightning protection system is to be designed to prevent lightning flashes from puncturing the radome and from damaging the antennareceiver system. High voltage (HV) testing that reproduces the lightning phase of the bidirectional leader development generally proves the functionality of the lightning protection system. However, discharge currents from HV generators do not exceed a few kA, so the ability of the protection system to divert and withstand the high current load cannot be demonstrated. The necessary experimental procedures for high current (HC) tests on coupons of the radome structure with the diverter strips mounted in a representative manner are discussed in the following. Moreover, successful HC test results for a generic Satcom A-Sandwich radome structure are presented. Keywords—Aircraft, Radome, Lightning, High Current Tests

I.

INTRODUCTION

This HC tests are required for radomes and antenna fairings. These tests are to demonstrate that the designed protection system can divert the lightning strike current to the metallic aircraft structure satisfactorily. It is also shown whether the particular protection scheme can withstand the lightning return stroke current and whether it can have a multiple strike capability. These HC tests shall verify the adequacy of mechanical and electrical fastening and grounding design. One way of conducting HC tests is to mount a full-scale radome in a return conductor rig and apply an arc with current components appropriate to the relevant lightning zone. However, full-sized radomes are expensive and it is therefore more convenient to perform HC tests with planar coupons with diverter strips mounted in a representative manner. Moreover, HC tests cannot really be performed on full-scale radomes since the voltage provided by the HC generator is not sufficient enough to ignite plasma channel above the whole length of segmented strips. The terminal voltage of most HC generators is approximately 20 to max.

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80 kV. Typical segmented strips have an ignition voltage of about 1 to 1.5 kV/cm [1]. In case of a hybrid solid-segmented protection system a particular attention shall be paid to the transition region between segmented and solid strips, to the metal fasteners, and to the electrical bonding to metallic airframe structure. II.

HIGH CURRENT TEST SETUP

The generic Satcom radome extends nearly from the aircraft’s nose, lightning zone 1A, up to the lightning zone 1C [2], see Fig. 1. Therefore, the current waveform A has to be applied in the lightning zone 1A. This waveform has a peak amplitude of 200 kA (±10%), a time duration of less than 500 μs and an action integral of 2x106 A2s (±20%) [3]. For HC tests of protected CFRP or GFRP airframe structures the combination of A/D+B+C* current component is generally used. However, the structural damage of dielectric structures is mainly caused by the magnetic and mechanical forces generated during the fast lightning current component A or D [4], [5]. It is therefore not necessary to apply the slow, low current amplitude components B and C* (here). The procedure of lightning protection design for the Satcom radome is described in detail in [6].

Fig. 1. Image of the generic Satcom radome

There are low level test generators that have the same wave shape as a component A waveform. This allows the induced voltage measured to be simply scaled up to full

threat. However, non-linear effects might falsify the results in a way that the extrapolated loads and the damage can be different to those obtained at full threat. Therefore, measurements at low level should be complemented by final measurements at high levels to fully understand the arising loads and thermo-mechanical damage. To simulate free space condition, the return conductors should be spaced around the current carrying parts of the radome protection system to form a basic coaxial system with the test piece as the inner conductor. Due to the possible complex geometry of the system under test, a simple concentric conductor configuration is not always possible. Procedures similar to those recommended in ED105 [7] should be used to design an appropriate return conductor rig.

of the specimen in the impulse current generator is shown in figure below.

A. High Current Generator The The high current tests are performed at the University of the Federal Armed Forces Munich, Faculty for Electrical Engineering [8]. The so-called tandem impulse current generator was used for the lightning impulse current tests with the lightning current component A, see Fig. 2.

Fig. 4. Current injection electrode with a thin copper starter wire

The high current tests are performed under the following environmental conditions: • Temperature: 22.7 °C, • Pressure: 946.8 hPa, • Humidity: 49.8 %. B. Circuit Design of the High Current Generator A high current generator usually consists of a set of large parallel high voltage capacitors forming a capacitor bank (CS). The capacitor bank is slowly charged by a D.C.-source to a high voltage (up to about 100 kV) and then rapidly discharged into the load (via a switch). The internal resistance Ri and inductance Li of the generator, the external wave forming elements Rex and Lex and the load characteristics RL and LL can be summed up to

Fig. 2. Image of the generic Satcom radome

R = Ri + Rex + RL

(1)

The tandem HC generator consists of two capacitor banks, each one with 30 μF charged up to 59 kV. In order to get a unidirectional current waveform, both capacitor banks are equipped with crowbar spark gaps. The currents of both capacitor banks are added up at the specimen under test. The equivalent circuit of the tandem HC generator circuit including starting spark gap (SG) and crowbar-switch gap (CSG) is shown in Fig. 3.

L = Li + Lex + LL

(2)

The current i of this RLC-circuit is given by the differential equation ∙

0

(3)

The resulting current waveform depends on the ratio of the circuit elements R, L and Cs. To obtain maximum current, RLC-current generators should be operated in an underdamped mode. However, the resulting oscillatory waveform is contrary to the unidirectional currents of natural lightning.

Fig. 3. Equivalent circuit diagram of the high current generator

The injected current was measured in the common return path to the generators using a wideband current monitor Pearson Model 2093. The current waveform was recorded by a digital measuring system High Volt MIAS. The test setup

A very effective way to obtain a unidirectional current is the use of a crowbar switch in the RLC-circuit. The basic circuit diagram is shown in Fig. 3, where the small internal inductance of the generator is neglected. An external inductance (6 μH) is connected to each of the circuits of the tandem generator. The discharge is initiated by a starting spark gap (SG) at t = 0, while the crowbar switch (CSG) is first open. To obtain maximum peak current, the generator is operated in a strong underdamped mode with low resistance. At the instant of the crest value of the current, tcr, the crowbar-switch is closed. Most of the energy initially stored

in the capacitor bank is then transferred both to the external inductance and to the inductance of the load. By shorting out the capacitor bank with the crowbar switch at tcr the current turns from an oscillatory to an exponentially decaying waveform, having a decay time constant τ = L/R. With τ being high, a long duration of the current is achieved. A crucial element of this generator type is the crowbar switch. At t = 0, when open, it has to withstand nearly the whole charging voltage. At the time of shorting, t = tcr, when the energy is transferred to the inductance (L), the voltage across the capacitor bank (CS) and therefore across the crowbar switch is near zero. Closing a crowbar switch near or at zero voltage requires advanced technology equipment. Furthermore, the crowbar switch must have the ability to handle currents in the 200 kA range with decay times of some 100 μs. Mechanical switches are usually too slow to operate at a microsecond time scale. That’s why a three-electrode crowbar switch in ambient air was developed and successfully applied. The crowbar switch is triggered by the high-voltage impulse of nearly 300 kV produced by a Marx generator using 4 stages. III.

B. Second Configuration The second configuration represents the hybrid segmented / solid protection system. Here, the solid diverter strip must be electrically connected to the metallic structure using a metallic grounding bolt. The solid diverter strip shall be securely fastened to the sandwich structure using e.g. metallic screws spaced approximately by about 15 cm. The mounting fasteners should be surrounded by a flexible seal that prevents moisture from entering the honeycomb core of the sandwich structure. The cross-sectional area of the solid strips is given by the pre-defined maximum temperature rise due to resistive heating of the lightning current waveform. A particular attention should be paid to the structural implementation of the transition region between segmented and solid diverter strips, see lower image of the sample in Fig. 7. The segmented as well as the sold diverter strip must be securely fastened to the radome structure. Moreover, the shape of the solid diverter tip influences the electric field distribution in this transition region. A non-appropriate design might enhance puncturing of the radome structure in this region [10]. The scheme of this radome coupon configuration is shown in figure below.

DESIGN OF RADOME COUPONS

Two different radome coupon samples that cover the representative protection diverter strip configurations of the hybrid lightning protection system on the surface of the radome prototype are prepared. A. First Configuration The first configuration represents the connection of the segmented diverter strip to the metallic airframe structure. The test samples are made from a planar radome structure and a representative part of the metallic airframe structure. The segmented diverters shall be bonded on the surface of the radome sandwich structure according to the LDS general guidelines for installation of segmented diverter strips [9]. The diverter strip shall be electrically connected to the metallic structure using a metallic grounding bolt and a metal bar. The scheme of this configuration is shown in Fig. 5.

Fig. 6. Scheme of a radome coupon sample that represents the transition between segmented and solid diverter strips

IV.

RESULTS

Two identical coupon samples are manufactured for each representative design configuration, see chapter above. Two lightning loads were applied to each sample in order to prove whether the designed protection scheme can withstand a single lightning current component A or whether it can even have a multiple strike capability. The images of one of the manufactured test samples for the design configuration no. 1 and 2 are shown in Fig. 7.

Fig. 5. Scheme of a radome coupon sample that represents the connection of a segmented diverter strip to the metallic airframe structure

According to EUROCAE ED-84 [3], the transient lightning current component A is defined by the • Peak Amplitude (IP): 200 kA ±10%, • Action Integral (AI): 2×106 A2s ± 20%, • Charge (Q): not specified for direct effect testing, • Time Duration (TE): ≤ 500 µs, • Time to Crest: (T1): ≤ 25 μs,

•

Decay Time: (T2): not specified for direct effect testing.

a function of time. It is shown that the recorded lightning currents of the first and the second shot are almost identical.

Fig. 8. Recorded lightning currents for the calibration sample R1

Fig. 7. Front and rear side images of the test sample configurations no. 1 (upper part) and 2 (lower part)

The estimated total electrical charges and the action integrals for the calibration sample R1 are plotted in Fig. 9. The amplitudes of the total charges and of the action integrals of the second shot are negligibly higher than those of the first shot, compare the values given in Table 1.

The samples including the reference/calibration samples and the results related to each test are summarized in Table 1 and 2. The parameters listed for the charges, the action integrals and the time constants are numerically derived from the recorded current amplitudes. TABLE I.

SUMMARY OF THE TEST RESULTS FOR THE COUPON SAMPLE CONFIGURATION NO. 1

IP [kA]

Q [C]

T1 [µs]

T2 [µs]

TE [µs]

R1_1

207

19,7

15,7

78,5

388,9

ΑΙ [MJ/Ω ] 2,04

R1_2 S1_1

207

20,1

15,9

78,8

408,1

2,06

207

19,8

15,0

78,1

396,8

2,03

S1_2

207

19,8

16,0

78,4

397,7

2,04

S2_1

207

19,4

15,7

77,8

382,5

2,00

S2_2

98

5,6

15,3

32,4

40,17

0,26

TABLE II.

SUMMARY OF THE TEST RESULTS FOR THE COUPON SAMPLE CONFIGURATION NO. 2

IP [kA]

Q [C]

T1 [µs]

T2 [µs]

TE [µs]

S3_1

204

19,6

16,2

78,6

388,1

ΑΙ [MJ/Ω ] 1,99

S3_2

203

20,2

16,4

78,6

422,7

2,02

S4_1

204

19,1

16,2

77,3

392,8

1,92

S4_2

204

18,2

16,3

43,7

409,1

1,77

The lightning current time behaviour, the amplitude and the action integral of the high current generator are calibrated on a reference gauging test sample. The recorded lightning currents for the calibration sample R1 are plotted in Fig. 8 as

Fig. 9. Calculated charges and action integrals from the recorded currentsfor the calibration sample R1

The test set-up with the gauging sample R1 is shown in figure below.

Fig. 10. Test set-up with the calibration sample R1

The current jet-diverting electrode with insulating ball is placed 50 mm above the test sample and slightly shifted from the tip of the segmented diverter strip in order to avoid plasma jets directed to the diverter strip, see Fig. 5. As shown in Tables 1 and 2, all parameters of lightning current waveforms, the magnitude, the action integral and the charge are not only within the tolerance limits, but very close to the given default values given by the standards, compare EUROCAE ED-84 [3]. The only exception is the second lightning shot on sample S1_2 showing a misfiring of the generator that resulted in an oscillating current wave, where the first negative half wave went out of the scale (and is not recorded). Therefore, the listed current amplitude and the calculated charge and action integral are significantly underestimated.

Fig. 11. Front side of the sample S1 after lightning strike loads

After the first lightning strike are, (as expected), visible traces of the thermal damage of the paint on the front side of the sample and visible traces of the thermal load on the segments of the diverter strip at the lightning plasma attachment area. This is exemplarily presented in Fig. 11 for sample S1. A part of the paint at the tip of the solid diverter strip is completely removed by the thermal effects and the acoustic shock waves of the lightning strikes, see Fig.12. Moreover, there is a visible thermal damage of the paint around the lightning strike area. In addition, there is a visible thermal damage on the surface damage of the fixing bolt and of the titanium screws used as electrical termination / grounding, see Fig. 12. However, there is no indication of damage of the radome sandwich structure and no visible damage on the rear side of the samples caused by the shock waves of the supersonic expansion of the plasma channel. The ultrasound C-scan tests performed after the lightning loads confirm this finding; neither delamination of the GFRP skins and the honeycomb core nor squeezing of the honeycomb core of the investigated sandwich samples has been found.

Fig. 12. Upper part: Front side of sample S4 after lightning strike loads. Lower part: Image of the damage at and around the transition region between the segmented and thesolid strip.

V.

SUMMARY

The following practical conclusions can be drawn from the performed lightning strike tests on two different coupon configurations representing the connection of the segmented diverter strip to the metallic airframe structure and the hybrid segmented / solid protection system: • The designed lightning strike protection system can withstand two lightning strike loads with the current waveform A (200 kA, 2 MJ/Ω). •

•

After the lightning strikes, traces of the thermal damage of the paint on the front side of the sample and traces of thermal load on the segments of the diverter strips at the lightning plasma attachment area are visible.

There is no indication of damage of the radome sandwich structure and no visible damage caused by the generated shock wave on the rear side of the samples, neither after the first nor after the second lightning strike load. After the second lightning strike load, the resin of glass fibre structure of skin of the sandwich structure is thermally loaded at the position of the segmented diverter strips. Nevertheless, the glass fibres of the glass skin are neither damaged nor broken. The Aerospace Recommended Practice ARP5412 / EUROCAE ED-84A und ARP5416 / EUROCAE ED-105A do not make any recommendation on how many high current lightning loads a segmented diverter strip has to withstand (without irreversible thermo-mechanical damage of the

underlying composite structure). Two lighting current loads during a flight mission might occur at the same position. However, the probability for such a case is extremely low. The obtained results demonstrate that the designed lightning strike system successfully passed the high-current lightning tests. REFERENCES [1]

C. Karch, “Lightning Protection of WB – SATCOM Radome – Maximum Length and Distance of Diverter“, Technical Report TX3RP1620081, Munich, 2016 [2] EUROCAE ED-91, “Aircraft Lightning Zoning”, 2006 [3] EUROCAE ED-84A, “Aircraft Lightning Environment and Related Test Waveforms”, 2013 [4] C. Karch and C. Metzner, “Lightning Protection of Carbon Fibre Reinforced Plastics – An Overview”, Proceedings of ICLP 2016, Estoril, Portugal, 2016, DOI: 10.1109/ICLP.2016.7791441 [5] C. Karch et al., “Modelling and Simulation of Lightning-Induced Damage of CFRP Structures”, International Conference on Lightning and Static Electricity, Nagoya, Japan, 2017 [6] C. Karch et al., “FFS: Lightning Strike Protection of Radomes – An Overview”, Deutscher Luft- und Raumfahrtkongress, Munich, Germany, 2017 [7] EUROCAE ED-105A, “Aircraft Lightning Test Methods”, 2013 [8] F. Heidler, W. Zischank, and A. Camara, “Lightning Current Test of Segmented Diverter Strips for the Protection of Sandwich Materials”, Protocol, University of the Federal Armed Forces Munich, Faculty for Electrical Engineering, 2017 [9] Application Note AN10007 Rev. B, “General Guidelines for Installation of Segmented Diverter Strips”, Lightning Diversion Systems 2015 [10] D. Balitrand, “A400M EV03 Radome lightning qualification test plan” Airbus, EYYBR, M53PL1503100, Issue 2.0, Jan. 2016