Extensive measurements of induced voltages and currents were made using a CFC (carbon fiber composites) horizontal stabilizer from the A320 as a test bed. The work was done to investigate the efficacy of various protection schemes to reduce the magnitudes of the induced voltages and validate a computer program INDCAL. Results indicate that a good understanding of the various induced voltage mechanisms including the long wave effect due to current redistribution was obtained

Haigh, S. J.

Hardwick, C. J.

English

NASA Technical Reports Server

N 9 1 - 3 2 6 3 8  ‘ 
THE ELECTROMAGNETIC ENVIRONMENT IN CFC STRUCTURES 
C a n d S J H a i g h  
Cdhm Lightning Test and Technology ,Abingdon, Oxford, UK, OX14 3DB 
ABSTRACT 
Extensive measurements of induced voltages and currents have been made using a CFC horizonfa1 
stabilizer from thc A320 as a test bed .The work was done m investigate the efficacy of various protection 
schemes to reduce the magnitudes of the induced voltages and validale a computer program INDCALResuIts 
indicate that a good understanding of Ihe various induced voltage mechanisms including the long wave effm 
due u, current nxhsttibution has been obtained 
1 INTRODUCTION 
“he use of CFC in ai-e construction produces indirect effects which have generated much 
interest in recent years. In particular, the relatively high resistivity of composite comparcd U) 
aluminium leads to significant redistribution of cumnt fmm the carbon fibre to any metallic 
components of the airfhme during later times of the lightning waveform (References 1 and 2). This 
is because after the initial fast current rise, the di/dt magnitude decays and the current distribution is 
no longer inductively dominated. 
waveform 5 is specified for equipment tests u, take account of the phenomenon. This current 
waveform, which will occur on conducting conduits or cable screens in hybrid struclurcs (structures 
containing both metallic and CFC components) has a rise time of 50p and time to 
S o O p .  Peak amplitudes specified are fmm 3-20kA. 
waveforms respectively which have the same shape as a component A lightning strike, waveform 2 
which is the derivative of waveform I and waveform 3 which is a damped sinwave. 
were reponed in a previous paper of this conference scries (Reference 4); no protection was installed 
on the box resulting in conduit currents of up 10 17kA. of a waveform 5 type due to cumnt 
redistribution. 
This present paper coven work investigating the effects of various protection melhods such 
as aluminium strips bonded to the box and of surface coatings of metallic foil. Induced voltages wcrc 
measured on wires muted inside and outside of conducting conduits, as well as the current 
distribution around the structure. 
Section 2 describes the experimental configuration, Section 3 presents the resulw and Section 
4 presents some limited comparisons with IMxlAL (Refercnce 5 )  computer predictions. Section 5 
makes predictions for more general hybrid strucIures. 
The work has been supported by thc Cullam Lightming Club. 
In the latest FAA advisory circular AC2@ 136 (Reference 3), a long duration waveform, 
height of 
This waveform 5 is supplementary to waveforms 1 and 4 which are current and voltage 
Some measurements on induced voltages and currents in a CFC horizontal stabiliser box 
2 EXPERIMENTAL CONRGURATlON 
The basic experimental configuration is shown in Figure 1 and this is similar to that uscd in 
the previous work reported in Reference 4. A 1.5m long metallic braided conduit was installcd insidc 
the stabiliser and linked two aluminium boxes bolh of which were well bonded to the CFC; induccd 
voltage diagnostics wefe installed in one of Lhc boxes. The extent of the aluminium pmlection strips 
and foil are also indicated in the figure. Figure 2 indicates the wire circuits on which the induccd 
voltage measurements were made. Each wire was s h o d  to the metallic box at one end and 
terminated in a high impedance at the other end. In addition to the unprotected structure (Figure 2a), 
data for three different protection conftgurations wen: measured; these were: 
A.uruX 
2.5cm wide, 2mm thick A1 strips bonded along their cntire length with M4 scrcws using 
existing captive nuts. These nuts were present for attaching aluminium strips on thc production 
aircraft (Figure 2b). 
45-1 
https://ntrs.nasa.gov/search.jsp?R=19910023324 2020-03-17T15:04:04+00:00Z
Epil 
25p thick aluminium foil of resistivity 2 . 5 m m  was placed over the upper and lower skins 
and extended as far as the entire length of the strips. The foil was clamped near the leading and 
trailing edges by the aluminium strips. It was isolated from the CFC surface by a layer of polythene - 
this was in order to ensure the current paths to the foil were well defined (Figure 2c). 
inboard and outboard extent, and held down with several self tapping screws. This was to enable 
current to pass straight from the CFC to the foil without having to deviate to the aluminium strips at 
the leading and txailing edges first. 
Thin aluminium strip was also used to sandwich the foil edges to the CFC at its furthest 
2.5cm A I  strips 
Strip current 
Upper skin 
Lower skin 
Density x resistivity x 1.5 
ICORE 
ICORE current x 3.8- 
ViR inner 
M m d B  
ViR outer 
The I S m ,  conduit used hitherto was ioined by a 360' joint to a similar Diece of conduit 
installed + ICORE 
VALUE A T 2 W  RISE TIME J.LS 
21 kA 4 
7.3 kA 2 
6.6 kA 2 
1110 v 
11.8 kA 20 
4 4 v  
46 V 18 
9 o v  2.5 
0.4 
giving a 3m long conduit of total mistance d 9.6mn: The ends of th is  assemcly were again bonded 
to the termination boxes A and B with 36o'joints, but the cable was routed external to, and its centre 
4cm away from the front spar of the stabiliser (Figure 2d). 
3 RESULTS 
The measurements are tabulated in Table 1 for the unprotected configuration, Table 2 for the 
ltstrip" configuration, Table 3 for the "foil and strip" and Table 4 for the "external conduit". The data 
values have been linearly extrapolated to 2MM peak cunent, but were generally obtained with a 
driving waveform of 12kA peak current, rise time 1.3ps, time to */,height on falling tail, 12p. For 
a Component A driving waveform, because of the longer decay time (70ps), larger redistribution 
currents would be obtained. 
The capacitor bank voltage was 12OkV, which with the 2.2pH inductance of the structure 
gave a peak value of di/dt of 5 3 W p .  Hence the appropriate scaling factors to 2OOkA peak current 
and l4OkA/ps peak di/dt are 18 and 2.64 respectively. 
INDCAL calculations were made using the cross section of the structure shown in Figure 2e 
and comparisons with data in the different configurations are made in Figures 3-6. The agreement is 
very favourable (3dB). Aspects of these waveforms are discussed in Section 4. 
Current at box corner 
Upper skin 
Lower skin 
Lower skin density * resistivity x 1.5m 
ICORE current 
ICORE c m t  x 3.8mR (ICORE mistance) 
IMU wire voltage 
Ouler wire voltage 
Inner, ICORE disconnected 
'ABLE 1 
VALUE I RISETIME 
of Peak Cmrent/Voltage at 200kA 
1 1.87 kA 
1.85 kA 
7.15 kA 
1185 V 
17.25 kA 
66 V 
52 V 
V 
1500 lo9 v 
BLS 
1 
1.7 
2.0 
23 
24 
2.6 
2.5 
45-2 
TABLE 3 
VALUE AT 
2ookA 
Strips 16 kA 
Upper skin 7.1 kA 
ICORE 7.6 kA 
ICORE current x 9 . 6 m  77 v 
ViR inner 9 o v  
ViR outer 94 (648)p 
ViR outer ICORE discannected I O  t V  
Strips and-foil installed 
I VALUEAT 
RISE TIME. 
WS 
2.5 
2 
2 
2.5 
.8 - .1 
- 
strip c m t  
uppet skin 
Lower skin 
Density x resistivity x 1.5m 
ICORE 
ICORE current x 3.8mR 
ViR inner 
V i  outer 
ICORE cumnt x 3.8mR 
UxlkA 
18 kA 
7.3 kA 
6.2 kA 
8.5 kA 
2 0 6 v  
32 V 
33 v 
5 4 v  
32 V 
RISE TIME, 
US 
2 
2 
30 
26 
4 
Notes: 
+ Bracketed figure is figure that would be obtained by scaling di/dt by same factor as current 
For waveforms with high HF content we have scaled LO peak dddt (factor 2.6) not current (x18) as 
the coupling mechanism is proportional to Wdt rather than c m t .  
4 DISCUSSION 
Measurements of voltages on wires in many different internal positions weE made in 
addition to those illustrated in Figure 2, however space precludes their inclusion; derails are available 
in the Culham Lightning Club reports (Reference 6). Nevertheless the a 
shown in Figure 7. These trends can be understood in terms of the resistive voltage drop and 
diffusion flux penetration through the box walls. 
Circuits using the box structure as a return path will have an induced voltage due to the sum 
of the resistive voltage drop along the stmcture itself plus a contribution fmm the rate of change of 
flux threading the circuit of w i ~  and st~uctuial return Well inside the CFC structure and away from 
any apertws this will be difhsion flux whose rate of change, roughly follows the cumnt wave 
shape. Depedmg on direction of the flux it may add or subtract to the structural voltage. At 
conducting elements in the box, the structuddiffusion flux voltage will drive redistribution current 
into the conduit; this cumnt will in tum reduce the total flux near to the conduit. 
The relative screening offered by the ddfwent configurations have been summarised in Figure 8. The 
results have been derived from the peak voltage data h m  the tables normalised lo 1 A, or to 1401rA/Clsn00kA 
depending on the coupling mechanism, to give a screening figure of merit in dBR. Shown on the same 
graph am thc A-136 threat levels. Computations show that with an AC20-136 component A driving 
waveform (longex than our experimental waveform) the values of induced voltage on wires inside the conduit 
which carry a redistribution current would be 7dB higher. Hence with a component A waveform, the ICORE 
routed inside givw a wavefam 5 type of induced voltage 368 below level 2 (redistrib~tion will effect peak 
value and 7dB correction added). Tht externally mutcd ICORE would give a Waveform 4 type of induced 
voltage, 3dB below level 2 (redistribution cumnt will not effect peak value which occurs with the peak of the 
driving cumnt and no Cortection ma&). Far lamger cables 6dB should be added f a  each doubling of length. 
Computa studies (see next section) have shown that provided the resistance per unit length of the conduit is 
not too high the current is largely determined by the CFC resistance and structure geometry which leads to 
the induced voltage developed being roughly propOmonal to conduit resistance. Differential voltages on two 
ment with INDCAL 
predictions for voltages was generally within about 6dB and the t m d  o $" values and waveshapes is 
45-3 
win  circuits of co~pse would be some UMB lower. Simple computations like these can thus be used to 
define h e  shepes and magnitudes of cumnts and voltages on circuits in the structure that will occur. 
D e t e i k a c a n ~  betwetndataandpedictionsartnoWdisc~- 
The CUcIlent density at the comers of the wing box, mar to the position of the aluminium 
a) -
strips is initially higher than elsewhere but decays more quickly than for example the current on the 
top skin. This is understood in terms of an initial inductive current distribution followed by resistive 
redisuibution. The INDCAL/data comparison is made in Figure 4. 
b) SIRIPS 
current waveform is higher in amplitude and shows a much later zero crossin than the ament 
more readily and the ICORE current has a lower amplitude (-3dB) and decays earlier. Figure 5 
shows that INDCAL is acxwale to within 3dB. 
is pmportional to structure current) threading between the "outer" wire and the ICORE, the "outer" 
voltage is higher than the inner and has a shorter time to peak. 
c) EQIL 
of the skin and into the strips is not now so marked. Consequently the strip current is less and 
decays more quickly. 'Ihe diffusion flux takes longer to penetrate the more conductive skin and 
hence. The cufient in the ICORE now has a longer rise time and its amplitude is Feduced by a further 
2dB compared to the "strips only" configuration. A comparison with INDCAL is made in Figure 6 
and again agreement with INDCAL is good. 
It was difficult to make a good joint between the foil and the CFC, and the joint mistance 
was substantial compared to the 2.5mW0 foil mistance. Since the ICORE c m n t  is a sensitive 
function of the foil resistivity the computation includes a value of foil resistivity that takes account of 
the measured pint resistance. INDCAL predicted an ICORE cumnt of 3% for an effective skin 
resistivity of 4mWn. which is close to the observed value of about 4%. 
than by the addition of strips since the diffusion flux penetrating to the interior will be lower due to 
the lower effective skin resistance. H e w  foil is very effective at reducing common mode resistive 
voltage that would occur on unscreened wires in CFC stnrctures. 
d) -
the ICORE and structure so the current waveform follows more nearly the driving waveform except it 
remains higher at late times due to current redistribution. INDCAL predictions an shown in Figure 7 
together with measured values. The computation includes a reduced cross section for the conduit to 
give a 50% increase in the self inductance of the ICORE 
length of smcm it spans). Because of the large amplitude of di/dt of the current in the conduit there 
is a big difference between inner and outer voltages. 
5 COMPUTATIONAL STUDY 
To find how the redistribution currents were affected by the variation of several parameters 
some compulational studies were made. Table 5 shows the result of such a study on a box of similar 
ccoss section and make up to the experimental structure. As the resistivity of the structure and hence 
total resistance is lowered (a similar effect would result from increasing the size of the box) the peak 
cuclwlt in the conduit talres a longer time to reach peak but the magnitude decreases rapidly. As the 
resistance of the conduit becomes lower, its cumnt also increases as does the time to peak but it is 
limited eventually by its inductance and these are a range of values below which the conduit current - not V a r y  m y .  
As expected the Aluminium strips drain current from the structure at late times so the strip 
Momver as the strips are more conductive than the ICORE cumnt redistributes inlo them 
Due to flux generated by redistribution current flowing in the conduit (whose rate! of change 
measured inthe carbon fibre at the same position before tbe strips were d d  
Since the average resistivity of the skin has been reduced considerably the redistribution out 
The "outer" wire voltage has been reduced by a larger factor (6dB) by the addition of foil 
The flux driving the current into the conduit is mainly fast aperture flux threading between 
ICORE conduit is longer than the 
46-4 
MOR studies of different structures indicated that in general the more important parameters, 
axe the CFC resistivity as noted in this section and the relative proportion of CFC to metal as noted in 
section 4b). A range of rise times and fall times for the long wave are possible and the Solls/SOOps 
is only om example of the form of redistribution cu~~wlts, longer and shorter wavefonns can occur. 
In particular if a substantial proportion of the structure is a poorly conducting metal such as would be 
found in a CFC covered titanium engine, very long pulses due to redistribution from dw: titanium to a 
good conducting conduit could be produced. 
TABU5 
6 CONCLUSIONS 
6.1 
redistribution mechanism in hybrid smctum. The agreement on the structure studied was within 
3dB fw m n t s  in the struchm and 6dB for voltages. 
6.2 
maximum lesistive voltage drop and these occur for wins muted near the composite skin particularly 
skin with high cuwative; for other wires the voltage will in general be lower especially those routed 
near to intemal low resistance m n t  carrying conductors. 
Cumts on internal oonduits will have long rise times and the induced voltage waveform of 
a wire insidc a 360' bonded conduit follows the current waveform pmisely and has a magnitude 
6.3 compltational pdictions show that the current canied by conduits or cable shields within a 
hybrid structure has a peak magnitude and a rise time-to-peak which is dependent on several factors 
relating to tbe geometry and resistivity of the skin and intemal structure; the more important factors 
being the relative proportion of metal LO CFC in the outer skin and the CFC resistivity. 
A mistive INDCAL analysis gives a good understanding of the induced voltage and cumnt 
The most severe common mode voltages inside hybrid s t r u m  are of the order of the 
equal to t h e c o n d u i t ~ X  Dcresistance. 
7 REFERENCES 
A Hummer et al. The Behaviour of Electric Currents in GraphWEpxy Structures. 
ICOLSE 1988, Oklahoma, USA. 
C Jones. The Slow Equipment Test Waveform for Systems in CFC Structures. ICOLSE 
1989, Bath, UK. 
FAA AC20-136, Rotection of Aircraft ElectricavElectomic Syssems against the Indirea 
Effects of Lightning, 1991. 
C Hardwick et al. Induced Effects Associated with Lightning Strikes to Hybrid Structures. 
ICOLSE 1989, Bath, UK. 
C Hardwick et al. A Filamentary Method for calculating Induced Voltages within Resistive 
Structures in either the Frequency Doman or Time Domain. ICOLSE 1988,Oklahoma,USA. 
C Hardwick et al. The Electromagnetic Enviment  Inside the CASA CFC Horizontal 
Stabilizer. Culham Lightning Club Report 613. 
1. 
2. 
3. 
4. 
5. 
6. 
8 ACKNOWI-EDGJMEMS 
The authors would l i e  to acknowledge financial suprt from the Culham Lightning Club; 
comprising: British Aemspace, Rolls Royce, Westlands, CASA. SAAB, Shorts, CAA, DTI. 
45-5 
46-6 
C 
0 
X 
0 
P 
r u 
4 
t4 
0 
& 
0 
(Y 
(.? c 
lec 
0 
Figure 2: Different configurations whose results are 
plotted in figure 8 
, I 0.30 a 
0.20 -. 
0.1 0 -. 
0.00 -. 
-- "f" 
? 
! 0 
a ,  
- I  
1. Basic structure & CORE 
L '  
2 - 
.-5i 
r 
2. AI strips & CORE 
- 
3. AI strips, CORE & foil 4. AI strips, CORE & foil with 
CORE routed outside box 
Key : Wire a - Structural return 
For configuration 4 datum for structural return plotted in figure 8 is 
for wire d with the CORE disconnected 
b - External to CORE 
d - lnternal to CORE 
e) Typical filamentary representation for INDCAL 
45-7 
I 
1 
w a $1 
: /  
I '  1 
c3 
r4 
U 
1 
1 
I 
r + 
I , 
I 
L 
I I 
5 rn l  
3 
v) w a 
w a 
3 e u 
Y co 
0 .'1 
- 0  
U 
VI 
0 1  
m 
LLl 
f: 
H e 
0 
c\1 
0 - 
0 
0 
V 
0 
m 
0 
hl 
0 
c 
0 
e 
I 
0 
I 
I 
/ I 
i 
l o  
I I i "  I 
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I I I  
I 
I 
I I 
I 
I 
/ 
/ 
/ 
I 
I 
I 
45-9 
600 
1' 
300 
TI!lE US 
Figure 7: Voltage waveforms for wires at different 
positions as shown in insert. The longer rise times 
for the wires 2-4 are due to current taking time to 
penetrate to the internal structure 
AC 20-1 36 
Waveform 2 8 4 
-20 voltage levels 
-40 L I 
m C -60 I U 
Level 
-5 
- 4  
-3 -
- 2  
- 1  
U b 
d 
-100 
- * O I  
1 I I 
-1 1157 
1 2 3 4 
-120 L l  
Configuration 
Figure 8: Time domain peak voltage data 
from tables 1 to 4 plotted relative to driving 
current in dBR 
45-10 


The electromagnetic environment in CFC structures

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