Tests designed to investigate the lightning-generated electromagnetic environment affecting aircraft are discussed. An F-106B aircraft specially instrumented for lightning electromagnetic measurements was used. The instrumentation system is reviewed and typical results recorded by the instrumentation during simulated-lightning ground tests performed for a safety survey are presented. Several examples of data obtained during summer flight testing are presented and future plans are discussed

Pitts, F. L.

Thomas, M. E.

English

NASA Technical Reports Server

IN-FLIGHT DIRECT-STRIKE LIGHTNING RESEARCH 
Felix 1. P i t t s  and Mitchel E .  Thomas 
NASA Langley Research Center 
SUMMARY 
The NASA Langley Research Center is performing in-fl ight direct-s t r ike 
lightning research to  bet ter  define the lightning-generated electromagnetic 
environment affecting a i r c ra f t .  
which operates in a thunderstorm environment and is specially instrumented 
for  lightning electromagnetic measurements. The instrumentation system i s  
reviewed and typical resul ts  recorded by the research instrumentation during 
simulated-lightning ground t e s t s  performed for a safety survey are presented. 
Several examples of data obtained during the summer of 1980 are  presented and 
future plans are  discussed. 
The research program uses an F-106B a i r c ra f t  
INTRODUCTION 
The NASA Langley Research Center i s  performing in-f l ight  direct-s t r ike 
lightning research u s i n g  a specially instrumented F-106B a i r c ra f t .  
of t h i s  research i s  to  ref ine the characterization of  the lightning-generated 
electromagnetic environment affecting a i r c ra f t .  The projected use of digi ta l  
avionic systems, along w i t h  composite a i r c ra f t  s t ructure ,  compounds lightning- 
related problems. Digital avionic systems are  potentially more susceptible to  
upset by e lec t r ica l  transients than previous generation systems and the compos- 
i t e  s t ructure  may not provide electr ical  shielding equivalent t o  t h a t  provided 
by metal a i r c r a f t  i n  the pas t .  Future design processes will thus require 
lightning-protection assessment techniques for d ig i ta l  avionic systems operat- 
ing i n  electromagnetically nonoptimum structures.  
of potential assessment techniques (which may range from purely analytical ,  
through simulation, t o  actual hardware t e s t s )  i s  a refined definit ion of the 
lightning electromagnetic hazard.  Recent discussion and review ( r e f .  1) 
supplemented by ground-based measurements ( r e f .  2 )  has indicated that  the 
r i s e  times of lightning electromagnetics are around one order of magnitude 
fas te r  than those used i n  current lightning-protection c r i t e r i a .  
The intent  
A necessary requirement 
The  in ten t ,  rationale,  and design goals of the instrumentation system 
developed a t  NASA Langley Research Center t o  a i d  in-f l ight  lightning-hazard 
characterization are  described i n  reference 3. The present report contains 
a review of the actual instrumentation system implementation, presents resul ts  
recorded by the research instrumentation during simulated lightning tes t ing,  
which was par t  of a safety survey of the a i r c r a f t ,  and presents and compares 
the i n i t i a l  direct-s t r ike lightning data w i t h  the simulation data. 
3 59 
https://ntrs.nasa.gov/search.jsp?R=19810010528 2020-03-21T13:39:04+00:00Z
INSTRUMENTATION SYSTEM OVERVIEW 
The instrumentation concept shown i n  f igure 1 consists of a number of 
electromagnetic sensors which measure the electromagnetic f ie lds  d u r i n g  the 
lightning process. 
instrumentation enclosure. A photograph of the instrumentation system with 
i t s  cover removed i s  shown i n  figure 2. 
bay of the F-106B. 
The data are  then recorded i n  a shielded, isolated 
The system i s  mounted i n  the missile 
Specially expanded Biomation transient waveform recorders ( r e f .  4)  , which 
operate a t  a 10-nanosecond sample interval and 6 - b i t  resolution, provide a 
unique capabili ty for  recording 1 i g h t n i n g  electromagnetic transients.  The 
basic Biomation Model 6500 recorder memory was increased by over two orders 
of magnitude t o  allow a s ignif icant  d a t a  window recording of 1.3 milliseconds 
of d a t a  a t  10-nanoseconds resolution. Photographs of the expanded Biomation 
f r o n t  panel and the memory expansion are shown i n  figures 3 and 4. 
acquisition of s t r i k e  d a t a  by the transient waveform recorder, the data are  
automatically transferred t o  the data formatter and thence t o  the instrumen- 
t a t i o n  recorder for  permanent storage. The transient recorder i s  then auto- 
matically reset  t o  acquire d a t a  from the next s t r ike .  
Upon 
The wide-band RCA video recorder has 6-MHz bandwidth and i s  capable of 
This continuous recorder is  used t o  recording continuously for 24 minutes. 
provide information on the overall lightning scenario. 
The sensors used in the lightning instrumentation system are derived 
The sensors measure the rate  of change of s t r ike  current t o  the 
from designs developed for  nuclear electromagnetic pulse measurements. (See 
ref. 5 . )  
noseboom (I-Dot) along w i t h  the ra te  of change of e l ec t r i c  and magnetic flux 
density (D-Dot, 6-Dot) a t  a number o f  locations as shown i n  f igure 5. The 
sensor response to  rates of change of  the lightning electromagnetic character- 
i s t i c s  (as  opposed t o  the current and  f i e lds ,  direct ly)  accentuates the 
recording of the higher frequency components of the lightning process. 
the magnitudes of induced voltages (and currents) are proportional t o  ra tes  
of change of the lightning electromagnetic character is t ics ,  enhanced definit ion 
of the more interesting (from an induced-effects viewpoint) p o r t i o n  of  the 
spectrum i s  obtained. The sensor sensi t ivi ty  is  calculated based on sensor 
geometry ( r e f .  6 )  and then checked using a parallel-plate transmission-line 
calibrator.  Figures 6 ,  7 ,  and 8 are  photographs of the I-Dot, B-Dot, and 
D-Dot sensors, respectively; figure 9 is a photograph of the f la t -p la te  
Since 
- transmission-line cal ibrator .  
Power system isolation f o r  the instrumentation i s  obtained using a 
generator se t .  A 3-phaseY 208-V, 400-Hz, 13-kVA e lec t r i c  motor externa 
the enclosure drives a nonconducting f lexible  coupler t o  a 12.5-kVA, 
120/208-V, 3-phase, 400-Hz AC generator located within the enclosure t o  
the system. 
The sensor-recorder configuration 
is shown i n  table I ,  which also summar 
holds, and ful l -scale  values. 
360 
used i n  the i n i t i a l  f l i gh t  conf 
zes the data channel bandwidths 
motor- 
t o  
power 
guration 
thres- 
SIMULATED LIGHTNING GROUND TEST 
A simulated lightning safety survey t e s t  was performed on the a i r c ra f t  to  
assess potential problems concerning a i r c ra f t  systems safety. These t e s t s  
were conducted by L i g h t n i n g  Technologies, Inc., and no safety hazards were 
disclosed by the tests. The safety survey t e s t s  a lso provided an opportunity 
for  "end-to-end" instrumentation system f ide l i t y  and noise immunity t e s t s .  
The t e s t s  were performed w i t h  the a i r c r a f t  engine r u n n i n g  and a l l  f l i gh t  
systems operating on a i r c r a f t  power. The instrumentation system measured and 
recorded the f ie lds  and currents on the a i r c ra f t  i n  response t o  a current 
pulse o f  known amplitude and  waveform (as  determined from an external current 
transformer measurement) which was generated with a high-vol tage capacitor 
discharge apparatus attached t o  the noseboom. The current exited from the 
a i r c ra f t  t a i l  and was returned t o  the generator using symmetrical return wires 
as  shown i n  the t e s t  setup photograph of figure 10. 
instrument system sens i t iv i ty  was increased by one order of magnitude over 
the i n i t i a l  f l i gh t  configuration to  accommodate the relat ively low capabili ty 
of  the t e s t  generator compared w i t h  the nominal lightning character is t ics  
assumed for data channel scaling. I n  addition, the channel assignments were 
altered as  shown i n  table I1 to  u t i l i z e  the ihcreased time resolution of  the 
transient recorder for monitoring the I-Dot sensor. 
During these t e s t s ,  the 
Typical responses of the I-Dot, D-Dot forward, and B-Dot longitudinal 
sensors are shown i n  figures 11 t o  14. Figure 11 shows I-Dot sensor response 
t o  an i n p u t  current t o  the noseboom. 
current osci l la t ing a t  86 000 Hz w i t h  a peak amplitude of 10 500 amperes. The 
I-Dot measurement agrees w i t h  the ra te  of change of i n p u t  current w i t h i n  a b o u t  
10 percent. 
current increasing o u t  of the noseboom, as was the case for  this t e s t .  
Figures 12  and 13 show i n  greater detail  the response o f  the I-Dot and D-Dot 
forward sensors t o  the f i rs t  cycle of i n p u t  current. 
The input was a damped sinusoidal 
Polarity of the I-Dot measurement i s  negative for  conventional 
Figure 14 shows the response of the B-Dot sensor t o  a damped sinusoidal 
i n p u t  current t o  the a i r c r a f t  noseboom. 
current osci l la t ing a t  86 000 Hz w i t h  a peak amplitude of 14 250 amperes. 
The magnitude of the B-Dot measurement i s  about 35 percent of the amplitude 
calculated for  this input current t o  a simple cylinder with a radius of 1 
meter (approximate fuselage radius). 
the magnetic f i e ld  which would be obtained from a simple cylinder. 
re la t ive position of the return wires causes intensification of current i n  
the wing, thereby reducing the f i e ld  a t  the location of the B-Dot sensor. 
The input was a damped sinusoidal 
The presence of the a i r c r a f t  wing a l t e r s  
The 
During subsequent t e s t s ,  sensor cables were terminated i n  dummy 50-ohm 
loads, i n  l ieu of the sensors, t o  investigate the noise immunity o f  the 
instrumentation system. The system d i d  n o t  respond d u r i n g  these t e s t s ,  which 
indicates that  the noise level would be a t  l eas t  one order of magnitude below 
the f l i g h t  configuration threshold. 
361 
LIGHTNING DATA 
The a i r c ra f t  sustained three s t r ikes  while i n  the vicini ty  of the National 
Severe Storms Laboratory, Norman, Oklahoma, on June 17, 1980. 
occurred w i t h  the a i r c ra f t  a t  an a l t i tude  of 4800 meters a t  a speed of 
300 knots; the approximate freezing level was a t  the a i r c ra f t  operating 
al t i tude.  
these s t r ikes  were not particularly energetic i n  t ha t  the magnetic character- 
i s t i c s  (I-Dot ,  B-Dot) d i d  not exceed system thresholds, and  only information 
from the forward D-Dot sensor was recorded. 
s t r ike  d a t a  records w i t h  the largest  ra tes  of change of e l ec t r i c  flux density 
are shown i n  figures 15 and 16. 
The s t r ikes  
The detailed waveforms of these s t r ikes  were reported in reference 7; 
The  portions of the preceding 
The a i r c ra f t  was struck a total  of ten times dur ing  the 1980 campaign 
w i t h  f ive of these occurring i n  r a p i d  succession d u r i n g  one f l i g h t  on 
September 3, 1980. Data processing, interpretation and analysis of the 
data are  continuing; the following observations a re  offered a t  this p o i n t :  
(1) the u t i l i t y  of the derivative (D-Dot) sensor is clearly demonstrated by 
examining the amplitude resolution of the "faster"  changing port ons of 
figures 15 and 16 during the f i r s t  microsecond of the event; and ( 2 )  the d a t a  
indicate s ignif icant  changes i n  the s t r ike  e l ec t r i c  characterist  cs during 
submicrosecond intervals.  
FUTURE PLANS 
The capability of the recording system i s  being significantly increased. 
A new transient  recorder w i t h  12  data channels of similar characterist ics t o  
the expanded Biomation t ransient  recorder i s  being o b t a i n e d  as i s  an ana log  
video recorder w i t h  a 15-MHz bandwidth. This increased capability will allow 
a much more complete characterization o f  the lightning-generated electro- 
magnetic environment 
Upon acquisition of a s t a t i s t i c a l l y  s ignif icant  data base for  s t r ike  
characterization (which i s  not near a t  hand), considerable e f fo r t  will be 
required t o  assess the significance and interpret  the data as concerns future 
ai rcra f t des i gns . 
362 
REFERENCES 
1. C l i f f o r d ,  Don W . ;  Kr ider ,  E .  P h i l i p ;  and Uman, Martin A . :  A Case f o r  
Submicrosecond Rise-Time Lightning Current Pulses f o r  Use i n  A i r c r a f t  
Induced Coupling S tud ie s ,  1979 IEEE I n t e r n a t i o n a l  Symposium on 
Electromagnet ic  Compat ib i l i ty .  San Diego, C a l i f . ,  Oct. 9-11, 1979, 
pp. 143-149. 
2. Weidman, C. D . ;  and Kr ider ,  E .  P . :  Submicrosecond Risetimes i n  
Lightning Radiat ion F ie lds .  Lightning Technology, NASA CP-2128, 
1980, pp. 29-38. 
3. Pi t ts ,  F. L . ,  e t  a l . :  In -F l igh t  Lightning C h a r a c t e r i s t i c s  Measurement 
System. Federal  Aviat ion Adminis t ra t ion - Flo r ida  I n s t i t u t e  of  
Technology Workshop on Grounding and L i g h t n i n g  Technology, 
FAA-RD-79-6, Mar. 1979, pp. 105-111. 
4. Thomas, Robert M. , J r .  : Expanded In t e r l eaved  S o l i d - s t a t e  Memory f o r  a 
Wide Bandwidth Trans i en t  Waveform Recorder. Lightning Technology, 
NASA CP-2128, 1980, pp.  119-129. 
5. Baum, C. E . ,  e t  a l . :  Sensors  f o r  Electromagnet ic  Pulse Measurements 
Both I n s i d e  and Away from Nuclear Source Regions. IEEE Trans.  
Electromagn. Compat., vol .  EMC-20, no. 1, Feb. 1978. 
Sensors  f o r  A i r c r a f t  L i g h t n i n g  Research. 
6. T r o s t ,  Thomas F . ;  and Zaepfel ,  Klaus P . :  Broadband Electromagnet ic  
L i g h t n i n g  Technology, 
NASA CP-2128, 1980, pp. 131-152. 
7. Pitts, Felix L . ;  and Thomas, Mitchel E.: I n i t i a l  D i rec t  S t r i k e  
L i g h t n i n g  Data. NASA TM-81867, 1980. 
363 
TABLE I .  - INITIAL FLIGHT CONFIGURATION 
Sensor  
6 Forward 
b Longitudinal 
6 Right Wing 
I 
Recorder 
T rans i en t  
Transient 
Wi de-band 
Analog 
Wi de-band 
Analog 
Sample Rate 
(Bandwidth) 
100 MHz 
(50* MHz) 
100 MHz 
(50* MHz) 
( 6  MHz) 
( 6  MHz) 
P-P 
F u l l  Sca le  
k24.5 A/m2 
+8695 T/s 
128 A/m2 
54.7 x lo1' A/s 
*Four-pole l i n e a r  phase low pass  f i l t e r  3 db p o i n t  a t  50 MHz. 
TABLE 11. - GROUND TEST CONFIGURATION 
Thres hol d 
t.4.9 A/m2 
+1739 T/s 
12.8 A/m2 
24.7 x 109 AIS 
Sensor  
b Forward 
6 Longitudinal 
b Tai l  
I 
Recorder 
T rans i en t  
W i de- band 
Analog 
Wi de-band 
Anal og 
Transient 
Sample Rate 
( Bandw i d t h ) 
100 MHz 
(6  MHz) 
( 6  MHz) 
100 MHz 
(50" MHz) 
(50" MHZ) 
P-P 
F u l l  Sca le  
k2.45 A/m2 
+870 T/s 
t2 .45  A/m2 
t 4 .7  x 109 A/S 
Thresh01 d 
t0 .49  A/m2 
+87 T/s 
t .245  A/m2 
*Four-pole l i n e a r  phase low pass f i l t e r  3 db p o i n t  a t  50 MHz. 
364 
Figure 1.- Instrumentation system concept. 
36 5 
Figure 3.- Expanded waveform recorder. 
Figure 4.-  Expanded memory. 
366 
Figure 5.- Sensor locations. 
367 
Figure 7 .- B-Dot  sensor.  
Figure 8 . -  D-Dot sensor.  
368 
Figure 9.- Flat-plate transmission-line calibrator. 
369 
x 103 
t 
-6 ~ I I I I I I I I I ~ I I I I I I I I I ~ I I I I I I I I I ~ I I I I I I I I I ~ I I I I I I I I I ~ I I I I I I I I I  
550 575 600 625 650 675 700 
MICROSECONDS 
Figure 1 1 . -  I-Dot sensor response. 
Figure 12 . -  I-Dot sensor response, expanded time scale. 
370 
I I I I I I I I f I I I I I I I I I I I l I I I I I I I I I I I I I I I I I I I~ I I I I I I I l1 l I I I I11 l1 I I I l I l I I I I l l  
653 65'1 655 656 657 658 659 660 
MICROSECONDS 
Figure 1 3 . -  D-Dot sensor response. 
435 
V 
W 
v) -. 
I- 
-1 
2 0  
Q -2
I- 
W 
;iE 
0 -J
I- 
O 
Ca 
- 
Lo 
-435 
I I I I I I I I 1 
MICROSECONDS 
0 10 20 30 40 50 60 70 80 
ot sensor response. 
3 77 
583 58Y 585 586 587 588 583 
MICROSECONDS 
Figure 15.- Direct-strike D-Dot record A. 
-15 
.? 
372 


In-flight direct-strike lightning research

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