analysis of direct effects of lightning on composite

7/31/2019 Analysis of Direct Effects of Lightning on Composite

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IX International Symposium on

Lightning Protection

26th-30th November 2007 Foz do Iguau, Brazil

Abstract

The increasing aviation world market demand for more

economical aircraft has motivated, since the 50s, a search

for lighter and with lower operational cost materials. In this

context, the development of aircraft parts made of composite

materials is mandatory for aircraft manufacturers. Hence,

the overall objective of this work is to analyse the impact of a

composite structure from the lightning effects point of view

and its main consequences, focusing on some possible design

considerations of carbon fiber composite wings.

The basic effects of lightning mechanisms will be explained

as the chief electrical characteristics of carbon fibercomposite materials. Furthermore, a few possible structural

configurations of a carbon fiber wing will be analysed with

the aid of a computational tool.

Thereby, different current distributions, based on different

structural approaches, such as the materials of ribs, spars

and possible alternative current paths (trailing and leading

edges) will be analysed and compared, provided the lower

power dissipated into carbon fiber structure, the lower is the

chance of arching inside of the tank, all these points have a

relevant role in this context.

The results are quite conclusive, showing that some

configurations decrease the amount of current which flows

through the carbon fiber structure considerably and othersprevent high surface current levels from occurring, which

may reduce the chance of arching in metal carbon joints,

mostly along bolts lines.

1 INTRODUCTION

Structure weight and the use of light materials have

always been an important issue as far as commercial or

military aircraft are concerned. When a modern transport

aircraft takes off, only about 20% of its total weight is

payload. The remaining 80%, roughly half is aircraft

empty weight and the other half, fuel. Hence, any saving

in structural weight can lead to a corresponding increasein payload and a decrease in power requirements, which

means a consequent reduction in fuel consumption and

operational costs.

Besides the weight reduction effort, the selection of

composite materials will have significant impacts on the

operational and final aircraft costs, since composite

materials are more inexpensive and have easier

manufacturing and maintenance processes.

Despite all advantages, numberless challenges come out

when the main parts of aircraft structure are made of CFC.

Moreover, as far as a CFC wing is concerned, one of theultimate issues is how to design lightning protections to

avoid sparks inside of the fuel tank. In this context, the

analysis of lightning current distribution is vital, since it is

known either additional protections can be used in order

to mitigate the risk of spark or different design approaches

can be taken into account.

The use of a computational tool to work out which design

better meets the requirements of a project is crucial. In the

following study, the chosen toll was Microwave Studio

[1], owing to its accuracy and friendly interface.

Additional information concerning the numerical methods

used and details about a specific simulation will be

omitted, but can be found in [4].

2 LIGHTNING BASIC MECHANISMS

According to [2], idealized lightning waveforms can be

depicted as below:

ANALYSIS OF DIRECT EFFECTS OF LIGHTNING ON COMPOSITE

STRUCTURES OF AIRCRAFT

L F. Nunes de Souza and H. Librantz J. Amorim and G. AdaboEmpresa Brasileira de Aeronutica, AvenidaBrigadeiro Faria Lima 2170, 12227-901 So Josdos Campos, SP Brasil

Instituto Tecnolgico de Aeronutica, Comando-Geral de Tecnologia Aeroespacial, Pa MarechalEduardo Gomes 50, 12228-900, So Jos dosCampos, SP, Brazil

[email protected] ;[email protected]

[email protected], [email protected]

Main authors address: [email protected]


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A whole description of the waveforms and its correlated

effects goes beyond the scope of this text (see [4]);

however, since we are dealing with current transfer and

possible arching related to that, it is important to noticethat the main concern is the waveforms A and D, which

contain the vast majority of the energy of a lightning

strike. Hence, the focus of this study will be only on

waveform A; nevertheless, owing to its similarity, it could

be extended to waveform D.

3 REDISTRIBUTION EFFECT

Redistribution effect is a process in which the division of

the current changes from an initial state governed mostly

by inductive effects to one governed by resistive effects.

A simple example of redistribution can be seen in the

structure below.

Fig. 1 CFC flat panel and an aluminium tube

Fig. 2 Front view

This structure is didactical, provided there is no current

transfer along both elements (tube and blade) and

consequently they could be considered as lumped

elements, as depicted below:

Fig 3 Blade Electrical Equivalent Circuit

Both simulations agree and shed some light on the

phenomenon: During the very first microseconds of

waveform A the inductive effect is hefty and as a

consequence the vast majority of the current flows

through the CFC part of the structure (blade), however, as

time passes the resistive effect becomes more prominent

and more and more current flows through the metal part of

the structure (tube)

The pictures below show the redistribution effect,

according to the electrical model and a 3D model

simulated by MWS.

Fig 4 Electrical Circuit Simulation


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0 10 20 30 40 50 60 70

Time(us)

Ampere

s(A)

Carbon Current

Metal Current

Waveform A

Fig 5 3D Simulation, using MWS

Both pictures depict perfectly the idea of redistribution,

showing how it might be strong in structures with metal

and CFC, nonetheless, more complex structures, as a wingbox for instance, have a much more complicated

interaction between the two materials, making unfeasible

any electrical representation. As can be noted in both

pictures, the transition between the resistive and inductive

states occurs around 45s. From that on the metallic

structure will carry the majority of the current.

Furthermore, since waveform A lasts 500s, the metallic

part of the structure carries the major part of waveform A

energy.

4 WING BOX

A more actual example is shown below. A simple wingbox (3m long x 2.2m wide) has four CFC spars and two

CFC skin shells, with no metallic structure inside of it.

Fig 6 CFC Wing Box

The CFC chosen has a resistivity of 15.000 S/m. The

current was injected and drained along the spars. It is

interesting, because only due to geometric factors, the fast

components of the waveform have the tendency to flow

through the periphery of the box; however, the slow

components have the same behaviour in the inner and

outer parts of the box. Moreover, in this case the skin

shells carry far more energy than the spars, as expected.

0

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0 5 10 15 20 25 30 35 40

Time(us)

Amperes(A)

Waveform ASkin Shells

Side Spars

Central

Spars

Fig 7 Current Waveforms in a CFC Wing Box

In the next example, the metallic parts were simulated as

PEC and the CFC side spars were exchanged by metallic

side spars having 15% of the entire cross section area,

keeping the same dimensions of the previous ones. The

results are shown below:

Fig 8 CFC Wing Box with Metallic Spars

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Amperes(A)

WaveformSkin

Side Spars

Central Spars

CFC Parts

Fig 9 Current Waveforms in a CFC Wing Box with metal side

spars

MetalSideSpars


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In the figure above, it is important to notice that in spite of

the fact the peak of current flowing into the skin shells

reaches 140kA, its decay time (27s) is short whencompared to waveform A decay time (70s), consequently

the proportion of energy that this pulse carries is small.

On the other hand, the side spars are responsible for

carrying a good part of waveform A energy, having a

much longer pulse and carrying the vast majority of

waveform A energy.

As a last example, instead of metal spars four metal strips

were located along the central spars, on the top and

bottom of the box on the skin shells. The strips are 60mm

wide and 2mm thick and represent less than 1% of the

whole area of the cross section.

Fig 10 CFC Wing Box with Metallic Strips

0

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-20 0 20 40 60 80 100

Time(us)

Amperes(A)

Waveform A

Skin Shells

Side S ars

Central Spars

CFC Parts

Metal Strips

Fig 10 Current Waveforms in a CFC Wing Box with Metal

strips

Comparing figures 9 and 10, it is important to notice the

amount of current carried by the metallic structure.

Despite its higher value in figure 10, the total energy

contained in this pulse is just 15% higher than the one

contained in Fig 9, on the other hand the cross section

area of the metallic structure in figure 8 is fifteen timeshigher than the one in figure 10.

5 CONCLUSION

The results are quite conclusive, indicating that, as far as

CFC structures are concerned, an electrical analysis of the

design of the wing box is vital, since the usage of metal is

inevitable, this approach could lead to the choose of the

right design and consequently mitigate the risk of arching

inside of the fuel tank (the lower the energy dissipated

into the CFC structure, the lower the chance of arching

inside of the tank). Otherwise, a wrong design

consideration, from this point of view, could jeopardize

the certification process of an aircraft.

Furthermore, it is clear that in a development process the

different designs taken into account must be carefully

analyzed. Since the correct amount of metal and its

correct location, from the lightning point of view, are

crucial in order to increase the amount of energy

dissipated into the metallic parts of the structure and then

mitigate the risk of arching inside of the fuel tank.

6 REFERENCES

[1] MWS Web site www.cst.com[2] Plumer, Perala, Fisher: Lightning protection of aircraft:

Lightning, 2 edition 1999.

[3] Rupke, Ed Lightning Direct Effects Handbook.

[4] Krietenstein, B.; Schuhmann, R.; Thoma, P.; Weiland,

T.: The Perfect Boundary Approximation technique facing the

challenge of high precision field computation: Proc. of the XIX

International Linear AcceleratorConference (LINAC98),

Chicago, USA, pp. 860-862, 1998.[5]Aerospace recommended practice: ARP5412A Aircraft

Lightning

Metal Strips

analysis of direct effects of lightning on composite

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