Final Report Summary - ARROW (Aircraft lightning thtRreat Reduction thrOugh Wiring optimization)
Executive Summary:
New generation aircraft make large use of composite materials (such as carbon fiber) because of their advantage in terms of weight and structural strength.
However, composite materials are generally less conductive than metals that replaced; therefore they cannot assure the necessary electrical conductivity to work as current return pathway as it happened with aircraft metallic bodies.
Therefore, the potential damage of Lightning (in terms of direct and indirect effects) on modern aircraft is very high.
The metallic bodies of “standard” aircrafts are commonly used as conductive electrical pathways for the return of direct and alternating currents, faults currents, lightning currents and also other functions related to voltage differentials, electrostatic charge draining, electromagnetic shielding etc.
Such a procedure is not applicable on aircrafts made of composite materials because of their low conductivity. A dedicated conductive electrical pathway, named “Almost Equipotential Network (ALEEN)” or “Current Return Network (CRN)” has therefore to be integrated into the aircraft body. Such networks can be practically realized in several different ways, mainly exploiting both structural metallic parts of the aircraft (beams, seats rails, etc.) and also dedicated paths, but anyway they can never be an ideal ground and worse performance than those currently obtained on metal aircraft may be expected.
Accurate electrical/electromagnetic characterization of ALEEN structure is therefore important:
1. to be able to correctly design electrical systems such as EWIS, reducing risks and saving mass
2. to estimate how the ALEEN configuration works with respect to other required functions (e.g. faults currents, lightning currents, electromagnetic shielding, etc.)
3. to optimize the ALEEN configuration itself without needing expensive (and sometime practically unfeasible) repeated bread boarding.
The ARROW project aims at developing a numerical methodology and a CAE tool – named ALEEN-L – suited to model user defined cable-harness configurations installed aboard an aircraft made of composite and conductive materials, equipped with an Almost Equipotential Electrical Network.
Fig. 1 – ALEEN-L Tool
In particular the Tool was suited to evaluate the Voc(t) and Isc(t) waveforms (Voc=Open Circuit voltage; Isc=Short Circuit current) at equipment loads, induced by lightning strikes on the aircraft, in order to allow the verification of their compatibility with the design and qualification values.
Simulation methods (based on S-PEEC: Surface-Partial Element Equivalent Circuit formulation for 3d analysis and CRIPTE for cable analysis) having special “low-frequency stability” and “high-fidelity modelling” features have been customized and validated for this kind of applications.
Furthermore an advanced Field-to-Transmission Line coupling procedure was developed to take into account the effects of common mode impedance for low conductivity current return paths.
Project Context and Objectives:
Context
The “System for Green Operations” (SGO) is one of the six Integrated Technology Demonstrators of the Clean Sky program. The main driver of SGO is represented by an eco-compatible design, aiming at reducing CO2 production and at optimizing aircraft energy management – see Fig. 2.
Fig. 2 – SGO ITD main drivers and goals
Work package 2.3.3 “Optimized Electrical Wiring Interconnection System for More Electric Aircraft & More Composite Aircraft” will address very ambitious challenges for Electrical Wiring Interconnection System (EWIS) for future Aircraft.
Project concept and objectives
The metallic bodies of “standard” aircrafts are commonly used as conductive electrical pathways for the return of direct and alternating currents, faults currents, lightning currents and also other functions related to voltage differentials, electrostatic charge draining, electromagnetic shielding etc. Such a procedure is not applicable in composite aircrafts because their bodies don’t assure the necessary conductivity, and a dedicated conductive electrical pathway (currently named “current return network”, hereafter referred as ALEEN, ALmost Equipotential Electrical Network) has to be integrated into the aircraft body.
Such networks can be practically realized in several different ways and can therefore have several features, e.g.:
a. It can be a network of interconnected longitudinal and lateral paths extending for the whole aircraft fuselage. Parallel and non-parallel paths can occur.
b. The paths may be made of wires, strips or also extruded metal sheets. Copper, aluminium and other types of conductive metals can be used.
c. The paths shall be fastened to the structure of the aircraft.
d. In case part of the aircraft structure is made of metal (e.g. structural beams, cages, rails to fix passengers seats, or also a whole sub-part of the aircraft like the nose) the current return network may be intentionally connected to such structures, which for this reason, become part of the return network.
e. The different parts of the network can be connected together by electrical joints (rivets, bolts and other connecting strategies).
Apart the implementation details it is obvious that not only the network can never be an ideal ground but also that worse performance than those currently obtained on metal aircraft may be expected. Therefore it is of interest to evaluate the effects the network has on connected electrical systems such as EWIS and more in general to estimate how it works with respect to each required function (e.g. return of direct and alternating currents, faults currents, lightning currents…).
In this frame, not only the compatibility of the critical/essential electrical and electronic systems with respect to indirect effects of lightning has to be assured , but also the aircraft-structure, the ALEEN, the cable-harness configuration and the cable-harness protection must be considered as a whole in order to optimize the overall wiring design and therefore to avoid over-dimensioning which could lead to undesired mass increase.
The ARROW project aims at developing a numerical methodology and a CAE tool (hereafter referred as “ALEEN-L Modelling Tool”) suited to model user defined cable-harness configurations installed aboard an aircraft made of composite and conductive materials, equipped with an Almost Equipotential Electrical Network.
In particular the ALEEN-L Tool has to be suited to evaluate the Voc(t) and Isc(t) waveforms (Voc=Open Circuit voltage; Isc=Short Circuit current) at equipment loads, induced by lightning strikes on the aircraft, in order to allow the verification of their compatibility with the design and qualification values.
The tool is able to:
a) provide fully 3D modelling capability (detailed shape of the aircraft/ALEEN parts, 3D shapes etc.).
b) provide cable harness modelling capability.
c) consider the composite parts of the aircraft, characterized by finite conductivity. In particular the Tool must be able to accurately model complex engineered materials (e.g. expanded foils on CFRP layers) in terms of impedance, skin and proximity effects. Characterization of the materials in terms of measured Surface Impedance must be allowed.
d) accept measured data as partial characterization of ALEEN and cable-bundle sub-parts (because some parts are impossible or too costly to model);
e) Interface with electrical DB for cable data import
f) manage the effect of the electrical connections among parts of the equipotential network (ALEEN) and of the aircraft.
g) accurately manage small elements in the frame of big models (for example the whole aircraft or the main part of it). Some other requirements about the modelling accuracy shall be derived considering the scope of ALEEN-L (e.g. maximum acceptable uncertainty about Voc and Isc).
h) accurately work in the very low frequency region (i.e. in the quasi-static region), where the well-known “low-frequency breakdown” makes a large number of numerical solvers ill-conditioned.
i) Solve the numerical model in the frequency domain
j) Advanced output data post processing in frequency domain and time domain.
Project Results:
Main Scientific and Technical Results
The electromagnetic modelling and simulation of ALEEN structures and harnesses installed on aircrafts can be considered a very challenging task. The electrical characterization of ALEEN, and the simulation of indirect effects of lightning, requires a great accuracy to correctly model all the involved phenomena (ohmic losses, skin effect, reactive and radiative coupling). Impedances and mutual coupling are strongly variable in the frequency range of interest (from DC to some MHz), and we have also to deal with large structures containing a lot of small parts (strong multi-scaled problem). Each of these points makes the problem hard to solve from a computational point of view.
Even if it is theoretically possible to carry out an electromagnetic analysis on a model comprising all the structures and all the wires present in an aircraft, a hybrid field-to-wire coupling procedure can advantageously be employed in order to lighten the models and to make easier parametric analysis on the internal cabling. Such a hybrid procedure was developed and implemented in the ARROW project. It consists by solving the 3D EM model of the structural parts, through a full wave method without explicitly considering the cables. A Multiconductor Transmission Network analysis (MTLN) is then carried out on the harnesses, by considering a distributed excitation term given by the incident electric field produced by the structural parts onto the cable routes. In particular, in the ARROW project, the application of such a hybrid method was formulated, implemented and validated (with respect to measurements) specifically in the frequency band pertinent to lightning problems, also in case of non-fully low-impedance metallic structures, like composite fuselages and/or aircraft equipped with a current return network.
In the frequency range of interest, classical 3D full wave numerical methods of analysis usually suffer by the well-known low frequency breakdown phenomena and are not able to correctly model complex configurations. Geometrical simplifications are usually carried out by expert modelers in order to possibly reduce numerical problems. Apart the complex “over-work” which is required, it is well known that this is only applicable at the expense of the possibility of correctly evaluate the whole set of electrical/electromagnetic phenomena of interest in the whole frequency range. A surface formulation of the PEEC method (S-PEEC) was adopted in ARROW for the 3D full wave analysis of ALEEN structures. By using triangular patches it allows to manage a realistic model of the structure (high fidelity modelling), and, by separating electric current and charge unknowns, it allows to mitigate the low frequency breakdown problem. In the ARROW project a particular preconditioner was introduced to be used with an iterative solver, and the Adaptive Cross Approximation technique was employed to allow an efficient analysis of large structures.
Concerning the MTLN analysis, it was developed a technique to consider the impedance of current return network paths, and the common mode voltages on ALEEN structures (both evaluated by the 3D full wave electromagnetic solver). Also the interaction between the cables and the aircraft (i.e. the p.u.l. [C], [L] parameters) is not evaluated anymore by referring to a return path directly located under the cable. The whole hybrid procedure was validated by means of an extended measurement campaign.
From the technological point of view, the whole modelling Procedure has been integrated in a software environment devoted to support the user in all the phases of the lightning threat analysis, from the input data import to the advanced output data post processing. The main functions implemented by the integrated tool are briefly summarized in the following:
- Import of platform geometry and material properties from CAD and the cable Harness representation from Harness Design Electric CAD;
- Guided definition of the Lightning analysis;
- Three-dimensional frequency domain Method of Moments full wave electromagnetic solver (MultiResolution and S-PEEC) reliable from the low frequency range;
- MultiLine Transmission Network solver able to consider a not perfect ground by means of a common mode impedance and/or a common mode voltage;
- Use of FFT/IFFT procedures to evaluate the time domain responses.
It is worth noted that by using the field-to-wire coupling approach it is possible to carry out sensitivity analysis by varying cable harness characteristics (i.e. cable path, cross section, material, loads) with just one full wave computation. A further automation of these procedures is recommended.
Potential Impact:
Impacts of ARROW project are expected both in the more general frames of “airplane security improvement” and “environmental aspect” and with regard to important technical aspects.
The impacts on “environmental aspects” and “airplane security” are related to the fact that composite aircrafts are considered. As known, the application of composite materials allows several benefits with respect to the usage of “standard” metallic structures, mainly in terms of maintenance cost and fuel consumption reduction. On the contrary composite materials have worse electrical and electromagnetic performance because of their lower conductivity (which means higher induced voltages, lower shielding effectiveness etc.), so augmenting the vulnerability of the electrical/electronic systems to both internal (intra and inter systems) and external threats (lightning, HIRF etc.).
ALEEN concept has been identified as an innovative mean to control and reduce such kind of problems, but it is worth noting that also innovative tools are needed to support aircraft manufacturers in the analysis, design and verification of such a complex 3D element, which moreover strongly interacts with the whole aircraft.
ARROW project will provide such a support, in form of a validated CAE industrial tool properly interfaced to CAD design procedures. More in particular the following advantages are expected through ARROW:
• higher confidence in the design solution (risks reduction) of More Electrical Aircraft combined with full composite aircrafts;
• reduced design cost and “time to market”, allowing for “virtual prototyping of the whole aircraft configuration and therefore allowing reduced bread-boarding activities;
• reduced cost related to certification, allowing a minor number of certification tests by reducing the number of configurations to be tested at aircraft level and helping a better design of the test-plan.
The technical aspects involved in ARROW will impact the European competitiveness not only at aircraft industry level, but also the:
• ships and naval industry (: “below deck” analysis and design)
critical infrastructures protection against pulse and CW threats (for example: Intentional Electro-Magnetic Interference, IEMI)
List of Websites:
http://arrowproject.univaq.it/
Ingegneria Dei Sistemi S.pA.
Via Enrica Calabresi, 24
56121 Pisa – Italy
m.bandinelli@idscorporation.com
a.bonsignore@idscorporation.com
mori@idscorporation.com
g.sammarone@idscorporation.com
New generation aircraft make large use of composite materials (such as carbon fiber) because of their advantage in terms of weight and structural strength.
However, composite materials are generally less conductive than metals that replaced; therefore they cannot assure the necessary electrical conductivity to work as current return pathway as it happened with aircraft metallic bodies.
Therefore, the potential damage of Lightning (in terms of direct and indirect effects) on modern aircraft is very high.
The metallic bodies of “standard” aircrafts are commonly used as conductive electrical pathways for the return of direct and alternating currents, faults currents, lightning currents and also other functions related to voltage differentials, electrostatic charge draining, electromagnetic shielding etc.
Such a procedure is not applicable on aircrafts made of composite materials because of their low conductivity. A dedicated conductive electrical pathway, named “Almost Equipotential Network (ALEEN)” or “Current Return Network (CRN)” has therefore to be integrated into the aircraft body. Such networks can be practically realized in several different ways, mainly exploiting both structural metallic parts of the aircraft (beams, seats rails, etc.) and also dedicated paths, but anyway they can never be an ideal ground and worse performance than those currently obtained on metal aircraft may be expected.
Accurate electrical/electromagnetic characterization of ALEEN structure is therefore important:
1. to be able to correctly design electrical systems such as EWIS, reducing risks and saving mass
2. to estimate how the ALEEN configuration works with respect to other required functions (e.g. faults currents, lightning currents, electromagnetic shielding, etc.)
3. to optimize the ALEEN configuration itself without needing expensive (and sometime practically unfeasible) repeated bread boarding.
The ARROW project aims at developing a numerical methodology and a CAE tool – named ALEEN-L – suited to model user defined cable-harness configurations installed aboard an aircraft made of composite and conductive materials, equipped with an Almost Equipotential Electrical Network.
Fig. 1 – ALEEN-L Tool
In particular the Tool was suited to evaluate the Voc(t) and Isc(t) waveforms (Voc=Open Circuit voltage; Isc=Short Circuit current) at equipment loads, induced by lightning strikes on the aircraft, in order to allow the verification of their compatibility with the design and qualification values.
Simulation methods (based on S-PEEC: Surface-Partial Element Equivalent Circuit formulation for 3d analysis and CRIPTE for cable analysis) having special “low-frequency stability” and “high-fidelity modelling” features have been customized and validated for this kind of applications.
Furthermore an advanced Field-to-Transmission Line coupling procedure was developed to take into account the effects of common mode impedance for low conductivity current return paths.
Project Context and Objectives:
Context
The “System for Green Operations” (SGO) is one of the six Integrated Technology Demonstrators of the Clean Sky program. The main driver of SGO is represented by an eco-compatible design, aiming at reducing CO2 production and at optimizing aircraft energy management – see Fig. 2.
Fig. 2 – SGO ITD main drivers and goals
Work package 2.3.3 “Optimized Electrical Wiring Interconnection System for More Electric Aircraft & More Composite Aircraft” will address very ambitious challenges for Electrical Wiring Interconnection System (EWIS) for future Aircraft.
Project concept and objectives
The metallic bodies of “standard” aircrafts are commonly used as conductive electrical pathways for the return of direct and alternating currents, faults currents, lightning currents and also other functions related to voltage differentials, electrostatic charge draining, electromagnetic shielding etc. Such a procedure is not applicable in composite aircrafts because their bodies don’t assure the necessary conductivity, and a dedicated conductive electrical pathway (currently named “current return network”, hereafter referred as ALEEN, ALmost Equipotential Electrical Network) has to be integrated into the aircraft body.
Such networks can be practically realized in several different ways and can therefore have several features, e.g.:
a. It can be a network of interconnected longitudinal and lateral paths extending for the whole aircraft fuselage. Parallel and non-parallel paths can occur.
b. The paths may be made of wires, strips or also extruded metal sheets. Copper, aluminium and other types of conductive metals can be used.
c. The paths shall be fastened to the structure of the aircraft.
d. In case part of the aircraft structure is made of metal (e.g. structural beams, cages, rails to fix passengers seats, or also a whole sub-part of the aircraft like the nose) the current return network may be intentionally connected to such structures, which for this reason, become part of the return network.
e. The different parts of the network can be connected together by electrical joints (rivets, bolts and other connecting strategies).
Apart the implementation details it is obvious that not only the network can never be an ideal ground but also that worse performance than those currently obtained on metal aircraft may be expected. Therefore it is of interest to evaluate the effects the network has on connected electrical systems such as EWIS and more in general to estimate how it works with respect to each required function (e.g. return of direct and alternating currents, faults currents, lightning currents…).
In this frame, not only the compatibility of the critical/essential electrical and electronic systems with respect to indirect effects of lightning has to be assured , but also the aircraft-structure, the ALEEN, the cable-harness configuration and the cable-harness protection must be considered as a whole in order to optimize the overall wiring design and therefore to avoid over-dimensioning which could lead to undesired mass increase.
The ARROW project aims at developing a numerical methodology and a CAE tool (hereafter referred as “ALEEN-L Modelling Tool”) suited to model user defined cable-harness configurations installed aboard an aircraft made of composite and conductive materials, equipped with an Almost Equipotential Electrical Network.
In particular the ALEEN-L Tool has to be suited to evaluate the Voc(t) and Isc(t) waveforms (Voc=Open Circuit voltage; Isc=Short Circuit current) at equipment loads, induced by lightning strikes on the aircraft, in order to allow the verification of their compatibility with the design and qualification values.
The tool is able to:
a) provide fully 3D modelling capability (detailed shape of the aircraft/ALEEN parts, 3D shapes etc.).
b) provide cable harness modelling capability.
c) consider the composite parts of the aircraft, characterized by finite conductivity. In particular the Tool must be able to accurately model complex engineered materials (e.g. expanded foils on CFRP layers) in terms of impedance, skin and proximity effects. Characterization of the materials in terms of measured Surface Impedance must be allowed.
d) accept measured data as partial characterization of ALEEN and cable-bundle sub-parts (because some parts are impossible or too costly to model);
e) Interface with electrical DB for cable data import
f) manage the effect of the electrical connections among parts of the equipotential network (ALEEN) and of the aircraft.
g) accurately manage small elements in the frame of big models (for example the whole aircraft or the main part of it). Some other requirements about the modelling accuracy shall be derived considering the scope of ALEEN-L (e.g. maximum acceptable uncertainty about Voc and Isc).
h) accurately work in the very low frequency region (i.e. in the quasi-static region), where the well-known “low-frequency breakdown” makes a large number of numerical solvers ill-conditioned.
i) Solve the numerical model in the frequency domain
j) Advanced output data post processing in frequency domain and time domain.
Project Results:
Main Scientific and Technical Results
The electromagnetic modelling and simulation of ALEEN structures and harnesses installed on aircrafts can be considered a very challenging task. The electrical characterization of ALEEN, and the simulation of indirect effects of lightning, requires a great accuracy to correctly model all the involved phenomena (ohmic losses, skin effect, reactive and radiative coupling). Impedances and mutual coupling are strongly variable in the frequency range of interest (from DC to some MHz), and we have also to deal with large structures containing a lot of small parts (strong multi-scaled problem). Each of these points makes the problem hard to solve from a computational point of view.
Even if it is theoretically possible to carry out an electromagnetic analysis on a model comprising all the structures and all the wires present in an aircraft, a hybrid field-to-wire coupling procedure can advantageously be employed in order to lighten the models and to make easier parametric analysis on the internal cabling. Such a hybrid procedure was developed and implemented in the ARROW project. It consists by solving the 3D EM model of the structural parts, through a full wave method without explicitly considering the cables. A Multiconductor Transmission Network analysis (MTLN) is then carried out on the harnesses, by considering a distributed excitation term given by the incident electric field produced by the structural parts onto the cable routes. In particular, in the ARROW project, the application of such a hybrid method was formulated, implemented and validated (with respect to measurements) specifically in the frequency band pertinent to lightning problems, also in case of non-fully low-impedance metallic structures, like composite fuselages and/or aircraft equipped with a current return network.
In the frequency range of interest, classical 3D full wave numerical methods of analysis usually suffer by the well-known low frequency breakdown phenomena and are not able to correctly model complex configurations. Geometrical simplifications are usually carried out by expert modelers in order to possibly reduce numerical problems. Apart the complex “over-work” which is required, it is well known that this is only applicable at the expense of the possibility of correctly evaluate the whole set of electrical/electromagnetic phenomena of interest in the whole frequency range. A surface formulation of the PEEC method (S-PEEC) was adopted in ARROW for the 3D full wave analysis of ALEEN structures. By using triangular patches it allows to manage a realistic model of the structure (high fidelity modelling), and, by separating electric current and charge unknowns, it allows to mitigate the low frequency breakdown problem. In the ARROW project a particular preconditioner was introduced to be used with an iterative solver, and the Adaptive Cross Approximation technique was employed to allow an efficient analysis of large structures.
Concerning the MTLN analysis, it was developed a technique to consider the impedance of current return network paths, and the common mode voltages on ALEEN structures (both evaluated by the 3D full wave electromagnetic solver). Also the interaction between the cables and the aircraft (i.e. the p.u.l. [C], [L] parameters) is not evaluated anymore by referring to a return path directly located under the cable. The whole hybrid procedure was validated by means of an extended measurement campaign.
From the technological point of view, the whole modelling Procedure has been integrated in a software environment devoted to support the user in all the phases of the lightning threat analysis, from the input data import to the advanced output data post processing. The main functions implemented by the integrated tool are briefly summarized in the following:
- Import of platform geometry and material properties from CAD and the cable Harness representation from Harness Design Electric CAD;
- Guided definition of the Lightning analysis;
- Three-dimensional frequency domain Method of Moments full wave electromagnetic solver (MultiResolution and S-PEEC) reliable from the low frequency range;
- MultiLine Transmission Network solver able to consider a not perfect ground by means of a common mode impedance and/or a common mode voltage;
- Use of FFT/IFFT procedures to evaluate the time domain responses.
It is worth noted that by using the field-to-wire coupling approach it is possible to carry out sensitivity analysis by varying cable harness characteristics (i.e. cable path, cross section, material, loads) with just one full wave computation. A further automation of these procedures is recommended.
Potential Impact:
Impacts of ARROW project are expected both in the more general frames of “airplane security improvement” and “environmental aspect” and with regard to important technical aspects.
The impacts on “environmental aspects” and “airplane security” are related to the fact that composite aircrafts are considered. As known, the application of composite materials allows several benefits with respect to the usage of “standard” metallic structures, mainly in terms of maintenance cost and fuel consumption reduction. On the contrary composite materials have worse electrical and electromagnetic performance because of their lower conductivity (which means higher induced voltages, lower shielding effectiveness etc.), so augmenting the vulnerability of the electrical/electronic systems to both internal (intra and inter systems) and external threats (lightning, HIRF etc.).
ALEEN concept has been identified as an innovative mean to control and reduce such kind of problems, but it is worth noting that also innovative tools are needed to support aircraft manufacturers in the analysis, design and verification of such a complex 3D element, which moreover strongly interacts with the whole aircraft.
ARROW project will provide such a support, in form of a validated CAE industrial tool properly interfaced to CAD design procedures. More in particular the following advantages are expected through ARROW:
• higher confidence in the design solution (risks reduction) of More Electrical Aircraft combined with full composite aircrafts;
• reduced design cost and “time to market”, allowing for “virtual prototyping of the whole aircraft configuration and therefore allowing reduced bread-boarding activities;
• reduced cost related to certification, allowing a minor number of certification tests by reducing the number of configurations to be tested at aircraft level and helping a better design of the test-plan.
The technical aspects involved in ARROW will impact the European competitiveness not only at aircraft industry level, but also the:
• ships and naval industry (: “below deck” analysis and design)
critical infrastructures protection against pulse and CW threats (for example: Intentional Electro-Magnetic Interference, IEMI)
List of Websites:
http://arrowproject.univaq.it/
Ingegneria Dei Sistemi S.pA.
Via Enrica Calabresi, 24
56121 Pisa – Italy
m.bandinelli@idscorporation.com
a.bonsignore@idscorporation.com
mori@idscorporation.com
g.sammarone@idscorporation.com
