TU Darmstadt / ULB / TUbiblio

Model-based Optimization, Control and Assessment of Electric Aircraft Taxi Systems

Re, Fabrizio (2017):
Model-based Optimization, Control and Assessment of Electric Aircraft Taxi Systems.
Darmstadt, Technische Universität, [Online-Edition: http://tuprints.ulb.tu-darmstadt.de/6239],
[Ph.D. Thesis]

Abstract

Aircraft ground operations are one important source of emissions in airports as taxi is conventionally performed by exploiting the inefficient idle thrust of the main jet engines. On-board Electric Taxi Systems (ETS) have been proposed featuring electric motors fitted in the landing gears in order to perform ground movements electrically while the main engines are off. While benefits can be expected on the ground due to the use of the Auxiliary Power Unit (APU) as power source which is more efficient in the required power range, the new system brings additional weight to the aircraft, resulting in a lower efficiency in flight and possibly even worsening the overall fuel consumption in a whole gate-to-gate mission. However, trade-offs and concrete figures regarding the expected benefits are difficult to identify in the state of the art because assessing methods for the taxi phase are often too coarse and based on too generic data and assumptions such as Thrust Specific Fuel Consumption tables, constant thrust settings and estimated taxi times.

This thesis contributes to the state of the art by presenting an integrated, model-based methodology for the assessment of aircraft systems at aircraft level in the conceptual design phase and its application to ETS. The proposed model-based process is shown to be necessary for answering key questions regarding the design of innovative aircraft subsystems in general, for performing solid comparisons and for determining suitable trade-offs while keeping the aircraft type and the specificities of the observed missions into account.

A substantial methodological contribution in the framework of the proposed approach is given by the automatic generation of energetically optimal ground path following profiles for electric taxiing based on convex optimization. Because an optimal path following profile exists for each given system architecture and variant, a sound performance comparison of different system variants is only possible if each of them can be operated according to its own optimal profile. Convex optimization permits to find a global optimum for each given problem in short computational time thanks to dedicated solving toolboxes. Convex formulations of path following problems studied in robotics and vehicle dynamics were adapted to the aircraft taxi problem. Moreover, convex formulations of relevant constraints in this problem, such as time constraints on passing predefined waypoints, were determined. The result of the convex optimization is used as input in the simulation of the mission ground phases with the integrated aircraft model.

The proposed system design methodology based on integrated simulation was instrumental for the following findings in connection with ETS. Firstly, a small system — which is lighter, but also less powerful — does not necessarily result in a further improvement of the benefits compared to larger, heavier systems because ground performance would be affected negatively. Secondly, the physical (e.g. thermal) behavior of the system during a given mission is a key factor as it has an immediate impact on the associated benefit. The optimal system architecture specifically depends on the aircraft and the missions flown; both must be taken into account in the early design phase. Thirdly, the prevailing interest for the ETS technology may be an economic one rather than an environmental one, as electric taxi may be economically viable even in case of increased mission block fuel.

Item Type: Ph.D. Thesis
Erschienen: 2017
Creators: Re, Fabrizio
Title: Model-based Optimization, Control and Assessment of Electric Aircraft Taxi Systems
Language: English
Abstract:

Aircraft ground operations are one important source of emissions in airports as taxi is conventionally performed by exploiting the inefficient idle thrust of the main jet engines. On-board Electric Taxi Systems (ETS) have been proposed featuring electric motors fitted in the landing gears in order to perform ground movements electrically while the main engines are off. While benefits can be expected on the ground due to the use of the Auxiliary Power Unit (APU) as power source which is more efficient in the required power range, the new system brings additional weight to the aircraft, resulting in a lower efficiency in flight and possibly even worsening the overall fuel consumption in a whole gate-to-gate mission. However, trade-offs and concrete figures regarding the expected benefits are difficult to identify in the state of the art because assessing methods for the taxi phase are often too coarse and based on too generic data and assumptions such as Thrust Specific Fuel Consumption tables, constant thrust settings and estimated taxi times.

This thesis contributes to the state of the art by presenting an integrated, model-based methodology for the assessment of aircraft systems at aircraft level in the conceptual design phase and its application to ETS. The proposed model-based process is shown to be necessary for answering key questions regarding the design of innovative aircraft subsystems in general, for performing solid comparisons and for determining suitable trade-offs while keeping the aircraft type and the specificities of the observed missions into account.

A substantial methodological contribution in the framework of the proposed approach is given by the automatic generation of energetically optimal ground path following profiles for electric taxiing based on convex optimization. Because an optimal path following profile exists for each given system architecture and variant, a sound performance comparison of different system variants is only possible if each of them can be operated according to its own optimal profile. Convex optimization permits to find a global optimum for each given problem in short computational time thanks to dedicated solving toolboxes. Convex formulations of path following problems studied in robotics and vehicle dynamics were adapted to the aircraft taxi problem. Moreover, convex formulations of relevant constraints in this problem, such as time constraints on passing predefined waypoints, were determined. The result of the convex optimization is used as input in the simulation of the mission ground phases with the integrated aircraft model.

The proposed system design methodology based on integrated simulation was instrumental for the following findings in connection with ETS. Firstly, a small system — which is lighter, but also less powerful — does not necessarily result in a further improvement of the benefits compared to larger, heavier systems because ground performance would be affected negatively. Secondly, the physical (e.g. thermal) behavior of the system during a given mission is a key factor as it has an immediate impact on the associated benefit. The optimal system architecture specifically depends on the aircraft and the missions flown; both must be taken into account in the early design phase. Thirdly, the prevailing interest for the ETS technology may be an economic one rather than an environmental one, as electric taxi may be economically viable even in case of increased mission block fuel.

Place of Publication: Darmstadt
Divisions: 16 Department of Mechanical Engineering > Institute of Flight Systems and Automatic Control (FSR)
16 Department of Mechanical Engineering
Date Deposited: 02 Jul 2017 19:55
Official URL: http://tuprints.ulb.tu-darmstadt.de/6239
URN: urn:nbn:de:tuda-tuprints-62395
Referees: Klingauf, Prof. Dr. Uwe and Rinderknecht, Prof. Dr. Stephan
Refereed / Verteidigung / mdl. Prüfung: 2 May 2017
Alternative Abstract:
Alternative abstract Language
Der Bodenbetrieb der Flugzeuge ist der Hauptverursacher von Emissionen in Flughafengebieten. Hierzu tragen insbesondere die Rollbewegungen im Leerlaufschub bei, da die Triebwerke in diesem Arbeitspunkt eine niedrige Effizienz aufweisen. Mehrere Akteure haben unter Anderem den Einsatz elektrischer Fahrwerksantriebe vorgeschlagen, die ein triebwerkloses Rollen ermöglichen. Die Meisten dieser Konzepte sehen das Hilfstriebwerk als elektrischen Energieerzeuger vor. Da diese im benötigten Leistungsniveau deutlich effizienter arbeitet als die Haupttriebwerke, ist ein Vorteil bezüglich Treibstoffverbrauch und Emissionen in der Bodenphase zu erwarten. Nachteilig wirkt sich jedoch das Zusatzgewicht des elektrischen Fahrwerkssystems in den Flugphasen aus. Die Bilanz über die ganze Flugmission muss somit ermittelt werden. Bisher wurden solche Untersuchungen nur anhand gemittelter Parameter und vereinfachter Annahmen insbesondere bei der Betrachtung der Bodenphase durchgeführt, was eine zuverlässige Abschätzung der Einsparungen und deren Sensitivität auf systemparametrische und fahrdynamische Änderungen verhindert. Die vorliegende Arbeit stellt eine modellbasierte integrierte Methodik zur Bewertung elektrischer Fahrwerkssysteme in der frühen Entwurfsphase vor. Diese Methodik lässt sich zudem auf die Bewertung und Optimierung neuartiger Flugzeugsystemtechnologien verallgemeinern. Sie ermöglicht somit die Beantwortung zentraler Fragen bezüglich der Adoption neuer Technologien, den Vergleich unterschiedlicher Systemarchitekturen und die Ermittlung der zu erwartenden Vorteile unter Beachtung der spezifischen Flugzeug-, System- und Missionseigenschaften. Die wesentlichen Schritte der Methodik für die betrachteten Fahrwerkssysteme beinhalten die Feststellung qualitativer Anforderungen an die elektrischen Komponenten, die Erstellung parametrischer Flugzeug- und Antriebsmodelle, die dynamische Simulation ganzer Flugmissionen (Gate-to-Gate), und die Anwendung relevanter Metriken an die ausgewerteten Simulationsdaten zur ganzheitlichen Bewertung. Ein wesentlicher Beitrag zum Stand der Technik ist durch die Generierung optimaler Fahrprofile für eine vorgegebene, zu simulierende Rollstrecke anhand eines konvexen Optimierungsverfahrens gegeben. Dank dieses Verfahrens kann die wirtschaftlichste Fahrweise für jede betrachtete Systemvariante mit geringem Rechenaufwand berechnet und als Fahrvorgabe bei der jeweiligen dynamischen Rollsimulation verwendet werden, was einen fundierten Vergleich der Vorteile unterschiedlicher Systeme unter jeweils besten Betriebsbedingungen ermöglicht. Die Anwendung der vorgestellten Methodik hat folgende Erkenntnisse zum Thema „elektrische Fahrwerksantriebe“ geliefert: - Ist die Reduktion des Systemgewichts mit einer Verschlechterung der Systemeigenschaften (Leistung, Drehmoment) verbunden, kann die Gesamteffizienz der Bodenphase überproportional sinken. In der Folge muss die Gewichtsreduktion nicht zwingend eine weitere Senkung des Treibstoffverbrauchs auf Missionsebene hervorrufen. - Das physikalische (z.B. thermische) Systemverhalten ist entscheidend für die Ermittlung des erwarteten Vorteils. Die optimale Systemarchitektur hängt stark von der Mission ab. Dies muss in frühen Entwurfsphasen berücksichtigt werden. - Der gesamte wirtschaftliche Vorteil durch die Benutzung elektrischer Fahrwerksantriebe kann deutlicher ausfallen, wenn weitere Aspekte zusätzlich zur Treibstoffersparnis berücksichtigt werden. Das Interesse der Akteure an elektrischen Fahrwerkssystemen kann primär wirtschaftlich sein.German
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