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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Serie: Aerospace Series (PEP)
- Sprache: Englisch
Although the overall appearance of modern airliners has not changed a lot since the introduction of jetliners in the 1950s, their safety, efficiency and environmental friendliness have improved considerably. Main contributors to this have been gas turbine engine technology, advanced materials, computational aerodynamics, advanced structural analysis and on-board systems. Since aircraft design became a highly multidisciplinary activity, the development of multidisciplinary optimization (MDO) has become a popular new discipline. Despite this, the application of MDO during the conceptual design phase is not yet widespread.
Advanced Aircraft Design: Conceptual Design, Analysis and Optimization of Subsonic Civil Airplanes presents a quasi-analytical optimization approach based on a concise set of sizing equations. Objectives are aerodynamic efficiency, mission fuel, empty weight and maximum takeoff weight. Independent design variables studied include design cruise altitude, wing area and span and thrust or power loading. Principal features of integrated concepts such as the blended wing and body and highly non-planar wings are also covered.
The quasi-analytical approach enables designers to compare the results of high-fidelity MDO optimization with lower-fidelity methods which need far less computational effort. Another advantage to this approach is that it can provide answers to “what if” questions rapidly and with little computational cost.
Key features:
- Presents a new fundamental vision on conceptual airplane design optimization
- Provides an overview of advanced technologies for propulsion and reducing aerodynamic drag
- Offers insight into the derivation of design sensitivity information
- Emphasizes design based on first principles
- Considers pros and cons of innovative configurations
- Reconsiders optimum cruise performance at transonic Mach numbers
Advanced Aircraft Design: Conceptual Design, Analysis and Optimization of Subsonic Civil Airplanes advances understanding of the initial optimization of civil airplanes and is a must-have reference for aerospace engineering students, applied researchers, aircraft design engineers and analysts.
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Contents
Cover
Aerospace Series List
Title Page
Copyright
Foreword
Series Preface
Preface
Acknowledgements
Chapter 1: Design of the Well-Tempered Aircraft
1.1 How Aircraft Design Developed
1.2 Concept Finding
1.3 Product Development
1.4 Baseline Design in a Nutshell
1.5 Automated Design Synthesis
1.6 Technology Assessment
1.7 Structure of the Optimization Problem
Bibliography
Design Development and Methodology
Transport Aircraft Technology
Computer-Assisted Synthesis and Design Integration
Chapter 2: Early Conceptual Design
2.1 Scenario and Requirements
2.2 Weight Terminology and Prediction
2.3 The Unity Equation
2.4 Range Parameter
2.5 Environmental Issues
Bibliography
Initial Orientation
Energy Efficiency
Emissions and Atmospheric Pollution
Chapter 3: Propulsion and Engine Technology
3.1 Propulsion Leading the Way
3.2 Basic Concepts of Jet Propulsion
3.3 Turboprop Engines
3.4 Turbofan Engine Layout
3.5 Power Plant Selection
Bibliography
Aero Engine Technology
Aircraft Noise
Chapter 4: Aerodynamic Drag and Its Reduction
4.1 Basic Concepts
4.2 Decomposition Schemes and Terminology
4.3 Subsonic Parasite and Induced Drag
4.4 Drag Polar Representations
4.5 Drag Prediction
4.6 Viscous Drag Reduction
4.7 Induced Drag Reduction
Bibliography
Theory and Prediction of Drag
Induced Drag
Drag Reduction Technology
Laminar Flow Control
Chapter 5: From Tube and Wing to Flying Wing
5.1 The Case for Flying Wings
5.2 Allocation of Useful Volume
5.3 Survey of Aerodynamic Efficiency
5.4 Survey of the Parameter ML/D
5.5 Integrated Configurations Compared
5.6 Flying Wing Design
Bibliography
Flying Wing Controversy
Flying Wing Design
Blended Wing Body
Span-Distributed Loading Cargo Aircraft
Chapter 6: Clean Sheet Design
6.1 Dominant and Radical Configurations
6.2 Morphology of Shapes
6.3 Wing and Tail Configurations
6.4 Aircraft Featuring a Foreplane
6.5 Non-Planar Lifting Systems
6.6 Joined Wing Aircraft
6.7 Twin-Fuselage Aircraft
6.8 Hydrogen-Fuelled Commercial Transports
6.9 Promising Concepts
Bibliography
Advanced Concepts and Technology
Flying Qualities and Airplane Design
Airplanes with Strut-braced Wings
Two-Surface and Three-Surface Airplanes
Aircraft with Non-planar Lifting Surfaces
Joined Wing Aircraft
Multi-Body and Lifting-Body Aircraft
Liquid Hydrogen-Fuelled Aircraft
Chapter 7: Aircraft Design Optimization
7.1 The Perfect Design: An Illusion?
7.2 Elements of Optimization
7.3 Analytical or Numerical Optimization?
7.4 Large Optimization Problems
7.5 Practical Optimization in Conceptual Design
Bibliography
Optimization Techniques
Multidisciplinary Design and Optimization
Geometry Representation
Chapter 8: Theory of Optimum Weight
8.1 Weight Engineering: Core of Aircraft Design
8.2 Design Sensitivity
8.3 Jet Transport Empty Weight
8.4 Design Sensitivity of Airframe Drag
8.5 Thrust, Power Plant and Fuel Weight
8.6 Take-Off Weight, Thrust and Fuel Efficiency
8.7 Summary and Reflection
Bibliography
Design Sensitivity of Weight
Structures Technologies and Materials
Chapter 9: Matching Engines and Airframe
9.1 Requirements and Constraints
9.2 Cruise-Sized Engines
9.3 Low Speed Requirements
9.4 Schematic Take-Off Analysis
9.5 Approach and Landing
9.6 Engine Selection and Installation
Bibliography
Take-Off and Landing Performance
Matching Engines to the Airframe
Engine Integration
Chapter 10: Elements of Aerodynamic Wing Design
10.1 Introduction
10.2 Planform Geometry
10.3 Design Sensitivity Information
10.4 Subsonic Aircraft Wing
10.5 Constrained Optima
10.6 Transonic Aircraft Wing
10.7 Lift Coefficient and Aspect Ratio
10.8 Detailed Design
10.9 High Lift Devices
Bibliography
Conventional and Supercritical Wing Design
Forward Swept Wings
Winglets, Sheared Tips and Crescent Moon-Shaped Wings
High Lift Systems
Variable Camber
Chapter 11: The Wing Structure and Its Weight
11.1 Introduction
11.2 Methodology
11.3 Basic Wing Box
11.4 Inertia Relief and Design Loads
11.5 Non-Ideal Weight
11.6 Secondary Structures and Miscellaneous Items
11.7 Stress Levels in Aluminium Alloys
11.8 Refinements
11.9 Application
Bibliography
Weight Prediction Methods
Chapter 12: Unified Cruise Performance
12.1 Introduction
12.2 Maximum Aerodynamic Efficiency
12.3 The Parameter ML/D
12.4 The Range Parameter
12.5 Range in Cruising Flight
12.6 Cruise Procedures and Mission Fuel
12.7 Reflection
Bibliography
Optimum Cruise Performance and Prediction of Range
Appendix A: Volumes, Surface and Wetted Areas
A.1 Wing
A.2 Fuselage
A.3 Tail Surfaces
A.4 Engine Nacelles and Pylons
A.5 Airframe Wetted Area
Bibliography
Appendix B: International Standard Atmosphere
Appendix C: Abbreviations
Index
Aerospace Series List
Advanced Aircraft Design: Conceptual Design, Analysis and Optimization of Subsonic Civil AirplanesTorenbeekJune 2013Design and Analysis of Composite Structures: With Applications to Aerospace Structures, Second EditionKassapoglouApril 2013Aircraft Systems Integration of Air-Launched WeaponsRigbyApril 2013Design and Development of Aircraft Systems, Second EditionMoir and SeabridgeNovember 2012Understanding Aerodynamics: Arguing from the Real PhysicsMcLeanNovember 2012Aircraft Design: A Systems Engineering ApproachSadraeyOctober 2012Introduction to UAV Systems, Fourth EditionFahlstrom and GleasonAugust 2012Theory of Lift: Introductory Computational Aerodynamics with MATLAB and OctaveMcBainAugust 2012Sense and Avoid in UAS: Research and ApplicationsAngelovApril 2012Morphing Aerospace Vehicles and StructuresValasekApril 2012Gas Turbine Propulsion SystemsMacIsaac and LangtonJuly 2011Basic Helicopter Aerodynamics, Third EditionSeddon and NewmanJuly 2011Advanced Control of Aircraft, Spacecraft and RocketsTewariJuly 2011Cooperative Path Planning of Unmanned Aerial VehiclesTsourdos et alNovember 2010Principles of Flight for PilotsSwattonOctober 2010Air Travel and Health: A Systems PerspectiveSeabridge et alSeptember 2010Unmanned Aircraft Systems: UAVS Design, Development and DeploymentAustinApril 2010Introduction to Antenna Placement and InstallationsMacnamaraApril 2010Principles of Flight SimulationAllertonOctober 2009Aircraft Fuel SystemsLangton et alMay 2009The Global Airline IndustryBelobabaApril 2009Computational Modelling and Simulation of Aircraft and the Environment: Volume 1 - Platform Kinematics and Synthetic EnvironmentDistonApril 2009Handbook of Space TechnologyLey, Wittmann HallmannApril 2009Aircraft Performance Theory and Practice for PilotsSwattonAugust 2008Aircraft Systems, Third EditionMoir and SeabridgeMarch 2008Introduction to Aircraft Aeroelasticity and LoadsWright and CooperDecember 2007Stability and Control of Aircraft SystemsLangtonSeptember 2006Military Avionics SystemsMoir and SeabridgeFebruary 2006Design and Development of Aircraft SystemsMoir and SeabridgeJune 2004Aircraft Loading and Structural LayoutHoweMay 2004Aircraft Display SystemsJukesDecember 2003Civil Avionics SystemsMoir and SeabridgeDecember 2002© 2013 Egbert Torenbeek All rights reserved 2013 John Wiley and Sons, Ltd.
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Library of Congress Cataloging-in-Publication Data
Torenbeek, Egbert. Advanced aircraft design : conceptual design, analysis, and optimization of subsonic civil airplanes / Egbert Torenbeek. pages cm Includes bibliographical references and index. ISBN 978-1-118-56811-8 (cloth) 1. Transport planes–Design and construction. 2. Jet planes–Design and construction. 3. Airplanes–Performance. I. Title. TL671.2.T668 2013 629.133’34–dc23 2013005449
A catalogue record for this book is available from the British Library.
ISBN: 9781119969303
Foreword
Aircraft design is a very fascinating and motivating topic for pupils, students and young researchers. They are interested in the engineering subject, knowing that this is a complex subject with the aerodynamics to make the aircraft fly, with the structural layout to accommodate some sort of payload and keep the integrity of the vehicle, and with the aspects of flight mechanics to stabilize and control the aircraft, just to mention the basic aspects. In the scientific world, the faculties of aerospace engineering follow this principle and consider the basic disciplines such as aerodynamics, lightweight structures, flight mechanics and space technologies as the fundamentals to provide the envelope for aeronautics and space for the engineering students. Aircraft design is normally not considered a specific discipline worthy of inaugurating a specific chair. Some exceptions, however, do exist. The Delft University of Technology was one of the first Technical Universities in Europe to inaugurate a specific chair for aircraft design, and with the nomination of Egbert Torenbeek in 1980 they found a very strong personality who has further developed the scientific approach and methodology for preliminary aircraft design. The Technical University of München (TUM) in 1995 established a new chair for aeronautical engineering with the specific focus on aircraft design and I was nominated for this chair. This shows that the focus of integrated aircraft design has only slowly found its role in the scientific world.
A similar view can also be seen in industry. During my time at Airbus, the Technical management was not fully convinced that the aircraft design had the same importance and role as the big engineering departments like aerodynamics, structures, systems, propulsion and cabin. On the other hand, Airbus suddenly discovered about some ten years ago with some urgency that they did not have enough engineers with sufficient global knowledge to understand the total aircraft as a complex system. A huge push was then started to develop within the company ‘aircraft architects’ and ‘aircraft integrators’, also highlighting, that the discipline ‘aircraft design’ with its specific knowledge and experience is of prime importance.
There is, however, a huge discrepancy between industry and research centres or universities with regard to integrated aircraft design. Industry claims and wishes that universities as well as research centres should not look too closely at aircraft integration; this is seen as the unique role of industry. Industry claims to be the only partner, who knows the market demand and who has to consider the right design approach with respect to time, cost, quality and risk before deciding on a new product and its introduction onto the market. Industry therefore would like to keep the universities out of the domain of aircraft design, and do not want to give too many details to the scientific community, on how to prepare an innovative aircraft design. On the other hand, students and young engineers have to be trained and have to learn and understand the basic features of aircraft design at university during their studies. Students are primarily not so much fascinated by details of low speed aerodynamics or the detailed design of a fuselage frame compared to designing an aircraft. They are motivated to develop aircraft models, sailplanes and want to know how to design this sort of flying vehicles and what is the approach to defining the size of the wing, tailplane and engines. The scientific approach to aircraft design is therefore a major topic for the universities and has to be part of the aeronautical engineering curriculum.
There are several good books on the market, one of the best in my view written by Egbert Torenbeek. But these books were written mainly in the years 1980 to 1990 and have established a lot of design data, collected from aircraft designs of the 1960s to the 1980s. Also at that time the focus was on the preliminary aircraft design, starting from the weight breakdown, defining wing and tailplane areas and checking stability and controllability.
Over the past twenty years, computer capabilities have improved considerably and a lot of aircraft design software programs are distributed on the market with some quite good success and good results as long as the aircraft design follows the classical design features. The new dimension which has been added to the aircraft design process is called multidisciplinary optimization (MDO) methodologies. The continuous increase in computer speed and capacity has first allowed FEM methods for all sort of structural layout and CFD methods for the aerodynamic design of aircraft components and the total aircraft to be developed. The next steps were then multidisciplinary tools, first, to integrate the different design boundaries such as high-speed and low-speed aerodynamics, and in a next step, today the multidisciplinary methods permit an aircraft to be designed by using the integration of aerodynamic, structural and flight mechanics design constraints and by using multidisciplinary optimization methodologies. MDO is the new design methodology for all aircraft design features and nearly all papers in aircraft design are now using some sort of multidisciplinary optimization approach.
I remember that some five years ago – sitting on the Wolga beach in Samara (Russia) during a seminar for aircraft design professors – we had some lively discussions on some aircraft optimization problems. We also learned that Egbert Torenbeek was working on a new book about advanced aircraft design. However, he had some doubts whether there were still enough people interested in learning about the complex aspects of advanced aircraft design, while all institutions are just working with big and complex software tools. He was not sure whether the aircraft community would like to see such a book. We encouraged him very much to continue. Egbert Torenbeek has a very high reputation among the aircraft design professors and I am very happy to see that he finally managed to finish his book. Having read several chapters, I really believe that his way of addressing a quasi-analytical approach to aircraft design is very valuable and an excellent complementary way to the common normal approach of computerized analysis.
In the next decades, the aeronautical industry will be faced with considerable new environmental challenges. The past success of air transport will be confronted with new questions like ‘Which optimal flight altitude will have minimum impact on the atmosphere?’ or ‘How can new aircraft concepts with new engine options like Open Rotors improve fuel efficiency and also the environmental footprint for a given mission?’ I am convinced that new aircraft concepts for the future will be required to cope better with the increasing environmental restrictions which air transport will have to face. This book will be of great help and interest for these sorts of questions where the impact of new boundary conditions will have to be analyzed and investigated and where the large industrial computer software is not yet properly validated and verified. The physics-based approach of this book will help to better qualify the dominant parameters for different new and unconventional aircraft concepts and also help the reader to understand the assessment of benefits and risks of these concepts.
I wish this book a lot of success and hope that my colleagues from industry and the scientific community and especially the young scientists will appreciate this book as well.
Prof.h.c. Dr.-Ing. Dr.h.c. Dieter Schmitt Aeronautical consultant. Former Head of Future Projects at Airbus SASFormer Professor at TU München, Institute of Aeronautical EngineeringBlagnac, 25th November 2012
Series Preface
The Aerospace Series covers a wide range of aerospace vehicles and their systems, comprehensively covering aspects of structural and system design in theoretical and practical terms. This book complements the others in the Series by looking at the concept phase of design of the aircraft.
Aircraft Design is an early stage of activity in the evolution of an aircraft project starting at the concept and enduring until the preliminary design. It is time for broad thinkers, for people prepared to take risks and to understand the big picture. At this stage of an aircraft project the important issues are the shape of the aircraft, its fuel and load carrying capability and its mass leading to an assessment of its suitability to perform a mission. From ideas generated during this process will gradually emerge a solution that can be committed to design and manufacture.
The author introduces the topic with an overview of the advanced design process, considering design requirements and methodologies, considerations driving a design, followed by an example of early design mass prediction. The next stage deals with the selection of the aircraft general arrangement, an essential but complex issue which concerns new technology applications and operational properties. Decisions made at this stage involve and affect many disciplines in a project – many of those dealt with in other books in the Series. This is the challenging stage of integration and the role of the Chief Designer. Then an approach to explicit optimization by means of quasi-analytic relations is developed and the book concludes with analytical examples that are essential to advanced design in general and optimization in particular.
This is performed in a clear and concise manner to make the book a comprehensive treatise on the subject of advanced design of subsonic civil aircraft from initial sizing through to final drag calculations. There are lessons to be learned here also for military aircraft designers. It will be of great use to undergraduate and postgraduate students as well as to practitioners in the field of aircraft design and scientists in aerospace research and development. The author has given his work authority by basing it on many years of research at the Delft University of Technology where this subject is taught under the auspices of a Chair in the subject.
Peter Belobaba, Jonathan Cooper and Allan Seabridge
Preface
I don’t know why people are frightened by new ideas.
It’s the old ones that frighten me.
—John Cage, American composer
Advanced Design (AD) is the name for the activity of a team of engineers and analysts during the early stages of an aircraft design and development process. The point of departure is a set of top level requirements specifying payload/range capabilities, cabin accommodation, flight performance, operational, and environmental characteristics. The first design activity generates a conceptual baseline configuration defined by (electronic) drawings of its layout, a database specifying the physical characteristics and the essential technological assumptions, and an assessment of the feasibility of complying with the requirements. Designers may propose one or several concepts which are subsequently refined and compared during the second advanced design stage called the preliminary design. Conceptual design and preliminary design are crucial phases in the development process during which creativity and ingenuity are of paramount importance to support the far-reaching decisions that can make or break the programme as a whole.
Since the 1970s, aircraft design has become the subject of academic education and research at an increasing number of academic institutions which have an aerospace curriculum. Many topics typical of aircraft design projects are nowadays covered in academic courses, and educational handbooks, and an abundance of software tools have become available to support students in their design exercises. Although many academic courses pay modest attention to aircraft design, a design-oriented approach to the traditional aeronautical disciplines can contribute to an improved understanding of aeronautical science as a whole. However, design handbooks are essentially based on existing or even obsolete technology and may produce unrealistic results when applied to future advanced aircraft design projects. And design technologies are becoming more complicated due to the introduction of integrated product design technology and multidisciplinary design optimization, subjects not covered in most handbooks.
In writing this book it has been the author’s aim to contribute to the advancement of aircraft design (teaching) by emphasizing clear design thinking rather than sophisticated computation or using a huge collection of statistical information. Another orientation came from industrial design staff and academic teachers who indicated that they would be particularly interested in assessments of unusual aircraft concepts and examples of practical optimization in the early design stage. It was decided to focus on subsonic transports and executive (business) aircraft. The present text combines the author’s academic teaching approach with numerous results from in-depth investigations on advanced technologies and innovative aircraft configurations reported since the 1970s. Particular attention is paid to research by staff of the aircraft design chair at Delft University of Technology between 1980 and 2000. Although some information about design methodologies and statistical data of recent airplane models are included, the result is not intended to be used as a handbook in the first place. Most of the material presented is readily understood by those who have previous experience with airplane design. The niche market for this book is formed by MSc and PhD students doing design-oriented research, academic staff teaching design, advanced airplane designers and applied scientists at aeronautical research laboratories.
The contents of this book can be subdivided into the following groups of chapters.
The quasi-analytical character of the present approach to conceptual design optimization cannot replace rigorous numerical methods. Intended primarily to support advanced designers and researchers and help them to understand the complex relationships between the effects on airplane characteristics of varying design parameters, the results may also be useful to validate complex design sizing and optimization programs. Moreover, the simplicity of the analytical criteria is useful to quickly estimate the effects of introducing alternative technologies for propulsion and airframe design. If used judiciously, quasi-analytical relationships can be sufficiently accurate to successfully answer ‘what-if’ questions and make trade-off studies such as weight growth problems, specification changes and considering derivative aircraft. From this perspective, the present book can be seen as a tribute to prominent scientists and designers from the past – such as I.H. Ashkenas, R.T. Jones, D. Küchemann, and G.H. Lee – who pioneered this approach during the era when computer-based aircraft design technology did not yet exist. The author hopes that this effort will contribute to the way of thinking of those who consider conceptual design as an art rather than a science: the art of conceiving and building well-tempered aircraft.
Acknowledgements
I am indebted to chair holders Michel van Tooren and Theo van Holten who offered me the hospitality of their disciplinary group SEAD and to Michiel Haanschoten for his professional assistance with ICT problems. I am grateful to the staff of DAR – in particular, Arvind Gangoli Rao, Gianfranco la Rocca, Dries Visser, Roelof Vos and Mark Voskuijl – for frequent interesting communications on propulsion and aircraft design and for giving valuable feedback after reading draft versions of chapters. Thanks are also due to Evert Jesse of ADSE who has been my prime consultant on the subject of weight prediction.
This book would never have been realized without the support of my wife. Dear Nellie Volker, considering my weakness to find a proper balance between the dedication you deserve and my insatiable fascination for aeronautical engineering, I am eternally grateful that you tolerated my periods of distraction and continued to respect me during the more than 10 years of writing this book.
E. Torenbeek Delft University of Technology, The Netherlands, April 2013
1
Design of the Well-Tempered Aircraft
Let no new improvement in flying and flying equipment pass us by.
—Bill Boeing (1928)
As our industry has matured … we have become increasingly enslaved to our data bases of past successful achievements. Increased competitive pressures and emphasis on control of rapidly escalating costs have combined to preclude the level of bold risks taking in exploring possible new configuration options that might offer some further increase in performance, etc., but for which no adequate data exist to aid development.
—J.H. McMasters [57] (2005)
1.1 How Aircraft Design Developed
1.1.1 Evolution of Jetliners and Executive Aircraft
The second half of the twentieth century has been truly revolutionary. In particular, the period 1945–1960 produced some highly innovative projects which demonstrated that propulsion of transport aircraft by means of jet engines had become feasible. In combination with the appearance of the sweptback wing, this resulted in a jump in maximum cruising speeds from about 550 to more than 850 km/h (Figure 2.1). Having pioneered the B-47 swept-wing bomber, Boeing introduced its basic jet concept to the 367-80 tanker transport and later to the 707 passenger transport; see Figure 1.1(a). This concept proved successful and has been adopted for jetliners almost universally since the 1960s. When one realizes that in the early 1950s designers did not yet avail themselves of the advantage of electronic computers, it will be appreciated that this revolution in design technology was a monumental achievement.
Modern jetliners are mostly low-wing designs with two or four engines installed in nacelles mounted underneath and to the fore of the wing leading edge. It should not be concluded, however, that since the Boeing 707 little progress has been made in configuration design. An early example of an unusual mutation was the Sud-Est Caravelle, see Figure 1.1(b), the airliner that pioneered jet engines attached to the rear fuselage. Even though this was a patented concept, several short-haul designs soon emerged with a similar layout and some of these were very successful. The introduction of bypass engines (~1960) and large turbofans (~1970) further improved the productivity and economy of jetliners. In combination with the strong worldwide economic expansion, this resulted in an unprecedented growth of air traffic and the almost complete extinction of competing modes of transportation over long distances, including the long-haul piston-powered and even the brand-new turboprop-powered propeller airliners.
Short-range jets initially suffered from poor low speed performances and high fuel expenditure. This market niche was filled by the four-engine Vickers Viscount and other turboprops designed in the 1950s. The twin-engine Fokker Friendship – see Figure 1.1(c) – had its Rolls-Royce Dart turboprop engines mounted to the high-set wing. This configuration was difficult to improve on and became the standard for similar propeller aircraft appearing later. Short-range turboprops have survived the twentieth century thanks to their excellent fuel economy and low operating costs. The idea of producing economy-size jets for large companies and wealthy individuals came around 1960. A prime example of a successful business jet was the Learjet depicted in Figure 1.1(d). Seating six in a slim fuselage (‘no-one walks about in a Cadillac’), it outperformed jetliners of its time in maximum speed. Learjet’s general arrangement, a low-wing design with engines attached to the rear fuselage and a high-set horizontal tail, has been adopted on most executive jets.
Figure 1.1 Prime examples of early post-WW II passenger aircraft. (a) Boeing 707 (1954): the first jet-powered airliner of US design. (b) Sud-Est Caravelle (1959): the first airliner with rear fuselage-mounted jet engines. (c) Fokker F 27 (1955): turboprop designed as a regional aircraft; still operational in 2012. (d) Gates Learjet 24B (1963): business jet designed in the early 1960s
Since the introduction of the first jetliners, subsonic civil airplane technology development has advanced in an evolutionary way. During the time span between 1950 and 2000, considerable improvement has been accomplished in all technical areas, but none could be regarded as revolutionary. The basic properties of traditional designs – such as lift, drag, weight and flight performances – have become well understood. Computational methods supporting advanced design (AD) have steadily developed over a long period of time and a wealth of empirical evidence confirms their accuracy. Consequently, aircraft with a conventional layout can be developed with a high degree of confidence in the analysis. Though designing an innovative configuration will always be challenging from an engineering viewpoint, its application in an industrial project entails many challenges. This may lead to the situation that, after several years of costly configuration development, the project has to be terminated by a show stopper. It is also observed that airline management tends to avoid the uncertainties of an unusual general arrangement and prefers the purchase of a traditional configuration.
The conformity between modern airliners is not caused by the lack of conceptual creativity of designers; arguments supporting this statement can be found in publications such as [12] and [14]. In fact, several innovative designs proposed during the last decennia of the twentieth century have not been developed into a for-sale aircraft because airlines were reluctant to order them for non-technical reasons. The following projects serve as examples.
The Boeing 7J7 project of the 1980s – Figure 1.2 (a) – was a 150-seat airliner in which new technologies were integrated: a fly-by-wire control system, unducted fan (UDF) engine technology, advanced system and flight deck technologies, and advanced aluminium alloys. The 7J7 did not find favour with the airlines mainly because the anticipated spike in fuel prices did not occur.Boeing’s Sonic Cruiser – Figure 1.2 (b) – was designed to connect typical long-range city pairs at Mach 0.95 or above. In a business class layout for 100 seats it would attract passengers who would be willing to pay a fare premium to save several hours on long distance flights with increased comfort. The 300-seat version would be used for continental flights circumventing the large hubs. The Sonic Cruiser became the victim of the aftermath of the events following September 2001, when airlines began to re-evaluate their business models resulting in a preference for a more economical (slower) design which became the 787 [41]. The Sonic Cruiser was not developed into a for-sale product because potential customers would rather see its advanced technology developed for integration into an airplane optimized for lower Mach numbers.Figure 1.2 Boeing design projects which were not put into production. (a) 7J7 open rotor-powered narrow body airliner of the 1980s. (b) Sonic Cruiser long-range wide body Mach 0.95 airliner 1999–2002
1.1.2 A Framework for Advanced Design
The non-recurring costs of a commercial aircraft development programme are so enormous that even a relatively minor technical hiccup may be magnified into an unacceptable commercial risk. Consequently, a certain amount of conservatism is inherent in the development of civil aircraft design. In spite of this, conservatism in design is risky because it can lead to missed opportunities when maturing aerodynamic, structural and propulsive technologies are becoming available which find their best application in concepts different from the current dominant configuration.
In civil aircraft development programmes, far-reaching decisions concerning top level specifications, general arrangement, propulsion and enabling technologies are made before and during the concept finding and the conceptual design phases. The preliminary design phase is then entered during which the aircraft’s characteristics are defined in more detail, initial assumptions are verified and the feasibility and risk level of the project are investigated. A year or more may elapse before management will decide to give the green light or withdraw from further development. The next phase consists of design verification (testing) and detail design during which major modifications of the basic configuration can be very labour-intensive and costly. Clearly, ESDU’s trademark phrase, ‘get it right the first time' is highly relevant for the initial aircraft system design process.
The observation has frequently been made that no more than a few percent of the pre-production costs are attributed by a few designers committing to a large fraction of total aircraft programme cost. In some cases this observation was made in favour of strengthening the advanced design capability of the aeronautical industry and/or the effort in academia to offer excellent aircraft design teaching. Although these arguments are fully justified, it is not always acknowledged that a large portion of aircraft programme costs is committed by merely specifying the need for the particular vehicle rather than by defining its technical and operational characteristics. If a new airplane has been developed for which no market exists, the project will be doomed to fail. The project design team cannot be blamed for a wrong go-ahead/exit decision and devoting more manpower to advanced design is not necessarily a panacea for avoiding misjudgement of the market. Although concept finding is not, in general, considered a part of the design project, it is at least as crucial to the success of a programme as the actual concept development phase.
1.1.3 Analytical Design Optimization
Since advanced design is highly relevant to the company’s viability, one would expect that the discipline of design optimization has traditionally received a great deal of attention from the aeronautical community – in fact, this is not the case. Until the time of large-scale computer applications, only a few systematic efforts were made to develop a fundamental framework for non-intuitive decision-making. Most of these were small-scale programs initiated by individuals in research institutes and academia and their impact on the actual practice in design offices has not become entirely clear. Nevertheless, from the educational point of view, several approaches and trends from the past still deserve to be mentioned even though not all of them have received widespread recognition.
Early parametric surveys were made on a limited scale in the industry by experienced designers. Until the 1960s, efforts to include optimization in conceptual design were based on relatively simple methods with minimum take-off gross weight (TOGW) considered as the criterion for the figure of merit. The analytical approach to sizing and improving a design in the conceptual stage was discussed in 1948 by Cherry and Croshere Jr [19]. Though their methodology was based on experience with propeller airplanes, its systematic character appeared useful for jet aircraft as well. In 1958, G. Backhaus proposed a comprehensive (quasi-)analytical optimization of jet transports [20]. His article did not get the recognition it deserved, probably because it was published in German. Another pioneer of the analytical approach to concept optimization was D. Küchemann. During the 1960s, he and his co-workers at the Royal Aircraft Establishment in the UK developed analytical design methods of aircraft intended to fly over widely different ranges at different (subsonic, supersonic and hypersonic) speeds [21]. Part of this work was based on research in connection with the conception of Concorde and was compiled in a unique book [1]. The elegance and lucidity of Küchemann’s analysis inspired the present author to initiate a systematic study of fundamental design considerations [27]; some of its results are included in the present book in a modified form. After the advent of computational design analysis and optimization technology in the 1970s, the (quasi-)analytical approach has appealed to only a few researchers; see, for example, W.H. Mason and B. Malone in [34, 35].
1.1.4 Computational Design Environment
During the first decennia after WWII, aircraft design was performed manually with the use of hand calculators and drawing boards. Despite the commercial success of several excellent airliners and business airplanes developed during this period, the ‘paper method’ is nowadays considered too labor-intensive and ineffective. Since the 1970s, the advancement of design technology changed fundamentally due to the availability of powerful computers and interactive graphics devices. Simultaneously, significant progress was made in the fields of computational engineering methods and numerical optimization, a trend set by early applications in astronautics and chemical engineering. The aeronautical community initially paid most attention to developing complex computer-based design synthesis programs such as those reported in [62] and [68]. Although automated design optimization has attracted much attention from research institutes such as NASA, reputed designers initially viewed these efforts with apprehension for reasons to be discussed in Chapter 2.
The penetration of ICT into all fields of aeronautics since 1980 has drastically changed the aircraft development scene. Whereas designs were traditionally almost exclusively produced by the aircraft company’s design offices, reports presented at scientific conferences indicate that research institutions and universities have become new actors in the aircraft configuration design field. The remarkable expansion of multidisciplinary design optimization (MDO) and concurrent engineering methodologies have brought about a design climate change in aeronautics as well as in other engineering disciplines. Since the 1990s, the EU Framework Programs have stimulated the industry, research institutions and academia to cooperate in order to improve aircraft design technology. These efforts have resulted in improved possibilities for designers to gain insight into the impact of new technologies and concepts on the design quality in pre-competitive phases before excessive resources have to be committed.
With the intention of offering a fresh and practical approach, the present book emphasizes the fundamentals of aircraft conceptual design sizing and optimization. The treatment of advanced computational systems and the presentation of design data collections is considered to be outside its scope. Fortunately, those involved in design teaching, students and practising designers can avail themselves of an abundance of detailed guidelines for drawing up a conceptual aircraft design in excellent books quoted in the bibliography of this chapter. Several of these and other publications have been used to compile this overview. The author is also indebted to J. van Toor for his permission to quote freely from personal correspondence [43].
1.2 Concept Finding
How an engineer generates good design concepts remains a mystery that researchers from engineering, computer and cognitive sciences are working together to unravel.
—P. Raj [95]
1.2.1 Advanced Design
The essential transportation properties of a new aircraft type, its overall system concept, design data and detailed geometry are defined by the company’s advanced design (AD) office which is responsible for the generation of aircraft concept proposals including the technical, technological, competitive and commercial aspects. Focussing on new product development, the AD team is active in the overall concept development and in defining its technical and operational properties. AD is a vital and essential part of product development and has a substantial influence on the company’s competitiveness and effectiveness. Dependent on the internal organization, most of the AD tasks can be categorized into the following activities.
Future projects. The prime task of a future projects team is carrying out pre-conceptual studies, conceptual design and proof of concept for a new (‘clean sheet’) design and making proposals for novel configurations. This complex activity has a highly multidisciplinary character which requires that individuals from functional groups such as flight physics, structures/materials and systems integration are involved in the AD process. The team must accomplish the projected task subject to boundary conditions such as top level requirements, certification rules, technical capabilities and economic environment of the company, customer operational aspects and other considerations.Tool development. Software tools for aircraft sizing, performance analysis, weight and cost prediction and optimization techniques are of vital importance for a successful design effort. Reflecting the expertise of the company, these tools are in general not available on the commercial market. The capability to investigate a wide variety of vehicles and alternative concepts requires the design tools to be continuously improved by making them more reliable and versatile and by incorporating and expanding design databases. Advanced designers will also be active in merging new results from the (applied) research field with available methods and procedures. Chapter 11 illustrates how a design tool can be developed.Enabling technologies. Most of the company’s R & D activities aim at applications with one of its (future) production programmes. AD identifies the required key technologies in accordance with the company’s technology objectives and gives guidance in the development of new technologies enabling competitive products. Included activities are assessment of operational research and market analysis, and available manufacturing capabilities.Competition evaluation. The technical, technological and economical situation of the company’s products is judged versus competing products and developments. This requires year-round exploration and modelling of competing airplanes under consideration by the same potential customers and creation of a well organized competition database.In addition to these focussed activities, AD is responsible for highly constrained temporary tasks. These may entail, for instance, interaction with the company’s sales department and with (potential) customers, external suppliers and partners. The engine selection process requires that regular contacts are made with engine manufacturers. During the validation and detailed design phases of an ongoing project, AD specifies and coordinates the peripheral activities carried out externally such as wind tunnel, structural and system testing. Another activity is developing proposals for upgrade programmes and future derivatives or modifications of the company’s existing product line.
1.2.2 Pre-conceptual Studies
The starting point for any project development is an understanding of market requirements and answering the question why – rather than how – a new product will be developed. The underlying reason for any commercial aircraft programme is its ability to provide a profit to the company that designs and builds it as well as the customer that uses it. Reliable forecasts about the demand for new aircraft are obtained from continuously monitoring and assessing the advancements in aeronautical research and technology. The pre-conceptual study phase is intended to identify a product line within the company’s capabilities that fits a potential market. This entails a complex process which ideally includes a dialogue between design, management, marketing and customer support. The pre-conceptual phase includes aircraft configuration trade studies identifying techniques and technology requirements suitable for integration into the new product. This is accomplished by initial aircraft sizing, engine matching, weight estimation and evolution of a family of aircraft with a given set of payload versus range combinations. At the end of the process, a management decision is expected for a selected configuration to be visualized in a provisional three view drawing.
Different terms exist for the pre-conceptual phase: companies may call it the pre-feasibility, concept finding or architectural phase. In fact, the notion of architecture refers to a transportation system – this could be an airline, a number of airlines or some other transportation service of which the future plane will be a constituent part – rather than to the characteristics of the aircraft itself. Pre-conceptual studies produce an agreed and binding set of definitions that will drive the design, generally known as top level requirements (TLRs). Together with airworthiness certification rules, these will form the principal framework of objectives and constraints for the following design phases, eventually leading to a new product development. TLRs also include the criteria for a go-ahead or exit decision at the end of the product design phase. Although the concept finding phase may eventually lead to a new product, it is not usually considered as part of a design project; hence, concept finding entails a more continuous activity than project development.
Top level requirements identify characteristics that should not be subject to significant variations during project design since this could entail a violation of the transportation system architecture that has been identified as desirable for the new product. An illustration is the selection of the design cruise speed or Mach number. This parameter has a major impact on the aircraft geometry, propulsion, weight, operating costs, as well as on the way airline operations are carried out. Compared to a high cruise Mach number, a reduced speed is likely to result in a lighter aircraft structure, reduced installed engine power and less fuel consumption. But the low block speed may be detrimental to efficient and flexible operation, as well as commercial productivity.1 Similar arguments apply to available field lengths for take-off and landing. Since these basic performance requirements are selected at the transportation system level, they should be considered as design constraints during conceptual sizing.
1.3 Product Development
The potential customer of a new airliner thinks that the life of a design begins with drawing up its requirements. However, advanced designers consider the conceptual design phase as the starting point of product development. The schedule of the development phases of a commercial or business airplane depicted in Figure 1.3 is helpful for understanding the design effort which is essential for a successful project. The complete process is subdivided into product design, manufacturing and testing phases. The product design process is broken down into conceptual design, preliminary design and detail design.
Conceptual design encompasses sizing of the most promising overall aircraft concept and proof of its feasibility. Having a typical duration between 4 to 6 months for a business aircraft and 9 to 12 months for a mid-size airliner, conceptual design is characterized by cyclic design improvements and complexity increasing in time.The preliminary design phase aims at specifying the design concept at the main component level, sometimes including subsystem trades. Preliminary design typically lasts between 12 and 16 months.The detail design phase is entered when a management decision is taken to continue and give the project go-ahead.2 This development phase is entered soon after the aircraft is committed to production and lasts between two and three years. The decision to freeze the configuration is taken early in the detail design phase when changes in the product definition are no longer appropriate.Figure 1.3 Schedule of the civil airplane development process [40]. Courtesy of J.H. McMasters
As the aircraft goes through these phases, the level of detail and the confidence that the design will work are steadily increasing. For instance, during conceptual design, the interaction between major components such as fuselage frames, wing spars, fuel tanks, and landing gears is more important than their detailed geometry which materializes during preliminary design.
Aircraft manufacturers are active in different product lines in different markets and operate with different management methods; hence, the schedule and terminology discussed in this overview are far from universal. Dependent on the level of detail exerted in AD, the distinction between design phases is somewhat blurred, whereas some of the phases may overlap. Additional information on product development is given in the time schedule of well-defined events having the character of milestones. Figure 1.4 shows an example indicating major events such as programme go-ahead, configuration freeze, first flight, certification and first delivery. The development time span between the first concept studies and certification is typically between three years for a light business aircraft and six years for a clean sheet wide body airliner. This illustrates why an aircraft must be conceived at least a decade ahead of the anticipated utilization period. Therefore, flexibility and extended duration have a strong impact on many application alternatives and the growth potential which a good design must have from its inception.
Figure 1.4 Boeing 777 project development milestones [36]
1.3.1 Concept Definition
The concept definition phase can be characterized as the highly creative and imaginative idea stage during which the component geometry, placement and connectivity of a future aircraft designed to fulfil the needs of a specific market are defined. Conceptual design also entails the development of a novel aircraft concept at an overall system level in competition with a more traditional layout. The objective is to explore a preferred configuration3 to determine a layout which is technically superior and economically viable. This involves preliminary performance predictions and provision of three-dimensional electronic drawings with several cross-sections, an inboard profile showing the approximate placement and size of the major vehicle components. A weight and balance diagram provides another essential proof of concept.
There is an intimate relationship between the design objectives of an aircraft and the configuration concept capable of fulfilling these objectives. The overall concept describes a highly complex system which has to reach a compromise between contradicting requirements. Application of new technologies affecting all sub-systems is indispensable for economic success of the new product. The conceptual design phase is intended to generate a credible proposal of a feasible baseline design in order to convince management that it is worth the substantial resources required to develop and improve the design in further detail. Design tools are semi-empirical and low to medium fidelity methods used in trade-off studies and basic optimizations – most of the geometry is provisional. Validated design tools developed by the AD office are calibrated with statistical data bases, handbooks and historical trends, taking into account improvements expected from new technologies. The amount of data generated for a baseline design will be moderate and prediction errors are around 5%, typically.4
In an environment where designs are developed which fit into an existing product line, designers may investigate different fuselage cross-sections, wing positions and planforms, number and/or location of engines, empennage and undercarriage concepts. A few promising concepts are analyzed and selected for further study. Conceptual studies may also be carried out to investigate potential gains expected from new aerodynamic devices, structural concepts, materials and/or system technologies and/or an advanced engine concept. All of these features have a far-reaching effect when integrated into the design. Designers who are supposed to explore a radical concept may consider an integrated configuration such as a blended wing body (BWB) or an all-wing aircraft (AWA) (Chapter 5). Less radical alternatives are a canard or three-surface aircraft, a twin-fuselage aircraft concept, a strut-braced or a nonplanar wing (Chapter 6). A detailed assessment of advantages and disadvantages must then be made by comparison with more conventional solutions. During all the stages of concept definition, the technical risks and costs of possible failure must be closely examined.5
Typical of concept design is its iterative character: primary components – wing, fuselage, nacelles, tailplane, landing gear, propulsion and other systems – are sized provisionally to result in a baseline design. Dependent on where improvements are desirable, the process may recycle to an earlier definition level at each point in time. A baseline design is not necessarily an optimized airplane and, for a traditional layout, the combined application of active constraints will normally give an adequate approximation to the best feasible design. If a novel solution is tried, the estimation of the aircraft effectiveness will be based in some areas on slender evidence and simple mathematical models. The best available model may then change rapidly with time and will probably be too crude to warrant rigorous treatment. In any case a comprehensive design optimization at the conceptual stage is of little value [66].
1.3.2 Preliminary Design
After selection of a baseline airplane concept, the design and analysis process will enter the preliminary design phase. As opposed to conceptual design which deals with the whole aircraft system, preliminary design aims at defining subsystems, making component trade-offs and optimization. Specialists from different functional groups contribute to this process of refining the initial vehicle concept – AD remains responsible for coordination. This team will (re)design the delivered baseline vehicle in sufficient detail to carry out supporting analysis and specify peripheral testing programmes but not with enough detail to specify each sub-assembly. Information to establish the programme feasibility is generated by means of sophisticated computational aerodynamical, mechanical and structural analysis, prediction of the economics and expected market penetration. Preliminary design can be characterized as setting goals for the extensive efforts to be made in the downstream detail design phase. Typical subjects are categorized as follows:
Design definition. The baseline design team is committed to elaborate detailed analysis and sensitivity studies, with the aim of developing the best feasible configuration. This includes finding a balance between required volumes, main dimensions, weight distribution and engine performances. Details of the aircraft geometry, aerodynamic properties, structural loads and deformations and flying qualities have to be settled.Design validation. The predicted characteristics of the preferred configuration are verified by high-fidelity supporting analysis, simulations and test data. This becomes the final step of the preliminary design cycles.A detailed analysis is usually made to determine the sensitivity of the configuration to technology inputs, performance objectives and design constraints. This makes sense since during preliminary design there still exists considerable freedom for refinements and improvements through optimization of variables such as detailed wing design, engine thrust, location of the power plant and empennage design. These trade studies have a widespread effect on most areas of the design and must therefore be carried out with scrutiny using high-fidelity analysis. Particular attention will be paid to the following issues:
Detailed volumetric sizing and mass breakdown, centre of gravity (CG) location and loading restrictions, and moments of inertia, resulting in a considerable expansion of the design database.Definition of the aerodynamic shape of lifting surfaces, including high-lift devices, using computational fluid dynamics (CFD) methods and wind tunnel testing.Layout and sizing of main mechanical and structural concepts, aero-elastic analysis by means of finite element methods (FEM), and structural testing.Layout of the basic flight control system and control surfaces and devices, including prediction of flying qualities.Drawing up the specifications for buy-out components to be subcontracted to suppliers. This concerns the power plant (engines, propellers, nacelles) and other major aircraft systems such as the auxiliary power plant, environmental control, fuel system, hydraulic system, electrical system and avionics.Economic analysis in terms of operating costs. Commercial prospects are then predicted by means of economic analysis and a market penetration model.Analysis of environmental issues such as internal/external noise and engine emissions.All activities are based on standards of the selected airworthiness codes and regulations as the primary measure for acceptance of design solutions. The scope and depth of the physics-based analysis and the fidelity of the computational models are increased to such a level that the impact on aircraft performance and cost of proposals to modify the baseline configuration can be quantified. Although CFD and FEM codes provide high-fidelity results, computer simulations cannot always be relied on for an accurate prediction of operational properties. Design validation will therefore require extensive wind tunnel testing6 and testing of structural models to ensure that achieved performances will be no more than a few percent off the requirements.
Whereas the engineers involved in conceptual design generally belong to AD, specialists from various functional disciplines become involved when the preliminary design stage is entered. From then on the approach to be taken relies heavily on system engineering techniques. This phase may span a year or more with a team of dozens up to hundreds of engineers working in a multidisciplinary environment. The amount of detailed data generated is substantial, prediction errors should amount to no more than few percent. The end product is an optimized and verified airplane configuration resulting in the technical description of prototypes to be tested and the type specification of the aircraft. When the design project is sufficiently mature, it may get an authorization to be offered for sale with written (contractually binding) guarantees on cost and performance. When market prospects appear to be promising, management authorizes the project go-ahead. In view of the high costs incurred by major configuration changes during the following design stage, this decision brings the iterative design cycle to an end by freezing the configuration.
1.3.3 Detail Design
The pieces of hardware that will actually be built and installed in the airframe are conceived during the detail design phase which is entered after the aircraft is committed to production. The objective of detail design is to specify the geometry of all components and plan their manufacturing processes. This encompasses drawing up instructions for the production department and the development of a careful plan for assembling the aircraft. Throughout this activity, drawings are progressively released for production. Results are complete production descriptions and specifications of all structures, systems and subsystems. Compared to preliminary design, detail design is much more labour-intensive and far more staff are involved. The participation of AD is restricted to coordinating measures to be taken when detail design appears to have an effect on the technical specifications. A typical example is when a significant empty weight growth has to be addressed by a weight reduction programme. After completion of the detail design phase, the manufacturing of components begins, followed by the final assembly of one or more flight-test vehicles, roll-out, first flight, flight testing, and certification. In the case of an all-new civil airplane development, the completion of the airworthiness certification process and first delivery to the customer will occur some five to eight years after the initial design efforts (Figure 1.4).
1.4 Baseline Design in a Nutshell
This book focusses on the conceptual and preliminary design definition rather than on the design validation and detail design. Although the described sizing procedure for conceiving a baseline design of a civil jet airplane is by no means a standard process, it contains a set of essential activity clusters. Conceptual design is a continuous iterative process that begins with an initialization of a few major characteristics to be discussed in Chapter 2. The following bird’s eye view of a baseline design sizing process describes the logical order of activities visualized in Figure 1.5 leading to a converged design in as few iterations as possible. The result is a non-optimized but feasible jetliner design which complies with TLRs – the sizing process of a turboprop airplane is similar although sizing relationships are different. The diagram indicates an inner loop for satisfying payload/range and high speed performance requirements and an outer loop for making sure that low-speed properties comply with TLRs and other constraints. This approach is in the interests of an efficient iteration process but does not necessarily lead to the best feasible design. The next steps are optimization and trade-off studies as discussed in later sections.
Figure 1.5 Schematic outline of a jetliner baseline design process
1.4.1 Baseline Sizing
A baseline design is materialized in the form of a fairly comprehensive definition of its basic geometry, using a computer assisted design (CAD) system. Defining the geometry by a parametric description helps the designer to predict the variation of mass distribution and aerodynamic properties due to variation of independent design variables such as wing area and aspect ratio, engine power or thrust, and empennage size. Figure 1.5 illustrates that initial ‘guesstimates’ are made of several important design characteristics.
