Egbert Torenbeek

Synthesisof SubsonicAirplane Design

Delft University PressKluwer Academic Publishers

Synthesis of Subsonic.Aitplane Design

An introduction to the preliminary design of subsonicgeneral aviation and transport aircraft, with emphasis onlayout, aerodynamic design, propulsion and performance

Egbert Torenbeekwith a foreword byH. Wittenberg

1982

15 Kluwer Academic Publishersa Dordrecht / Boston / London

Library of Congress Cataloging In Publication Data

Torenbeek, Egbert.Synthesis of Subsonic Airplane Design.

Includes bibliographical references and index.1. Airplanes-Design and Construction.I. Title.TL671.2.T67 1982 629.134'1 82-12469

ISBN 90-247-2724-3

Joint edition published byDelft University Press,Mijnbouwplein II,2628 RT Delft, The Netherlandsand byKluwer Academic PublishersP.O. Box 173300 AA Dordrecht, The Netherlands.

Sold and distributed in the U.S.A. and Canada byKluwer Academic Publishers101 Philip DriveNorwell, MA 02061U.S.A.

In all other countries, sold and distributed byKluwer Academic Publishers Group,P.O. Box 3223300 AH DordrechtThe Netherlands.

Reprinted 1984, 1985, 1987, 1988, 1990, 1993, 1995, 1996, 1999

Copyright 1982 by Delft University Press, Delft, The NetherlandsAll rights reserved. No part of this publication may be reproduced, stored in a retrieval system,or transmitted in any form or by any means, mechanical, photocopying, recording, orotherwise, without written permission of the copyright owner, Delft University Press,Mijnbouwplein 11,2628 RT Delft, The Netherlands

Contents

FOREWORDby Professor H. Wittenberg

AUTHOR'S PREFACEACKNOWLEDGEMENTSUNITS

CHAPTER 1. GENERAL ASPECTS OF AIRCRAFT CONFIGURATION DEVELOPMENT

1.1. Introduction1.2. Aircraft design and development1.3. Configuration development

1.3.1. The design concept1.3.2. Initial configuration design and configuration variations1.3.3. Baseline configuration development1.3.4. The preliminary design department

1.4. The initial specification1.4.1. The need for a new type of aircraft1.4.2. Transport capacity1.4.3. Design cruising speed and range1.4.4. Low-speed characteristics and field performance1.4.5. Other requirements

1.5. A continuous thread running through the design process1.5.1. The iterative character of design1.5.2. Searching for the optimum1.5.3. A suggested scheme for preliminary design

1.6. Impact of civil airworthiness requirements, and operating and flight rules1. 6.1. General1.6.2. Federal Aviation Regulations1.6.3. British Civil Airworthiness Requirements1.6.4. Airworthiness standards and design

1.7. Conclusion

CHAPTER 2. THE GENERAL ARRANGEMENT

2.1. Introduction

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10101213141516161718191921222325

27

2e

V

2.2. H~gh. low or mid wing?2.2.1. High wing2.2.2. Mid win

CHAPTER 4. AN APPRECIATION OF SUBSONIC ENGINE TECHNOLOGY

4.1. Introductory comparison of engine types4.2. Current reciprocating engines

4.2.1. Some characteristics of the four stroke engine4.2.2. Engine design and its influence on flight performance4.2.3. Engine classification by cylinder arrangement4.2.4. Two-stroke and Rotary Combustion engines

4.3. Basic properties of aircraft gas turbines for subsonic speeds4.3.1. The gas producer4.3.2. The propulsive device4.3.3. The pure jet engine4.3.4. The turbofan engine4.3.5. The turboprop engine4.3.6. Overall efficiency, specific fuel consumption and specific thrust

(power)4.3.7. Analysis of the engine cycle

4.4. Assessment of turbojet engines4.4.1. Overall Pressure Ratio4.4.2. Turbine Entry Temperature4.4.3. Bypass ratio4.4.4. Engine noise4.4.5. Summary and prognosis for the turbofan engine4.4.6. / Engine performance in non-standard atmosphere

4.5. Assessment of turboprop engines4.5.1. Performance4.5.2. Weight and drag4.5.3. Turboprop engine configurations

CHAPTER 5. DESIGN FOR PERFORMANCE

5.1. Introduction5.2. Initial weight prediction

5.2.1. Stages in the estimation of airplane weight5.2.2. Examples of weight "guesstimates"

5.3. Initial estimation of airplane drag5.3.1. Drag breakdown5.3.2. Low-speed drag estimation method5.3.3. Compressibility drag5.3.4. Retracing a drag polar from performance figures5.3.5. Drag in takeoff and landing

5.4. Evaluation of performance requirements5.4.1. High-speed performance5.4.2. Range performance5.4.3. Climb performance5.4.4. Stalling and minimum flight speeds5.4.5. Takeoff5.4.6. Landing

pag'"

97

99101101106110III

112113116116116117

118119120121123125131133133134135137137

141

143144144145148148149152153153155155157160165167170

VII

CHAPTER 6. CHOICE OF THE ENGINE AND PROPELLER AND INSTALLATION OF THE POWERPLANT 181

CHAPTER 7. AN INTRODUCTION TO WING DESIGN

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171171172173174178178

182183183184186187190190191195199204204205205206206207209210210213

215

217219219224227227228229231232232233235

5.5. Aircraft synthesis and optimization5.5.1. Purpose of parametric studies5.5.2. Basic rules5.5.3. Sizing the wing of a long-range passenger transport5.5.4. Wing loading and thrust (power) loading diagrams5.5.5. Optimization for low operating costs5.5.6. Community noise considerations

6.1. Introduction6.2. Choice of the

6.2.1. Engine6.2.2. Engine6.2.3. Engine6.2.4. Choice

number of engines and the engine typeinstallation factorsfailureperformance and weight variationsof the engine type

6.3. Characteristics, choice and installation of propellers6.3.1. General aspects6.3.2. Propeller coefficients and diagrams6.3.3. Blade angle control6.3.4. Propeller geometry

6.4. Installation of propeller engines6.4.1. Location of the propellers6.4.2. Tractor engines in the nose of the fuselage6.4.3. Wing-mounted tractor engines

6.5. Installation of turbojet engines6.5.1. General requirements6.5.2. Fuselage-mounted podded engines6.5.3. Wing-mounted podded engines

6.6. Miscellaneous aspects of powerplant installation6.6.1. Thrust reversal6.6.2. Auxiliary Power Units (APU)

7.1. Introduction and general design requirements7.2. Wing area

7.2.1. Wing loading for optimum cruising conditions7.2.2. Wing loading limits and structural aspects

7.3. Some considerations on low-speed stalling7.3.1. Stall handling requirements and stall warning7.3.2. Design for adequate stall characteristics7.3.3. Stalling properties of airfoil sections7.3.4. Spanwise progression of the stall

7.4. Wing design for low-subsonic aircraft7.4.1. Planform7.4.2. Aspect ratio7.4.3. Thickness ratio

VIII

7.4.4. Wing taper7.4.5. Airfoil selection7.4.6. Stalling characteristics and wing twist

7.5. Wing design for high-subsonic aircraft7.5.1. Wing sections at high-subsonic speeds7.5.2. Wing design for high speeds7.5.3. Low-speed problems of high-speed wings7.5.4. Planform selection

7.6. High lift and flight control devices7.6.1. General considerations7.6.2. Trailing-edge flaps7.6.3. Leading-edge high lift devices7.6.4. Flight control devices

7.7. Dihedral, anhedral and wing setting7.7.1. The angle of dihedral (anhedral)7.7.2. Wing/body incidence

7.8. The wing structure7.8.1. Types of wing structure7.8.2. Structural arrangement in plan

CHAPTER 8. AIRPLANE WEIGHT AND BALANCE

8.1. Introduction; the importance of low weight8.2. Weight subdivision and limitations

8.2.1. Weight subdivision8.2.2. Weight limitations and capacities8.2.3. Operational weights and the payload-range diagram8.2.4. The choice of weight limits

8.3. Methodology of empty weight prediction8.4. Weight prediction data and methods

8.4.1. Airframe structure8.4.2. The propulsion group8.4.3. Airframe services and equipment8.4.4. Useful Load and the All-Up Weight

8.5. Center of gravity8.5.1. The load and balance diagram8.5.2. Loading flexibility and restrictions8.5.3. Effects of the general arrangement and layout8.5.4. Design procedure to obtain a balanced aircraft

CHAPTER 9. PRELIMINARY TAILPLANE DESIGN

9.1. Introduction to tailplane design, control systems and stabilization9.2. Static longitudinal stability and elevator control forces

9.2.1. Stick-fixed static stability and neutral point9.2.2. Stick-free static stability and neutral point; the stick force

gradient9.2.3. Stick-fixed and stick-free maneuver points and maneuver control

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263

265268269271273274275277277285286293294296297299300

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305308308

310

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CHAPTER 10. THE UNDERCARRIAGE LAYOUT 341

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342343343345348349349

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351353355356356358359360360361362

365

10.1. Introduction10.2. Tailoring the undercarriage to the bearing capacity of airfields

10.2.1. Runway classification10.2.2. The Equivalent Single Wheel Load (ESWL)10.2.3. Multiple wheel undercarriage configurations

10.3. Disposition of the wheels10.3.1. Angles of pitch and roll during takeoff and landing10.3.2. Stability at touchdown and during taxying: tricycle under-

carriages10.3.3. Gear length, wheelbase and track: tricycle undercarriages10.3.4. Disposition of a tailwheel undercarriage

10.4. Type, size and inflation pressure of the tires10.4.1. Main wheel tires10.4.2. Nosewheel tires10.4.3. Inflation pressure

10.5. Gear geometry and retraction10.5.1. Energy absorption on touchdown10.5.2. Dimensions of the gear10.5.3. Gear retraction

x

11.1. Introduction11.2. Terminology in relation to the determination of drag

CHAPTER 11. ANALYSIS OF AERODYNAMIC AND OPERATIONAL CHARACTERISTICS

forces9.2.4. Reduction of control forces 3139.2.S. Effects of compressibility and powerplant operation 316

9.3. Some aspects of dynamic behavior 3179.3.1. Characteristics of the SP oscillation 3179.3.2. Criteria for acceptable SP characteristics 3199.3.3. A simple criterion for the tailplane size 3209.3.4. The phugoid 323

9.4. Longitudinal control at low speeds 3239.4.1. Control capacity required to stall the aircraft 3249.4.2. Control capacity required for takeoff rotation and landing flareout 3259.4.3. Out-of-trim conditions 326

9.5. Preliminary design of the horizontal tailplane 3269.5.1. Tailplane shape and configuration 3269.5.2. Design procedures 327

9.6. Design of the vertical tailplane 3319.6.1. Control after engine failure: multi-engine aircraft 3329.6.2. Lateral stability 3359.6.3. Crosswind landings 3389.6.4. The spin 3389.6.5. Preliminary design of the vertical tailplane 339

11.2.1. Pressure drag and skin friction drag11.2.2. Wake drag, vortex-induced drag, and wave drag11.2.3. Form drag, profile drag, and induced drag11.2.4. Zero-lift drag and lift-dependent drag11.2.5. Breakdown for drag analysis11.2.6. Bodies with internal flow

11.3. Determination of aerodynamic characteristics11.3.1. Reynolds number effects11.3.2. Mach number effects11.3.3. Low-speed polars

11.4. The flight envelope11.5. Flight profile analysis and payload-range diagram

11.5.1. Operational climb11.5.2. Cruise performance11.5.3. Descent11.5.4. Payload-range diagram and block time

11.6. Climb performance11.6.1. Maximum rate of climb, time to climb and ceilings11.6.2. Takeoff and landing climb

11.7. Airfield performance11.7.1. Takeoff field length11.7.2. Landing field length

11.8. Some aspects of operating economy11.8.1. Economic criteria11.8.2. Estimation of DOC

CHAPTER 12. EVALUATION AND PRESENTATION OF A PRELIMINARY DESIGN

12.1. Presentation of the design12.2. External geometry and structural arrangement12.3. Layout drawings12.4. Conclusion

REFERENCES

APPENDIX A. DEFINITIONS RELATING TO THE GEOMETRY AND AERODYNAMIC CHARACTERISTICSOF AIRFOILS

A-I. GeneralA-2. Wing sections

A-2.1. Geometric definitionsA-2.2. Aerodynamic definitionsA-2.3. Nomenclature for some NACA sections

A-3. WingsA-3.1. Wing planformA-3.2. (Wing) twist and incidenceA-3.3. Aerodynamic definitions

References

page

368368369369370371371371372372373375375375377378378378378380380381382382384

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390390397398

435

43643643643743843843844044U

443

XI

APPENDIX C. PREDICTION OF WING STRUCTURAL WEIGHT 451

APPENDIX D. THE WEIGHT PENALTY METHOD FOR FUSELAGE STRUCTURAL WEIGHT PREDICTION 457

452452454454

469469470470

C-l. IntroductionC-2. Basic wing structure weightC-3. High lift devices, spoilers and speedbrakesC-4. Wing group weight

B-1. Fuselage 446B-l.l. General method 446B-l.2. Quick method for bodies of revolution 447

B-2. Wings and tailplanes 447B-3. Fuel tank volume 448B-4. Engine nacelles and air ducts 449References 449

APPENDIX B. THE COMPUTATION OF CIRCUMFERENCES, AREAS AND VOLUMES OF CURVES,SECTIONS AND BODIES 445

page

XII

D-l. Survey of the methodology 4580-2. Gross shell weight 458

D-2.1. Gross skin weight 458D-2.2. Gross stringer and longeron weight 459D-2.3. Gross standard frame weight 459

D-3. Gross shell modifications 460D-3.1. Removed material 460D-3.2. Doors, hatches, windows and enclosures 460

D-4. Flooring 462D-4.1. Passenger cabin and freight hold floors 462D-4.2. Various other floors 463

D-5. Pressure bulkheads and frames 463D-5.1. Pressure cabin bulkheads 463D-5.2. Wheelbays for retractable undercarriages 463

D-6. Support structure 464D-6.1. Wing/fuselage connection 4640-6.2. Engine support structure 464D-6.3. Other support structures 464

0-7. Additional weight items 465References 465

E-l. Applicability of the methodsE-2. Contributions to the liftE-3. Lifting properties of airfoil sections

E-3.1. The zero-lift angle

APPENDIX E. PREDICTION METHODS FOR LIFT AND PITCHING MOMENT OF AIRCRAFT IN THEEN ROUTE CONFIGURATION 467

E-3.2. Lift-curve slopeE-3.3. Maximum lift

E-4. Wing lift and lift distributionE-4.1. Lift-curve slopeE-4.2. Spanwise lift distributionE-4.3. Zero-lift angleE-4.4. Maximum lift

E-5. Pitching moment of the wingE-5.1. Aerodynamic centerE-5.2. Pitching moment (Cm )ac w

E-6. Wing/fuselage interference effects on liftE-7. Wing/fuselage pitching moment

E-7.1. Aerodynamic centerE-7.2. Pitching moment (Cm ) facw

E-8. Nacelle and propeller contributionsE-9. Lift of the complete aircraft

E-9.1. Tailplane liftE-9.2. Total trimmed airplane liftE-9.3. Wing/body incidenceE-9.4. Trimmed lift curve

E-IO. Airplane pitching moment and neutral point (stick fixed)E-10.1. The stick-fixed neutral pointE-10.2. Horizontal stabilizer incidenceE-10.3. Pitching moment curve

References

APPENDIX F. PREDICTION OF THE AIRPLANE POLAR AT SUBCRITICAL SPEEDS IN THEEN ROUTE CONFIGURATION

F-l. Drag componentsF-2. Primary components of vortex-induced drag

F-2.1. Untwisted plane wingsF-2.2. Drag due to twistF-2.3. Wing tip correction on vortex-induced dragF-2.4. Vortex drag induced by fuselage liftF-2.5. Nacelle contributionF-2.6. Horizontal tailplane contribution

F-3. Profile drag of smooth, isolated major componentsF-3.1. The flat plate analogyF-3.2. Wing sectionsF-3.3. WingsF-3.4. Fuselages and tail boomsF-3.5. Engine nacellesF-3.6. Tailplane profile drag

F-4. Subcritical interference effects and correctionsF-4.1. Wetted area correctionsF-4.2. Wing/fuselage interferenceF-4.3. Nacelle/airframe interference

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489492492493496496496496497497499500501505508509509509510

Xli

APPENDIX G. PREDICTION OF LIFT AND DRAG IN THE LOW-SPEED CONFIGURATION 525

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H-l. Scope of the methodH-2. The gas generatorH-3. Specific performance of straight jet engines

F-4.4. Tailplanejairframe interference 511F-5. Protuberances, surface imperfections and other extra's 512

F-5.1. Fixed undercarriages 513F-5.2. Canopies and windshields 513F-5.3. Wheel-well fairings and blisters 514F-5.4. External fuel tanks 514F-5.5. Streamlined struts 514F-5.6. Powerplant installation drag 515F-5.7. Excrescences, surface imperfections and other extra's 516

References 519

G-l. Introduction 527G-2. Effect of trailing-edge flap deflection on airfoil section lift 528

G-2.1. General aspects 528G-2.2. Lift increment at zero angle of attack 529G-2.3. Maximum lift coefficient 534G-2.4. Lift-curve slope 536

G-3. Lift of aircraft with deflected trailing-edge flaps 537G-3.1. Wing lift 537G-3.2. Various contributions 540G-3.3. Contribution of the horizontal tailplane 540

G-4. Prediction of the low-speed drag polar 543G-4.1. Profile drag 543G-4.2. Vortex-induced drag 545G-4.3. Trim drag 545

G-5. Leading-edge high-lift devices 547G-5.1. Sections with plain leading-edge flaps 547G-5.2. Sections with slats and Krueger flaps 548G-5.3. Wing lift with leading-edge devices 549G-5.4. Drag due to leading-edge devices 549

G-6. Drag due to extension of a retractable undercarriage 550G-7. Ground effects 551

G-7.l. Ground effect on lift 551G-7.2. Ground effect on drag 553

G-8. Drag due to engine failure 553G-8.1. Engine windmilling drag 553G-8.2. Propeller drag 554G-8.3. Drag due to the asymmetric flight condition 555

References 556

APPENDIX H. PROCEDURES FOR COMPUTING TURBO-ENGINE PERFORMANCE FOR AIRCRAFTPROJECT DESIGN WORK 561

page

APPENDIX J. PRINCIPAL DATA OF THE US AND ICAO STANDARD ATMOSPHERES 571

H-4. Specific performance of turbofan engines 565H-5. Thrust lapse rates, intake and exhaust areas of turbojet and turbofan

engines 566H-6. Specific performance of turboprop engines 567H-7. Cycle efficiencies and pressure losses 568References 568

xv

573

574576578579579580581581583586

589

The ground run from Vx to VRThe rotation phaseThe airborne phaseThe stopping distance

References

INDEX

K-l. Reference distance definitionsK-2. Reference speedsK-3. Procedure for determining the takeoff field lengthK-4. Methods and data for the analysis of the takeoff

K-4.1. The ground run from standstill to Vx

K-4.2.K-4.3.K-4.4.K-4.5.

APPENDIX K. THE DEFINITION AND CALCULATION OF THE TAKEOFF FIELD LENGTH REQUIREDFOR CIVIL TRANSPORT AIRCRAFT

Foreword

Since the education of aeronautical engineers at Delft University of Technology startedin 1940 under t~e inspiring leadership of Professor H.J. van der Maas, much emphasis hasbeen placed on the design of aircraft as part of the student's curriculum. Not only isaircraft design an optional subject for thesis work, but every aeronautical student hasto carry out a preliminary airplane design in the course of his study. The main purposeof this preliminary design work is to enable the student to synthesize the knowledge ob-tained separately in courses on aerodynamics, aircraft performances, stability and con-trol, aircraft structures, etc.

The student's exercises in preliminary design have been directed through the years by anumber of staff members of the Department of Aerospace Engineering in Delft. The authorof this book, Mr. E. Torenbeek, has made a large contribution to this part of the studyprogramme for many years. Not only has he acquired vast experience in teaching airplanedesign at university level, but he has also been deeply involved in design-oriented re-search, e.g. developing rational design methods and systematizing design information. Iam very pleased that this wealth of experience, methods and data is now presented in this

book.

In the last twenty years of~university education for engineers much attention has beendevoted to the fundamental sciences such as mathematics and physics. Recent years haveseen a revival of the interest in IIdesign" and a number of general textbooks have nowbeen published on this subject. However, very few modern textbooks on the scienc& and tneart of aircraft design, are available. It is my sincere hope that Mr. Torenbeek's bookwill contribute to a renewed interest in airplane design in many parts of the aeronauti-cal world, both inside and outside universities.

In view of the immense increase of knowledge in the aeronautical sciences and engineeringsince the Second World War, it seems a formidable task, requiring much courage on theauthor's part, to write a textbook on airplane design. It is well-nigh impossible to dealwith all problems of airplane design at the same depth and undoubtedly personal choicehas to prevail in many areas with regard to the material to be presented. In my view, Mr.TQrenbeek has made an excellent choice of his subjects, preserving a careful balance be-tween the presentation of a design manual and a general textbook on airplane design.This volume will therefore be a most worthwhile guide to everybody who in the course of

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his professional training or career, is interested in the initial design phase of air-plane projects, an activity which is very important for shaping the future of aviation.

Delft University of Technology

August 1975

H. Wittenberg

Professor of Aerospace Engineering

XVII

Author's preface

This textbook is intended to offer readers with a professional interest in airplanedesign a general survey of the layout design process. It contains a large amount of

data and numerous methods which will be useful for carrying out the initial design cal-culations associated with the dimensioning of all major airplane parts. To a certain ex-tent it has the character of a design manual, but considerable attention is also devotedto qualitative background information.Several of the design methodologies and procedures presented have already appeared in theliterature on the subject, while others have been developed recently by the author. Theyhave been chosen on the basis of two criteria: they are not overdependent upon the stateof the art and they give reliable results with a minimum of information. Most of theprocedures have been extensively tested and considerably improved during the decade forwhich the author was responsible for students' design courses and projects in the Depart-ment of Aerospace Engineering of the Delft University of Technology. Emphasis is laidon conventional subsonic airplane designs in the civil category, i.e., broadly speakingthe airplane types to which the American FAR Parts 23 and 25 and equivalent BeAR require-ments apply (light and transport-type aircraft). Although many of the aspects to be dis-cussed are equally relevant to V/STOL and military aircraft, other complicattng factorsare involved in the design of these types, resulting in a radically different approachto the design process. The large variety of design specifications and configurations inthese categories prohibits a general treatment.

The author makes no apology for the fact that his approach to airplane design may bebiased by a university environment, probably not the ideal One in which to carry out de-sign studies. The teaching of design in the aeronautical departments of universities andinstitutes of technology has, unfortunately. not kept pace with developments in industri-al design practice. Aircraft design and development have become a matter of large in-vestments, even in the case of relatively small projects. The manhours required have in-creased considerably in recent years and the time is almost past when a single designercould consider himself the spiritual father of a new type.In contrast with the increased sophistication to be observed in industrial design veryfew regular design courses at technological universities and institutes have been able tosurvive the process of continuous curriculum evaluation and revision.Although experienced designers in the industry may possibly be the only authors qualifiedto write an authoritative textbook on airplane design, they are usually not in a positionto devote enough of their time to a task which is not felt to be in the direct interestof their employers. The reader may therefore conclude that the present book will be most

XVIII

ruseful for teaching and study purposes and for people who need a general introduction tothe vast field of initial aircraft design and development. Nevertheless, some of the pro-cedures and data presented will certainly be of some assistance to design departments inindustry.

A knowledge of the principles of applied aerodynamics, airplane structures, performance,stability, control and propulsion is required to derive the utmost from this book. Itsusefulness for degree design courses will therefore be greatest in later stages of thecourse. In the presentation of the individual subjects the need to balance design con-siderations is frequently stressed. This is particularly the case in the second chapter,where the initial choice of the general arrangement is discussed, the basis adopted beinga synthesis of many considerations of widely differing character. The main body of thebook is devoted to the rationale behind layout de~ign and although estimation methods forlift, drag, geometry, etc. are considered essential parts of the design process, theyhave been brought together in a separate set of appendices with a limited amount of text.Considerable attention is devoted in all the chapters to the impact of airworthiness re-quirements on design and to subjects that have been covered only very briefly by otherauthors. Particular emphasis is laid on the interior layout of }he fuselage (Chapter 3),a survey of the present and future potentials of aircraft engines (Chapter 4), system-atic design studies based on performance requirements (Chapter 5), and weight estimationmethods (Chapter 8). The complex interaction of wing location, center of gravity range,and horizontal tailplane design is treated in Chapters 8 and 9. The consistent collectionof prediction methods for lift, drag and pitching moment estimation will, it is thought,be useful as a general survey and as a tool for wing design (Chapter 7) and performancecalculations (Chapter 11). A large collection of statistical data, illustrations anddiagrams is added to this presentation, which aims at providing the individual student/designer or the small design team with reliable guidelines. For industrial applicationssome of the methods may have to be refined and/or extended.A large and systematic list of references to literature is presented, which will help thereader to find more information on the subjects specifically dealt with and on other re-lated subjects. As he glances through these references the reader's attention may bedrawn to a particular subject that interests him, possibly stimulating him to add anotherinnovat1on to the design synthesis of his project and thereby contribute to the overallquality of aircraft design technology.

ACKNOWLEDGEMENTS

As is the case in the preparation of most technical books, the author of this volumeis indebted to many persons who have aided in its completion.For many years Professor H. Wittenberg has been the promotor of courses in preliminaryaircraft design, and the idea of writing this book came about as a direct consequenceof his activities. The author wishes to express his appreciation to him for his generalsupport, for his critical revision of the text and for his willingness to write theforeword.I am indebted to Mr. G.H. Berenschot, who has given general and technical assistanceby collecting information and data, preparing many figures and tables, compiling theindex and reVising the text in detail. His perseverance, friendship and the moral supporthe has given me for many years have been particularly. invaluable.I would like to express my appreciation to Professor J.H.D. Blom, chief aerodynamicist,and Ir. P.F.H. Clignett, preliminary design engineer, both of Fokker-VFW International

XIX

as well as to Ir. C.H. Reede, head of the scientific departmeht. of the Royal Dutch Air-lines (KLMl, and to my colleagues Ir. F.W.J. van Deventer and Dr. Th. van Holten, whoprovided valuable and detailed suggestions together with actual text after readingparts of the book for their technical content. In addition, many students have usedforerunners of the present text during their studies and their useful feedback hasresulted in many improvements.Thanks are due to the Department of Aeronautics and Space Engineering of DelftUniversity of Technology for granting permission to prepare and publish this book, andfor providing the necessary typing and duplication facilities. Many members of thisDepartment and of the Photographic Office of the Central Library have given professionalhelp in producing the illustrative material. I am also indebted td the Delft UniversityPress, and in particular to Ir. P.A.M. Maas and Mrs. L.M. ter Horst-Ten Wolde for theirsupport, encouragement and assistance in editing the publication.The author wishes to express his gratitude to Messrs. J. van Hattum, J.W. Watson M.A.,and D.R. Welsh M.A., who have made admirable contributions to the translation of theDutch text and to improving the readability of the book. I am extremely indebted toMrs. C.G. van Niel-Wilderink, for the excellence with which she performed the formidabletask of typing not only the manuscript, but also the final copy for photoprinting. Iwould like to thank my uncle, the Rev. E. Torenbeek, for his painstaking efforts inchecking the typographic accuracy of the copy, and mr. P.K.M. De Swert for preparingthe final layout and for his methodical checking.The following individuals, companies and organizations have kindly provided data anddrawings.

Advisory Group for Aerospace Research and Development: Figs. 5-24, 10-1, 10-2, 10-3,F-17 and F-22;American Institute of Aeronautics and Astronautics: Figs. 2-1, 4-39, 7-20, &-1, 9-11,10-5, F-12 and G-25;Aeronautical Research Council: Figs. 2-24, G-16, G-17 and K-6;Aerospatiale: Figs. 2-3, 2-10 and 7-25;Airbus Industries: Figs. 11-5 and 11-9;Aircraft Engineering: Figs. 3-1, 3-11, 3-21, 3-26, 6-16, 7-25, 10-17, 10-16, 12-3 and12-5;Alata Internazionale: Fig. 2-3;The Architectural Press Ltd.: Fig. 3-6;Avco Lycoming: Fig. 4-14;Aviation Magazine: Fig. 2-8;Avions Marcel Dassault-Breguet Aviation: Figs. 3-3, 3-11 and 10-20;Boeing Aircraft Company: Figs. 2-11, 2-15, 3-11, 5-18, 7-25 and 10-6;Mr. S.F.J. Butler: Fig. F-22;Canadair Limited: Fig. 3-21;Canadian Aeronautics and Space Institute: Fig. 3-27;Centre de Documentation de l'Armement: Figs. 2-5 and 10-21;The De Havilland Aircraft of Canada Ltd.: Fig. 3-11,Detroit Diesel Allison Division of General Motors Corporation: Fig. 4-46;Dowty Group: Fig. 6-27;Engineering Sciences Data Unit Ltd.: Figs. F-23, G-26 and K-9;Flight Control Division of U.S. Air Force Flight Dynamics Laboratory: Figs~ A-2, A-3, E-3and E-9;Fokker-VFW International: Fig. 11-14;

xx

XXI

Finally, I would like to thank my wife, Nel, for her unstinting help without which thisbook would hardly have been possible.

In accordance with the convention used in publications such as Jane's All the World'sAircraft and Flight International, all data and most of the figures have been given inthe technical unit system, both in British and metric units. Hence, lb and kg refer topound and kilogram forces, respectively. An exception is made in Appendix J, where sealevel data of the Standard Atmosphere have been given both in the technical and SIsystems.

Ir. E. Torenbeek

UNITS

September 1975Delft University of Technology

Flight International: Figs. 2-2, 2-10, 2-11, 3-8 and 7-30,Hamilton Standard Division of United Technologies Corporation: Fig. 6-5,Hartzell Propeller Inc.: Fig. 6-7;Hawker Siddeley Aviation: Figs. 2-20, 3-21, 12-2 and 12-4,International Civil Aviation Organisation: Figs. 10-4 and 10-5;De Ingenieur: Fig. 5-22,Institute of Aerospace Sciences: Fig. 6-24;Lockheed Aircraft Corporation: Fig. 3-17,McDonnell Douglas Corp.: Figs.3-14, 6-2, 7-25;McGraw-Hill Book Cy.: Fig. 6-19,Messerschrnitt-Bolkow-Blohm: Fig. 2-7;(British) Ministry of Aviation: Fig. 10-6,National Aeronautics and Space Administration: Figs. 2-25, 6-5, 7-5b, 7-22, 9-8, E-2,E-5, E-6, G-ll, G-12, G-19 and G-22;National Research Council of Canada: Fig. 3-27;Pergamon Press: Fig. 5-11,Mr. D.H. Perry: Fig. K-6,Piper Aircraft Corporation: Fig. 4-10,Polytechnisch Tijdschrift: Figs. 2-10 and 7-25f,Pratt and Whitney Aircraft Division of United Technologies Corp.: Fig. 4-46,The Royal Aeronautical Society: Figs. I-I, 2-14, F-6, F-22 and K-8,Rolls Royce 1971 Ltd., Derby Engine Division: Figs. 4-15, 4-19, 4-20, 4-37, 4-46 and6-25;Dr. W. Schneider; Figs. C-2 and C-3;Ir. G.J. Schott Jr.: Fig. 5-22,Society of Aeronautical Weight Engineers: Figs. 11-7 and D-l,Society of Automotive Engineers, Inc.: Figs. 4-7, 4-8, 4-9, 6-2, 6-7, 6-23, 6-27, 7-5and 11-13;Mr. W.C. Swan: Fig. F-17,Turbomeca Bordes: Fig. 4-46;Vereinigte Flugtechnische Werke-Fokker Gmbh: Fig. 3-11;Mr. R.E. Wallace: Fig. 5-24;John Wiley and Sons Ltd.: Fig. F-8.

Preface to the student Edition

Textbooks on the rapidly advancing subject of aircraft design tend to becomeobsolete within a few years. In spite of this the first edition has proved itsvalue up to the present time, as a reference source for design efforts andpublications in many places allover the world.It therefore pleases me that the publishers have decided to launch this new edition.aimed at an expansion of the market into the university classroom, thereby makingthe book affordable by many more individuals.This has given me the opportunity to further refine some of the methods andformulations, mainly on the basis of suggestions and comments of attentive students.In spite of the reduction in size, the contents are not abbriviated.

Delft, May 1981E. Torenbeek

XXII

Chapter 1. General aspects of aircraft configurationdevelopment

SUMMARY

It is shown that there is an interaction between the development work for a new designand the various factors determining the need for a new type of aircraft. Preliminary air-craft design is an essential part of this development phase; its aim is to obtain the in-formation required in order to decide whether the concept will be technically feasibleand possess satisfactory economic possibilities.Attention is paid to the impact of the design requirements laid down in the initial spec-ification and the airworthiness regulations. General observations aimed at illustratingthe aircraft design and optimization process are presented.

1-1. INTRODUCTION

In the pioneering era of civil aviation theaircraft designer had only a very limitedchoice. There was practically only one cat-egory of powerplant at his disposal, namelythe - nearly always aircooled - piston en-gine, which gave very limited power. Eitherthere were no aerodynamic aids to augmentthe lift of the wing at low speeds, or thosethat did exist were, for various reasons,seldom used, with the result that wingloadings were kept low and high speeds wereconsequently unobtainable. Low wing loadingfavored the biplane layout which, with itshigh parasitic drag, formed another obsta-cle to high speeds. Flight was rarely above10,000 feet (3,000 meters), since there wereno pressurized cabins. During this epochaircraft design was generally the work ofone or a very few designers in each factoryand the scope of development work for eachnew aircraft type was limited. In the twen-ties it was possible to design and producea new aircraft for delivery to the customerwithin half a year, one of the reasons be-ing that series were relatively small. Thisenabled Anthony Fokker and his staff tobuild fourteen entirely different commer-cial designs during the relatively shortperiod of eighteen years (1918-1936).But the'nature of aircraft project designhas undergone a radical change since theSecond World War. Development of the jetengine and subsequently the turbofan, nowsupplying a thrust up to about 50,000 lb(22,500 kg), has greatly widened the choiceof powerplants. Transport aircraft nowcruise at altitudes of 30,000 to 40,000feet (9,000 to 12,000 m) at speeds not farbelow that of sound. The takeoff and land-ing speeds of the largest aircraft have, ofcourse, risen steadily and runways haveconsequently become longer and longer, butmeans to call a halt to this trend havemeanwhile proved technically feasible.Air transport has passed through an era ofunprecedented growth. During the periodfrom 1950 till 1970 the average yearly in-crease in the passenger-miles flown reached

2

14%, a growth figure which was only exceed-ed by the sale of plastics. The increase intransport productivity (payload times speed)of the largest transport aircraft has beenequally impressive. In addition, modernaircraft have to satisfy an ever increasingnumber of'severe safety regulations, whileeconomic requirements, resulting from in-tense competition, have steadily becomemore exacting, with the result that the de-velopment and construction of new types ofaircraft - even relatively small ones - de-mand a very high capital outlay and entailconsiderable financial risk. An aircraftindustry nowadays is generally unable toproduce an entirely new type of aircraftoftener than once every 12 years (roughly),quite apart from the question of whetherthere is any need for more rapid replace-ment of existing types. An exception to this"rule" must be made for the giants in theaircraft industry, e.g. Boeing and McDonnellDouglas, who are able to bring out one ortwo additional new designs during the sameperiod. The sheer size and long leadtime ofnew projects have led various firms toshare the risks by cooperation, while inEurope they have resulted in internationaljoint ventures.Although new concepts have been and willcontinue to be proposed from time to timeby talented designers, the time is pastwhen a chief designer could be regarded asthe spiritual father of a new type of air-craft. A possible exception to this mightbe made in the case of private aircraft andsmall transports. Preliminary design de-partments nowadays have staffs numberingsome dozens to several hundreds of highlytrained technicians and engineers and com-puting facilities have increased immensely,while some preliminary design teams evenhave wind tunnels permanently at their dis-posal. More manhours are now being investedin the project design phase than were for-merly spent on the entire detail design.Work in the design department has developedinto a professional occupation, carried outin teams, with regular consultations be-tween specialists in various disciplines.

I DEMAND I TRAFFICI BUSINESS r- VOLUME -PRIVATEt

I FARES II PASSENGER APPEALt

SIZE AND NUMBER OF II SPEED, COMFORT, AIRCRAFT REQUIREDRELIABILITY, SAFETYI

OPERATING ICOSTt

...... OPERATIONALTECHNICAL REGULARITY, FREQUENCY, FINANCIAL

AERODYNAMICS ROUTESPROPULSION

~ INDIRECT COSTS UNIT I PRODUCTIONSTRUCTURES/MATERIALS

-

DIRECT COSTS--

COST I DEVELOPMENTSYSTEMS, EQUIPMENT

Fig. 1-1. Factorscontributing tothe growth of airtransport, accord-ing to Morgan(Ref. 1-26)

3

*For example, the role of the government'saeronautical activities has been omitted.

tors contributing to the growth of airtransport. Growth in air traffic stems fromreduction of fares, improved quality of theaircraft (speed, comfort), increased busi-ness activity and growth of private incomes,aircraft capacity growth, increasing numberof routes, increasing frequency on existingroutes, and greater utilization of aircraftand ground facilities. The contribution ofresearch and development to this process,indicated in the lower left-hand corner ofthe diagram, is unique because this blockshows only output lines. Although the dia-gram is obviously a simplification of thereal situation and, at the same time, mustnot be considered as a control system*,it does indicate that aircraft developmentis a primary cause of growth. As a corol-lary to this, it is necessary in launchinga new development program to appreciate theinteracting effects of the "aeronauticalenvironment" in advance and thereby ensurethat there will be no conflict with the(future) needs of operators, passengersand the general public. Moreover a numberof industrial constraints set limits to thefeasibility of new projects, namely:a. the available project development organ-ization and production capacity;

1.2. AIRCRAFT DESIGN AND DEVELOPMENT

This does not imply that the man in ov~rallcharge of preliminary design has himself tobe a specialist. As will be shown furtheron, he must be able to take a wide interestin and have a sound insight into a greatmany disciplines related to design as awhole.

Although the technical aspects of aircraftdesign form the subject of this book, itwill be appreciated that this is not an ac-tivity carried on in remote offices by spe-cialists generating designs of any kindthat may occur to their imaginations. Thereis close interaction between the develop-ment work for a new aircraft type and theother factors which together determine thegrowth of and/or changes in aeronauticalactivities. These interrelationships beingdifferent for the various fields of aero-nautics (passenger and cargo transport,business aviation, tourism, flying instruc-tion, etc.), we will deal with the matterby quoting a single example here.The development of new airliners has alwaysbeen stimulated mainly by the growth of thetraffic volume and the improvement of tech-nical and operational standards. Fig. 1-1is a scheme for allocating the various fac-

Fig. 1-2. Airplane design and development

ENGINEERINGDESIGN

*Frequently referred to as "preliminarydesign e~gineering".

first deliveries to customers will takeplace. The information gathered during thisperiod generally leads to engineering modi-fications, which will occupy the design of-fice for a long time to come.After the first production series has leftthe factory, the company will continue todevelop its product. These developments maytake the form of an increase in the air-plane's transport capacity (stretching),installation of an improved type of engine,improvement of performance by introducingaerodynamic refinements, such as "cleaningup" the aircraft, etc. Successful aircraftgenerally go through a process of growthwhich offers the customer a choice of anumber of variants, each suitable for aspecific transport assignment, and this con-siderably strengthens the company's abilityto face up to competition. In Fig. 1-2these activities have been arranged in threegroups: the confiquration development* phase,the detail design phase and the service en-gineering phase. The first two of thesehave been taken as separate phases, sincethe decisions taken during the first stageare still partly based on the statisticalprobabilities that specific technical aimswill be achieved and the actual construc-tion is only defined in broad general terms,whereas in the detail design phase the air-craft is designed "down to the last rivet"and the detailed production schedule islaid down. During this period the number of

b. the technical and industrial knowhow re-quired for developing a new category of air-craft;c. the prospects as regards competition;d. the availability of adequate financialbacking.

4

The initiative for a design study does notalways stem from any specific person (chiefdesigner) or department (preliminary designoffice) and does not necessarily take theform of an order issued by the management.The idea is elaborated by the preliminarydesign department during an initial specu-lative design phase in a feasibility study.The object of this conceptual design phaseis to investigate the viability of the pro-ject and to obtain a first impression ofits most important characteristics.If the results seem encouraging both from atechnical point of view and as regards themarket prospects, a decision may be takento develop the design further in order toinitiate a new-design aircraft developmentprogram (Fig. 1-2). Comparisons will bemade with some alternatives, preferably ona systematic basis. The design that hasscores the highest rating will be elaboratedin greater detail in the preliminary designphase. A characteristic of this phase isthat modifications are made continuouslyuntil a decision can be taken to "freeze"the configuration*, and this marks the endof the preliminary design phase. If themarket is considered likely to receive thedesign favorably and finance for the proj-ect is assured, the management may givepermission for further development (go-ahead approval). The subsequent phases ofdetail design, construction and testingwill lead to the granting of a Certificateof Airworthiness and some time later the

*The expression "configuration' as usedin this chapter refers to the general lay-out, the external shape, dimensions andother relevant characteristics. It is notintended to indicate the actual airplaneconfiguration as characterized by the po-sition of the flaps, landing gear, etc.

1 ASSESSMENT or CONTRACTOR & AIRWORTHINESSCUSTOMER REQUIREMENTS REQUIREMENTSI Tange. payload, speed. etc. materials, production. process-~I es, handling qualities, etc.

:IIIt' INITIAL DESIGN SPECIFICATION",I

iu!Z' I DESIGN CONCEPT I81 I ENGINE(S) SELECTION Jt--- !INITIAL BASELINE DESIGN!i PARAMETRIC "DESIGN" PHASE

effects studies, trade-of f II.

conf iguration variabJ esi

ilASELINE CONFIGURATION DEVELOPMENT

'"~I winl , high.speed characteristics.high-lift system!! fuselage: contours. mock ups~ handling qualities..

structural analysis!::l:5 systems design~,., powerplant installation~ performance assessment

operating economy. noise characteristics

I TECHNICAL DESCRIPTION I

documents and draWings will increase rapid-ly and the development costs will show anearly proportional rise.

1.3. CONFIGURATION DEVELOPMENT

The principal aim in this phase of design(Fig. 1-3) is to obtain the informationrequired in order to decide whether theconcept will be technically feasible andhave satisfactory economic possibilities.In contrast to the detail design phase,neither the actual construction processnor the detailed production schedule playsa dominant part here.An important aspect of the entire develop-ment of a new type of aircraft is that ittakes place in a succession of design cy-cles. In the course of each of these cy-cles the aircraft is designed in its en-tirety and investigation is carried out

Fig. 1-3. Configurationdesign and developmentof a high-subsonic trans-port aircraft.

into all the main groups and airframe sys-tems and equipment to a similar degree ofdetail. The extent of this detailing stead-ily increases as the design cycles succeedeach other, until finally the entire air-craft is defined in every detail. On thebasis of the terminology given in Figs.1-2 and 1-3, the subsequent basic designstages might be designated as follows:a. conceptual design;b. initial baseline design;c. baseline configuration development;d. detail design.These design stages might also be referredto as the speculative design, the feasibledesign, the best conceivable design and thefinal hardware design cycles. A number ofaspects of the first three of these desi~ncycles will be further elaborated and dis-cussed in the course of this book. The co,.{-ceptual design will be the subject of thesecond chapter. Chapters 3 through 10 dis-

5

cuss the procedure followed in evolving aninitial baseline configuration and consti-tute the main portion of this book. Chap-ters 11 and 12, finally, present a surveyof several items which are related to fur-ther elaboration and presentation of a de-sign. Since the information on these itemsavailable elsewhere is abundant and exceedsthe scope of our textbook, a representativechoice has been made from the multitude ofsubjects, while some have been further e-laborated in the form of appendices. Inview of the fact that the author has beenmainly concerned with aspects of aerodynam-ic design and performance, it was decidedto present methods required to estimatesome aerodynamic and flight characteris-tics.

1.3.1. The design concept

In the initial phase the probable demandfor a new type of aircraft is furtherspecified with the aid of market surveys,further inquiries and discussions withpotential customers. Market research formsa specialized discipline for which thelarge aircraft companies employ a separatedepartment or office, whil~ in smallerfactories the work is often done by thedesign team. In either case, it is essen-tial that the designer or the design teamis closely involved since there is nosense in starting a design before the na-ture of the design requirements has beenstudied from all angles and a clear picturehas emerged On which to base the generaldesign philosophy.The market survey will lead to an initialspecification which will mainly define thetransport performance - payload and maxi-mum range, often also the cruising speed -as well as the most relevant field andclimb performance, cabin arrangement, air-frame services and equipment, etc. A deci-sion will also have Lo be made as to whichset of airworthiness and operational re-quirements the design will have to complywith. Fig. 1-3 oversimplifies the situationby presenting the airworthiness require-

6

ments in a separate box whereas in actualfact they influence the entire developmentof the aircraft and are a dominant factorin every design decision down to the verylast detail. Special attention is there-fore devoted to airworthiness requirementsin a separate section at the end of thischapter. Prior to and also during the pro-ject, the designer will closely study oth-er types of aircraft or aircraft projectsor certain design aspects which most close-ly fit the specification as regards thetransport requirements. A summary and crit-ical assessment are made of the principaldata of competing designs, literature, pre-vious experience, etc.Brainstorming sessions are arranged in or-der to generate "new and wonderful ideas",most of which will generally be discardedagain. The reader will find some interest-ing creative attempts to stimulate newconceptions in Ref. 1-13, while Ref. 1-35presents a most fascinating account of theactivities which take place in a large air-craft industry during the project designphase. Nevertheless it remains most unlike-ly that procedures developed during thisextremely speculative first design phasewill offer any certainty of leading to asuccessful new conception. There is no realsubstitute for the originality of the en-gineer who is capable of forcing a break-through with a unique brainwave such asthe monocoque construction, the sweptbackwing, j~t engines at the rear of the fuse-lage (a la Caravelle), area ruling andother inspired innovations, sometimesgenerated in an environment where projectdesign is engaged in only incidentally, ifat all. Design concepts are therefore beingdeveloped continuously, while only very fewactually result in a preliminary design andsubsequent development program (Fig. 1-2).The conception phase will result in prelim-inary layout sketches of the kind shown inFig. 1-4, including a summary of the prin-cipal characteristics and basic designphilosophy on which subsequent designstages will be founded. Although hardly anythought has been given at this stage to

details, a complete aircraft has alreadybeen put on paper and the designer may navea strong feeling that it could be the an-swer.

1.3.2. Initial configuration design andconfiguration variations

Since design is not a deterministic process,particularly in the early stages in whichthe conception is realized, various solu-tions to attain the desired goal will pre-Sent themselves. If it proves impossible to

Fig. 1-4. Initial design con-cept of an ultra-short-haulairliner (30 passengers)

weigh up the pros and cons and arrive arealistic answer, based on the intuitionand experience of the designer(s), compar-ative studies will have to be undertaken.Since it will generally not be found fea-sible to transform all likely configura-tions into fully developed projects, a pa-rametric design phase, as shown in Fig.1-3, is often decided upon. This first en-tails the development of an initial base-line design (or point design), using rela-tively easily applied sizing methods, pro-vided these are availabl~. In this book the

7

reader will find a number of these methods,based partly on theoretical interrelation-ships, partly on statistical material (see,for example, Chapters 5, 6 and 7). A com-plete layout is made of this baseline de-sign, showing three views and some princi-pal cross-sections. The next step is tocheck to what extent the characteristicsand performance of the design will meetthe design requirements.Changes are now made in this baseline de-sign, preferably in a systematic manner,following predetermined and clearly de-fined working rules. There will thus e-merge a family of designs which are easilycomparable with each other as well as withthe baseline design. The object of thisexercise is twofold: first to improve t.hedesign where it does not meet the require-ments, and second to investigate the mostlikely possibilities and see whether othervariants may prove a better proposal. Itmay also show that changes in the designrequirements would yield a better overallbalance. Although the diagram in Fig. 1-3suggests that the type of engine has al-ready been chosen before the initial base-line design has been put on paper, the pa-rametric design phase may nevertheless in-clude studies of variants with differenttypes of engines and even a different num-ber of engines per aircraft. When a numberof variants are studied, a systematic ap-proach is essential in order to obtain asound basis for comparison. Although theabsolute accuracy of the methods usedshould be as high as possible, the mainobjective is to differentiate between thedesigns. Final judgement at the end of thisphase will result in a baseline configura-tion which, subject to approval by all con-cerned, will be chosen for further develop-ment. It can be presumed that this design,after detailed engineering, will probablymeet the initial specification while, inaddition, it will be the best conceivabledesign. On the other hand, in the absenceof complete certainty as to specific aero-dynamic characteristics of the consequencesof variations in weight as between differ-

8

ent structural designs, the parametricstudy may not lead directly to a definiteconclusion. In such an advent it is advis-able to consider whether it would not bebetter to carry out a further detailed in-vestigation of, say, two alternative con-figurations before an irrevocable choiceis made. This point is treated more fullyin Ref. 1-38.To give some idea of the magnitude of thisphase of a preliminary design, the initialbaseline design of a small transport willrequire something in the order of severalthousands of manhours, whereas the subse-quent design phase of variants and param-etric studies will demand a multiple ofthis. These figures will naturally show aconsiderable spread and will depend on thetype of aircraft and on the extent towhichthe company is prepared to pursue the in-vestigation. A number of examples of param-etric studies are given in this book (Sec-tion 5.5) and also in the referencesappended to Chapter 5. Examples of methodsfor estimating weights, aerodynamic charac-teristics and the sizing of tail surfacescan be found in Chapters 8 and 9 and in theappendices.

1.3.3. Baseline configuration development

During this phase the baseline design isfurther developed to a depth of detailwhich can be regarded as meaningful. Vari-ous sections of the design department willbe called in to contribute to the aerody-namic design, the stressing of the mainstructure, design of the airframe systemsand equipment, etc. At the earliest possi-ble stage a start is made with tests in thewind tunnel, while the external lines aredetermined and mockups showing the inter-nal layout of the fuselage, cable runs andinstallations, are built and all the re-maining tasks involved in arriving at acomplete definition of the project arecarried out.During the development of the baseline con-figuration, errors will be observed whichhave been made during the phases already

described and which are usually caused bylack of data. Correction of these errorswill entail corrections in the design whichwill have repercussions for all the disci-plines involved.Coordination during this phase will be theresponsibility of the preliminary designdepartment, for this team will be mostfamiliar with the project and is conse-quently best able to visualise the conse-quences of the corrections. One of the mostfrequent jobs during this phase is thesetting-up of a weight control program,particularly for those weight componentswhich have been estimated solely on thebasis of statistical mate=ial. These rec-tifying programs and corrections may insome cases be covered by feedbacks in Fig.1-3, but specific mention of them therehas been omitted to preserve the clarityof the illustration.As soon as the project can be regarded assufficiently mature and any doubts regard-ing its essential characteristics have beenremoved, the project manager may take thedecision to freeze the configuration andthis means the end of the preliminary de-sign phase. The characteristics of the de-sign are surrmarized in a technical descrip-tion which serves as a basis for discus-sions with potential customers.Some idea of the scope of a configurationdevelopment program can be obtained fromRef. 1-36, which gives the following in-formation concerning the Lockheed L-1011:"In the two years of the configuration de-velopment, over two million man hours wereexpended to investigate various configura-tions and approaches to determine the op-timum design. More than 10,000 hours oftesting have been completed in seven dif-ferent wind tunnels to establish the mostefficient overall configuration".

1.3.4. The preliminary design department

When the development of a new type of air-craft is to be undertaken, the generalpractice nowadays is to form a projectgroup, containing not only preliminary de-

sign engineers but also experts in otherdisciplines, such as:- aerodynamicists who are directly c~ncerned with the design of the externalshape,- structural engineers, dealing with pre-liminary research into the overall struc-tural layout and carrying out the dimen-sioning and optimisation,- production experts and experts in thematerials field who investigate what typesof production methods should be adopted,- service experts, to ensure easy mainte-nance and overhaul,- weight engineers, whose job is to dealwith the prediction and control of theweight (distribution) and moments of iner-tia,- engineers to design the flight controlsystem and analyse flying qualities,- designers of airframe systems and equip-ment, and

- financial and economic experts who arenot only able to estimate the first andoperating costs of the aircraft, but alsokeep a close check on the financing of theentire design project.Since this book does not deal primarilywith the organizational aspect of projectdevelopment, but rather with the technicalaspects of airplane design, we will specifyonly the various tasks of the preliminarydesign team. Unlike the other departmentsof the design office, this team is perma-nently engaged in project work and its workmainly consists of the following activi-ties:- market analysis and the drawing-up ofinitial specifications for new types ofaircraft in close cooperation with thesales department;- devising various solutions to a givendesign problem;- evaluation of different design proposalsusing preliminary design methods in orderthat decisions are taken on the basis ofasound assessment of the pros and cons;- setting up and coordinating detail re-search oriented on aerodynamical, struc-tural and other problem areas. These tasks

9

may be of a general character, such as thedevelopment of design methods for esti-mating drag, weights, etc., or project-oriented, such as the aerodynamic designof a flap system;- discussions with potential customers and(future) subcontractors for main compo-nents such as engines, landing gear, air-frame services, avionics, etc.:- assisting the sales department by supply-ing technical data;- making product development studies, aimedat increasing the utility of existing air-craft.

1.4. THE INITIAL SPECIFICATION

There is certainly no need to prove thatsufficient material on a subject such as"market analysis aimed at the developmentof new aircraft" exists to warrant thepublication of a separate volume. Thepres-ent paragraph will of necessity have to berestricted to a few general observationswith civil aviation as their main back-ground. The example used will be an ini-tial design specification for a hypothet-ical short-haul airliner for 180 passengersin the all-tourist layout, referred to as"Project M-184". A design evaluation ofthis project can be found in Ref. 1-64; itwas intended as a highly simplified exam-ple for the purpose of illustrating thedesign process in a series of lectures. Anapology is due for the fact that most ofthe considerations which follow in thepresent section apply to this particulardesign, intended for introduction intoservice around 1980.

In civil aviation the specification of anew aircraft type is generally drawn up bythe manufacturer. Airlines are usuallymore content to evaluate projects offeredto them for use on their own route network,though in a few caSes they themselves havetaken the initiative and written the spec-ification which they felt was required.The designer will, however, realize that

10

a project can only be justified when it islikely to find a worldwide market. A spec-ification issued by an airline will onlybe interesting provided it also appeals tocompetitors. It should also be realizedthat operators do not necessarily possessthe best insight into the technical capa-bilities and knowhow of the airplane manu-facturer. Nevertheless, there are examplesof successful aircraft which have been de-signed to an operator's specification or aspecification written with the customer'sactive cooperation (Viscount, Tristar andDC-10). All the same, the responsibilityfor the specification and the resultingproject will still rest squarely on theshoulders of the aircraft manufacturer.This procedure is quite different from thecase of military aircraft, where the spec-ification will nearly always be issued bythe customer: the armed forces.The term "initial specification", as opposed to

the more detailed type specification of a design,

is used here to emphasize that there is an inter-

action between the technical design work and the

development of the design requirements as a result

of market analysis, engine development and various

assessments during the development phase.

1.4.1. The need for a new type of aircraft

The following are some good reasons forinitiating a new aircraft design:a. EXisting aircraft are becoming eithertechnically or economically obsolescent,and a new type may do the job better. Newstandards for equipment, maintenance, op-erational use, noise suppression, passen-ger comfort, etc. may make renovation ofthe operator's fleet desirable.b. Certain developments in traffic patternshave created a need for new types of airtransport. For example, the growth oftraffic may, as explained in Section 1.2,result in a new class of (larger) trans-port aircraft, or new travel habits (hometo work and back) may open up the possi-bility of a new class of commuter aircraf~Air transport may fulfil the needs of de-veloping countries, where the infrastruc-

Fig. 1-5. Initial specification of a hypothetical short-haul airliner for introductioninto service around 1980

HUMBER OF PASSENGEIlS IN AN ALL-TOURIST LAYOUT(SEAT PITCH 34 IN 87 K): 180 ORIlRE.COIlRESPONDING DESIGN PAYLOAD: 20.000 KG(44,100 LB). AN UNDEIlFLOOR FREIGHTHOLD VOLUKJlOF AT LEAST 50 K3 0,762 CU.FT) WILL BEREQUIRED. STANDARD SIZE BELLY CONTAINEIlS AREPREFEIlRED.

RANGE, WITH ABOVE MENTIONED PAYLOAD: 2,200

KK (1.200 NK) IN A HIGH-SPEED CRUISE, ATADOKESTIC RESERVES. KAXIKUK RANGE (REDUCEDPAYLOAD): 3,200 KK 0,726 NK) AT LONG-RANGECRUISE TECHNIQUE.

KAX. CRUISING SPEED AT 9,150 K (30.000 FT) AL-

ture is inadequate for surface transport.c. A new type of aircraft is built andtested in order to give added impetus toan important new technical development,such as a V/STOL demonstrator prototype.Since experimental aircraft nearly alwayslead to a financial loss, at least in thefirst stages, there will have to be govern-ment funding, e.g. in the form of a devel-opment contract.Manufacturers should be wary of aiming atfilling the "gap in the market". That gapmay well have remained unfilled for thesimple reason that the need for an air-plane of the kind was insufficient. Anotherdanger which should be warned against isthe adoption of a particular technical nov-elty which in itself may be a very cleverachievement but is unlikely to contributeto profitable operation of the aircraft.Nevertheless, the design office will becontinually involved in studies aimed atdetermining the potentialities of newtechnical developments and innovations.Any new type designed will have to bemarketed in accordance with a properlythought-out time schedule. It is importantto remember that if it is offered too earlythe production rate will increase too slow-

TITUDE: K .82. DESIGN LIKITS: '\to .85.VIl 704 KKII (380 KTS) EAS.

FIELD LENGTH REQUIRED FOR TAREO", AND LANDING.ACCORDING TO AIRWORTHINESS RULES: 1.800 K(5.900 FT) AT SEA LEVEL, ISA + 20C (95 OF).AT KAXIKUK(CERTIFICATED) TAREO", WEIGHT.RUNWAY LOADING: LCN 3D, RIGID PAVEKENT, 18

CK (7 IN.) THIC1CNESS.

REGULATIONS: FAR PARTS 25, 36 AND 121. THE NOISE

CHARACTERISTICS MUST SHOW AN IKPROVEKENT RELA-

TIVE TO THE 1969 VEIlSION OF FAR PART 36 OF 10

EPNd8.

ly, resulting in a productivity loss oninvestments which the company has put intothe project. A launching delayed too longmay be equally disadvantageous, either be-cause the market has meanwhile been satura-ted by competitors' products or because theproduction line has to expand too fast andexcessive manpower has to be (temporarily)hired and additional investments made.

The initial specification shown in Fig. 1-5was drawn up for an airliner intended toaugment and replace the current class ofhigh-subsonic short-haul passenger trans-ports: the BAC 1-11, McDonnell Douglas DC-9and Boeing 737, and to some extent alsoaircraft designed for medium ranges: theHawker Siddeley Trident, A~rospatialeCara~elle and Boeing 727. The category con-sidered does not include smaller aircraftsuch as the Fokker F-28 or the VFW-6l4. Theaircraft mentioned above are powered bylow-bypass turbofans and have a capacityof 80-120 passengers (short-haul) or 120-180 (medium-haul). The need for a new typestems from the following considerations:a. The increased traffic volume requireslarger-capacity aircraft.b. The new standard of passenger comfort

11

introduced by the wide-body jets will un~doubtedly be extended to short-haul traf-fic.c. Reduction of noise production will be aprerequisite in the eighties.d. Technology improvements in the fields ofhigh-speed and low-speed wing aerodynamics,new structural materials (composite struc-tures), lightweight avionics and improvedflight control systems may be consideredfor application in this new aircraft cat-egory.In view of the large volume of short-haultraffic the market seems to offer scopefor a new aircraft with smaller capacityas compared to the Airbus A-300, for ex-ample.

1.4.2. Transport capacity

When a new specification is being drawn up,the first step will have to be a forecastof the traffic and the transport demandover the route sector concerned during theperiod under review. A technique commonlyused here is a statistical analysis of theyearly growth percentage of the total dis-tance covered by passengers in terms ofpassenger-miles (passenger-kilometers). Onthe basis of.an extrapolation of thisgrowth percentage, the total transport de-mand for the period considered may be es-timated. Assumptions will next have to bemade regarding the frequency of the flights,the average load factor and yearly utili-zation and from these the desired produc-tivity (number of passengers times blockspeed) can be deduced. A rule of the thumbsometimes used states that the most favor-able time between successive flights overa particular route is about equal to thetime taken to fly the route. Hence, if theblock speed increases, the frequency of theservice should also be stepped up. The fol-lowing are some other aspects to be con-sidered:a. For a large capacity aircraft the oper-ating costs per aircraft-mile will be high,but those for a seat-mile will be low,since certain costs do not rise proportion-

12

ally to the size of the aircraft, e.g. to-tal salaries of the flight crew and thecost of avionics and certain serVices, andwill therefore decline with each addition-al seat.b. A comparatively small aircraft willshow a low cost per aircraft-mile and itscritical load* will be smaller than thatof a large aircraft. This does not neces-sarily apply to the critical load factor(critical load/maximum load).Generally speaking, large aircraft arebest suited to routes with high trafficdensity, provided the frequency of opera-tions is compatible with the market re-quirements.

In drawing up the specification for the M-184 project (Fig. 1-5), the following con-clusions were arrived at:a. During the 1960-1970 period short-haultraffic grew considerably, both in theUnited States and in Europe. A yearlygrowth of 15 percent, resulting in adoubling in five years, was no exceptionand the growth was even more marked duringthe 1965-1970 period. Charter traffic infact underwent an explosive expansion dur-ing that same period, with growth percent-ages as high as 25 to 30. Factors whichcontributed to this growth were: regulartariff decreases, a rising level of pros-perity, and the greatly improved comfortof jet aircraft compared with other meansof transport.b. A gradual decrease in the yearly growthcan be expected for the period 1975-1985as a result of a slackening-off or declinein the economy, a certain measure of satu-ration of the transport market, and una-voidable increases in tariffs. The latterare a result of the rapidly increasingcosts of fuel and the measures which haveto be taken to meet the certification re-quirements regarding noise levels. Assumingan annual growth of 10 percent for theyears 1973-1980 the total yearly production

*The number of passengers required to paythe cost of the flight.

on short routes will have to rise to 195percent of the 1973 value, while duringthe first three years after the airplane'sintroduction the traffic demand will riseto about 250 percent.c. On very busy routes the Airbus A-300and possibly also the Trijets McDonnellDouglas DC-I0 and Lockheed 1011 will takeover a large share of the short/medium-haul traffic. On routes where the growthwill be less progressive, however, thejump in capacity from current short-haulaircraft to the A-300 will probably be toogreat and there will be an opening foraircraft with a capacity some 80 to 100percent greater than that of the DC-9,provided it offers good possibilities forfurther growth.d. For the M-184 a capacity of at least180 passengers has been chosen for an all-tourist layout with a possible later"stretch" to about 250 passengers, whilethe cargo holds require a total volume ofat least 1800 cu. ft (50 m3 ). Compared tothat of current airliners the passengeraccommodation must show an improvement inthe level of comfort, but this need notnecessarily be achieved by the use of twoaisles. A very close watch will have to bekept on the economical consequences of anincreased level of comfort.

1.4.3. Design cruising speed and range

The speed factor has constituted an out-standing contribution to the developmentof aviation; the aircraft has proved to bethe only means of transport in which in-creased speed does not necessarily lead toan increase in fuel consumption. Althougha fast means of transport will be attrac-tive to the passenger, the air transportcompanies in particular rate the speedelement highly because, broadly speaking,it means that more trips can be made perday and production is increased. It is notonly the cruising speed, however, that isimportant; equally vital is the time de-voted to taxying, takeoff, climb, descent,approach and landing, which means that the

block speed is a better yardstick than thecruising speed. Any new type of short-haulaircraft will have to possess a consider-ably higher cruising speed than the one itis intended to replace, in order to savethe time needed for an extra flight.In the case of smaller general aviationaircraft the value of speed mainly dependson how the aircraft is used. A top execu-tive whose working hours are assumed to beextremely valuable will be prepared to payconsiderably more for speed than the ownerof a small utility aircraft which is usedfor tourism or in regions with an under-developed infrastructure where reasonablesurface transport is lacking.

In drawing up the specification for theM-184 project (Fig. 1-5), it has been as-sumed that the design crUising speed mustnot be less than that of existing aircraft.In the high-subsonic speed bracket, how-ever, any increase in speed will consid-erably influence the external shape (angleof sweep, airfoil shape and thickness),generally resulting in an empty weight in-crement, extra development costs and in-creased fuel consumption. The extent towhich the economic advantages of the high-er block speeds will outweigh these lossescannot be predicted offhand; this wouldhave to be ascertained by a tradeoff study,which could also take into account thepossibilities of rec~nt developments inhigh-speed wing aerodynamics.In the case considered here a design Machnumber of .82 in high-speed cruise hasbeen chosen on the basis of conventionalsection shapes, while the possible gainresulting from the use of an advanced wingshape may be either the use of a thickerairfoil - and hence a lighter structure -,a larger wing span, or a higher economicalcruising speed.

As regards the choice of the design rangeof the M-184 it was concluded from a sur-vey of route distributions that a peakoccurs for traffic on ranges of about 280nm (500 km), e.g. Los Angeles - San Fran-

13

Fig. 1-6. Trends in the takeoff performance of civil aircraft (only trend-setting typeshave been plotted)

1.4.4. Low-speed characteristics and fieldperformance

Two starting-points may be used for speci-fying the runway length for takeoff andlanding:a. The aircraft is optimized for cruisingflight. The shape and dimensions of thewing, as well as the cruising altitude, areso chosen that the fuel consumption is aminimum for the design range flown at thedesign cruising speed. The thrust of theengines will be based either on the re-quired climb performance or on the designcruising spee~ requirement. The takeoff andlanding performance will now become more orless derived values and can be influencedonly to a limited extent by the design ofthe flaps and the wheel brakes. The contin-uous growth in aircraft weight and conse-

1I.~20-:---1I::':30-:---:,7MO'="--1IIO~:---:IIIO:'::-:---:lI:'1O::---:lIIO::'0YEAIl'UO"""--::lI3O"=--:l94O~'---"'1::::9!>O~--::lIIO"=--:I'-}.1O=---lIIO~I.0

YEAIl

cisco. Another peak, although less pro-nounced, is observed around 500 nm (900km). An aircraft designed to fly ranges be-tween 110 and 1,200 nm (200 and 2,200 km)will cover 87 percent of the traffic mar-ket. Although a decrease to 600 nm (1,100km) in the range for maximum payload maylead to a slight improvement in the directoperating costs at short ranges, 25 percentof the short-haul routes are longer and aconsiderable number of operators would notchoose the aircraft. A design range of1,200 nm (2,200 km) at high-speed cruisewas decided for the M-184. In view of thespecified field performance there may bean opening for a version with increasedall-up weight and fuel capacity to suitoperators who require a longer range ver-sion and put less emphasis on low-speedperformance.

14

140

.7..81t N 120~'2 ~SI~ ..-..:~..j' "10~+ ~

!. 10or:..

I. 403.0 lo.000~

20 + ...+ Il'Ol*" ~ i jet" ...l:l

2.5~ 0 Z...0 ..... 0>t ..

~ ...u:2.0! +~ 5,000 ;:- )( ~C ...2 ..1.5 :r Schiphp! Airport + ~

++

+ + ........... M: jet M:

quent increase in wing loading (Fig. 1-6)have resulted in increased takeoff dis-tances which have demanded a steady length-ening of the runways, in some cases andcertainly at the principal internationalairports to as much as about 13,000 feet(4,000 m). The approach speeds for thelanding have risen to 160 to 170 kts (300to 315 km/h) , although the landing distanceis not critical for most long-range trans-ports.Any further continuation of this trendwould only be justified if adaptation ofthe aircraft to eXisting runways led to aconsiderable increase in operating costsand, moreover, the lengthening of the run-ways was environmentally acceptable. If wealso take into account the 1969 require-ments regarding noise production (FAR 36)and a possible tightening up of these inthe future, it would not appear very likelythat future generations of transport air-craft will require any appreciable length-ening of runways which are now being usedby aircraft like the DC-a, Boeing 707 andBoeing 747.b. The runway performance of the new de-sign will be adapted to the airports fromwhich the future customer is now operatingthe aircraft that the new product will haveto replace. For a new short-haul aircraftthis means that the runway length shouldnot exceed that used for the category towhich the DC-9, BAC 1-11 and Boeing 737 be-long and that the design of the landinggear should be adapted to the strength ofthese runways. Any increase in operatingcosts resulting from these requirementsshould be carefully watched and realisticdata should be available when it comes todiscussing the tradeoff between shorterrunways and cost increase. The design studywill therefore have to include an investi-gation into the effect of field require-ments on the deaign characteristics, directoperating costs and noise characteristics.

With a specified runway length for the M-184 project(Fig. 1-5) of not more than 6,000 ft (1,800 m) atsea level (standard atmosphere), it is anticipated

that the majority of potential customers will beable to operate the aircraft from the runways now

being used, provided that the runway Load Classi-

fication Number at Maximum Takeoff Weight does not

exceed 30 on a rigid pavement 7 inches (18 em)thick*.

1.4.5. Other requirements

a. The engine constitutes an important fac-tor in the reduction of the operating costsand its choice should be carefully matchedto the aircraft. In the case of transportaircraft the design range is particularlyimportant, while the noise level has tosatisfy exacting requirements if restric-tions are not to be applied to the use ofthe aircraft. Fuel consumption has to becarefully watched.

b. Much attention should be paid to an op-timum cabin arrangement to enable the op-erator to use different layouts. In generalthe distance between fixed partitions atthe front and rear of the cabin should beas great as possible.

c. Equipment and instruments. The specifi-cation will state the amount of NAV/COM e-quipment to be carried and its degree ofduplication. This will result from discus-sions with customers and will be based onthe mode of operation of the aircraft (VFRand/or IFR flights category of landings),and a distinction will generally be madebetween standard and optional equipment.

d. Construction, inspection and maintenance.Apart from the airworthiness requirements(Section 1.6) the specification will gen-erally also feature special requirementssuch as a fail-safe or safe-life designphilosophy and a service life of the struc-ture, expressed in terms of the maximumnumber of flight hours or flight cycles orboth. The manufacturing and productionprocesses, etc., may also be subject tospecial requirements which can have far-

*cf. Chapter 10.2.1.

15

reaching effects when certain structuralparts or even main structures are adoptedfrom types already in use. A case in pointis the Boeing 707, 727 and 737 family ofaircraft all of which have almost identicalfuselage cross-sections.

e. Airframe services and noise level. Theprincipal design requirements to be met bythe air-conditioning and pressurizationsystem are related to the air supply, tem-perature and degree of humidity, cabinpressure differential, etc. Noise levels,both internal and external, are also de-cided upon. Requirements may also bewritten into the specification with respectto the electrical, hydraulic and pneumaticsystems, anti-icing equipment and possiblyalso the AUXiliary Power Unit (APU).

1.5. THE "CONTINUOUS THREAD" RUNNINGTHROUGH THE DESIGN PROCESS

1.5.1. The iterative character of design

The creation of an airplane configurationcannot be laid down in a universal, de-tailed procedure. However, some generalcharacteristics of the design process maybe amplified with the help of Fig. 1-7,

r-----IIIIIIIIIIIIIIIIIIIII: II r---..J------,L __ -l CHANGED I

I REQUIREMENT IL J

Fig. 1-7. General design procedure

16

which shows the principal phases schemati-cally. This diagram deals with technicaland computational elements and could applyequally well to the design of other tech-nical products, unlike Fig. 1-3 which re-fers specifically to an aircraft develop-ment.An essential element of the design processis that it is always made up of iterations.After a trial configuration has been sub-jected to a first analysis of its charac-teristics (weights, mass distribution, per-formance, flying qualities, economy, etc.),it will be seen either that it does notmeet all the requirements, or that it doescomply with them but improvements in somerespects are possible. Only after a numberof configuration changes have been incor-porated will the designer be able to de-termine whether the final configurationsatisfies the requirements in every respectand may also be regarded as the best con-ceivable design, bearing in mind the inev-itable uncertainties which are peculiar tothe preliminary design phase. The conver-gence test has been incorporated in thediagram to indicate that a situation mayarise in which, despite all the improve-ments made in the design, no configurationcan be found which entirely meets all re-quirements simultaneously.The reason may be that certain requirementsin the specification and other constraintshave proved to be contradictory or too ex-treme, taking into account the state of theart, or that the basic conception has notbeen chosen properly. For example, the de-signer may be confronted with a situationwhich, to ensure that the engines selectedwill supply the power reqUired to keep theaircraft in the air after engine failure,would necessitate leaVing a large part ofthe payload back at the airport. The con-vergence test in Fig. 1-7 is therefore ageneral indication shoWing whether the at-tempts to improve the design have broughtit closer to the requirements or not.

1.5.2. Searching for the optimum

The search for the best conceivable designmay be illustrated by a hypothetical casein which the quality of the design isjudged on the basis of a single numericalcriterium, referred to as the "merit func-tion" or "objective function". In the caseof transport aircraft this may be the Di-rect Operating Costs (DOC, see Section11-8) at the design range but it may alsobe the Maximum Takeoff Weight.In Fig. 1-8 it has been assumed that vari-

N

II:...

~...

~CII:C...

PARAMETER 1

Fig. 1-8~ Graphical representation of de-sign optimization

ations in the design are limited to twoindependently variable parameters, such asthe wing loading and the thrust loading.Each point on the diagram represents afully defined design, the merit functionof which can in principle be determined.In the case considered in Fig. 1-7 it isexpressed as a percentage of the minimumvalue which can be obtained. The require-ments of the specification have been in-corporated in this overall picture as fol-lows:a. Requirements which sharply define thetransport capacity and/or other aspects ofperformance should be used as a basis forthe aircraft's general arrangement, layoutand shape, serving as uniquely defined, ex-plicit conditions.b. Requirements which are put in the form

of constraints in the sense of minima forflying speeds, maxima for the runwaylength, etc. appear as boundaries of thearea within whi~ the design parametersmay be chosen.In the shaded area we find those combj"a-tions of the design variables for which itwill not be possible to satisfy certainrequirements laid down in the form of con-straints. In the example it can be seenthat the trial configuration (point A) liesin the region of unacceptable combinations.However, all reqUirements can be met bychanging just one parameter, although atthe cost of a less favorable merit function(point B). If the second parameter is nowalso changed (point C), we find that themerit function has been improved. Point 0,where one of the limitations is tangent tothe line of the constant merit function,indicates the combination with which allrequirements can be met and, at the sametime, the most favorable assessment ob-tained. Nevertheless, the designer may de-cide to choose point E for the final con-figuration since in general the positionsof the design boundaries are still subjectto some doubts and design E offers a cer-tain margin which considerably reduces therisk of crossing the borderline.Although the "absolute optimum" (point 0)is of no immediate importance in the caseunder review, it may still be useful to ex-plore this design in somewhat greater de-tail, because the difference between themerit functions of designs 0 and 0 is anindication of the price that has to bepaid for a requirement which makes it im-possible to achieve the theoretical abso-lute optimum. If this disadvantage were toprove serious, the incorporation of moreadvanced techniques might be considered,enabling the designer to approach the op-timum more closely. Although it is truethat these will generally lead to an in-crease in development costs and influencethe overall evaluation, the result mightwell be a saving in operating costs and/oran increase in productivity.Another approach might be a certain relax-

17

craft designer. '!be method outlined is nothing more

than a tool to arrive at a better justification fordecisions. An aircraft is never evaluated on the 8basis of a single quantifiable criterion, while the 9number of variable parameters will always be much

Fig. 1-9. Survey of the initial baselineconfiguration design

..0(

GIVEUP

WING DESIGN; CAT 1 LIFTW~NG LOCATION

ENGINE 81 PROPElLER CHOICE

2

5

3

4

CHAPTER4 ,---------,1

67

10

12

concept was right.This book has been composed in such a waythat the reader will be able to use thissimplified procedure with the help of themethods presented in the text. Not onlywill he find the necessary formulas and re-lationships, but his attention will also bedrawn tc considerations which precede de-cisions. No attempt has been made tostreamline the design process as such, al-though the sequence of the chapters doesshow some affinity to the continuous threadthat runs through the design process for aconventional subsonic transport aircraft.This is illustrated in Fig. 1-9, subject tothe following reservations:a. The diagram, although representative incharacter, does not possess universal va-lidity. Designers do not always consciouslywork according to a set program.

larger than two. For instance, increasing the fuse-lage diameter will generally lead to greater com-

fort and increase the passenger appeal with a pos-

sible increase in yield, but it will also increase

the empty weight and the drag and hence the oper-

ating costs. It will be almost impossible to find

a way out of this dilemma without relying upon the

sound judgement of the designer.

1.5.3. A suggested scheme for preliminarydesign

18

In the above we have made no reference tohow a designer lays out a trial configura-tion and how he introduces changes. Whenthe design problem lends itself to quan-ti