Aircraft Design: A Systems Engineering Approach

3.3 Aircraft Configuration Alternatives 52 3.3.1 Wing Configuration 53 3.3.2 Tail Configuration 55 3.3.3 Propulsion System Configuration 55 3.3.4 Landing Gear Configuration 56 3.3.5 Fuselage Configuration 58 3.3.6 Manufacturing-Related Items Configuration 58 3.3.7 Subsystems Configuration 59 3.4 Aircraft Classification and Design Constraints 62 3.5 Configuration Selection Process and Trade-Off Analysis 68 3.6 Conceptual Design Optimization 74 3.6.1 Mathematical Tools 74 3.6.2 Methodology 76 Problems 86 References 92 4 Preliminary Design 93 4.1 Introduction 93 4.2 Maximum Take-Off Weight Estimation 94 4.2.1 The General Technique 94 4.2.2 Weight Build-up 95 4.2.3 Payload Weight 96 4.2.4 Crew Weight 97 4.2.5 Fuel Weight 100 4.2.6 Empty Weight 108 4.2.7 Practical Steps of the Technique 112 4.3 Wing Area and Engine Sizing 113 4.3.1 Summary of the Technique 113 4.3.2 Stall Speed 118 4.3.3 Maximum Speed 120 4.3.4 Take-Off Run 131 4.3.5 Rate of Climb 136 4.3.6 Ceiling 140 4.4 Design Examples 145 Problems 155 References 158 Contents ix 5 Wing Design 161 5.1 Introduction 161 5.2 Number of Wings 164 5.3 Wing Vertical Location 165 5.3.1 High Wing 165 5.3.2 Low Wing 168 5.3.3 Mid-Wing 169 5.3.4 Parasol Wing 169 5.3.5 The Selection Process 169 5.4 Airfoil Section 170 5.4.1 Airfoil Design or Airfoil Selection 171 5.4.2 General Features of an Airfoil 173 5.4.3 Characteristic Graphs of an Airfoil 176 5.4.4 Airfoil Selection Criteria 182 5.4.5 NACA Airfoils 183 5.4.6 Practical Steps for Wing Airfoil Section Selection 188 5.5 Wing Incidence 195 5.6 Aspect Ratio 198 5.7 Taper Ratio 203 5.8 The Significance of Lift and Load Distributions 206 5.9 Sweep Angle 209 5.10 Twist Angle 223 5.11 Dihedral Angle 226 5.12 High-Lift Device 230 5.12.1 The Functions of a High-Lift Device 230 5.12.2 High-Lift Device Classification 232 5.12.3 Design Technique 235 5.13 Aileron 241 5.14 Lifting-Line Theory 242 5.15 Accessories 246 5.15.1 Strake 247 5.15.2 Fence 247 5.15.3 Vortex Generator 248 5.15.4 Winglet 248 5.16 Wing Design Steps 249 5.17 Wing Design Example 250 Problems 259 References 264 6 Tail Design 265 6.1 Introduction 265 6.2 Aircraft Trim Requirements 268 6.2.1 Longitudinal Trim 270 6.2.2 Directional and Lateral Trim 276 6.3 A Review on Stability and Control 278 x Contents 6.3.1 Stability 278 6.3.2 Control 282 6.3.3 Handling Qualities 284 6.4 Tail Configuration 285 6.4.1 Basic Tail Configuration 285 6.4.2 Aft Tail Configuration 288 6.5 Canard or Aft Tail 294 6.6 Optimum Tail Arm 298 6.7 Horizontal Tail Parameters 301 6.7.1 Horizontal Tail Design Fundamental Governing Equation 301 6.7.2 Fixed, All-Moving, or Adjustable 304 6.7.3 Airfoil Section 306 6.7.4 Tail Incidence 308 6.7.5 Aspect Ratio 311 6.7.6 Taper Ratio 312 6.7.7 Sweep Angle 313 6.7.8 Dihedral Angle 313 6.7.9 Tail Vertical Location 314 6.7.10 Other Tail Geometries 315 6.7.11 Control Provision 316 6.7.12 Final Check 316 6.8 Vertical Tail Design 317 6.8.1 Vertical Tail Design Requirements 317 6.8.2 Vertical Tail Parameters 319 6.9 Practical Design Steps 329 6.10 Tail Design Example 331 Problems 336 References 340 7 Fuselage Design 341 7.1 Introduction 341 7.2 Functional Analysis and Design Flowchart 341 7.3 Fuselage Configuration Design and Internal Arrangement 345 7.4 Ergonomics 346 7.4.1 Definitions 346 7.4.2 Human Dimensions and Limits 348 7.5 Cockpit Design 350 7.5.1 Number of Pilots and Crew Members 351 7.5.2 Pilot/Crew Mission 353 7.5.3 Pilot/Crew Comfort/Hardship Level 353 7.5.4 Pilot Personal Equipment 354 7.5.5 Control Equipment 355 7.5.6 Measurement Equipment 356 7.5.7 Level of Automation 357 7.5.8 External Constraints 359 Contents xi 7.5.9 Cockpit Integration 359 7.6 Passenger Cabin Design 360 7.7 Cargo Section Design 368 7.8 Optimum Length-to-Diameter Ratio 372 7.8.1 Optimum Slenderness Ratio for Lowest fLD 372 7.8.2 Optimum Slenderness Ratio for Lowest Fuselage Wetted Area 378 7.8.3 Optimum Slenderness Ratio for the Lightest Fuselage 380 7.9 Other Fuselage Internal Segments 380 7.9.1 Fuel Tanks 381 7.9.2 Radar Dish 385 7.9.3 Wing Box 386 7.9.4 Power Transmission Systems 387 7.10 Lofting 388 7.10.1 Aerodynamics Considerations 388 7.10.2 Area Ruling 390 7.10.3 Radar Detectability 392 7.10.4 Fuselage Rear Section 392 7.11 Fuselage Design Steps 394 7.12 Design Example 395 Problems 406 References 410 8 Propulsion System Design 413 8.1 Introduction 413 8.2 Functional Analysis and Design Requirements 414 8.3 Engine Type Selection 416 8.3.1 Aircraft Engine Classification 417 8.3.2 Selection of Engine Type 428 8.4 Number of Engines 436 8.4.1 Flight Safety 437 8.4.2 Other Influential Parameters 438 8.5 Engine Location 439 8.5.1 Design Requirements 439 8.5.2 General Guidelines 441 8.5.3 Podded versus Buried 443 8.5.4 Pusher versus Tractor 444 8.5.5 Twin-Jet Engine: Under-Wing versus Rear Fuselage 446 8.6 Engine Installation 448 8.6.1 Prop-Driven Engine 450 8.6.2 Jet Engine 452 8.7 Propeller Sizing 456 8.8 Engine Performance 461 8.8.1 Prop-Driven Engine 461 8.8.2 Jet Engine 462 8.9 Engine Selection 462 xii Contents 8.10 Propulsion System Design Steps 464 8.11 Design Example 467 Problems 471 References 478 9 Landing Gear Design 479 9.1 Introduction 479 9.2 Functional Analysis and Design Requirements 481 9.3 Landing Gear Configuration 484 9.3.1 Single Main 484 9.3.2 Bicycle 485 9.3.3 Tail-Gear 487 9.3.4 Tricycle 487 9.3.5 Quadricycle 488 9.3.6 Multi-Bogey 489 9.3.7 Releasable Rail 489 9.3.8 Skid 489 9.3.9 Seaplane Landing Device 490 9.3.10 Human Leg 491 9.3.11 Landing Gear Configuration Selection Process 492 9.3.12 Landing Gear Attachment 493 9.4 Fixed, Retractable, or Separable Landing Gear 494 9.5 Landing Gear Geometry 497 9.5.1 Landing Gear Height 498 9.5.2 Wheel Base 503 9.5.3 Wheel Track 508 9.6 Landing Gear and Aircraft Center of Gravity 516 9.6.1 Tipback and Tipforward Angle Requirements 516 9.6.2 Take-Off Rotation Requirement 518 9.7 Landing Gear Mechanical Subsystems/Parameters 524 9.7.1 Tire Sizing 524 9.7.2 Shock Absorber 525 9.7.3 Strut Sizing 526 9.7.4 Steering Subsystem 527 9.7.5 Landing Gear Retraction System 527 9.8 Landing Gear Design Steps 528 9.9 Landing Gear Design Example 529 Problems 539 References 544 10 Weight of Components 547 10.1 Introduction 547 10.2 Sensitivity of Weight Calculation 549 10.3 Aircraft Major Components 553 10.4 Weight Calculation Technique 556 10.4.1 Wing Weight 559 Contents xiii 10.4.2 Horizontal Tail Weight 561 10.4.3 Vertical Tail Weight 561 10.4.4 Fuselage Weight 562 10.4.5 Landing Gear Weight 563 10.4.6 Installed Engine Weight 564 10.4.7 Fuel System Weight 564 10.4.8 Weight of Other Equipment and Subsystems 565 10.5 Chapter Examples 565 Problems 570 References 573 11 Aircraft Weight Distribution 575 11.1 Introduction 575 11.2 Aircraft Center of Gravity Calculation 578 11.3 Center of Gravity Range 585 11.3.1 Fixed or Variable Center of Gravity 585 11.3.2 Center of Gravity Range Definition 586 11.3.3 Ideal Center of Gravity Location 587 11.4 Longitudinal Center of Gravity Location 590 11.5 Technique to Determine the Aircraft Forward and Aft Center of Gravity 598 11.6 Weight Distribution Technique 606 11.6.1 Fundamentals of Weight Distribution 607 11.6.2 Longitudinal Stability Requirements 609 11.6.3 Longitudinal Controllability Requirements 611 11.6.4 Longitudinal Handling Quality Requirements 613 11.7 Aircraft Mass Moment of Inertia 615 11.8 Chapter Example 620 Problems 624 References 630 12 Design of Control Surfaces 631 12.1 Introduction 631 12.2 Configuration Selection of Control Surfaces 637 12.3 Handling Qualities 638 12.3.1 Definitions 640 12.3.2 Longitudinal Handling Qualities 643 12.3.3 Lateral-Directional Handling Qualities 647 12.4 Aileron Design 654 12.4.1 Introduction 654 12.4.2 Principles of Aileron Design 656 12.4.3 Aileron Design Constraints 664 12.4.4 Steps in Aileron Design 669 12.5 Elevator Design 670 12.5.1 Introduction 670 12.5.2 Principles of Elevator Design 672 12.5.3 Take-Off Rotation Requirement 676 xiv Contents 12.5.4 Longitudinal Trim Requirement 680 12.5.5 Elevator Design Procedure 683 12.6 Rudder Design 685 12.6.1 Introduction to Rudder Design 685 12.6.2 Fundamentals of Rudder Design 688 12.6.3 Rudder Design Steps 709 12.7 Aerodynamic Balance and Mass Balance 713 12.7.1 Aerodynamic Balance 715 12.7.2 Mass Balance 722 12.8 Chapter Examples 723 12.8.1 Aileron Design Example 723 12.8.2 Elevator Design Example 729 12.8.3 Rudder Design Example 738 Problems 745 References 752 Appendices 755 Appendix A: Standard Atmosphere, SI Units 755 Appendix B: Standard Atmosphere, British Units 756 Index 757 Preface Objectives The objective of this book is to provide a basic text for courses in the design of heavier- than-air vehicles at both the upper division undergraduate and beginning graduate levels. Aircraft design is a special topic in the aeronautical/aerospace engineering discipline. The academic major of aeronautical/aerospace engineering traditionally tends to have four main areas of expertise: aerodynamics, flight dynamics, propulsion, and structure. A qualified aircraft designer employs all these four scientific concepts and principles and integrates them using special design techniques to design a coordinated unique system; an aircraft. Design is a combination of science, art, and techniques. A designer not only must have sufficient level of knowledge in these four areas, but also needs to employ mathematics, skills, experiences, creativity, art, and system design techniques. It is true that aircraft design is not completely teachable in classrooms, but combining class lectures with a semester-long aircraft design project provides the best opportunity for students to learn and experience aircraft design. Every aeronautical engineering discipline offers at least one course in aircraft design or aerospace system design. The lack of an aircraft design textbook with academic features – such as full coverage of all aspects of an air vehicle, aeronautical concepts, design methods, design flowcharts, design examples, and end-of-chapter problems – combined with the newly developed systems engineering techniques was the main motivation to write this book. In the past several years, I have talked to various aircraft design instructors and stu- dents at conferences and AIAA Design/Build/Fly design competitions. I came to the conclusion that the great design books published by such pioneers as Roskam, Toren- beek, Nicolai, Stinton, and Raymer need more development and expansion. This is to meet the ever-increasing need of universities and colleges for aircraft design education, and of industries for design implementation. The new text should possess significant features such as systems engineering approaches, design procedures, solved examples, and end-of-chapter problems. This book was written with the aim of filling the gap for aeronautical/aerospace engineering students and also for practicing engineers. xvi Preface Approach The process of air vehicle design is a complex combination of numerous disciplines which have to be blended together to yield the optimum design to meet a given set of requirements. The systems engineering approach is defined as an interdisciplinary approach encompassing the entire technical effort to evolve and verify an integrated and lifecycle-balanced set of system people, products, and process solutions that satisfy customer needs. Multi-discipline system engineering design involves the application of a systems engineering process and requires engineers with substantive knowledge of design across multiple technical areas and improved tools and methods for doing it. Complex aircraft systems, due to the high cost and the risks associated with their development, become a prime candidate for the adoption of systems engineering methodologies. The systems engineering technique has been applied in the development of many manned airplanes. An aircraft is a system composed of a set of interrelated components working together toward some common objective or purpose. Primary objectives include safe flight achieved at a low cost. Every system is made up of components or subsystems, and any subsystem can be broken down into smaller components. For example, in an air transportation system, the aircraft, terminal, ground support equipment, and controls are all subsystems. Throughout the text, the systems engineering approach is examined and implemented. The book has been arranged to facilitate the student’s gradual understanding of design techniques. Statement proofs are provided whenever they contribute to the understanding of the subject matter presented. Special effort has been made to provide example problems so that the reader will have a clear understanding of the topic discussed. The reader is encouraged to study all such solved problems carefully; this will allow the interested reader to obtain a deeper understanding of the materials and tools. Features Some of the unique features of this textbook are as follows. It: • follows a systems engineering approach; • is organized based on components design (e.g., wing design, tail design, and fuselage design); • provides design steps and procedures in each chapter; • derives a number of design equations that are unique to the book; • provides several fully solved design examples at the component level; • has many end-of-chapter problems for readers to practice; • includes a lot of aircraft figures/images to emphasize the application of the concepts; • describes some real design stories that stress the significance of safety in aircraft design; • provides various aircraft configurations, geometries, and weights data to demonstrate real-world applications and examples; • covers a variety of design techniques/processes so that the designer has freedom and flexibility to satisfy the design requirements in several ways; • encourages and promotes the creativity of the reader. Preface xvii For these reasons, as aeronautical/aerospace engineering students transit to practicing engineers, they will find that this text is indispensable as a reference text. Some materials, such as “design optimization” and “design of control surfaces,” may be taught at the graduate level. The reader is expected to have a basic knowledge of the fundamentals and concepts of aerodynamics, propulsion, aero-structure, aircraft performance, and flight dynamics (stability and control) at aeronautical/aerospace engineering senior level. The following is a true statement: “design techniques are not understood unless prac- ticed.” Therefore, the reader is strongly encouraged to experience the design techniques and concepts through applied projects. Instructors are also encouraged to define an open- ended semester/year-long aircraft design project to help the students to practice and learn through application and experiencing the iterative nature of the design technique. It is my sincere wish that this book will help aspiring students and design engineers to learn and create more efficient and safer aircraft. Outline The text consists of 12 chapters and is organized in a standard fashion according to the systems engineering discipline: conceptual design, preliminary design, and detail design. In summary, Chapter 3 presents the aircraft conceptual design; Chapter 4 introduces the aircraft preliminary design; and Chapters 5–12 cover the aircraft detail design. The outline of this book is as follows. Chapter 1 is an introduction to design fundamentals and covers such topics as engi- neering design principles, design project planning, decision-making processes, feasibility analysis, and tort of negligence. Design standards and requirements such as Federal Avia- tion Regulations (FARs) and Military Standards are reviewed in this chapter, and addressed further throughout the text. Chapter 2 deals with the systems engineering approach. Major design phases accord- ing to systems engineering are introduced: conceptual system design, preliminary system design, and detail system design. In this chapter, several concepts and fundamental def- initions such as technical performance measures, functional analysis, system trade-off analysis, design review, and design requirements are reviewed. Implementations of sys- tems engineering into aircraft design via aircraft design phases, aircraft design flowcharts, aircraft design groups, and design evaluation and feedback loops are explained. At the end of the chapter, the overall aircraft design procedure in terms of design steps is outlined. Chapter 3 covers aircraft conceptual design, and examines the aircraft configuration selection. The primary function of each aircraft component such as wing, fuselage, tail, landing gear, and engine is introduced. Furthermore, various configuration alternatives for each component are reviewed. In addition, the aircraft classification and design constraints are addressed. In this chapter the design optimization and its mathematical tools are briefly reviewed. The chapter ends with a configuration selection process and methodology, and also a trade-off analysis technique. Chapter 4 discusses the topic of aircraft preliminary design. In this chapter, the tech- nique to determine three aircraft fundamental parameters is presented. These parameters are: maximum take-off weight, wing area, and engine thrust/power. The weight build- up technique is examined for estimation of the aircraft maximum take-off weight. The xviii Preface matching plot technique is utilized in the calculation of wing area, and engine thrust/power. These three parameters are computed based on the aircraft performance requirements such as range, endurance, maximum speed, take-off run, rate-of-climb, and ceiling. Two fully solved examples illustrate the application of the two techniques. Chapters 5–9 and 12 present detail design of the aircraft components of wing, tail, fuse- lage, propulsion system, landing gear, and control surfaces respectively. In these chapters, the techniques to calculate all aircraft components parameters such as wing/tail span, chord, airfoil, incidence, sweep angle, tail arm, tail area, landing gear height, wheel base, wheel track, fuselage diameter, fuselage length, cabin design, cockpit design, number of engines, and engine selection are examined. Furthermore, the features of various compo- nent configurations and their relationship with the design requirements (e.g., performance, stability, control, and cost) are addressed. Chapter 12 introduces the detail design of the conventional control surfaces of aileron, elevator, and rudder. In each chapter, the design flowchart and design step for each component is also presented. Each chapter is accom- panied by several examples, including a fully solved chapter example to demonstrate the applications of design techniques and methods. Chapter 10 introduces the technique to calculate the weight of the aircraft components, equipment, and subsystems. The technique is derived mainly based on past aircraft weight data and statistics. Chapter 11 addresses the topic of aircraft weight distribution, and weight and balance. The aircraft center of gravity (cg) calculation, aircraft most aft and most forward cg, and cg range are also covered in this chapter. In addition, the technique to determine the aircraft mass moment of inertia about three axes (i.e., x , y, and z ) is examined. Unit Systems In this text, the emphasis is on SI units or the metric system, which employs the meter (m) as the unit of length, the kilogram (kg) as the unit of mass, and the second (s) as the unit of time. It is true that metric units are more universal and technically consistent than British units. However, currently, many FARs are published in British units, where the foot (ft) is the unit of length, the slug is the unit of mass, the pound (lb) is the unit of force (weight), and the second (s) is the unit of time. In FARs, the pound is used as the unit for force and weight, the knot for airspeed, and the foot for altitude. Thus, in various locations, the knot is mainly used as the unit of airspeed, the pound for weight and force, and the foot for altitude. Therefore, in this text, a combination of SI and British unit systems is utilized. A common mistake in the literature (even in the Jane’s publications) is the application of kg for the unit of aircraft weight. Throughout the text, whenever the unit of kg is used, the term “aircraft mass” is employed. Some texts have created the pound-mass (lbm) as the unit of mass, and the pound-force (lbf) as the unit of weight. This initiative may generate some confusion; so in this text, only one pound (lb) is employed as the unit of weight and force. Series Preface The field of aerospace is wide ranging and multi-disciplinary, covering a large variety of products, disciplines and domains, not merely in engineering but in many related supporting activities. These combine to enable the aerospace industry to produce exciting and technologically advanced vehicles. The wealth of knowledge and experience that has been gained by expert practitionersin the various aerospace fields needs to be passed onto others working in the industry, including those just entering from University. The Aerospace Series aims to be a practical and topical series of books aimed at engineering professionals, operators, users and allied professions such as commercial and legal executives in the aerospace industry. The range of topics is intended to be wide ranging, covering design and development, manufacture, operation and support of aircraft as well as topics such as infrastructure operations and developments in research and technology. The intention is to provide a source of relevant information that will be of interest and benefit to all those people working in aerospace. Aircraft design brings together the key aeronautical engineering disciplines: aerody- namics, flight dynamics, propulsion and structures, which must be combined to produce designs that meet today’s stringent performance, economic and environmental demands. As such, aircraft designis a key component of all undergraduate aerospace engineering courses, and all aerospace students usually tackle some form of aircraft design project. This book, Aircraft Design: A Systems Engineering Approach, extends the classical aircraft design approaches through the implementation of systems engineering techniques for the conceptual, preliminary and detailed design of heavier-than-air vehicles. As a very readable and informative text reference, with plenty of examples from a wide range of contemporary aircraft designs, and solved examples at the end of each chapter, it is a worthy addition to the Wiley Aerospace Series. Peter Belobaba, Jonathan Cooper, Roy Langton and Allan Seabridge Acknowledgments I am enormously grateful to the Almighty for the opportunity to serve the aerospace community by writing this text. The author would like to acknowledge the many con- tributors and photographers who have contributed to this text. I am especially grate- ful to those who provided great aircraft photographs: Anne Deus (Germany); Jenney Coffey (UK); Anthony Osborne (UK); A J Best (UK); Vlamidir Mikitarenko (Ger- many); Rainer Bexten (Germany); Hideki Nakamura (Japan); Akira Uekawa (Japan); Luis David Sanchez (Puerto Rico); Tom Houquet (Belgium); Toshi Aoki (Japan); Miloslav Storoska (Slovakia); Tom Otley (Panacea Publishing International, UK); Jonas L¨ovgren (SAAB, Sweden); Jeff Miller (Gulfstream Aerospace Corporation, USA); Michael de Boer (Netherland); Konstantin von Wedelstaedt (Germany); Augusto G. Gomez R. (Mex- ico); Randy Crew (Singapore); Robert Domandl; Serghei Podlesnii (Moldova); Orlando J. Junior (Brazil); Bal´azs Farkas (Hungary); and Christopher Huber and www.airliners.net. In addition, the efforts of the author were helped immeasurably by the many insights and constructive suggestions provided by students and instructors over the past 16 years. Unattributed figures are held in the public domain and are from either the US Government Departments and Agencies, or Wikipedia. Putting a book together requires the talents of many people, and talented people abound at John Wiley & Sons, Inc. My sincere gratitude goes to Paul Petralia, commissioning editor, for coordinating the whole publication process; Clarissa Lim for coordinating the production project; Sarah Lewis for editing the manuscript; Jayashree Saishankar for typesetting; and Sandra Grayson for helping in the copyright process. I am particularly grateful to my editors, Liz Wingett and Sophia Travis, for their comments and guidance. My special thanks go to the outstanding copyeditors and proofreaders who are essential in creating an error-free text. I especially owe a large debt of gratitude to the reviewers of this text. Their ideas, suggestions, and criticisms have helped me to write more clearly and accurately and have influenced the evolution of this book markedly. Symbols and Acronyms Symbols Symbol Name Unit a Speed of sound m/s, ft/s a Acceleration m/s2 , ft/s 2 A Area m2 , ft 2 AR Aspect ratio – b Lifting surface/control surface span m, ft B Wheel base m, ft C Specific fuel consumption N/h kW, lb/h hp C Mean aerodynamic chord m, ft C D , C L, C y Drag, lift, and side-force coefficients – Cl , Cm , Cn Rolling, pitching, and yawing moment coefficients – Ch Hinge moment coefficient – C Dy Aircraft side drag coefficient – C Dβ Rate of change of drag coefficient w.r.t. sideslip angle; ∂C D /∂β 1/rad Cm ac_wf Wing/fuselage pitching moment coefficient (about the wing/fuselage aerodynamic center) – Cmα Rate of change of pitching moment w.r.t. angle of attack 1/rad Cm q Rate of change of pitch rate w.r.t. angle of attack 1/rad C LδE ∂C L /∂δE 1/rad CmδE ∂Cm /∂δE 1/rad ClδA ∂Cl /∂δA 1/rad CnδR ∂C n /∂δR 1/rad Cnβ Rate of change of yawing moment coefficient w.r.t. sideslip angle 1/rad Cnr Rate of change of yawing moment coefficient w.r.t. yaw rate 1/rad xxiv Symbols and Acronyms Symbol Name Unit C Do Zero-lift drag coefficient – C Di Induced drag coefficient – Cf Skin friction coefficient – C Lα Wing/tail/aircraft (3D) lift curve slope 1/rad Clα Airfoil (2D) lift curve slope 1/rad C Lmax Maximum lift coefficient – D Drag force, drag N, lb D Diameter m, ft dc Distance between the aircraft cg and center of the projected side area m, ft E Endurance h, s E Modulus of elasticity N/m2 , Pa, lb/in2 , psi e Oswald span efficiency factor – F Force, friction force N, lb FC Centrifugal force N, lb FOM Figure of merit – g Gravity constant 9.81 m/s2 , 32.17 ft/s 2 G Fuel weight fraction – GR Gearbox ratio – GC Ratio between the linear/angular movement of the stick/wheel to deflection of the control surface deg/m, deg/ft, deg/deg H Altitude m, ft h, ho Non-dimensional distance between cg (h) or ac (ho ) and a reference line – H Height, wheel height m, ft H Control surface hinge moment Nm, lb ft ih Tail incidence deg, rad iw Wing incidence deg, rad l Length, tail arm m, ft I Mass moment of inertia kg m 2 , slug ft 2 I Second moment of area m4 , ft 4 I Index (e.g., design, performance) – K Induced drag factor – L, LA Rolling moment Nm, lb ft L Length m, ft L Lift force, lift N, lb (L/D) max Maximum lift-to-drag ratio – M Mach number – M , MA Pitching moment Nm, lb ft m mass kg, slug • m Engine air mass flow rate kg/s, lb/s MTOW Maximum take-off weight N, lb MAC Mean aerodynamic chord m, ft n Load factor – n Number of rows in cabin – Symbols and Acronyms xxv Symbol Name Unit n Rotational speed rpm, rad/s N Normal force N, lb N Number of an item – N , NA Yawing moment Nm, lb ft P Pressure N/m 2 , Pa, lb/in2 , psi P Power W, kW, hp, lb ft/s Ps Seat pitch m, ft Preq Required power W, kW, hp, lb ft/s Pav Available power W, kW, hp, lb ft/s Pexc Excess power W, kW, hp, lb ft/s P , p Roll rate rad/s, deg/s q, q Dynamic pressure N/m2 , Pa, lb/in2 , psi Q, q Pitch rate rad/s, deg/s Q Fuel flow rate kg/s, lb/s R Range m, km, ft, mile, mi, nmi R Air gas constant 287.26 J/kg K R Radius m, ft R Rank – Re Reynolds number – ROC Rate of climb m/s, ft/min, fpm R, r Yaw rate rad/s, deg/s s Semispan (b/2) m, ft S Planform area of lifting/control surface m 2 , ft 2 SA Airborne section of the take-off run m, ft SG Ground roll m, ft STO Take-off run m, ft SFC Specific fuel consumption N/h/kW, lb/h/hp, 1/s, 1/ft SM Static margin – t Time s, min, h T Engine thrust N, lb T Temperature ◦C, ◦R, K T Wheel track m, ft T , t Thickness m, ft t/c Airfoil thickness-to-chord ratio – T /W Thrust-to-weight ratio – U Forward airspeed m/s, ft/min, km/h, mi/h, knot V Velocity, speed, airspeed m/s, ft/min, km/h, mi/h, knot V Volume m 3 , ft 3 Vmax Maximum speed m/s, ft/min, km/h, mi/h, knot Vmc Minimum controllable speed m/s, ft/min, km/h, mi/h, knot Vmin D Minimum drag speed m/s, ft/min, km/h, mi/h, knot Vmin P Minimum power speed m/s, ft/min, km/h, mi/h, knot VR Rotation speed m/s, ft/min, km/h, mi/h, knot VROCmax Maximum rate of climb speed m/s, ft/min, km/h, mi/h, knot Vs Stall speed m/s, ft/min, km/h, mi/h, knot VT True speed m/s, ft/min, km/h, mi/h, knot xxvi Symbols and Acronyms Symbol Name Unit VTO Take-off speed m/s, ft/min, km/h, mi/h, knot VW Wind speed m/s, ft/min, km/h, mi/h, knot V H , V V Horizontal/vertical tail volume coefficient – W Weight N, lb W Width m, ft Wf Fuel weight N, lb WTO Maximum take-off weight N, lb W /P Power loading N/W, lb/hp W /S Wing loading N/m2 , lb/ft2 x , y, z Displacement in x , y, and z direction m, ft Y Side force N, lb y Beam deflection m, ft Greek symbols Symbol Name Unit α Angle of attack deg, rad β Sideslip angle deg, rad γ Climb angle deg, rad θ Pitch angle, pitch attitude deg, rad λ Taper ratio – φ Bank angle deg, rad δ Pressure ratio – δ Control surface deflection deg, rad σ Air density ratio – σ Sidewash angle deg, rad ρ Air density, materials density kg/m3 , slug/ft 3 μ Dynamic viscosity kg/m s, lb s/ft 2 μ Friction coefficient – μ Mach angle rad, deg η Efficiency, dynamic pressure ratio – Sweep angle deg, rad ω Angular velocity rad/s, deg/s ωn Natural frequency rad/s, deg/s ω Frequency rad/s, deg/s ψ Yaw angle, heading angle deg, rad π 3.14 –  Spin rate rad/s, deg/s, rpm τ Control surface angle of attack effectiveness –  Dihedral angle deg, rad ε Downwash angle degr, rad ∂ε/∂α Downwash slope – ∂σ /∂β Sidewash slope – •• θ Take-off rotation angular acceleration deg/s2 , rad/s 2 x cg Non-dimensional range of center of gravity – Symbols and Acronyms xxvii Subscripts Note AR, S , b, λ, , , and C without a subscript indicate a wing property 0, o Zero-lift, sea level, about aerodynamic center 0.25 Quarter chord 1 Steady-state value a, A Aileron aft The most aft location A Aerodynamic ac Aerodynamic center avg Average a Aircraft b Baggage c/4 Relative to the quarter chord c/2 Relative to the 50% of the chord cs Control surface cross Cross-section C Crew, ceiling, cruise, cabin d Design D Drag e, E Elevator, equivalent, empty, exit eff Effective E Engine f Fuel, fuselage, flap, friction for The most forward location GL Glide h Horizontal tail i Item number, inboard, ideal, initial, inlet ISA International Standard Atmosphere L Lift, left, landing LG Landing gear max Maximum min Minimum m Pitching moment mg Main gear mat Materials o Outboard opt Optimum ot Overturn p Propeller PL Payload r, R Rudder R Rotation r Root ref Reference s Stall, stick ss Steady-state SL Sea level S Side SR Spin recovery xxviii Symbols and Acronyms t Tip, tab, twist, horizontal tail T True TO Take-off tot Total ult Ultimate v, V Vertical tail VT Vertical tail w, W Wing, wind wet Wetted wf Wing/fuselage x , y, or z In the x , y, or z direction xx , yy, or zz About the x -, y-, or z -axis Acronyms ac or AC Aerodynamic center ca Center of area, center of action cg or CG Center of gravity APU Auxiliary power unit CAD Computer-aided design CAM Computer-aided manufacturing CDR Conceptual design review CFD Computational fluid dynamics cp Center of pressure DOF Degrees of freedom DOD Department of Defense EASA European Aviation Safety Agency ETR Evaluation and test review FDR Final (critical) design review FAA Federal Aviation Administration FAR Federal Aviation Regulations FBW Fly-by-wire GA General aviation HALE High-altitude long-endurance HLD High-lift device IATA International Air Transport Association ISA International Standard Atmosphere JAR Joint aviation requirements KTAS Knot true air speed KEAS Knot equivalent air speed LG Landing gear LE Leading edge MAC Mean aerodynamic chord MDO Multidisciplinary design optimization MIL-STD Military Standards NACA National Advisory Committee for Aeronautics NASA National Aeronautics and Space Administration NTSB National Transportation Safety Board Symbols and Acronyms xxix np or NP Neutral point OEI One engine inoperative PDR Preliminary design review rpm Revolutions per minute rad Radian RCS Radar cross-section STOL Short take-off and landing TE Trailing edge Turboprop Turbopropeller VTOL Vertical take off and landing WWII World War II Conversion Factors Length, Altitude, Range 1 in = 2.54 cm = 25.4 mm 1 ft = 0.3048 m = 12 in 1 statue mile (mi) = 5280 ft = 1.609 km 1 nautical mile (nmi) = 6076 ft = 1.852 km 1 km = 0.6214 mi = 3280.8 ft 1 m = 3.281 ft = 39.37 in Area 1 m2 = 10.764 ft 2 1 ft2 = 0.093 m2 Volume 1 l = 0.001 m3 = 1000 cm3 = 0.0353 ft 3 = 0.264 US gal 1 ft3 = 0.0283 m3 = 7.481 US gal 1 US gal = 0.1337 ft 3 = 3.785 l 1 m3 = 1000 l = 264.17 US gal = 35.315 ft 3 Speed, Airspeed, Rate of Climb 1 knot = 0.514 m/s = 1.151 mi/h = 1.852 km/h = 1.688 ft/s = 101.27 ft/min 1 mi/h = 1.609 km/h = 1.467 ft/s = 0.447 m/s = 0.869 knot = 88 ft/min 1 km/h = 0.6214 mi/h = 0.2778 m/s = 0.9113 ft/s = 0.54 knot = 54.68 ft/min 1 ft/min = 0.01 knot = 0.011 mi/h = 0.018 km/h = 0.0051 m/s = 0.017 ft/s Mass 1 slug = 14.59 kg 1 kg = 1000 g = 0.0685 slug xxx Symbols and Acronyms Force, Weight, Thrust 1 N = 0.225 lb 1 lb = 4.448 N Mass and Weight 1 N (weight) = 1 kg m/s 2 (weight) → 0.102 kg (mass) 1 lb (weight) = 1 slug ft/s 2 (weight) = 4.448 N (weight) → 0.454 kg (mass) 1 kg (mass) → 9.807 N (weight) = 2.205 lb (weight) Work, Energy 1 J = 0.7376 ft lb 1 BTU = 1055 J = 778.17 ft lb 1 cal = 4.187 J = 3.09 ft lb Power 1 hp = 550 ft lb/s = 745.7 W = 33 000 lb ft/min 1 kW = 737.56 ft lb/s = 1.341 hp Mass Moment of Inertia 1 kg m2 = 0.738 slug ft 2 1 slug ft 2 = 1.356 kg m2 Pressure 1 Pa = 1 N/m 2 = 0.00015 lb/in2 = 0.00015 psi 1 atm = 101 325 Pa= 1.013 bar = 14.7 lb/in 2 = 14.7 psi 1 psi = 6895 Pa = 0.068 atm Time, Endurance 1 day = 24 h = 1440 min = 86 400 s 1 h = 60 min = 3600 s Angle 1 rad = 180/π deg = 57.3 deg 1 deg = π/180 rad = 0.01745 rad 1 Aircraft Design Fundamentals 1.1 Introduction to Design Aircraft design is essentially a branch of engineering design. Design is primarily an ana- lytical process which is usually accompanied by drawing/drafting. Design contains its own body of knowledge, independent of the science-based analysis tools usually coupled with it. Design is a more advanced version of a problem-solving technique that many people use routinely. Design is exciting, challenging, satisfying, and rewarding. The gen- eral procedure for solving a mathematical problem is straightforward. Design is much more subjective, there is rarely a single “correct” answer. The world of design involves many challenges, uncertainties, ambiguities, and inconsistencies. This chapter is intended to familiarize the reader with the basic fundamentals and overall process of design. This book has been written primarily to provide the basic tools and concepts required to create an optimum/efficient aircraft design that will meet the necessary design requirements. A very basic and simplified model of a design process is shown schematically in Figure 1.1. In general, a design process includes three major operations: analysis, synthe- sis, and evaluation. Analysis is the process of predicting the performance or behavior of a design candidate. Evaluation is the process of performance calculation and comparing the predicted performance of each feasible design candidate to determine the deficiencies. The noun synthesis refers to a combination of two or more entities that together form something new. In this text, synthesis is employed interchangeably with design. Hence, synthesis is defined as the creative process of putting known things together into new and more useful combinations. Synthesis is the vehicle of the design, with evaluation being its compass. The candidate designs that fail to satisfy (partially or completely) the requirements are reiterated. That is new values, features, characteristics, or parameters are determined during synthesis operation. The redesigned candidate is reanalyzed again for compliance with the design requirements. This iterative process is continued until the design requirements are met. A design process requires both integration and iteration, invoking a process that coordinates synthesis, analysis, and evaluation. These three oper- ations must be integrated and applied iteratively and continuously throughout the lifecycle of the design. Aircraft Design: A Systems Engineering Approach, First Edition. Mohammad H. Sadraey.  2013 John Wiley & Sons, Ltd. Published 2013 by John Wiley & Sons, Ltd. 2 Aircraft Design Analysis Evaluation Synthesis Figure 1.1 Interrelationship between synthesis, analysis, and evaluation Aircraft Design Engineering/Mathematical Calculations Decision Making/ Logical Selections Figure 1.2 Two main groups of design activities in aircraft design A design operation often involves two activities: (i) problem solving through math- ematical calculations and (ii) choosing a preferred one among alternatives (Figure 1.2). The first activity is performed in Chapters 4–12 in designing various aircraft components. The second design activity is in general a decision-making process. The fundamentals of decision making are reviewed in Section 1.4; and employed entirely in aircraft conceptual design (Chapter 3). In addition, there are various decision-making processes in aircraft components design (e.g., wing design, tail design, and propulsion system design), as will be discussed in several chapters. The major components that comprise a conventional air- craft are wing, fuselage, horizontal tail, vertical tail, engine, landing gear, and equipment. The decision-making process plays a significant role in the configuration design of these primary components. The traditional engineering education is structured to emphasize mathematics, physical sciences, and engineering sciences. The problem is the lack of sufficient concentration on design and creativity. Creative thinking and its attitudes are essential to design suc- cess. Producing a new design requires an ability to be creative and overcome strong barriers. To address this significant issue a new organization, CDIO,1 was established in the late 1990s. The CDIO initiative is defined to be an innovative educational frame- work for producing the next generation of engineers. The framework provides students with an education stressing engineering fundamentals set within the context of conceiv- ing/designing/implementing/operating real-world systems and products. This textbook has been written with a strong emphasis on creativity, and the freedom of the designer to go beyond current aircraft designs. 1 www.cdio.org. Aircraft Design Fundamentals 3 Throughout this text, various techniques for generating creative design alternatives are introduced. An effective approach in creative design as a source of new ideas is brainstorming. Brainstorming is a structured group-oriented technique for conceiving design alternatives. It consists of a group of individuals letting their imaginations run wild, but in accordance with central procedural rules. The ultimate goal is that the group members will inspire and support each other. The outcome is that the group will be able to conceptualize design alternatives that are more elegant than those the individuals could have achieved independently. In order to encourage members to describe their ideas, even totally impractical ones, a crucial brainstorming rule is that no criticism of individuals or ideas is permitted. The emphasis is on generating as many ideas and concepts as possible, without worrying about their validity. Rectifying, organizing, and combining the ideas suggested in a brainstorming session is performed out of the group meeting. The brainstorming technique is mainly applicable at the conceptual design phase (see Chapters 2 and 3). In general, aircraft design requires the participation of six (Figure 1.3) fundamental disciplines: (i) flight dynamics, (ii) aerodynamics, (iii) propulsion, (iv) aero-structure, (v) management skills, and (vi) engineering design. The first four items are primary expertise areas of aeronautical engineering. This text has no particular chapters on any of these four topics; so the reader is expected to be familiar with the fundamentals, concepts, technical terms, and engineering techniques in such areas. Management is defined [1] as coordinating work activities so that they are completed efficiently and effectively with and through other people. An aircraft designer needs to be equipped with managerial skills and act as a manager throughout the design process. This topic is not covered in this text; however, a few aspects of management – such as project planning and decision making – are reviewed in this chapter (Sections 1.3 and 1.4). Finally, engineering design [2–4] is at the heart of the design process and is assumed as the sixth discipline necessary for design of an air vehicle. Section 1.2 briefly examines various aspects of engineering design. It must be noted that aircraft engineering design has its own science, concepts, fundamentals, technical terms, and techniques. Chapters 3–12 all address various aspects of designing aircraft components as well as introducing aircraft design procedures. This chapter will first examine the engineering design profession. Next, design project planning is addressed and tools such as Gantt charts are introduced. Then the princi- ple of decision making, a very significant section of any design process, is presented. Feasibility study is also discussed in Section 1.5. Finally, the tort of negligence will be described to warn aircraft design engineers to take the utmost care in order to prevent liability. Engineering Design Aircraft Design Flight Dynamics Propulsion Aerodynamics Structure Management Figure 1.3 Aircraft design required tools and expertise 4 Aircraft Design 1.2 Engineering Design Aircraft design is essentially a branch of engineering design. Design is the culmination of all engineering activities, embodying engineering operations and analysis as tools to achieve design objectives. Many engineering professors find it more difficult to teach design than to teach traditional engineering science-based analytical topics. Every under- graduate engineering curriculum has a design component, although the extent and structure of that component may vary widely. Engineering design fundamentals are common to all engineering disciplines – aeronautical, mechanical, electrical, civil, and computer. Engi- neering design is a methodical approach to dealing with a particular class of large and complex projects. Engineering design provides the design engineer with a realistic design process. Design is the central activity of the engineering profession, and it is concerned with approaches and management as well as design techniques and tools. In this section, the fundamentals of engineering design as well as the definitions of a few technical terms are presented. There is a clear distinction between classical mathematics and science problem-solving techniques, and design operation. There is inherently a beauty embedded in the design process which is usually felt after the design output is created. The mathematics and science problems have three main features: (i) the problems are well-posed in a compact form, (ii) the solutions to each problem are unique and compact, and (iii) the problems have an identifiable closure. However, a real-world engineering design problem does not share these characteristics. In fact, engineering design problems are usually poorly posed, do not have a unique solution, and are also open-ended. The Accreditation Board of Engineering and Technology (ABET) [5] defines engineering design as follows: Engineering design is the process of devising a system, component, or process to meet desired needs. It is a decision making process (often iterative), in which the basic sciences and math- ematics and engineering sciences are applied to convert resources optimally to meet a stated objectives. Among the fundamental elements of the design process are the establishment of objectives and criteria, synthesis, analysis, construction, testing, and evaluation. Just as the ABET statement is only one of many definitions of engineering design, there are several approaches to describing how design is done. This text formalizes the ABET description into a simplified step-by-step model of the design process based on a systems engineering approach [6]. A very basic block diagram of the design process is shown in Figure 1.4. It represents the road from customer need to design output, including feedback based on evaluation. The problem formulation is discussed in this section, and project Design output Customer need Problem formulation Project planning Design Operation Evaluation Figure 1.4 Engineering design block diagram Aircraft Design Fundamentals 5 Deorbit Orbit insertion ET separation Main engine cut off SRB separation SRB splashdown Landing Re-entry Liftoff On-orbit operations Figure 1.5 The original Space Shuttle concept and mission profile. Reproduced from permission of NASA planning is examined in Section 1.4. A large part of this text is on design operations, including Chapters 3–12. The evaluation not only influences the design operation, but most of the time may affect problem formulation and project planning. A clear current example is the Space Shuttle, which started in 1981 but retired in 2011. After more than 30 years of successful operations (135 space missions), the National Aeronautics and Space Administration (NASA) figured out that the current design concept is not viable. Besides economic factors, two reasons that forced NASA to re-engineer the Space Shuttle (Figure 1.5) are the disasters that happened in 1986 and 2003. On January 28, 1986 Space Shuttle Challenger broke apart, just 73 seconds into its flight, leading to the deaths of its seven crew members. On February 1, 2003, shortly before it was scheduled to conclude its 28th mission, Space Shuttle Columbia disintegrated over Texas during re-entry into the Earth’s atmosphere, resulting in the death of all seven crew members. Until another US launch vehicle is ready, crews will travel to and from the International Space Station aboard Russian Soyuz spacecraft or possibly a future American commercial spacecraft. After the need is clearly defined, the designer has to turn his/her attention to describ- ing how he/she envisions meeting the need. This fundamental step requires achieving a delicate balance between establishing the general scope of the design efforts, and avoid- ing being so specific that opportunities are unnecessarily narrowed for creative design solutions. Problem formulation includes recognizing the need, identifying the customer, market assessment, defining the problem, functional analysis, and establishing design requirements. A problem statement needs to be constructed in such a way that it consists of three components: goal, objectives, and constraints (Figure 1.6). A goal statement is a brief, general, and ideal response to the need statement. The need describes the current, unsatisfactory situation, while the goal describes the ideal future 6 Aircraft Design Problem Statement Objectives Goal Constraints Figure 1.6 Three elements of a problem statement condition to which we aspire in order to improve on the situation described by the need. The goal is defined by describing the current situation that is unsatisfactory. Hence the goal is to improve the current situation to a higher level. The goal is generally so ideal that it could never be accomplished. The goal is usually revised through a process called benchmarking. Benchmarking involves explicitly comparing your design to that of the competitor which does the best job in terms of satisfying customer requirements. The objectives are quantifiable expectations of performance which identify those per- formance characteristics of a design that are of most interest to the customer. In addition, the objectives must include a description of conditions under which a design must per- form. In the lifecycle, the objective is to specify the whats and not the hows; that is, what needs to be accomplished versus how it is to be done. When the operating conditions are specified, the designer is able to evaluate the performance of different design options under comparable conditions. Each of the objectives must be defined using words that convey the desirable aspect of performance. The term “performance specification” is often a synonym for objectives. However, the term “design specification” refers to the detailed description of the completed design, including all dimensions, material properties, weight, and fabrication instructions. Restrictions of function or form are called constraints; they limit our freedom to design. Constraints define the permissible conditions of design features and the permissible range of the design and performance parameters. They are features that all design must have in order to be eligible for consideration. Most engineering design projects essentially include a variety of realistic constraints, such as economic factors, safety, reliability, aesthetics, ethics, and social impacts. For instance, the height of the new system cannot exceed 1.4 m; or its mass may not exceed 3.6 kg; or it must operate year-round during cold and hot days. The value-free descriptors associated with each objective are referred to as criteria. For instance, an objective for a design is that it must be “inexpensive.” The criterion associated with this objective is “cost.” The criteria are quantified using the same bases for measurement and the same unit as their corresponding objectives. In other words, the criteria are more compact ways of identifying objectives. Table 1.1 demonstrates a number of typical design objectives and related criteria to design a vehicle. Fundamentally, design products are developed and created to satisfy needs and wants and provide utility to the customer. The customer’s needs have to be translated into design requirements through goal and objectives. Design requirements mainly include customer requirements plus engineering requirements. The customer requirements refer to objectives as articulated by the customer or client. The engineering requirements refer to the design and performance parameters that can contribute to achieving the customer requirements. Aircraft Design Fundamentals 7 Table 1.1 Typical design objectives and related criteria for a vehicle design project No. Objective Basis for measurement Criterion Units 1 Inexpensive in market Unit manufacturing cost Manufacturing cost Dollar 2 Inexpensive in operation Fuel consumption per kilometer Operating cost l/km 3 Light Total weight Weight N 4 Small size Geometry Dimensions m 5 Fast Speed of operation Performance km/h 6 Maintainable Man-hours to maintain Maintainability Man-hour 7 Producible Required technology for manufacturing Manufacturability – 8 Recyclable Amount of hazardous or non-recyclable materials Disposability kg 9 Maneuverable Turn radius Maneuverability m 10 Comfortable Ergonomic standards Human factor – 11 Airworthiness Safety standards Safety – 12 No human casualty in operation Level of injury to passengers in a mishap Crashworthiness – Figure 1.7 illustrates conceptually the status of various design features during the design process. It indicates that there will be a large commitment in terms of configuration, manufacturing technology, and maintenance techniques at the early stages of a design program. In addition, it is at this point that major decisions are made and product-specific knowledge is limited. Moreover, it is estimated that about 70% of the projected lifecycle cost for a given product can be committed based on engineering design and management decisions during the early stages of design. As the design progresses, changes to the design get harder and harder. Therefore, the impact of a decision at the early stages of a design program is more profound than a decision at the later stages. Hence, it is crucial to be highly confident about any decision a designer makes at the conceptual design phase. The cost of aircraft design is about 1% of the total lifecycle cost; however, this 1% determines the other 99%. Furthermore, the design cost is about 20% of the production (acquisition) cost. Thus, any necessary investment in design team members is worth it. Most aircraft manufacturers do not make any profit in the first couple of years of production, in the hope that in the future, they will make money. The large aircraft manufacturers get back their money after about 10 years; after that, they will make a profit. In the past, there were a few examples where aircraft manufacturers were bankrupted and only resurrected by government through long-term loans. Wind-tunnel testing costs from 200 US$/hour for GA (General Aviation) small air- craft to 5000 US$/hour for large transport aircraft. The design and fabrication of some 8 Aircraft Design Commitment to configuration, manufacturing technology, and maintenance techniques Design progress Ease of change in design Product-specific knowledge Cost incurred 100% Percent Figure 1.7 Status of various design features during the design process aircraft – such as supersonic transport aircraft Aerospatiale-BAC Concorde (Figures 7.24 and 11.15) – was a great achievement, but when the international market does not pur- chase it, the production has to be stopped. 1.3 Design Project Planning In order for a design project schedule to be effective, it is necessary to have some pro- cedure for monitoring progress; and in a broader sense for encouraging personnel to progress. An effective general form of project management control device is the Gantt chart. It presents a project overview which is almost immediately understandable to non- systems personnel; hence it has great value as a means of informing management of project status. A Gantt chart has three main features: 1. It informs the manager and chief designer of what tasks are assigned and who has been assigned them. 2. It indicates the estimated dates on which tasks are assumed to start and end, and represents graphically the estimated ration of the task. 3. It indicates the actual dates on which tasks were started and completed and pictures this information. Like many other planning/management tools, Gantt charts provide the manager/chief designer with an early warning if some jobs will not be completed on schedule and/or if others are ahead of schedule. Gantt charts are also helpful in that they present graph- ically immediate feedback regarding estimates of personnel skill and job complexity. Table 1.2 illustrates a typical Gantt chart for the design of a light single-seat aircraft in the form of a combined bar/milestone chart. Such a chart provides the chief designer with Aircraft Design Fundamentals 9 Table 1.2 A typical Gantt chart for the design of a light single-seat aircraft Job/task Week/month/year Job Task January 2013 February 2013 March 2013 April 2013 May 2013 June 2013 Identification of design requirements Problem definition Feasibility analysis Configuration design Conceptual design Conceptual design review Calculations Preliminary design Preliminary design review Wing design Tail design Fuselage design Propulsion system Landing gear Equipment/ subsystems Integration Wind-tunnel testing Weight distribution Performance/stability analysis Control surfaces design Detail design Evaluation and test review Flight testing Production of prototype Flight tests Critical design review Certification Certification Critical design review 10 Aircraft Design Figure 1.8 Airbus A-380, the newest Airbus production. Reproduced from permission of Anne Deus a scheduling method and enables him/her to rapidly track and assess the design activities on a weekly/monthly basis. An aircraft project such as Airbus A-380 (Figure 1.8) will not be successful without design project planning. A preferred method of scheduling is through the use of program networks [2] such as the program evaluation and review technique (PERT) and the critical path method (CPM). The application of network scheduling is appropriate for both small- and large-scale design projects and is of particular value for a system development where there are several interdependencies. The definitions of new terms in Table 1.2, such as preliminary design and critical design review, and their associated techniques are addressed in Chapter 2. 1.4 Decision Making First and foremost, it must be emphasized that any engineering selection must be supported by logical and scientific reasoning and analysis. The designer is not expected to select a configuration just because he/she likes it. There must be sufficient evidence and reasons which prove that the current selection is the best. The main challenge in decision making is that there are usually multiple criteria along with a risk associated with each one. In this section, a few techniques and tools for aiding decision making under complex conditions are introduced. However, in most design projects there are stages where there are several acceptable design alternatives and the designer has to select only one of them. In such cases, there are no straightforward governing equations to be solved mathematically. Thus, the only way to reach the solution is to choose from a list of design options. There are frequently many circumstances in which there are multiple solutions for a design problem but one option does not clearly dominate the others in all areas of comparison. A simple example is a transportation design problem where a designer is required to design a vehicle to transfer one person from one city to another. It is assumed that the two cities are both seaports and located at a distance of 300 km. The design solution alter- natives are bicycle, motorbike, automobile, train, bus, ship, and aircraft. A traveler may select to travel using any of these vehicles. Three common criteria in most engineering design projects are: (i) cost, (ii) performance, and (iii) safety (and reliability). Table 1.3 shows a typical comparison of these design options and the ranking of each alternative. Aircraft Design Fundamentals 11 Table 1.3 A typical multi-criteria decision-making problem (1 is the most desirable) No. Design option Criteria (vehicle) Cost (of operation) Safety Performance (maximum speed) 1 Bicycle 1 1 7 2 Motorbike 2 7 3 3 Automobile 5 6 4 4 Bus 3 5 5 5 Train 4 3 2 6 Ship 6 4 6 7 Aircraft 7 2 1 As the ranking illustrates, no one option clearly ranks first with respect to all three criteria to dominate the other six alternatives. If the designer cares only for the cost of operation and safety, he/she has to select the bicycle, but if the only criterion was travel speed, the aircraft would be chosen as the vehicle. The bicycle is often the slowest vehicle; however it is the cheapest way to travel. In contrast, the aircraft does the best job in terms of speed (fastest to travel), but it is usually the most expensive option. It is evident that, for a typical traveler and designer, all the criteria matter. Thus, the question is how to come up with the best decision and the optimum vehicle. This example (Table 1.3) represents a typical multi-criteria decision- making problem that a design engineer frequently faces in a typical engineering design project. After the type of vehicle is selected, the calculations begin to determine geometry and other engineering characteristics. A designer must recognize the importance of making the best decision and the adverse consequences of making a poor decision. In the majority of design cases, the best decision is the right decision, and a poor decision is the wrong one. The right decision implies design success, while a wrong decision results in a failure of the design. As the level of design problem complexity and sophistication increases in a particular situation, a more sophisticated approach is needed. The approach for making the best decision to select/determine the best alternative is to take five steps, as follows. • Step 1. Specify all the alternatives to be included in the exercise. Try to generate as many design concepts as possible using the brainstorming technique. However, given the resources required to include and consider all alternatives, you need to give considerable thought to reducing the alternatives to a manageable number. • Step 2. The second step in selecting the best design is to identify and establish the criteria (e.g., Table 1.1). These criteria serve later as the guidelines for developing the options. Some design references employ the term “figures of merit” instead of criteria. 12 Aircraft Design • Step 3. The next step is to define the metrics. The metrics are defined as a shorthand way of referring to the criteria performance measures and their units. Metrics are the tool to overcome a non-comparable complex situation (e.g., comparing apples and oranges) by establishing a common evaluation scale and mapping each criterion’s metric onto this scale. A simple evaluation scale is to map each criterion as either excellent, adequate, or poor. So, each design option may be rated with respect to each criterion using this common scale. A better and more quantifiable scale is a numerical scale, as demonstrated in Table 1.4. Typical metrics for measuring performance of an aircraft are maximum speed, take-off run, rate-of-climb, range, endurance, turn radius, turn rate, and ceiling. • Step 4. The fourth step is to deal with criteria that have unequal significance. A designer should not frequently treat all criteria as being equally important. The designer must try to ascertain how important each requirement (i.e., criterion) is to the customer. The simplest approach is to assign numerical weights to each criterion (or even at a metrics level) to indicate its importance relative to other criteria. These weights ideally reflect the designer’s judgment of relative importance. Judgment as to whether one design alternative is superior to another may be highly dependent on the values and preferences of the evaluator. In some cases, the designer has no way other than relying on personal “feelings” and “judgments” for the basis of the numerical weights. As a starting point, you may pair up each criterion with every other criterion one at a time and judge which of the items in each pair is more important than the other. The weights may later be normalized (i.e., mathematically convert each number to a fraction of 1) in order to make them easier to compare. A prerequisite to identifying the weight of each criterion is prioritization. Table 3.6 demonstrates the priorities of various aircraft designers against 10 design criteria. When the number of criteria is small, this task is straightforward. For large and complex systems, a systems engineering approach must be employed (Chapter 2). A cookbook method is no substitute for experience and sound professional judgment in what is inherently a subjective process. Reference [2] describes a higher-level approach which is referred to as the analytical hierarchy process (AHP) method; it is worth considering for sophisticated systems. • Step 5. Select the alternative which gains the highest numerical value. It is expected that the output of the decision-making process will yield the most desirable result. The designer may conduct the decision-making process by developing a software package to minimize or maximize a specific index. In case there are uncertainties in evaluating criteria, a sophisticated robust decision rule should attempt to incorporate the uncertainties into the decision-making process. One of the difficulties of dealing with uncertainties is coming up with the probabilities of the uncertain parameters and factors. This is best performed in a process referred to as “sensitivity analysis.” 1.5 Feasibility Analysis In the early stages of design and by employing brainstorming, a few promising concepts are suggested which seem consistent with the scheduling and available resources. Prior to committing resources and personnel to the detail design phase, an important design activity – feasibility analysis – must be performed. There are a number of phases through Aircraft Design Fundamentals 13 Table 1.4 Common scale and criteria metrics and three examples No. Common scale Criteria metrics Preferred level Value Example 1: length (m) Example 2: maximum speed (km/h) Example 3: mass (kg) 1 Perfect 10 35 60 500 2 Excellent 9 29.1 52 550 3 Very good 8 25.7 41 620 4 Good 7 21.4 32 680 5 Satisfactory 6 18.4 27 740 6 Adequate 5 16.6 21 790 7 Tolerable 4 12.7 17 830 8 Poor 3 8.4 17 910 9 Very poor 2 6.7 14 960 10 Inadequate 1 4.3 10 1020 11 Useless 0 2.5 7 1100 which the system design and development process must invariably pass. Foremost among them is the identification of the customer-related need and, from that, the determination of what the system is to do. This is followed by a feasibility study to discover potential technical solutions, and the determination of system requirements. It is at this early stage in the lifecycle that major decisions are made relative to adapting a specific design approach and technology application, which have a great impact on the lifecycle cost of a product. At this phase, the designer addresses the fundamental question of whether to proceed with the selected concept. It is evident that there is no benefit or future in spending any more time and resources attempting to achieve an unrealistic objective. Some revolutionary concepts initially seem attractive but when it comes to the reality, they are found to be too imaginary. Feasibility study distinguishes between a creative design concept and an imaginary idea. Feasibility evaluation determines the degree to which each concept alternative satisfies the design criteria. In the feasibility analysis, the answers to the following two questions are sought: 1. Are the goals achievable, are the objectives realistic, or can the design requirements be met? 2. Is the current design concept feasible? If the answer to the first question is no, the design goal and objectives, and hence the design requirements, must be changed. Then, no matter what the source of the design requirements – either direct customer order or market analysis – they must be changed (Figure 1.9). When the answer to the second question is negative, a new concept must be selected. Finding the answers to these questions is not always easy. To determine the answers other professionals beside design engineers – such as financial experts or manufacturing engineers – must often be involved in the feasibility study. The feasibility 14 Aircraft Design Yes No No Yes Design requirements Market analysis Conceptual design Convergence test Concept feasible? Redesign for a new configuration (concept) Detail design Customer order Change design requirements Figure 1.9 Feasibility analysis process analysis will refine the design requirements and narrow down the initial promising design concepts to a few feasible ones. It is at this stage that uncertainties are identified. When several concepts are analyzed and the convergency test illustrates that none of the promising concepts are feasible, the customer is informed that the objectives are not achievable within the current limits of science and technology. At this time, it is recommended that the customer reduces the level of his/her expectations. In contrast, the results of a feasibility study will significantly impact the operational characteristics of the product and its design for producibility, supportability, disposability, and detectability. The selection and application of a given technology or given materials has reliability and maintainability implications, will influence manufacturing operations, and will affect the product operating cost. For instance, Boeing 787 Dreamliner (Figure 1.10) is the first commercial transport air- craft with full composite structure. The composite materials may have reduced the aircraft Figure 1.10 Boeing 787 Dreamliner. Reproduced from permission of A J Best Aircraft Design Fundamentals 15 weight, but will certainly influence the reliability, maintenance, and entire lifecycle. All these considerations should be dealt with during the feasibility study before a commitment is made to pursue extensive design activities. The systems engineering approach has a systematic view of feasibility analysis. Thus, a primary objective of systems engineering is to ensure the proper coordination and timely integration of all systems elements (and the activities associated with each) from the beginning. The systems engineering approach is introduced in Chapter 2. 1.6 Tort of Negligence The issue of legal liability is crucial to an aircraft design engineer. Liability is basically part of the system of civil law. In civil law, the issue is not one of innocence or guilt; it is a question of who is at fault in a dispute, or who violated an agreement, or who failed to fulfill obligations. Liability law belongs to that branch of civil law known as torts. The area of tort law known as negligence involves harm caused by carelessness, not intentional harm. Negligence is a failure to exercise the care that a reasonably prudent person would exercise in like circumstances. Designers and manufacturers who sell their products to the public face many uncertainties regarding the legal ramifications of their actions. Design engineers and manufacturers are responsible and liable for harm done by their product or design to a customer or third party. Thus a designer has the responsibility to act in a careful and prudent manner. The negligence is applied to a designer when the product was defective or a design created a concealed danger. Thousands of disasters have occurred throughout aviation history, for a great number of which the designers (not the pilots) have been responsible. Disasters include aircraft crashes, mishaps, and accidents. In all of these cases, harm (bodily or financially) has been done to a customer or to the public. The primary source of such incidents is the designer’s carelessness in design, error in calculations, or lack of prediction of the future. In the area of accident prediction, Murphy’s Law applies which states: If any event can happen, it will happen; or anything that can go wrong will go wrong. For instance, one application of this law relates to liquid containers. The direct applica- tion of the law is as follows: every system in an aircraft which carries a liquid will leak. An aircraft with an air-breathing engine carries fuel and a passenger aircraft carries water. Thus, the aircraft designer must avoid installing electrical wiring and avionic systems in the belly, below the toilet or liquid container or fuel tank. Reference [7] describes a num- ber of war stories based on actual events that happened in the design and development of aircraft programs. For instance, one story relates how the unacceptable field performance of the first F-18 fighter was traced to an error in the calculation of aerodynamic forces in the ground effect. Another war story describes the Fowler flaps crunching in the first flight of the General Dynamics strike aircraft F-111A, when the pilot engaged the wing sweep system to sweep the wing aft after landing. The accident was clearly the designer’s fault, in not expecting such an event. The solution was to employ an interlocking device to prevent a pilot from sweeping the wings with the flap down. One of the continuing functions of a design engineer is to compile development and operations “lessons learned” documents and 16 Aircraft Design ensure their integration into future systems development activities. Lessons learned files from previous projects are especially valuable in risk identification and characterization, and must be employed in feasibility studies. The following three aircraft-related cases arose out of tragic accidents occurring at different times, and where the relatives of the victims brought a wrongful death case to court. In all three cases the court found the company (i.e., the designer) negligent and liable. Once a judgment has been made in favor of the plaintiff in a liability case, a monetary award is made. However, in more serious cases, punitive damages may also be awarded. In the area of astronautics, most satellite mishaps stem from engineering mistakes. To prevent the same errors from being repeated, some references have compiled lessons that the space community should heed. • Case 1: United States versus “Weber Aircraft Corp.” in 1984. When the engine of an Air Force aircraft failed in flight, the pilot was severely injured when he ejected from the plane. After Air Force collateral and safety investigations of the incident had been completed, the pilot filed a damages action against respondents as the entities responsible for the design and manufacture of the plane’s ejection equipment. • Case 2: Jack King and 69 European plaintiffs versus “Cessna Aircraft Company” in a tragic plane crash that occurred at Linate Airport in Milan, Italy, on October 8, 2001. On that foggy morning, a private Cessna jet operated by Air Evex, a German charter company, made a wrong turn and taxied toward an active runway, causing it to collide with Scandinavian Airlines Flight 686, which was just taking flight. One hundred and eighteen people died, including everyone on board both planes and four people on the ground, and others on the ground were injured. • Case 3: Starting in 1991, a number of accidents and incidents involving the Boeing 737 were the result of the airplanes’ unexpected rudder movement. One incident occurred on September 8, 1994 when a Boeing 737-300 of USAIR Flight 427 crashed near Pittsburgh, PA, killing 132 people. Another incident was when the Boeing 737 Flight 185 of SilkAiron plunged from 35 000 ft into a muddy river in Indonesia on December 19, 1997, killing all 104 people aboard. The Los Angeles Superior Court jury decided defects in the rudder control system caused the crash and Parker Hannifin Corp., the world’s largest maker of hydraulic equipment, was told to pay US$43.6 million to the families of three people killed. On the contrary, the US National Transportation Safety Board (NTSB) concluded that there were no mechanical defects and the pilot intentionally caused the crash. The Federal Aviation Administration (FAA) ultimately ordered an upgrade of all Boeing 737 rudder control systems by November 12, 2002. • Case 4: A Continental Airlines Boeing 737 went off the runway during takeoff from Denver International Airport in Colorado, plunging into a ravine and shearing off its landing gear and left engine. At least 58 people were injured in the crash that happened on December 20, 2008. The entire right side of the plane was burned, and melted plastic from overhead compartments dripped onto the seats. Note that the plane’s left engine was ripped away along with all the landing gear. NTSB published that the probable cause of this accident was the captain’s error (cessation of right rudder input). Figure 1.11(a) shows a Tupolev Tu-154 which crashed while attempting to land in poor weather conditions on September 14, 1991 in Mexico City. Luckily all 112 occu- pants survived. Figure 1.11(b) illustrates the transport aircraft Ilyushin Il-76 freighter, Aircraft Design Fundamentals 17 (a) (b) Figure 1.11 Two aircraft in tragic accidents: (a) Tupolev Tu-154 crashed due to poor weather conditions; (b) An Ilyushin Il-76 freighter which caught fire on the ground. Reproduced from permission of (a) Augusto G. Gomez; (b) Serghei Podlesnii Part (a) reproduced from permission of Augusto G. Gomez which caught fire on the ground while it was being loaded in preparation for a flight to Brazzaville, Congo on May 10, 2007. The threat of liability law suits must spur on designers and manufacturers to be more sensitive to safety issues and to address them in more creative and innovative ways. The liability threat should not have a stifling effect on creative design and technological innovation. For this reason, the employment of safety factors is highly recommended. Federal Aviation Regulations have addressed this issue in many ways, but it does not suffice; aircraft designers and all involved engineers must be prudent and careful in the design process. A prudent design strategy is to employ the utmost care; to anticipate relevant wrongful events; and to incorporate some features into products to make them more robust. There is a famous 109 rule in aircraft design which is acceptable within society. This rule states that one death in 1 000 000 000 aircraft travelers is accepted. Even one human death is a great disaster to a community, but stupidity and negligence can sometimes lead to a deadly crash. In terms of statistics, about 300 people are killed every year in aviation- related accidents in the USA while about 45 000 are killed in car accidents. Therefore, the aircraft is much safer than the car, and air travel is 150 times safer than road travel. About one-third of aviation accidents are because of CFIT (controlled flight into terrain). When a pilot makes a mistake and hits a mountain, a designer has almost no influence on this incident. Not every pilot mistake has a solution by the aircraft designer; some mistakes may be avoided by design, but not all. Reference [7] describes several stories about pilot mistakes as well as designer mistakes. All stories are beneficial to aircraft designers and have lessens to be learned. References [1] Robbins, S.P. and Coulter, M. (2008) Management, 10th edn, Pearson Prentice Hall. [2] Dieter, G. and Schmidt, L. (2008) Engineering Design, 4th edn, McGraw-Hill. [3] Hyman, B. (2003) Fundamentals of Engineering Design, 2nd edn, Prentice Hall. [4] Eggert, R.J. (2005) Engineering Design, Pearson Prentice Hall. [5] ABET Constitution, Accreditation Board for Engineering and Technology (2012), www.abet.org. [6] Blanchard, B.S. and Fabrycky, W.J. (2006) Systems Engineering and Analysis, 4th edn, Prentice Hall. [7] Roskam, J. (2006) Roskam’s Airplanes War Stories, DAR Corporation. 2 Systems Engineering Approach 2.1 Introduction The systems engineering (SE) discipline was originally developed to help understand and manage complexity. The scale of complexity found in modern aircraft systems necessitates an approach different from that applied traditionally. The formal instruction in modern SE principles is cited as beginning more than 40 years ago. The applications of systems engineering began during the late 1950s, when the early Department of Defense (DOD) view of SE was documented. This was due to the race to space [1] and the development of the nuclear missiles program that were essential for US survival. The first attempt to formalize systems engineering within an engineering curriculum occurred at MIT in 1950. In the 1960s, systems engineering gained widespread acceptance within the DOD as the preferred approach to engineer military systems. The systems engineering approach was revitalized in the mid-1990s. Increased participation using systems engineering processes and practices during the system development and demonstration phase is seen as key to implementing this approach. The industry, academia, and government revitalization efforts include publishing systems engineering processes, methods, and templates to guide people to implement systems engineering. In 2003 and 2004, the DOD [2, 3] issued a number of policies that placed renewed emphasis on the application of systems engineering, stating that it is essential to the Department’s ability to meet the challenge of developing and maintaining needed capability. It was noted that this is especially true as systems become more complex in a family-of-systems, or system-of-systems. In addition, NASA [4] developed and published a systems engineering handbook in 2007. In general, systems may be classified as either natural or human-made. Human-made or technical systems (e.g., aircraft) come into being by human intervention in the nat- ural order utilizing pervasive technologies. System is an assemblage or combination of elements, members, components, and parts forming a complex or unitary whole. A ran- dom group of items in a room would not qualify as a system because of the absence of unity, functional relationship, and useful purpose. Systems are composed of components, attributes, and relationships. The purposeful action performed by a system is its function. Aircraft Design: A Systems Engineering Approach, First Edition. Mohammad H. Sadraey.  2013 John Wiley & Sons, Ltd. Published 2013 by John Wiley & Sons, Ltd. 20 Aircraft Design A system view is only one way of understanding complexity. The systems engineering approach is defined as “an interdisciplinary approach encompassing the entire technical effort to evolve and verify an integrated and lifecycle balanced set of system people, product, and process solutions that satisfy customer needs.” Multi-discipline SE design involves the application of a systems engineering process and requires engineers with substantive knowledge of design across multiple technical areas and improved tools and methods for doing it. Industry, government, and academia share responsibility for the development of the future engineers needed to keep aerospace products and capabilities at the leading edge of technology. One of the enablers is fundamental knowledge of systems engineering and its practical application to systems that involve multiple disciplines. Nonetheless, engineer- ing education programs continue to focus on the traditional educational product – highly qualified but single-discipline engineers and technologists. Meeting the demand for multi- disciplinary systems engineering designers requires teaching something different than is found in current textbooks. Education in systems engineering [5–7] is often seen as an extension of the regular engineering programs. The formal instruction in SE principles is cited as beginning more than 40 years ago. The applications of systems engineering began during the late 1950s because of the race to space and the development of the nuclear missiles program that were essential for national survival. There are a limited number of undergraduate univer- sity programs in systems engineering. The International Council on Systems Engineering (INCOSE) maintains a directory of systems engineering [8] academic programs world- wide. As per the 2006 INCOSE directory, there are about 75 institutions in the United States offering a total of about 130 undergraduate and graduate programs in systems engi- neering. Education in systems engineering can be broken down into two basic categories: systems engineering-centric or domain-centric. SE-centric programs treat systems engi- neering as a separate discipline, focusing their courses on systems engineering practice and techniques. In 2006, there were 31 institutions offering 48 degree programs in the systems engineering-centric category and 48 institutions offering 82 domain-centric SE degree programs across a number of engineering domains. This chapter is devoted to briefly introducing the fundamentals of the systems engi- neering discipline, principles, design phases, design flowcharts, design evaluation, and systems engineering application to the aircraft design process. The systems engineering approach is employed throughout this book to present aircraft design techniques. This chapter is organized as follows. In Section 2.2, the fundamentals of systems engineer- ing are presented. Sections 2.3–2.5 provide the features of the conceptual design phase, the preliminary design phase, and the detail design phase, respectively. In Section 2.6, design for operational feasibility is introduced which covers topics such as maintainabil- ity, producibility, detectability, usability, supportability, affordability, recyclability, and disposability. The design review, evaluation, and feedback are important steps in the sys- tems engineering technique and are covered in Section 2.7. Finally, the application of systems engineering for the design of an air vehicle or systems engineering approach in aircraft design is established in Section 2.8. 2.2 Fundamentals of Systems Engineering An aircraft is a system composed of a set of interrelated components working together toward some common aerial objective or purpose. Primary objectives include safe flight Systems Engineering Approach 21 achieved at a low cost. Aircraft are extremely complex products comprising many sub- systems, components, and parts. They are but one system operating within a global air transportation or defense “system-of-systems.” The conception, design, production, oper- ation, and maintenance of aircraft are influenced by many factors including technical, economic, political, organizational, financial, and regulatory. The engineering of an air- craft as a system requires methods, tools, and processes which can successfully address these many complexities. Aircraft systems, due to the high cost and the risks associated with their development, are a major user of systems engineering methodologies. Systems engineering is the fundamental discipline embodying these methods, tools, and processes. It also addresses the overall strategy for developing system-level requirements which meet users’ needs, meet investors’ expectations, incorporate knowledge from past experience, and satisfy regulatory and other constraints. Systems engineering, on the other hand, is the process used to develop integrated human, hardware, and/or software compo- nents such that the resulting system or product meets the system-level requirements. To apply the systems engineering technique, one must decide “what constitutes the system.” To provide a framework, the following levels are defined where (in this book) the level 2 or system level is addressed. • Level 1, System-of-systems level. The air transportation/defense system which inclu- des aircraft, missiles, satellites, ground stations, airports, air traffic management, etc. • Level 2, System level. The aircraft and/or related systems which include aircraft, users, operators, trainers, manufacturing plants, maintenance shops, etc. • Level 3, Subsystem level. Major aircraft subsystems which include the flight control mechanism, hydraulic, electric, avionic, powerplant, fuel, air conditioning, structure, seat, etc. • Level 4, Component level. Components which include the wing, fuselage, tail, landing gear, radar, pumps, nacelles, control surfaces, auxiliary power unit (APU), etc. • Level 5, Part level. Parts which include fittings, fasteners, blades, propeller, screws, nuts, ribs, spars, frame, stiffener, skin, shaft, wires, pipes, etc. To ensure economic competitiveness, engineering must become more closely associated with economics and economic feasibility, which is best accomplished through a lifecycle approach to engineering. The system lifecycle includes design, development, production, operation, support, and disposal. The design process is divided into three major phases: (i) conceptual design phase, (ii) preliminary design phase, and (iii) detail design phase. These are artificial categories that, along with test and evaluation, make up the four basic phases of system design. The summation of conceptual design, preliminary design, detail design, and production and/or construction is referred to as the acquisition phase (Figure 2.1), while the summa- tion of product use, support, phase-out, and disposal is called the utilization phase. It is essential that aircraft designers be sensitive to utilization outcomes during the early stages of the design and development process. They also need to conduct lifecycle engineering studies as early as possible in the design process. Figure 2.2 illustrates the relationship among four major design activities in a systems engineering approach. The design pro- cess primarily starts with the conceptual design phase, based on design requirements. The details of the conceptual design phase are presented in Section 2.3. The preliminary design begins right after the conceptual design phase and employs the output of this phase. 22 Aircraft Design Conceptual Design Preliminary Design Detail Design N E E D Product Use, Support, and Disposal Production/ Construction Acquisition phase Utilization phase Figure 2.1 The system life cycle Designed System Design Requirements Conceptual Design Preliminary Design Test and Evaluation Detail Design Figure 2.2 Relationship among four major design activities Section 2.4 is devoted to the preliminary design phase. The detail design phase begins right after the preliminary design phase and utilizes the output of this phase. Section 2.5 reviews the detail design phase. After each round of design, a test and evaluation is conducted to compare the character- istics of the designed system with the design requirements. If the system does not meet the requirements in any way, feedback is sent to the design groups to make the necessary cor- rections. This iteration is continued until all design requirements are satisfied. Figure 2.2 models the entire design process as a feedback control system where an error signal is produced if there is any difference between input (design requirements) and output (features of the designed system). The test and evaluation is introduced in Section 2.7. Although the overall design phases are generally accepted, a particular design pro- cess (such as the waterfall model, spiral model, or “Vee” model) must be tailored to a specific program need. The interested reader is encouraged to refer to Refs [1, 9, 10] for more details on various models. The systems engineering and aerospace engineering influences on design are illustrated in Figure 2.3. Both influences reverse as the design process progresses; the influence of systems engineering decreases, while the area influ- ence of aerospace engineering increases. Thus, there is a need to ensure that the aerospace engineering techniques are properly integrated. From the perspective of systems engineering, the design of aircraft should not only transform a need into an air vehicle, but also ensure the aircraft’s compatibility with related physical and functional requirements. Therefore, it should consider operational outcomes expressed as safety, producibility, affordability, reliability, maintainability, usability, sup- portability, serviceability, detectability, disposability, as well as the requirements on per- formance, stability, control, and effectiveness. A major objective of systems engineering Systems Engineering Approach 23 Systems engineering Development phase Conceptual design Preliminary design Detail design Aerospace engineering Design influence Low High Figure 2.3 Systems engineering and aerospace engineering influence on design is to develop a good set of requirements in order to define a single baseline from which all lower-level requir