Advanced Aircraft Design
Conceptual Design, Analysis and Optimization of Subsonic Civil Airplanes
Aerospace Series

Although the overall appearance of modern airliners has not changed a lot since the introduction of jetliners in the 1950s, their safety, efficiency and environmental friendliness have improved considerably. Main contributors to this have been gas turbine engine technology, advanced materials, computational aerodynamics, advanced structural analysis and on-board systems. Since aircraft design became a highly multidisciplinary activity, the development of multidisciplinary optimization (MDO) has become a popular new discipline. Despite this, the application of MDO during the conceptual design phase is not yet widespread.

Advanced Aircraft Design: Conceptual Design, Analysis and Optimization of Subsonic Civil Airplanes presents a quasi-analytical optimization approach based on a concise set of sizing equations. Objectives are aerodynamic efficiency, mission fuel, empty weight and maximum takeoff weight. Independent design variables studied include design cruise altitude, wing area and span and thrust or power loading. Principal  features of integrated concepts such as the blended wing and body and highly non-planar wings are also covered. 

The quasi-analytical approach enables designers to compare the results of high-fidelity MDO optimization with lower-fidelity methods which need far less computational effort. Another advantage to this approach is that it can provide answers to ?what if? questions rapidly and with little computational cost.

Key features:

• Presents a new fundamental vision on conceptual airplane design optimization
• Provides an overview of advanced technologies for propulsion and reducing aerodynamic drag
• Offers insight into the derivation of design sensitivity information
• Emphasizes design based on first principles
• Considers pros and cons  of innovative configurations
• Reconsiders optimum cruise performance at transonic Mach numbers

Advanced Aircraft Design: Conceptual Design, Analysis and Optimization of Subsonic Civil Airplanes advances understanding of the initial optimization of civil airplanes and is a must-have reference for aerospace engineering students, applied researchers, aircraft design engineers and analysts.

Series Preface xix

Preface xxi

Acknowledgements xxv

1 Design of theWell-Tempered Aircraft 1

1.1 How Aircraft Design Developed 1

1.1.1 Evolution of Jetliners and Executive Aircraft 1

1.1.2 A Framework for Advanced Design 4

1.1.3 Analytical Design Optimization 4

1.1.4 Computational Design Environment 5

1.2 Concept Finding 6

1.2.1 Advanced Design 6

1.2.2 Pre-conceptual Studies 7

1.3 Product Development 8

1.3.1 Concept Definition 10

1.3.2 Preliminary Design 11

1.3.3 Detail Design 13

1.4 Baseline Design in a Nutshell 13

1.4.1 Baseline Sizing 13

1.4.2 Power Plant 15

1.4.3 Weight and Balance 16

1.4.4 Structure 16

1.4.5 Performance Analysis 17

1.4.6 Closing the Loop 18

1.5 Automated Design Synthesis 19

1.5.1 Computational Systems Requirements 19

1.5.2 Examples 20

1.5.3 Parametric Surveys 21

1.6 Technology Assessment 22

1.7 Structure of the Optimization Problem 25

1.7.1 Analysis Versus Synthesis 25

1.7.2 Problem Classification 26

Bibliography 27

2 Early Conceptual Design 31

2.1 Scenario and Requirements 31

2.1.1 What Drives a Design? 31

2.1.2 Civil Airplane Categories 33

2.1.3 Top Level Requirements 35

2.2 Weight Terminology and Prediction 36

2.2.1 Method Classification 36

2.2.2 Basic Weight Components 37

2.2.3 Weight Limits 39

2.2.4 Transport Capability 39

2.3 The Unity Equation 41

2.3.1 Mission Fuel 43

2.3.2 Empty Weight 44

2.3.3 Design Weights 45

2.4 Range Parameter 46

2.4.1 Aerodynamic Efficiency 47

2.4.2 Specific Fuel Consumption and Overall Efficiency 48

2.4.3 Best Cruise Speed 49

2.5 Environmental Issues 51

2.5.1 Energy and Payload Fuel Efficiency 51

2.5.2 ‘Greener by Design’ 54

Bibliography 56

3 Propulsion and Engine Technology 59

3.1 Propulsion Leading the Way 59

3.2 Basic Concepts of Jet Propulsion 60

3.2.1 Turbojet Thrust 60

3.2.2 Turbofan Thrust 61

3.2.3 Specific Fuel Consumption 62

3.2.4 Overall Efficiency 63

3.2.5 Thermal and Propulsive Efficiency 63

3.2.6 Generalized Performance 65

3.2.7 Mach Number and Altitude Effects 66

3.3 Turboprop Engines 67

3.3.1 Power and Specific Fuel Consumption 67

3.3.2 Generalized Performance 68

3.3.3 High Speed Propellers 69

3.4 Turbofan Engine Layout 70

3.4.1 Bypass Ratio Trends 70

3.4.2 Rise and Fall of the Propfan 72

3.4.3 Rebirth of the Open Rotor? 74

3.5 Power Plant Selection 74

3.5.1 Power Plant Location 75

3.5.2 Alternative Fuels 76

3.5.3 Aircraft Noise 77

4 Aerodynamic Drag and Its Reduction 81

4.1 Basic Concepts 81

4.1.1 Lift, Drag and Aerodynamic Efficiency 82

4.1.2 Drag Breakdown and Definitions 83

4.2 Decomposition Schemes and Terminology 84

4.2.1 Pressure and Friction Drag 84

4.2.2 Viscous Drag 85

4.2.3 Vortex Drag 85

4.2.4 Wave Drag 86

4.3 Subsonic Parasite and Induced Drag 87

4.3.1 Parasite Drag 87

4.3.2 Monoplane Induced Drag 90

4.3.3 Biplane Induced Drag 91

4.3.4 Multiplane and Boxplane Induced Drag 94

4.4 Drag Polar Representations 95

4.4.1 Two-term Approximation 95

4.4.2 Three-term Approximation 96

4.4.3 Reynolds Number Effects 97

4.4.4 Compressibility Correction 98

4.5 Drag Prediction 99

4.5.1 Interference Drag 100

4.5.2 Roughness and Excrescences 101

4.5.3 Corrections Dependent on Operation 102

4.5.4 Estimation of Maximum Subsonic L/D 102

4.5.5 Low-Speed Configuration 104

4.6 Viscous Drag Reduction 106

4.6.1 Wetted Area 107

4.6.2 Turbulent Friction Drag 108

4.6.3 Natural Laminar Flow 108

4.6.4 Laminar Flow Control 110

4.6.5 Hybrid Laminar Flow Control 111

4.6.6 Gains, Challenges and Barriers of LFC 112

4.7 Induced Drag Reduction 114

4.7.1 Wing Span 114

4.7.2 Spanwise Camber 115

4.7.3 Non-planar Wing Systems 115

Bibliography 115

5 From Tube and Wing to Flying Wing 121

5.1 The Case for Flying Wings 121

5.1.1 Northrop’s All-Wing Aircraft 121

5.1.2 Flying Wing Controversy 123

5.1.3 Whither All-Wing Airliners? 124

5.1.4 Fundamental Issues 126

5.2 Allocation of Useful Volume 127

5.2.1 Integration of the Useful Load 128

5.2.2 Study Ground Rules 128

5.2.3 Volume Ratio 129

5.2.4 Zero-Lift Drag 130

5.2.5 Generalized Aerodynamic Efficiency 131

5.2.6 Partial Optima 132

5.3 Survey of Aerodynamic Efficiency 134

5.3.1 Altitude Variation 134

5.3.2 Aspect Ratio and Span 135

5.3.3 Engine-Airframe Matching 136

5.4 Survey of the Parameter ML/D 138

5.4.1 Optimum Flight Conditions 138

5.4.2 The Drag Parameter 139

5.5 Integrated Configurations Compared 140

5.5.1 Conventional Baseline 141

5.5.2 Is a Wing Alone Sufficient? 143

5.5.3 Blended Wing Body 144

5.5.4 Hybrid Flying Wing 146

5.5.5 Span Loader 147

5.6 Flying Wing Design 149

5.6.1 Hang-Ups or Showstopper? 149

5.6.2 Structural Design and Weight 150

5.6.3 The Flying Wing: Will It Fly? 151

Bibliography 152

6 Clean Sheet Design 157

6.1 Dominant and Radical Configurations 157

6.1.1 Established Configurations 157

6.1.2 New Paradigms 159

6.2 Morphology of Shapes 159

6.2.1 Classification 160

6.2.2 Lifting Systems 160

6.2.3 Plan View Classification 162

6.2.4 Strut-Braced Wings 163

6.2.5 Propulsion and Concept Integration 164

6.3 Wing and Tail Configurations 165

6.3.1 Aerodynamic Limits 165

6.3.2 The Balanced Design 167

6.3.3 Evaluation 168

6.3.4 Relaxed Inherent Stability 169

6.4 Aircraft Featuring a Foreplane 169

6.4.1 Canard Configuration 170

6.4.2 Three-Surface Aircraft 172

6.5 Non-Planar Lifting Systems 173

6.5.1 Transonic Boxplane 173

6.5.2 C-Wing 175

6.6 Joined Wing Aircraft 177

6.6.1 Structural Principles and Weight 178

6.6.2 Aerodynamic Aspects 179

6.6.3 Stability and Control 180

6.6.4 Design Integration 181

6.7 Twin-Fuselage Aircraft 182

6.7.1 Design Integration 185

6.8 Hydrogen-Fuelled Commercial Transports 186

6.8.1 Properties of LH2 187

6.8.2 Fuel System 188

6.8.3 Handling Safety, Economics and Logistics 189

6.9 Promising Concepts 189

Bibliography 190

7 Aircraft Design Optimization 197

7.1 The Perfect Design: An Illusion? 197

7.2 Elements of Optimization 198

7.2.1 Design Parameters 198

7.2.2 Optimal Control and Discrete-Variable Optimization 199

7.2.3 Basic Terminology 200

7.2.4 Single-Objective Optimization 201

7.2.5 Unconstrained Optimizer 202

7.2.6 Constrained Optimizer 204

7.3 Analytical or Numerical Optimization? 206

7.3.1 Analytical Approach 206

7.3.2 Multivariate Optimization 207

7.3.3 Unconstrained Optimization 209

7.3.4 Constrained Optimization 210

7.3.5 Response Surface Approximation 211

7.3.6 Global Models 212

7.4 Large Optimization Problems 213

7.4.1 Concept Sizing and Evaluation 213

7.4.2 Multidisciplinary Optimization 214

7.4.3 System Decomposition 215

7.4.4 Multilevel Optimization 217

7.4.5 Multi-Objective Optimization 218

7.5 Practical Optimization in Conceptual Design 219

7.5.1 Arguments of the Sceptic 219

7.5.2 Problem Structure 220

7.5.3 Selecting Selection Variables 220

7.5.4 Design Sensitivity 222

7.5.5 The Objective Function 222

Bibliography 223

8 Theory of Optimum Weight 229

8.1 Weight Engineering: Core of Aircraft Design 229

8.1.1 Prediction Methods 230

8.1.2 Use of Statistics 231

8.2 Design Sensitivity 232

8.2.1 Problem Structure 232

8.2.2 Selection Variables 233

8.3 Jet Transport Empty Weight 234

8.3.1 Weight Breakdown 234

8.3.2 Wing Structure (Item 10) 235

8.3.3 Fuselage Structure (Item 11) 236

8.3.4 Empennage Structure (Items 12 and 13) 237

8.3.5 Landing Gear Structure (Item 14) 238

8.3.6 Power Plant and Engine Pylons (Items 2 and 15) 238

8.3.7 Systems, Furnishings and Operational Items (Items 3, 4 and 5) 238

8.3.8 Operating Empty Weight: Example 239

8.4 Design Sensitivity of Airframe Drag 239

8.4.1 Drag Decomposition 240

8.4.2 Aerodynamic Efficiency 242

8.5 Thrust, Power Plant and Fuel Weight 243

8.5.1 Installed Thrust and Power Plant Weight 243

8.5.2 Mission Fuel 245

8.5.3 Propulsion Weight Penalty 245

8.5.4 Wing and Propulsion Weight Fraction 248

8.5.5 Optimum Weight Fractions Compared 249

8.6 Take-Off Weight, Thrust and Fuel Efficiency 249

8.6.1 Maximum Take-Off Weight 249

8.6.2 Installed Thrust and Fuel Energy Efficiency 251

8.6.3 Unconstrained Optima Compared 252

8.6.4 Range for Given MTOW 253

8.6.5 Extended Range Version 254

8.7 Summary and Reflection 254

8.7.1 Which Figure of Merit? 254

8.7.2 Conclusion 256

8.7.3 Accuracy 257

Bibliography 257

9 Matching Engines and Airframe 261

9.1 Requirements and Constraints 261

9.2 Cruise-Sized Engines 262

9.2.1 Installed Take-Off Thrust 262

9.2.2 The Thumbprint 263

9.3 Low Speed Requirements 265

9.3.1 Stalling Speed 265

9.3.2 Take-Off Climb 266

9.3.3 Approach and Landing Climb 266

9.3.4 Second Segment Climb Gradient 267

9.4 Schematic Take-Off Analysis 267

9.4.1 Definitions of Take-Off Field Length 268

9.4.2 Take-Off Run 269

9.4.3 Airborne Distance 270

9.4.4 Take-Off Distance 270

9.4.5 Generalized Thrust and Span Loading Constraint 271

9.4.6 Minimum Thrust for Given TOFL 273

9.5 Approach and Landing 273

9.5.1 Landing Distance Analysis 273

9.5.2 Approach Speed and Wing Loading 274

9.6 Engine Selection and Installation 275

9.6.1 Identifying the Best Match 275

9.6.2 Initial Engine Assessment 276

9.6.3 Engine Selection 277

Bibliography 278

10 Elements of Aerodynamic Wing Design 281

10.1 Introduction 281

10.1.1 Problem Structure 282

10.1.2 Relation to Engine Selection 283

10.2 Planform Geometry 283

10.2.1 Wing Area and Design Lift Coefficient 285

10.2.2 Span and Aspect Ratio 286

10.3 Design Sensitivity Information 286

10.3.1 Aerodynamic Efficiency 287

10.3.2 Propulsion Weight Contribution 288

10.3.3 Wing and Tail Structure Weight 289

10.3.4 Wing Penalty Function and MTOW 290

10.4 Subsonic Aircraft Wing 291

10.4.1 Problem Structure 291

10.4.2 Unconstrained Optima 292

10.4.3 Minimum Propulsion Weight Penalty 294

10.4.4 Accuracy 294

10.5 Constrained Optima 295

10.5.1 Take-Off Field Length 296

10.5.2 Tank Volume 296

10.5.3 Wing and Tail Weight Fraction 297

10.5.4 Selection of the Design 297

10.6 Transonic Aircraft Wing 298

10.6.1 Geometry 298

10.6.2 Wing Drag in the Design Condition 299

10.6.3 Modified Wing Penalty Function 300

10.6.4 Thickness Ratio Limit 301

10.6.5 WPF Affected by Sweep Angle and Thickness Ratio 303

10.7 Lift Coefficient and Aspect Ratio 304

10.7.1 Partial Optima 304

10.7.2 Constraints 306

10.7.3 Refining the Optimization 307

10.8 Detailed Design 309

10.8.1 Taper and Lift Distribution 309

10.8.2 Camber and Twist Distribution 310

10.8.3 Forward Swept Wing (FSW) 311

10.8.4 Wing-Tip Devices 312

10.9 High Lift Devices 313

10.9.1 Aerodynamic Effects 313

10.9.2 Design Aspects 314

Bibliography 315

11 The Wing Structure and ItsWeight 319

11.1 Introduction 319

11.1.1 Statistics can be Useful 319

11.1.2 Quasi-Analytical Weight Prediction 320

11.2 Methodology 321

11.2.1 Weight Breakdown and Structural Concept 321

11.2.2 Basic Approach 323

11.2.3 Load Factors 324

11.3 Basic Wing Box 326

11.3.1 Bending due to Lift 326

11.3.2 Bending Material 331

11.3.3 Shear Material 333

11.3.4 In-Plane Loads and Torsion 334

11.3.5 Ribs 334

11.4 Inertia Relief and Design Loads 335

11.4.1 Relief due to Fixed Masses 336

11.4.2 Weight-Critical UL and Design Weights 337

11.5 Non-Ideal Weight 338

11.5.1 Non-Taper, Joints and Fasteners 339

11.5.2 Fail Safety and Damage Tolerance 340

11.5.3 Manholes and Access Hatches 340

11.5.4 Reinforcements, Attachments and Support Structure 341

11.5.5 Dynamic Over Swing 342

11.5.6 Torsional Stiffness 342

11.6 Secondary Structures and Miscellaneous Items 344

11.6.1 Fixed Leading Edge 345

11.6.2 Leading Edge High-Lift Devices 345

11.6.3 Fixed Trailing Edge 346

11.6.4 Trailing Edge Flaps 346

11.6.5 Flight Control Devices 348

11.6.6 Tip Structures 348

11.6.7 Miscellaneous Items 349

11.7 Stress Levels in Aluminium Alloys 349

11.7.1 Lower Panels 350

11.7.2 Upper Panels 350

11.7.3 Shear Stress in Spar Webs 352

11.8 Refinements 352

11.8.1 Tip Extensions 352

11.8.2 Centre Section 353

11.8.3 Compound Taper 354

11.8.4 Exposed Wing Lift 355

11.8.5 Advanced Materials 355

11.9 Application 357

11.9.1 Basic Ideal Structure Weight 357

11.9.2 Refined Ideal Structure Weight 358

11.9.3 Wing Structure Weight 359

11.9.4 Accuracy 359

11.9.5 Conclusion 360

Bibliography 361

12 Unified Cruise Performance 363

12.1 Introduction 363

12.1.1 Classical Solutions 363

12.1.2 Unified Cruise Performance 364

12.1.3 Specific Range and the Range Parameter 365

12.2 Maximum Aerodynamic Efficiency 366

12.2.1 Logarithmic Drag Derivatives 368

12.2.2 Interpretation of Log-Derivatives 369

12.2.3 Altitude Constraint 370

12.3 The Parameter ML/D 371

12.3.1 Subsonic Flight Mach Number 371

12.3.2 Transonic Flight Mach Number 372

12.4 The Range Parameter 374

12.4.1 Unconstrained Optima 374

12.4.2 Constrained Optima 376

12.4.3 Interpretation of ηM 376

12.4.4 Optimum Cruise Condition 378

12.5 Range in Cruising Flight 379

12.5.1 Br´eguet Range Equation 379

12.5.2 Continuous Cruise/Climb 380

12.5.3 Horizontal Cruise, Constant Speed 381

12.5.4 Horizontal Cruise, Constant Lift Coefficient 381

12.6 Cruise Procedures and Mission Fuel 382

12.6.1 Subsonic Flight 382

12.6.2 Transonic Flight 383

12.6.3 Cruise Fuel 384

12.6.4 Mission Fuel 385

12.6.5 Reserve Fuel 387

12.7 Reflection 388

12.7.1 Summary of Results 388

12.7.2 The Design Connection 389

Bibliography 390

A Volumes, Surface and Wetted Areas 393

A.1 Wing 393

A.2 Fuselage 394

A.3 Tail Surfaces 395

A.4 Engine Nacelles and Pylons 395

A.5 Airframe Wetted Area 395

Bibliography 396

B International Standard Atmosphere 397

C Abbreviations 399

Index 403

Egbert Torenbeek, Delft University of Technology, The Netherlands
Egbert Torenbeek is Professor Emeritus of Aircraft Design at Delft University of Technology.
He graduated as an engineer in 1961 at TU Delft and in 1964 he became responsible for teaching the Aircraft Preliminary Design course at the department of Aerospace Engineering. After a sabbatical at Lockheed Georgia Company, he became a senior lecturer and full professor of the Aircraft Design chair at TU Delft, initiating research and teaching in computer-assisted aircraft design.