The Design of Aircraft Landing Gear

Abstract

The aircraft landing gear and its associated systems represent a compelling design challenge: simultaneously a system, a structure, and a machine, it supports the aircraft on the ground, absorbs landing and braking energy, permits maneuvering, and retracts to minimize aircraft drag. Yet, as it is not required during flight, it also represents dead weight and significant effort must be made to minimize its total mass.

The Design of Aircraft Landing Gear, written by R. Kyle Schmidt, PE (B.A.Sc. - Mechanical Engineering, M.Sc. - Safety and Aircraft Accident Investigation, Chairman of the SAE A-5 Committee on Aircraft Landing Gear), is designed to guide the reader through the key principles of landing system design and to provide additional references when available. Many problems which must be confronted have already been addressed by others in the past, but the information is not known or shared, leading to the observation that there are few new problems, but many new people.

The Design of Aircraft Landing Gear is intended to share much of the existing information and provide avenues for further exploration. The design of an aircraft and its associated systems, including the landing system, involves iterative loops as the impact of each modification to a system or component is evaluated against the whole. It is rare to find that the lightest possible landing gear represents the best solution for the aircraft: the lightest landing gear may require attachment structures which don’t exist and which would require significant weight and compromise on the part of the airframe structure design.

With those requirements and compromises in mind,The Design of Aircraft Landing Gear starts with the study of airfield compatibility, aircraft stability on the ground, the correct choice of tires, followed by discussion of brakes, wheels, and brake control systems. Various landing gear architectures are investigated together with the details of shock absorber designs. Retraction, kinematics, and mechanisms are studied as well as possible actuation approaches. Detailed information on the various hydraulic and electric services commonly found on aircraft, and system elements such as dressings, lighting, and steering are also reviewed. Detail design points, the process of analysis, and a review of the relevant requirements and regulations round out the book content.

The Design of Aircraft Landing Gear is a landmark work in the industry, and a must-read for any engineer interested in updating specific skills and students preparing for an exciting career.

Full text

R. KYLE SCHMIDT
Vols. I & II
The Design
of Aircraft
Landing Gear

The Design of Aircraft Landing Gear
Volumes 1 & 2

The Design of Aircraft Landing Gear
Volumes 1 & 2
R. KYLE SCHMIDT
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Contents v
v
©2020 SAE International
For my wife, Natalie, and my children, Jacob, Dylan, and Hunter.
dedication

Contents vii
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©2020 SAE International
contents
Acknowledgements xix
Preface xxi
A Note on Units xxiii
Volume 1
CHAPTER 1
Introduction 1
Brief History of Landing Gear 2
Design Process 10
Nomenclature 12
Book Outline 13
References 16
CHAPTER 2
Airfield Compatibility 17
Flotation/Ground Compatibility 18
Common Concepts in Ground Compatibility 20
General Overview 20
California Bearing Ratio 21
Modulus of Subgrade Reaction, k 25
Ground Compatibility Nomenclature 26
Ground Contact Pressure 28
Landing Gear Arrangement Nomenclature 29
Ground Compatibility (Flotation) Analysis 31
Unpaved Surfaces 31
Soil and Grass 32
Unpaved Analysis Method ASD-TR-68-34 33
Alternative Unpaved Analysis Methods 43
Gravel/Aggregate Airfields 44
Paved Surfaces 46
Pavement Design Analysis 47
Layered Elastic and Finite Element Analysis 47
Flexible Pavements-Historic Approach 52
Rigid Pavements-Historic Approach 56
Pavement Strength Reporting Methods 59
Load Classification Number/Load Classification Group Method 60

viii Contents
Modern Methods for Paved Runways—ACN/PCN and ACR/PCR 60
ACN/PCN 60
ACR/PCR 68
Membrane and Mat Surfaces 71
PCASE Software for Flotation Analysis 72
Engineered Materials Arresting Systems (EMAS) 75
Snow and Ice Runways 75
Prepared Snow Runways 76
Ice Runways 78
Helidecks and Heliports 82
Naval Vessels/Aircraft Carriers 83
Aircraft Carriers 85
Amphibious Warfare Ships 85
Example 86
Maneuvering 86
ICAO Airport Standards 86
Required Maneuvers—NAS3601 91
Required Maneuvers—Land-Based Military Aircraft 91
Required Maneuvers—Shipboard Military Aircraft 92
Surface Texture and Profile 92
Paved Runways 93
Micro/Macrotexture 93
Runway Roughness/Profile and Obstacles 96
Roughness Measurement Techniques 96
Power Spectral Density Approach 96
Boeing Bump Method 97
International Roughness Index 98
Short Wavelength Roughness 99
ProFAA Roughness Evaluation Tool 99
Industry Standard Roughness Profiles 99
Bomb Damage Repair 105
Arrestor Cables 108
Unsurfaced Runways 109
Deck/Helideck 109
References 112
CHAPTER 3
Tires 115
Tire Construction and Terminology 117
Construction Terminology 119
Tire Dimensions and Properties 121
Inflation Pressure 127
Tire Temperatures 130
Tire Classification 134

Contents ix
Selection between Bias and Radial Tires 136
Manufacturing, Certification, and Standardization 137
Tire Sizing 139
Tire Sizing Formulae 139
Tire Sizing Requirements 141
Tire Tables 142
Tire Performance and Modeling 158
Mechanics of Pneumatic Tires 158
Rolling Behavior 159
Turning Behavior 160
Vertical Stiness 161
Braking Behavior 163
Tire-Ground Friction 167
Wet Runways and Hydroplaning 169
Snow and Ice 175
Wear 175
Tire Property and Behavior Models 177
NASA Technical Report R-64 177
Brush Model and Fiala Model 177
Beam and String Models 179
Magic Formula Model 179
Undesirable Tire Behavior 180
Spray 181
Debris Lofting 185
Tire Failure Modes 188
Modeling Tire Failure Events 191
Model 1: Tire Debris Threat Model 192
Model 3E: Flailing Tire Strip Threat Model 193
Model 3R: Flailing Tire Strip Threat Model 193
Model 4: Tire Burst Pressure Eect Threat Model 194
Understanding the Impact of Tire Failures 197
References 199
CHAPTER 4
Wheels, Brakes, and Brake Control 203
Brakes 203
Aircraft Deceleration 208
Brake Sizing 213
Energy 213
Kinetic Energy Calculation 214
Rational Brake Energy Calculation 216
Torque 217

x Contents
Brake Design 219
Brake Actuation 225
Mechanical Connection to the Landing Gear Structure 228
Weight 230
Worked Example 231
Wheel and Brake Certification and Recommended Practices 235
Brake Issues and Concerns 247
Vibration 247
Failure and Degradation Modes 249
Braking Accessories 250
Brake Cooling Fans 250
Brake Temperature Measuring Systems 251
Retraction Braking 252
Wheels 252
Bearing Selection and Preload 257
Over Temperature and Over Pressure Relief 261
Wheel Mass 262
Failure Modes 262
Bearing Failure 262
Wheel Rim Release 263
Brake Control 264
Brake Control Architectures 266
Antiskid and Related Functions 273
Braking Eciency 274
Antiskid Dynamics 274
Antiskid Hardware 278
Autobrake 280
Failure Modes 281
References 281
CHAPTER 5
Layout, Stability, and Maneuverability 285
Tricycle Arrangement 287
Conventional (Taildragger) Configuration 301
Bicycle Configuration 304
Maneuvering 306
References 311

Contents xi
CHAPTER 6
General Arrangement 313
Energy Absorption 313
Aircraft Structural Arrangement 319
Landing Gear Topologies 329
Common Considerations 331
Caster 331
Wheel Alignment 333
Cantilever 335
Cantilever Gear Bearing Overlap 338
Pogo-Stick Design 340
Torque Links and Splines 340
Semi-Articulated 343
Articulated 345
Side-Hinged Articulated 349
Multi-Wheel Bogie Arrangements 349
Other Configurations 356
Wheel-Less Configurations 358
Skids and Skis 359
Adaptive Structure 362
Seaplanes, Floats, and Hydrofoils 363
Air Cushion 365
References 368
CHAPTER 7
Shock Absorbers 371
Damping 372
Friction Damping 372
Hydraulic Damping 372
Other Damping Types 380
Recoil Damping 381
Structural Spring Types 383
Coil Spring 383
Ring-Spring 383
Leaf Spring 385
Elastomeric 387
Pneumatic 391
Liquid Spring 394

xii Contents
Liquid Spring Sizing 396
Liquid Spring Examples and Issues 402
Oleo-Pneumatic 406
Oleo-Pneumatic Sizing 413
Refinements 416
Real Gas Model 416
Inflation Gas Solubility in Oil 417
Design for Real-World Operation 422
Example Single Stage Oleo-Pneumatic Shock Absorbers 422
Multiple Stage Oleo-Pneumatic Shock Absorbers 427
Active Shock Absorbers 434
Shock Absorber Design Considerations 442
Seals 442
Inflation and Fill Valves 444
Servicing 444
Shock Absorber Oil 447
Single Use Shock Absorbers: Crashworthiness and Space
Applications 447
Rotorcraft Ground Resonance 453
References 455
CHAPTER 8
Retraction, Kinematics, and Mechanisms 459
Retraction/Extension 460
Sliding Systems 460
Hinged Systems 462
Parallelogram Arrangements 465
Secondary Motion 469
Additional Hinge Axis 469
Wheel Rotation 471
Planing Mechanisms 473
Shortening 479
Bogie Positioning 486
Bogie-Controlled Articulation 487
Stabilization, Locking, and Unlocking 493
Planar Braces 493
Telescopic Braces 499
Rolling-Folding Braces 504
Dual Brace (Rolling-Folding) 505
Plunger Locks 507
Over-Center Locks 509

Contents xiii
Self-Breaking Locks 511
Latch Locks 516
Ground Locks 518
Springs 519
Door Mechanisms 522
Gear-Actuated Doors 522
Independently Actuated Doors 527
Ground Door Opening 527
Actuation Layout and Loads 529
Actuator Load Requirements 535
References 540
Volume 2
CHAPTER 9
Actuation 541
Manual Actuation 541
Hydraulic Actuation 543
Rotary Hydraulic 543
Linear Hydraulic 544
Retraction Actuators 546
Unlock Actuators 548
Bogie Pitch Trimmers 550
Internally Locking Actuators 559
Collect Lock Actuators 561
Segment Lock Actuators 562
Electric Actuation 564
Electro-Hydraulic 565
Electro-Mechanical 567
References 573
CHAPTER 10
Systems 575
Power Sources– Electrical, Hydraulic, and Pneumatic 575
Electrical 575
Hydraulic 578
Typical Central Hydraulic Systems 578
Dedicated Systems– Hydraulic Power Packs 582
Hydraulic Components 583
Pneumatic 587

xiv Contents
Sensors and Monitoring Systems 589
Proximity Sensing 589
Rotary and Linear Position Sensing 592
Pressure and Temperature Sensing 593
Electrical and Hydraulic Dressings 594
Electrical Dressings 594
Hydraulic Dressings 597
Weight on Wheels (Air/Ground) Detection 600
Extension and Retraction 604
Example Systems 607
Alternate Extension 611
Steering and Steering Control 615
Required Steering Torque 616
Single Wheel Scrubbing Torque, Ts 616
Dual Wheel Scrubbing Torque, Ts 617
Steering Arrangements 619
Centering 621
Steer Motors and Control 622
Rack and Pinion 623
Push-Pull 626
Rotary Steer Motor 627
Shimmy Damping 627
Nose Wheel Steering Examples 631
Tailwheel Steering 644
Main Gear Steering Examples 646
Towing Concerns 647
Landing and Taxi Lights 648
References 655
CHAPTER 11
Special Functions 659
Catapult and Holdback 659
Jump Strut 664
Hiking and Kneeling 665
Autonomous Taxi 672
Tire Pre-rotation 673
Tail Bumper 675
Weight and Balance 676
Skis 678
References 683

Contents xv
CHAPTER 12
Detail Design 687
Overview 687
Structural Materials 692
Steel and Corrosion Resistant Steel 696
Aluminum 696
Titanium 698
Composites 699
Surface Treatments 701
Surface Modification 701
Wear and Sealing Surface Coatings 701
Corrosion Protection Coatings 703
Inspection 705
Corrosion Avoidance 708
Stress Corrosion Cracking 710
Galvanic Corrosion Avoidance 712
Fasteners 716
Locking and Dual Locking 719
Clearance Requirements for Fastener Installation and
Maintenance 721
Pins, Lugs, Sockets, and Bushings 725
Initial Sizing 727
Bushings 727
Grease Grooves 729
Bushing Installation 730
Repair Allowance 733
Grease Fittings and Greasing Provisions 733
Grease Selection 735
Pin and Lug Joint Examples 735
Limits and Fits 741
Metric System 742
US Customary (Inch) System 744
Typical Fit Classes for Landing Gear Components 74 5
Springs 746
Seals 751
Electrical Bonding, Lightning, and Static Dissipation 755
Shock Absorber Bearings 757
Bogie Pivot Joint 760
Towing, Jacking, and Tie-Down Provisions 763
Tow Fittings 763
Jacking 766

xvi Contents
Tie-Down 767
Emergency Towing (Debogging) 768
Crashworthiness 769
Fuse Pins 771
Maintainability and Murphy Proofing 772
References 773
CHAPTER 13
Loads, Structural Analysis, and Testing 783
Loads 784
Ground Loads 785
Asymmetrical Loads on Multiple Wheel Landing Gears 785
Book Cases 787
Taxi, Takeo, and Landing Roll 787
Braked Roll Conditions 788
Turning 790
Tailwheel-Specific Cases 790
Nose-Wheel-Specific Cases 790
Pivoting 791
Reverse Braking 791
Rational Loads 792
Landing Loads 792
Level Landing Conditions 792
Tail-Down Landing Condition 794
One Gear Conditions 794
Lateral Drift Landing Case (Side Load) 794
Rebound and Free Extension 796
Shock Absorber Pressures 796
Sailplane Specific Loads 796
Level Landing Conditions 797
Tail-Down Landing Conditions 797
One-Wheel Landing Condition 798
Side Load Conditions 798
Nose-Wheel Conditions 798
Tail Skid Impact 799
Wingtip Landing 799
Towing Loads 799
Jacking Loads 799
Emergency Towing (Debogging) Loads 800
Tie Down Loads 801
Aerodynamic and Inertial Loads 801
Aerodynamic Loads 801
Retractable Landing Gears 803
Extension/Retraction Loads 804
Door Attachment Loads 804
Fixed Landing Gears 804

Contents xvii
Inertial Loads 807
Brake Application in Air 807
Gyroscopic Loads 807
Vibration and Shock Loads 807
Aircraft Maneuver Loads 808
Failure Case Loads 808
Bird Strike 808
Handling and Abuse Loads 811
Dynamic Behavior 812
Landing Analysis 813
Equations of Motion 813
Example Model Implementation 818
Simple Model for Early Estimates 819
Spin-Up and Spring Back 821
Model Validation—Drop Testing 826
Extension/Retraction 827
Ground Handling 829
Shimmy 830
Influences on Shimmy Behavior 831
Shimmy Analysis Models 833
Approximate and Heuristic Methods 833
Closed Form Solutions 835
Simulation-Based Methods 837
Shimmy Model Validation 838
Shimmy Suppression 838
Gear Walk 841
Acoustic Analysis 843
Static Strength 844
Static Strength Requirements 845
Beam Model 847
Classical Analysis 847
Lugs 848
Sockets 849
Pins and Tubes 850
Beams and Columns 852
Stress Concentration Factors 852
Interaction Equations 853
Finite Element Analysis 853
Static Strength Reporting 855
Strength Verification 855
Fatigue 857
Fatigue Requirements 859
Analysis Approach 860
Fatigue Loading Spectrum 860

xviii Contents
Scatter Factors 862
Fatigue Verification 862
Mass Properties 863
Mass Prediction 864
Landing Gear Mass Data 866
Mass Properties Evaluation 866
Mass Properties Validation 871
Safety and Reliability 873
Safety Assessment Overview 873
Reliability and Availability 876
References 877
CHAPTER 14
Requirements and Regulations 885
Civil 885
Military 938
Environmental 938
Aerospace Standards, Recommended Practices, and
Information Reports 968
References 971
Appendix A: 100 Busiest Airports Showing Runway
SizeandStrength 973
Appendix B: Example ACN Values for a Variety of Aircraft 981
Appendix C: Runway Roughness Profiles 989
Appendix D: Specific Volume of Nitrogen 1011
Appendix E: Gland Seal and Scraper Standard Dimensions 1015
Appendix F: Proposed Load Case Amendments for Part 25
Considering Aircraft with Multiple Landing Gears 1033
Index 1051

Contents xix
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©2020 SAE International
acknowledgements
I would like to thank my family: Natalie, Jacob, Dylan and Hunter, for their patience,
support, and encouragement, without which Iwould not have been able to dedicate the
time to writing this book. Iwould also like to thank my father, Bob Schmidt, who was
the rst to read and comment on each chapter as it was produced. Ithank my colleagues
in Canada, France, the USA, and the UK who have read sections and chapters of this
work and provided me with suggestions, corrections, and encouragement. In particular,
Iwould like to thank those who gave up their time to review and comment: Bruno
Aldebert, Steve Amberg, Rod Van Dyk, Andrew Ellis, Jack Hagelin, Dan Hetherington,
Marianna Lakerdas, Grant Minnes, Andy Paddock, Michael Saccoccia, Jon Smith, and
Peter Taylor. Monica Nogueira at the SAE has supported me from the outset of this
project, gently prodding to ensure that it was completed! Iwould also like to thank the
industry expert reviewers who reviewed portions of the book on behalf of the SAE: CB
Alsobrook, Gregg Buttereld, David Brill, Bob Knieval, and Henry Steele. Finally, Iwould
like to thank Ian Bennett and Mark Shea who reviewed the entire manuscript in detail
and provided a number of excellent comments and suggestions.

Contents xxi
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©2020 SAE International
preface
e author has been fortunate enough to work in the eld of aircra landing gear for
over twenty-ve years and in three countries: Canada, France, and the UK, and to have
held a variety of engineering roles relating to the development of new landing gears and
the sustainment of existing landing gears in ser vice. Landing gear provides an intriguing
and compelling challenge, combining many elds of science and engineering. is book
was born of the author’s desire to learn ever more about landing gear— their history
and the ways in which others have addressed their problems and challenges; in continu-
ously striving to learn more about the eld, it was considered advantageous to put these
learnings into print in the hope that they can assist others. e book is intended, broadly,
for two audiences: experienced aircra and landing gear design engineers, for whom it
is hoped that the book will act as a reference as well as an ‘idea book’, and for those new
to the eld who are, perhaps, working on their rst landing gear design (maybe as part
of their education). For the latter, it is hoped that the book provides all the information
needed to aid in their design and studies, and that they are as intrigued and compelled
by the beautiful complexity of landing gear to consider this challenging eld for their
future employment.
No single textbook can provide all the answers; throughout the chapters there are
a number of references to additional docu ments which can aid in t he design, development,
and support of landing gears and their associated systems. In particular, documents
produced by the SAE A-5 committees on aircra landing gear are widely referenced and
participation in these committees is highly recommended to readers of the book and
practitioners of landing system engineering.
e opinions and approaches outlined in this book are those of the author and do
not necessarily represent those of his employer (Safran Landing Systems). Although a
great deal of care has been taken in the preparation and review of this work to ensure
that the approaches, methods, and data provided are accurate, the author and publisher
are not liable for any damages incurred as a result of usage of this book, for ty pographical
errors, or for any misinterpretations.

Contents xxiii
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©2020 SAE International
a note on units
Wherever possible, units in this book follow the International System of Units (SI, also
known as the metric system) approach. However, aircraft and landing gear are
international in nature and many components and analysis approaches are conducted
in US Customary units. In particular, some empirical formulas are based on US
Customary measures and do not lend themselves to conversion to another system of
measure. In general, most calcu lations can beperformed using either SI or US Customary
units, provided t wo dierent measurement systems are not mixed in the same calculation
and that the units utilized are self-consistent. An area where attention needs to bepaid
is the use of the US customary unit of weight and force, the pound, which is oen
colloquially used as a unit of mass (with an implicit assumption of earthly gravity);
calculations conducted in US customary units which require units of mass can employ
the ‘slug’– which is dened as the mass that is accelerated by 1 foot per second per second
when a force of one pound is exerted on it. A familiarity with both systems of measure
is recommended due to the international nature of the aircra business.