CONCEPTUAL DESIGN OF HIGH-LIFT PROPELLER SYSTEMS FOR SMALL ELECTRIC AIRCRAFT A Dissertation Presented to The Academic Faculty by Michael D. Patterson In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the School of Aerospace Engineering Georgia Institute of Technology August 2016 Copyright c 2016 by Michael D. Patterson CONCEPTUAL DESIGN OF HIGH-LIFT PROPELLER SYSTEMS FOR SMALL ELECTRIC AIRCRAFT Approved by: Professor Brian J. German, Advisor Dr. Nicholas K. Borer School of Aerospace Engineering Aeronautics Systems Analysis Branch Georgia Institute of Technology NASA Langley Research Center Professor Marilyn J. Smith Dr. Erik D. Olson School of Aerospace Engineering Aeronautics Systems Analysis Branch Georgia Institute of Technology NASA Langley Research Center Professor Lakshmi N. Sankar Date Approved: 27 April 2016 School of Aerospace Engineering Georgia Institute of Technology To the One \who is able to do immeasurably more than all we ask or imagine"1 and to my son Isaac who is living proof of His immeasurable power 1Ephesians 3:20 (NIV) ACKNOWLEDGEMENTS This work would not have been completed without the help and encouragement of many people. There is simply not sufficient time or space to acknowledge everyone who has helped me make it through graduate school and finish this document. First and foremost, my wife, Laura, has helped keep me sane throughout the many stressful times and transitions that have occurred in my life since beginning grad school. From providing me encouragement when things were getting overwhelming, to picking up the slack around our home when I was slammed with work, to being an awesome mother to our kids, she has helped me more than any other person to complete this work. All the other members of my family have also provided love, support, encour- agement, and much-needed diversions from work throughout grad school. My boys, William and Isaac, have helped me keep perspective on what's really important in life (particularly recently). The rest of my extended family, not least of which are my parents and parents-in-law, have also been great sources of support throughout my life and in particular through this last year. Perhaps the single most influential person in helping to guide the general direction of my research has been Mark Moore of NASA Langley. I would certainly not be where I am today physically or professionally if Mark had not taken a chance in funding my advisor and one of his young grad students to help with the Zip Aviation study back nearly five years ago. Mark has also been instrumental in promoting electric aircraft and on-demand mobility research at NASA, and the SCEPTOR project (among many others) simply would not have happened without his tireless efforts. I would also like to thank my entire thesis committee for their guidance, feedback, iv and inspiration. I am very thankful for Dr. German's decision to bring me into his research group and for the many conversations and meetings we had together throughout my time in grad school. He has been a great advisor, and I am certainly a better researcher, engineer, and writer because of him. Dr. Borer has been an awesome mentor at NASA. Virtually every aspect of this dissertation has come either directly or indirectly from conversations I've had with him. Without his encouragement and pragmatic advice, I would not have completed this document. It was in Dr. Sankar's class where I first learned about rotor design, and many elements of this document would likely not have come about without his teaching. Dr. Smith's teaching and technical excellence have pushed me to perform as high-quality work as possible. I have been inspired by Dr. Olson's work in applying corrections to lower-order tools to obtain more accurate results, and I emulate some of his general techniques in this document. Big thanks are also due to all the members of the \German Research Group" (formal and honorary) for their friendship, doubt raising, humor, and all-around awesomeness throughout grad school. Y'all made life in asbestos-laden ESM and the dungeon of Weber much more enjoyable than it could have been. David Pate deserves special thanks, as I'm not sure if I would have made it through qualifying exams without him. The OVERFLOW CFD results presented in Chapter III would not have been possible without Dr. Joe Derlaga of NASA Langley. He was quite patient with me in helping get me set up to run cases for which I am very thankful. His practical aerodynamic knowledge has been a huge asset to me and the broader SCEPTOR project team. Joe has been a great colleague, and I look forward to continuing to work along side him into the future. I would also like to thank the entire SCEPTOR project team including everyone at Joby Aviation, Empirical Systems Aerospace (ESAero), and across NASA. In terms v of technical content in this document, the FUN3D CFD results from the LEAPTech configuration discussed in Chapter III were performed by Karen Deere, Salley Viken, and Steve Bauer of NASA Langley, and the STAR-CCM+ CFD results for the same configuration were run by Alex Stoll of Joby Aviation. I'm also grateful for Alex's help in tracking down and discussing various simplified methods of estimating lift increases due to propeller blowing. Brandon Litherland of NASA Langley performed the parasite drag estimation of the SCEPTOR aircraft without flaps and helped me get up and running in VSPAero. I would also like to acknowledge the funding that has made this work possible. First, the NASA Aeronautics Scholarship Program provided funding for my final years on campus. Second, much of this work has been performed while I have been working at NASA Langley Research Center on the Convergent Aeronautics Solutions (CAS) and Transformational Tools and Technologies (TTT) Projects, which are both part of the NASA Aeronautics Research Mission Directorate's Transformative Aeronautics Concepts Program. vi TABLE OF CONTENTS ACKNOWLEDGEMENTS .......................... iv LIST OF TABLES ............................... xi LIST OF FIGURES .............................. xii LIST OF SYMBOLS OR ABBREVIATIONS ............... xvii SUMMARY .................................... xxi I INTRODUCTION ............................. 1 1.1 Motivation . 1 1.1.1 Electric Aircraft Propulsion . 2 1.1.2 Integrated, Distributed Propulsion and Distributed Electric Propulsion . 11 1.1.3 Past NASA Distributed Electric Propulsion Research . 14 1.1.4 Potential Benefits of Distributed Electric Propulsion for Small Aircraft . 16 1.2 NASA's SCEPTOR Project . 25 1.2.1 Project Background . 25 1.2.2 Distributed Electric Propulsion System . 26 1.2.3 Research Challenges . 29 1.3 Scope and Organization of this Thesis . 30 1.3.1 Contributions . 30 1.3.2 Outline . 31 II TRADITIONAL PROPELLER ANALYSIS AND DESIGN METH- ODS AND PROPELLER-WING INTERACTION ......... 33 2.1 A Brief Overview of General Aerodynamic Modeling Techniques for Lifting Surfaces . 33 2.2 Propeller Analysis and Design Methods . 35 2.2.1 Momentum Theory . 35 2.2.2 Blade Element Momentum Theory . 39 vii 2.2.3 Vortex Theory and Minimum Induced Loss Design Methods . 42 2.3 Propeller-Wing Interaction . 45 2.3.1 Propeller Slipstream Flow Characteristics . 45 2.3.2 General Impacts of Tractor Propellers on Downstream Wings 47 2.3.3 Modeling Propeller-Wing Interaction . 51 2.3.4 Desire for New Theory . 58 2.4 Chapter Summary . 59 III A SIMPLE HIGH-LIFT PROPELLER LIFT AUGMENTATION THEORY ................................... 60 3.1 A Two-Dimensional Model for Predicting Wing Lift Augmentation from High-Lift Propellers . 61 3.1.1 Geometry . 61 3.1.2 General Model . 64 3.1.3 Specific Propeller Installations . 66 3.1.4 Extension to Three Dimensions . 71 3.1.5 Intermediate Summary of the Two-Dimensional Model . 75 3.2 Accounting for Propeller Slipstream Height . 77 3.2.1 Quantifying Slipstream Height Impacts . 80 3.2.2 Modifying the Simple Theory . 98 3.3 Comparison of Model to Experimental Results and Other Methods . 100 3.3.1 LEAPTech . 100 3.3.2 Kuhn and Draper Experiments . 105 3.3.3 Gentry et al. Experiments . 107 3.3.4 Summary of Simple Models . 111 3.4 Implications of Model . 112 3.5 Chapter Summary . 113 IV A SIMPLE METHOD FOR HIGH-LIFT PROPELLER CONCEP- TUAL DESIGN ............................... 115 4.1 Motivation for a New Propeller Design Method . 116 viii 4.2 High-Lift Propeller Design Method . 119 4.2.1 Blade Design for a Desired Induced Velocity Distribution . 120 4.2.2 Example Propeller Design and Comparison to a Minimum In- duced Loss Propeller . 128 4.2.3 Additional Comments on the Optional Steps . 138 4.3 Cautionary Statements . 142 4.4 Chapter Summary . 143 V DETERMINING THE DESIGN POINT FOR THE HIGH-LIFT PROPELLERS ............................... 145 5.1 Approach Considerations . 145 5.1.1 Regulations Related to Stall and Approach Speeds . 146 5.1.2 Exploration of Lift Coefficient Margin and Potential Approach Profiles of Aircraft with High-Lift Propellers . 153 5.2 Altitude Considerations . 188 5.3 Chapter Summary . 191 VI SELECTING THE NUMBER AND POSITION OF HIGH-LIFT PROPELLERS ............................... 193 6.1 High-Lift Propeller Installation Considerations . 194 6.1.1 Propeller Orientation Relative to the Wing . 194 6.1.2 Propeller Placement Relative to the Wing . 196 6.2 Design Space Exploration for Selecting the Number of High-Lift Pro- pellers . 201 6.2.1 Design Assumptions and SCEPTOR Aircraft Information . 202 6.2.2 Design Space Exploration Process . 205 6.2.3 Linking the Process to Wing Design . 219 6.3 Chapter Summary . 223 VII CONCLUSIONS .............................. 226 7.1 Summary and Implications of Results . 226 7.1.1 Summary of Contributions . 232 7.2 Potential Directions for Further Research . 233 ix APPENDIX A | OVERFLOW SIMULATION RESULTS AND SUR- ROGATE MODEL ............................