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Aerodynamic Centers of Arbitrary Airfoils Utah State University DigitalCommons@USU All Graduate Theses and Dissertations Graduate Studies 12-2017 Aerodynamic Centers of Arbitrary Airfoils Orrin Dean Pope Utah State University Follow this and additional works at: https://digitalcommons.usu.edu/etd Part of the Mechanical Engineering Commons Recommended Citation Pope, Orrin Dean, "Aerodynamic Centers of Arbitrary Airfoils" (2017). All Graduate Theses and Dissertations. 6890. https://digitalcommons.usu.edu/etd/6890 This Thesis is brought to you for free and open access by the Graduate Studies at DigitalCommons@USU. It has been accepted for inclusion in All Graduate Theses and Dissertations by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. AERODYNAMIC CENTERS OF ARBITRARY AIRFOILS by Orrin Dean Pope A thesis submitted in partial fulfillment of the requirements of the degree of MASTER OF SCIENCE in Mechanical Engineering Approved: ______________________ ____________________ Douglas Hunsaker, Ph.D. Stephan A. Whitmore, Ph.D. Major Professor Committee Member ______________________ ____________________ Geordi Richards, Ph.D. Mark R. McLellan, Ph.D. Committee Member Vice President for Research and Dean of the School of Graduate Studies UTAH STATE UNIVERSITY Logan, Utah 2017 ii Copyright © Orrin Dean Pope 2017 All Rights Reserved iii ABSTRACT Aerodynamic Centers of Arbitrary Airfoils by Orrin Dean Pope, Master of Science Utah State University, 2017 Major Professor: Douglas Hunsaker, Ph.D. Department: Mechanical and Aerospace Engineering A method for accurately predicting the aerodynamic center of an airfoil is presented based on a general form for the nonlinear lift and pitching-moment of an airfoil as a function of angle of attack. This method does not suffer from small-angle, small- camber, and thin-airfoil approximations, and is shown to match inviscid results to much higher accuracy than the traditional methods. It is shown that the aerodynamic center of an airfoil with arbitrary amounts of thickness and camber in an inviscid flow does not, in general, lie at the quarter-chord. Rather, it is a single, deterministic point, independent of angle of attack, which lies at the quarter chord only in the limit as the airfoil thickness and camber approach zero. Furthermore, it is shown that once viscous effects are included, the aerodynamic center is not in general a single point as predicted by traditional thin airfoil theory, but is a function of angle of attack. Differences between nonlinear predictions and those based on thin airfoil theory are on the order of 1-5%, which can be significant when predicting aircraft stability. (135 pages) iv PUBLIC ABSTRACT Aerodynamic Centers of Arbitrary Airfoils Orrin Dean Pope The study of designing stable aircraft has been widespread and ongoing since the early days of Orville and Wilbur Wright and their famous Wright Flyer airplane. All aircraft as they fly through the air are subject to minor changes in the forces acting on them. The field of aircraft stability seeks to understand and predict how aircraft will respond to these changes in forces and to design aircraft such that when these forces change the aircraft remains stable. The mathematical equations used to predict aircraft stability rely on knowledge of the location of the aerodynamic center, the point through which aerodynamic forces act on an aircraft. The aerodynamic center of an aircraft is a function of the aerodynamic centers of each individual wing, and the aerodynamic center of each wing is a function of the aerodynamic centers of the individual airfoils from which the wing is made. The ability to more accurately predict the location of the airfoil aerodynamic center corresponds directly to an increase in the accuracy of aircraft stability calculations. The Aerolab at Utah State University has develop new analytic mathematical expressions to describe the location of the airfoil aerodynamic center. These new expressions do not suffer from any of the restrictions, or approximations found in traditional methods, and therefore result in more accurate predictions of airfoil aerodynamic centers and by extension, more accurate aircraft stability predictions. v ACKNOWLEDGMENTS I would like to specifically thank Dr. Doug Hunsaker for his guidance, support, and unending passion for aircraft and aeronautics. I am also thankful to the Utah NASA Space Grant Consortium (award NNX15A124H) for funding this research. I give special thanks to my colleagues, friends, and especially my family who have always supported me throughout my Master’s program and through all of my engineering education. Orrin Dean Pope vi CONTENTS Page ABSTRACT ....................................................................................................................... iii PUBLIC ABSTRACT ....................................................................................................... iv ACKNOWLEDGMENTS ...................................................................................................v LIST OF TABLES ........................................................................................................... viii LIST OF FIGURES ........................................................................................................... ix NOTATION .........................................................................................................................x CHAPTER 1 INTRODUCTION .........................................................................................................1 1.1 Research Motivation ..............................................................................................1 1.2 Literature Review ...................................................................................................2 1.2.1 Traditional Thin Airfoil Theory Relations for the Aerodynamic Center .........2 1.2.2 General Relations for the Aerodynamic Center ..............................................5 2 ALTERNATIVE APPROACH TO FINDING THE LOCATION OF THE AERODYNAMIC CENTER .......................................................................................8 2.1 The Aerodynamic Center as a Function of Coefficient of Lift ..............................8 2.2 The Aerodynamic Center as a Function of Normal-force Coefficient .................12 2.2.1 Equivalence Proof ..........................................................................................15 2.3 Sample Results ......................................................................................................18 2.3.1 Vortex Panel Method .....................................................................................18 2.3.2 Finite Difference Method ...............................................................................20 3 THE AERODYNAMIC CENTER OF INVISCID AIRFOILS ..................................24 3.1 Classical Thin Airfoil Theory ...............................................................................24 3.2 General Airfoil Theory .........................................................................................27 3.2.1 Theory Development .....................................................................................28 3.2.2 Comparison to Inviscid Computational Results ............................................38 vii 3.3 Least Squares Regression Fit Coefficients-Inviscid Flow ....................................44 3.3.1 Fit to Thin Airfoil Theory Equations ........................................................44 3.3.2 Fit to General Airfoil Theory Equations ...................................................46 3.4 The Aerodynamic Center of Airfoils in Inviscid Flow .........................................48 3.4.1 Thin Airfoil Theory with Trigonometric Nonlinearities ................................48 3.4.2 General Airfoil Theory ..................................................................................49 4 THE AERODYNAMIC CENTER OF VISCOUS AIRFOILS ..................................54 4.1 The Aerodynamic Center of Airfoils in Viscous Flow .........................................54 4.2 Third Order Approximation ..................................................................................58 4.3 Sample Results ......................................................................................................60 4.4 Least Squares Regression Fit Coefficients-Viscous Flow ....................................70 5 CONCLUSION ...........................................................................................................74 REFERENCES ..................................................................................................................77 APPENDICES ...................................................................................................................80 A Viscous Least Squares Regression Fit Coefficients ...................................................81 B Vortex Panel Method Code .......................................................................................86 C Analytical Symbolic Solver Code ..............................................................................93 D Inviscid Aerodynamic Center NACA Airfoil Camber/Thickness Code ..................117 viii LIST OF TABLES Table Page 1 Vortex panel method data for NACA 8415 airfoil, − 7 ≤ α ≤10 ......................19 2 Vortex panel method data for NACA 8415 airfoil, −10 ≤ α ≤15 ....................40 3 Least squares regression coefficients for NACA 8415 airfoil ..........................41
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