CLOSED CYCLE PROPULSION FOR SMALL UNMANNED AIRCRAFT By THOMAS CHADWICK HAYS Master of Science in Mechanical Engineering Oklahoma State University Stillwater Oklahoma 2009 Submitted to the Faculty of the Graduate College of the Oklahoma State University in partial fulfillment of the requirements for the Degree of DOCTOR OF PHILOSOPHY July, 2015 CLOSED CYCLE PROPULSION FOR SMALL UNMANNED AIRCRAFT Dissertation Approved: Dr. Andrew S. Arena Dissertation Adviser Dr. Daniel Fisher Dr. Jamey D. Jacob Dr. Richard J. Gaeta Dr. Russel Rhinehart ii There has been no greater contributor to my education at Oklahoma State University than my advisor Dr. Andrew Arena. Working for, and with him on the Dragonfly project and my masters gave me the confidence to challenge continually harder questions as my studies progressed. Not enough can be said for his ability to simplify both engineering problems, and contractual politics. The lessons learned in both areas will guide me for years to come. I must also thank my parents for their approval of my studies. They have encouraged me during my years at Oklahoma State to go beyond the minimum requirements of the program, and have provided endless support in pursuing that goal. iii Acknowledgements reflect the views of the author and are not endorsed by committee members or Oklahoma State University. Name: Thomas Hays Date of Degree: JULY, 2015 Title of Study: CLOSED CYCLE PROPULSION FOR SMALL UNMANNED AIRCRAFT Major Field: DOCTORATE OF MECHANICAL AND AEROSPACE ENGINEERING Abstract: This study evaluates the merit of closed cycle propulsion systems for use in unmanned systems. The complexity and added weight of closed cycle engines is offset by benefits in high altitude performance, operation in polluted air environments, multi-fuel operation, and potential for flight in low oxygen environments using generic thermal heat sources. Although most closed thermal cycles cannot match the efficiency and power density potential of internal combustion engines (ICE) and turbomachines in aircraft propulsion applications, the addition of design requirements regarding noise output, and operation at high altitude results in IC and CC engine’s performance becoming much more comparable. Muffling devices increase backpressure on internal combustion engines thereby reducing power output and efficiency. Multi stage turbo- supercharging for operation at high altitude can in some cases increase efficiency of ICE’s, but at the result of significant additional complexity and cost that also reduces practical reliability because of the often intricate mechanisms involved. It is in these scenarios that closed cycle engines offer a comparable performance alternative that may prove to be simpler, cheaper, and more reliable than high altitude or low noise internal combustion or turbomachine propulsion systems. iv TABLE OF CONTENTS Chapter Page Introduction ............................................................................................................................................................... 1 Problem Statement, Scope, and Contributions ................................................................................................. 1 A. Problem Statements ................................................................................................................................. 2 B. Scope of Research...................................................................................................................................... 2 C. Unique Contributions .............................................................................................................................. 3 CHAPTER I .................................................................................................................................................. 5 Background and Literature Review ..................................................................................................................... 5 I. Technical Foundation ................................................................................................................................... 5 A. Low Power Aircraft Design and Flight ............................................................................................. 5 B. UAV Propulsion system survey ........................................................................................................... 6 C. Small Internal Combustion Engine Dynamometer ...................................................................... 7 D. Besler Brothers Aircraft ......................................................................................................................... 8 E. Robert McConaghy’s Stirling Airplane [3] ...................................................................................... 9 Acoustic Directionality Study ............................................................................................................................. 9 F. Finite Difference Time Derivative Acoustic Propagation Simulation ................................ 12 G. Conclusions ................................................................................................................................................ 26 Closed Cycle Engines for Quiet UAV propulsion ....................................................................................... 27 H. History of Closed Cycle Aircraft Propulsion Research ............................................................. 28 I. Horsepower ............................................................................................................................................... 33 J. Noise spectrum ........................................................................................................................................ 35 K. Energy as Function of Weight ............................................................................................................ 39 Theoretical Evaluation of Closed Cycle Suitability .................................................................................. 40 L. Cycle Downselect ..................................................................................................................................... 40 1. Closed Brayton .................................................................................................................................... 41 2. Rankine ................................................................................................................................................... 42 v 3. Stirling ..................................................................................................................................................... 44 M. Sensitivity of cycle efficiency to component performance ................................................ 46 Robert Mcconaghy’s Stirling engine .............................................................................................................. 47 N. Design of a stirling engine for model aircraft propulsion ...................................................... 48 O. An Improved Stirling Engine for Model Aircraft Propulsion................................................. 50 P. Flight Tests of a Stirling Powered Model Aircraft ...................................................................... 51 Q. Conclusions on Mcconaghy Flights: ................................................................................................. 52 R. Conclusions ................................................................................................................................................ 53 CHAPTER II ............................................................................................................................................................. 54 System Simulation Model ..................................................................................................................................... 54 Matlab Model of Closed Cycle Systems ......................................................................................................... 54 S. Rankine Cycle Model .............................................................................................................................. 55 T. Brayton Cycle Model .............................................................................................................................. 57 Comparison of Cycle efficiency ........................................................................................................................ 58 Non-Ideal Component Efficiency ..................................................................................................................... 61 U. 90% of Isentropic .................................................................................................................................... 61 V. 80% of Isentropic .................................................................................................................................... 65 W. 70% of Isentropic ............................................................................................................................... 67 X. 70% of Isentropic with Non-Ideal Regenerator ......................................................................... 70 Closed Cycle Future Growth Potential .......................................................................................................... 73 Y. Effects of operating fluid on efficiency ..........................................................................................