On the Low Speed Longitudinal Stability of Hypersonic Waveriders A Thesis submitted to the University of Sydney for the Degree of Doctor of Philosophy, from the School of Aerospace, Mechanical & Mechatronic Engineering Jonathan J. Jeyaratnam SID: May 2020 Abstract The development of hypersonic civilian transport aircraft requires solutions to a num- ber of challenging problems in the areas of aerothermodynamics, control, aeroelasticity, propulsion and many others encountered at high Mach number flight. The majority of research into hypersonic vehicle design has therefore rightly focused on solutions to these issues. The desire for good aerodynamic performance at high Mach numbers, most often defined by the lift to drag ratio, results in slender vehicle designs which minimise drag and take advantage of compression lift through attached shock waves along the leading edge. These so-called waverider designs show good promise for high aerody- namic efficiency and potential for long range transport applications. However a civilian transport aircraft that travels at hypersonic speeds requires satisfactory stability and handling qualities across the entire flight trajectory. The stability and handling of wa- verider shapes at the low speeds encountered at the take-off and landing phases of flight is not well studied. The stability of an aircraft is characterised by the aerodynamic stability derivatives. Only a few existing studies have looked at these low speed derivatives for waverider designs and no research has been found on the dynamic damping derivatives even though these are important to characterising the handling qualities of a vehicle. The work presented here covers the use of various high fidelity methods to extract these derivatives for a particular vehicle, the Hexafly-Int hypersonic glider. This vehicle has been chosen as it represents a mature example of a waverider vehicle that has been designed for flight testing at Mach 7.2. It is therefore typical of waverider designs optimised for high speed performance. The highly separated flowfield which slender vehicle designs exhibit at low speeds and high angles of attack requires the use of higher fidelity Computational Fluid Dynamics (CFD) simulations. i The work covered in this thesis includes static and dynamic CFD simulations of the Hexafly glider which have been used to obtain longitudinal stability derivatives at low speeds. Complementary static and free-to-pitch dynamic wind tunnel testing, which has not been previously used to test hypersonic designs are used to validate the CFD computations. A final chapter on the optimisation of waverider designs incorporating low speed longitudinal stability as a criteria is presented to provide insight into the impacts of this additional requirement on the hypersonic design space. The static wind tunnel testing has identified stability issues relating to the location of the centre of gravity. The design centre of gravity which is suitable for the Hexafly-Int vehicle at Mach 7.2 is found to be too far aft which results in instability at low speeds. This discrepancy between the centre of gravity location suitable for hypersonic cruise and the centre of gravity suitable for low speed operations with high angles of attack is found to be a significant issue for hypersonic waverider designs. In addition, the dynamic testing in the wind tunnel shows that the pitch damping is inadequate at low speeds. The CFD simulations agree well with the wind tunnel test results validating the use of CFD tools for determining dynamic stability derivatives of this class of slender vehicle in the design process. To alleviate the low speed stability issue of hypersonic vehicles, a waverider shape optimisation study has been carried out to understand what shapes will produce better low speed stability behaviour. These shapes are found to produce lower aerodynamic efficiency at high speeds which suggests that a design compromise between low speed stability and high speed performance is required at the outset of hypersonic waverider design. ii Declaration This thesis was completed between August of 2013 and July of 2019, under the supervi- sion of Dr. Dries Verstraete from the School of Aerospace, Mechanical and Mechatronic Engineering (AMME) at the University of Sydney. The content contained within this thesis is the product of the work of Mr. Jonathan J. Jeyaratnam, with guidance pro- vided by Dr. Dries Verstraete through discussions on research direction, interpretation of results and proofreading of the manuscript. No portion of the work referred to in this thesis has been submitted in support of an application for another degree or qualification at this or any other University or institution of learning. .................................................... ..................................................... Jonathan J. Jeyaratnam Dr. Dries Verstraete iii Acknowledgements I'll keep this as brief as I can. Thanks to the University of Sydney for providing top notch high performance comput- ing tools (USyd HPC), fabrication facilities, a wind tunnel and excellent support from workshop staff who were always ready to assist. Thank you to Dr Johan Steelant of ESA and the rest of the Hexafly-Int project team for providing a highly refined hypersonic vehicle design for me to play with. It's taken years of work and I appreciate being a small part of it. Thank you to Kai and Matt for teaching me the ways of dynamic wind tunnel testing, for letting me use your brilliant work and for being around for all my dumb questions. And Matt for teaching me how to solder and make a decent contact. Thank you to Nick and Tamas who helped me get my head around CFD simulations and the joys of solving convergence errors. And Nick for being there in June 2019 when we stared into the abyss...well not really, but we did lose a lot of sleep and I'm grateful to have not been alone in the office during those weekends and late nights. Thank you to Gareth for keeping us all sane with your wise counsel. You are the Gandalf of this school. Thank you to my supervisor, Dries, for always being available to discuss ideas with and for always providing the resources I needed to get this thing done. iv Thank you to my Aunty Sobhini for always seeing the possibilities and encouraging us all to excel. To my brothers for being a constant source of confidence. Thank you most of all to my mum, who has always been there for me, and has shown inconceivable patience, kindness and love at all times. I would not be much at all without her. She really is a picture of Jesus to me. And to dad, wish you were still with us, but know you are probably proud. v Contents Abstract i Declaration iii Acknowledgements v List of Figures xiii List of Tables xv 1 Introduction 1 1.1 An incomplete history of high speed flight research . 2 1.1.1 Horizontal Lifting Re-entry Design Flight Tests . 3 1.1.2 High Supersonic USAF Designs . 6 1.1.3 Hypersonic Airbreathing Vehicle Design Studies . 7 1.1.4 Focused Hypersonic Technology Demostrators . 10 1.1.5 Hypersonic Civilian Transport Design Studies . 11 1.1.6 Off-Design and Low Speed Studies . 15 1.2 Towards a Hypersonic Civilian Transport . 19 1.3 Research Proposal . 20 1.4 Thesis Outline . 21 2 Vehicle Design 22 2.1 Hypersonic Vehicles . 22 2.2 Hypersonic Waveriders . 27 2.2.1 Development of Waverider Designs . 29 2.2.2 Off-design Performance of Waveriders . 32 vi 2.2.3 Dorsal Engine Waveriders . 33 2.3 Hexafly-Int Hypersonic Glider . 34 2.4 Reference Systems and Stability Criteria . 36 2.4.1 Static Stability Criteria . 38 2.5 Conclusion . 39 3 Computational Results 40 3.1 Numerical Solver, Domain and Conditions . 42 3.1.1 ANSYS Fluent . 42 3.1.2 Simulation Conditions . 43 3.1.3 Computational Mesh . 45 3.1.4 Computing Tools . 47 3.2 Steady Simulations . 47 3.2.1 Post Processing . 48 3.2.2 Grid convergence . 48 3.2.3 Steady Results . 49 3.3 Dynamic Simulations . 58 3.3.1 Transient CFD calculations . 59 3.3.2 Data Reduction Methods . 60 3.3.3 Comparison of Unsteady and Steady Results . 62 3.3.4 Dynamic Pitching Results . 63 3.4 Conclusions . 71 4 Static Wind Tunnel Testing 72 4.1 Wind Tunnel Facilities . 72 4.2 Wind Tunnel Model . 75 4.3 Test Conditions . 77 4.4 Uncertainty, Corrections and Calibration . 78 4.4.1 Sensor Calibrations . 78 4.4.2 Wind Tunnel Corrections . 79 4.4.3 Measurement Uncertainty . 79 4.5 Results . 81 4.5.1 Longitudinal Results . 81 4.5.2 Lateral Results . 86 vii 4.6 Conclusions . 90 5 Dynamic Wind Tunnel Testing 92 5.1 Dynamic Wind Tunnel Testing Methods . 93 5.2 Data Processing methods . 96 5.3 Model Design . 99 5.3.1 Centre of Gravity and Mass Distribution . 99 5.3.2 Model Sizing . 100 5.3.3 Model Construction and Internal Components . 102 5.4 Data Acquisition System . 108 5.5 Manoeuvre Design and Test Conditions . 112 5.6 Experimental Procedure . 116 5.6.1 Calibration . 116 5.6.2 Initial Testing . 117 5.7 Results . 120 5.8 Handling Qualities Assessment . 129 5.9 Comparison of wind tunnel data and CFD data . 132 5.9.1 Static CFD Comparison . 133 5.9.2 Dynamic CFD Comparison . 134 5.10 Conclusions . 136 6 Waverider Study 137 6.1 Optimisation Problem . 137 6.2 Waverider Design . 139 6.2.1 Osculating Cones Design Method . 139 6.3 Low Speed Numerical Method . 145 6.3.1 Polhamus Suction Analogy . 145 6.3.2 Validation . 147 6.4 NSGA-II Algorithm . 148 6.5 Optimisation Results . 150 6.6 Conclusions . 158 7 Conclusions and Further Work 159 7.1 Completed Research Goals .