Tài liệu Master's thesis of engineering aerodynamic performance comparison of a conventional uav wing and a fishbac morphing wing

Aerodynamic Performance Comparison of a Conventional UAV Wing and a FishBAC Morphing Wing A thesis submitted in fulfilment of the requirements for the degree of Master of Engineering Arthur Wong Bachelor of Engineering (Aerospace Engineering) (Honours), RMIT University School of Engineering College of Science, Technology, Engineering and Maths RMIT University June 2021 Declaration I certify that except where due acknowledgement has been made, the work is that of the author alone; the work has not been submitted previously, in whole or in part, to qualify for any other academic award; the content of the thesis is the result of work which has been carried out since the official commencement date of the approved research program; any editorial work, paid or unpaid, carried out by a third party is acknowledged; and, ethics procedures and guidelines have been followed. I acknowledge the support I have received for my research through the provision of an Australian Government Research Training Program Scholarship. Signed Arthur Wong 08 June 2021 i Acknowledgments I would like to take this opportunity to thank all those who have supported me during this program. I thank RMIT University for allowing me to continue my development in the masters by research program. I would like to thank my senior supervisors Professor Cees Bil and Dr Matthew Marino for looking over my growth and providing guidance to me completing the program. I thank the technical staff team particularly Gil Atkin and Paul Muscat for assisting me in getting from a concept and design to a prototype of the morphing wing and a compliant morphing skin providing guidance, advice, teaching composite lay-up techniques necessary to reach the end goal. Additionally I thank Nhu Huynh, my long-time girlfriend for her endless encouragement and support during the program as well as my friends and family for their support. ii Table of Contents Declaration ............................................................................................................................................... i 1 Introduction ......................................................................................................................................... 2 2 Literature Review ................................................................................................................................. 5 2.1 Types of Morphing Wings ............................................................................................................. 7 2.1.1 Planform Morphing ................................................................................................................ 8 2.1.2 Out-of-Plane Morphing ........................................................................................................ 11 2.1.3 Airfoil Adjustment ................................................................................................................ 13 2.2 Morphing Wing Actuation .......................................................................................................... 14 2.2.1 Internal Mechanisms ........................................................................................................... 14 2.2.2 Piezoelectric Actuators ........................................................................................................ 15 2.2.3 Shape Memory Alloys .......................................................................................................... 15 2.3 Examples of Morphing Structures .............................................................................................. 15 2.3.1 Fish Bone Active Camber (FishBAC) ..................................................................................... 15 2.3.2 Zig-Zag Wingbox ................................................................................................................... 18 2.3.3 GNAT Spar ............................................................................................................................ 19 2.4 Morphing Wings in Industry ....................................................................................................... 20 2.5 Morphing Wing Concept Selection ............................................................................................. 21 2.6 Literature Review on Morphing Skins ......................................................................................... 22 2.6.1 Honeycomb and Honeycomb Variants ................................................................................ 24 2.6.2 Corrugated structures .......................................................................................................... 25 2.6.3 Flexible Matrix Composites (FMC) ....................................................................................... 26 2.6.4 Concept Selection ................................................................................................................ 27 2.6.5 Further Investigation into Flexible Matrix Composites (FMC) ............................................. 27 3 Motivations and Past Research.......................................................................................................... 28 3.1 Wing Concept and Conceptual Design ........................................................................................ 28 3.1.1 Morphing Wing Actuation Method...................................................................................... 32 3.2 Wing Design ................................................................................................................................ 34 4 Research Questions............................................................................................................................ 35 4.1 Project Scope .............................................................................................................................. 35 5 Research Methodology ...................................................................................................................... 36 5.1 Airfoil Development .................................................................................................................... 36 5.2 Simulations.................................................................................................................................. 37 5.2.1 XFLR5 .................................................................................................................................... 37 iii 5.2.2 Tornado ................................................................................................................................ 45 5.2.3 Wind tunnel Testing ............................................................................................................. 48 6 Building the Morphing Wing .............................................................................................................. 49 6.1 Morphing Skin Concept............................................................................................................... 50 6.3 Evolution of the Morphing Skin .................................................................................................. 51 6.3.1 Morphing Skin Manufacturing Process ................................................................................ 54 6.4 Summary of the Morphing Wing Design..................................................................................... 55 7 Results ................................................................................................................................................ 57 7.1 Flow Visualization ....................................................................................................................... 57 7.1.2 Summary of Flow Visualization Behaviour........................................................................... 57 7.2 Wind Tunnel Data Post Processing ............................................................................................. 59 7.3 Experimental Results and Discussion .......................................................................................... 60 7.3.1 Conventional T240 Wind Tunnel Results ............................................................................. 61 7.3.2 Morphing Wing Wind Tunnel Results .................................................................................. 66 7.3.3 Roll Results ........................................................................................................................... 76 7.3.4 Discussion of Results ............................................................................................................ 82 7.3.5 Summary of Comparison – Conventional T240 vs Morphing Wing ..................................... 84 8 Conclusions ........................................................................................................................................ 90 8.1 Recommendations/Further research.......................................................................................... 91 References ............................................................................................................................................ 92 APPENDIX A – Wind tunnel Calibration .............................................................................................. 100 APPENDIX B – Assembly of the Wing and Preparation of the Wing ................................................... 109 APPENDIX C – Flow Visualization for Various Morphing Deflections ................................................. 117 APPENDIX D – Experimental Results ................................................................................................... 137 APPENDIX E – Comparison between Wind Tunnel Test and Simulation ............................................ 151 APPENDIX F – XFLR5 Convergence ...................................................................................................... 159 APPENDIX G – Further Information on the Vortex Lattice Method .................................................... 163 iv List of Figures Figure 1 Principles of aircraft drag polar affected by airfoil camber variation in steady cruise flight [10]. ................................................................................................................................................................ 3 Figure 2 Precedent T240 aircraft and its wing’s dimensions in plan view. ............................................. 4 Figure 3 Morphing wing dimensions in plan view. ................................................................................. 5 Figure 4 Makhonine Mak-10 aircraft [4]. ................................................................................................ 6 Figure 5 Examples of variable sweep wings [4]. ..................................................................................... 6 Figure 6 Span morphing wing via telescopic wing [28]......................................................................... 10 Figure 7 Planform alteration types [2]. ................................................................................................. 11 Figure 8 Camber morphing concept visualization [2]. .......................................................................... 11 Figure 9 Span-wise bending morphing concept [2]. ............................................................................. 12 Figure 10 Wing twisting concept seen in the 1899 Wright Kite [36]. ................................................... 13 Figure 11 Airfoil adjustment morphing concept visualization [2]. ....................................................... 14 Figure 12 Airfoil adjustment via actuators inside the wing [37]. .......................................................... 14 Figure 13 A SMA spring actuator recovering its original shape after heating [47]............................... 15 Figure 14 FishBAC rib design [23]. ........................................................................................................ 16 Figure 15 FishBAC utilized as a morphing trailing edge and model parameters [49]. .......................... 17 Figure 16 Top-view of the zig-zag wingbox concept [25]. .................................................................... 18 Figure 17 Schematic of GNATSpar concept [24]. .................................................................................. 19 Figure 18 Rack and pinion actuation system for GNATSpar [6]. ........................................................... 20 Figure 19 Flexsys' Flexfoil deflected [22]. ............................................................................................. 21 Figure 20 Composite Cellular Material Morphing Wing [56]. ............................................................... 21 Figure 21 FishBAC and corrugated morphing trailing edge concept [60]. ........................................... 25 Figure 22 FMC fibre orientation for a) span morphing and b) for camber morphing [52]. .................. 26 Figure 23 Three-view of the initial rib design that connects to the trailing edge [69]. ........................ 28 Figure 24 Colour coded isometric view of morphing wing concept [69]. ............................................. 29 Figure 25 Revised rib design and its assembly [69]. ............................................................................. 29 Figure 26 Revised rib displacements [19]. ........................................................................................... 30 Figure 27 Velcro strips on revised rib [69]. ........................................................................................... 31 Figure 28 Step by step assembly of the wing [69]. ............................................................................... 31 Figure 29 Four view of the fuselage wingbox without the covering panel........................................... 32 Figure 30 Assembled fuselage wingbox. ............................................................................................... 33 Figure 31 Proposed servo locations in the fuselage wingbox and morphing wing. ............................. 33 Figure 32 CAD model of wing design (without stringers attached) [70]............................................... 34 Figure 33 Complete CAD model of 2nd wing design [70]....................................................................... 35 Figure 34 Construction of the T240 airfoil. ........................................................................................... 36 Figure 35 Morphing wing airfoil construction. ..................................................................................... 37 Figure 36 XFLR5 simulation process for wing aerodynamic analysis. ................................................... 38 Figure 37 T240 airfoil in XFLR5.............................................................................................................. 39 Figure 38 T240 airfoil with flap deflections in XFLR5. ........................................................................... 39 Figure 39 2D analysis results of T240 airfoil with Flaps applied at various Reynolds numbers in XFLR5. .............................................................................................................................................................. 42 Figure 40 XFLR5 Analysis for 𝑪𝑳 vs 𝜶 at various Reynolds numbers. ................................................... 45 Figure 41 Tornado simulation process.................................................................................................. 46 v Figure 42 TORNADO Analysis for 𝑪𝑳 vs 𝜶 at various Reynolds numbers. ............................................ 47 Figure 43 Schematic of the industrial wind tunnel at RMIT University. ............................................... 48 Figure 44 Electronic turntable aft of the contraction point in the wind tunnel. .................................. 48 Figure 45 Isometric view of the Morphing Wing. ................................................................................. 50 Figure 46 2D Morphing Wing splines from XFLR5. ............................................................................... 50 Figure 47 Initial morphing skin design A to morphing skin design C. ................................................... 51 Figure 48 Morphing skin design D to morphing skin design E. ............................................................. 52 Figure 49 Morphing skin F to Morphing skin G..................................................................................... 53 Figure 50 Morphing skin H to the Final Skin. ........................................................................................ 54 Figure 51 Compliant morphing skin demonstration. ............................................................................ 54 Figure 52 Exploded isometric view of the Morphing Wing. ................................................................. 56 Figure 53 Conventional T240 Wing results for 𝑪𝑳 vs 𝜶 at various Reynolds numbers. ....................... 63 Figure 54 Conventional T240 Wing Wind Tunnel results for 𝑪𝑫 vs 𝜶 at various Reynolds numbers. . 65 Figure 55 Morphing Wing results for 𝑪𝑳 vs 𝜶 at various Reynold numbers. ....................................... 68 Figure 56 Morphing Wing Experimental results for 𝑪𝑫 vs 𝜶 at various Reynold numbers. ................ 71 Figure 57 Wind tunnel results of 𝑳/𝑫 vs 𝑪𝑳 at various Reynolds numbers for Morphing Wing and Conventional Wing................................................................................................................................ 74 Figure 58 Wind tunnel results of 𝑳/𝑫 vs 𝑪𝑳 at various Reynolds numbers for Morphing Wing and Conventional Wing with flaps. .............................................................................................................. 75 Figure 59 TORNADO and wind tunnel results for 𝑪𝒍 vs 𝜹 at various Reynolds number. ...................... 78 Figure 60 𝑪𝑳 comparison for the Conventional T240 and Morphing wing at various Reynolds numbers. .............................................................................................................................................................. 79 Figure 61 Difference in TORNADO and wind tunnel testing for 𝒑 vs 𝜹 at various Reynolds numbers. 80 Figure 62 𝒑 comparison between Conventional T240 and Morphing Wing at various Reynolds numbers. ............................................................................................................................................... 81 Figure 63 𝒑 comparison between Conventional T240 and Morphing Wing at various Reynolds numbers. ............................................................................................................................................... 82 Figure 64 Calibration setup in the y-axis (drag axis) of the JR3 Load cell, measured at z= 1 m above the load cell. .............................................................................................................................................. 100 Figure 65 Calibration curve for the Lift axis of the JR3 load cell – “Lift” Force output vs “Lift” Force input. ................................................................................................................................................... 102 Figure 66 Calibration curve for the phantom outputs of the JR3 load cell – “Drag” Force output vs “Rolling” Moment output. .................................................................................................................. 102 Figure 67 Calibration curve for the Drag axis of the JR3 load cell – “Drag” Force output vs “Drag” Force input. ................................................................................................................................................... 103 Figure 68 Calibration curve for the Drag axis of the JR3 load cell – “Drag” Force output vs “Yawing” Moment input. .................................................................................................................................... 104 Figure 69 Calibration of the JR3 load cell by applying pure moments in the Yaw axis, at x= -0.3m via Tbeam. .................................................................................................................................................. 104 Figure 70 Calibration of the JR3 load cell by applying pure moments in the Yaw axis, at x= 0.3m via Tbeam. .................................................................................................................................................. 105 Figure 71 Pure Yaw moment configuration calibration in the Drag axis of the JR3 load cell. ............ 106 Figure 72 Pure Yaw moment configuration calibration in the Drag axis of the JR3 load cell. ............ 106 Figure 73 Using calibration data from Yaw moment calibration, calibration curve for the Drag axis of the JR3 load cell. ................................................................................................................................. 107 vi Figure 74 Calibration of the JR3 load cell by applying pure moments in the Roll axis, at y= -0.3m via Tbeam. .................................................................................................................................................. 108 Figure 75 Using calibration data from Rolling moment calibration, calibration curve for the Lift axis of the JR3 load cell. ................................................................................................................................. 108 Figure 76 Using calibration data from Rolling moment calibration, calibration curve for the Lift axis of the JR3 load cell. ................................................................................................................................. 109 Figure 77 Layout of Leading edge and Spar to be bonded. ................................................................ 110 Figure 78 Bonding Ribs to the Leading edge....................................................................................... 111 Figure 79 Bonding Ribs to the Trailing edge. ...................................................................................... 111 Figure 80 Bonding the thin Al sheet to the Wing................................................................................ 112 Figure 81 Bonding the thin Al sleeve to the Trailing edge. ................................................................. 112 Figure 82 Bonding reinforcing L shape carbon fibre angles to the ribs. ............................................. 112 Figure 83 Assembled morphing wing minus the wingtip.................................................................... 113 Figure 84 Isometric view of Wing tip post modifications; removal of spar box and addition of thin Al strips.................................................................................................................................................... 113 Figure 85 Bottom view of Wing tip post modifications; removal of spar box and addition of thin Al strips.................................................................................................................................................... 114 Figure 86 Isometric view of the Bonding of the wingtip cover to the wingtip. ................................. 114 Figure 87 Top view of the Bonding of the wingtip cover to the wingtip. ........................................... 115 Figure 88 Curing of the Epoxy resin applied to the foam components of the Morphing wing and wingtip. ............................................................................................................................................... 115 Figure 89 Morphing wing spray painted. ............................................................................................ 116 Figure 90 Wingtip spray painted. ........................................................................................................ 116 Figure 91 Morphing wing - post cure of the spray paint. ................................................................... 116 Figure 92 Flow visualization for 𝜹𝒎 = 0°. ........................................................................................... 118 Figure 93 Flow visualization for 𝜹𝒎 = 2°. .......................................................................................... 120 Figure 94 Flow visualization for 𝜹𝒎 = 3°. ........................................................................................... 121 Figure 95 Flow visualization for 𝜹𝒎 = 5°. ........................................................................................... 123 Figure 96 Flow visualization for 𝜹𝒎 = 10°. ......................................................................................... 125 Figure 97 Flow visualization for 𝜹𝒎 = 15°. ......................................................................................... 127 Figure 98 Flow visualization for 𝜹𝒎 = 20°. ........................................................................................ 128 Figure 99 Flow visualization for 𝜹𝒎 = 25°. ......................................................................................... 130 Figure 100 Flow visualization for 𝜹𝒎 = 30°. ....................................................................................... 132 Figure 101 Flow visualization for 𝜹𝒎 = 35°. ....................................................................................... 134 Figure 102 Flow visualization for 𝜹𝒎 = 40°. ....................................................................................... 136 Figure 103 Conventional T240 Wing Experimental results for 𝑪𝑳 vs 𝜶 and 𝑪𝑫 vs 𝜶 at Re 202000 and Re 269000. .......................................................................................................................................... 138 Figure 104 Morphing Wing Experimental results for 𝑪𝑳 vs 𝜶 and 𝑪𝑫 vs 𝜶 at Re 202000 and Re 269000. ............................................................................................................................................................ 140 Figure 105 Experimental results of 𝑳/𝑫 vs 𝑪𝑳 at various Reynolds numbers with error bars. ......... 143 Figure 106 Wind tunnel results of 𝑳/𝑫 vs 𝑪𝑳 at various Reynolds numbers for Morphing Wing and Conventional Wing with flaps and error bars. .................................................................................... 145 Figure 107 𝑳/𝑫 vs 𝑪𝑳 comparison of the ideal morphing deflection and conventional wing with flaps and error bars (using the agreeable data). ......................................................................................... 146 vii Figure 108 TORNADO and wind tunnel testing results for𝑪𝒍 vs 𝜹 and 𝒑 vs 𝜹 at Re 202000 and Re 269000. ............................................................................................................................................... 147 Figure 109 XFLR5 Analysis for 𝑪𝑳 vs 𝜶 at Re 202000 and Re 269000. ............................................... 148 Figure 110 TORNADO Analysis for 𝑪𝑳 vs 𝜶 at Re 202000 and Re 269000. ........................................ 148 Figure 111 𝑪𝑳 and 𝒑 comparison between Conventional T240 and Morphing Wing at Re 202000 and Re 269000. .......................................................................................................................................... 149 Figure 112 𝒑 comparison between Conventional T240 and Morphing Wing at various Reynolds Numbers.............................................................................................................................................. 151 Figure 113 Morphing Wing performance comparisons at 𝜹𝒎 = 0° to 𝜹𝒎 = 40° at various Reynolds numbers .............................................................................................................................................. 159 Figure 114 Lifting lines in both spanwise and chordwise directions superimposed onto a wing [33, 89] ............................................................................................................................................................ 163 Figure 115 Velocity (the direction is coming out of the paper) induced at point P by the infinitesimal segment of the lifting surface[33]. ..................................................................................................... 164 Figure 116 Single horseshoe to a system of horseshoe vortices (Vortex lattice) on a finite wing [33]. ............................................................................................................................................................ 166 Figure 117 Nomenclature for calculating induced velocity by a finite length vortex segment [89]. . 167 Figure 118 A typical horseshoe vortex [89]. ....................................................................................... 168 Figure 119 Vector elements for the calculation of induced velocities [89]. ....................................... 169 Figure 120 Nomenclature for tangency condition: (a) normal to element of mean camber surface, (b) section AA, (c) section BB [89] ............................................................................................................ 171 Figure 121 Dihedral angle [89]............................................................................................................ 171 List of Tables Table 1 Morphing Skin Concepts. ......................................................................................................... 22 Table 2 Material combinations tested by Kirn [66]. ............................................................................. 27 Table 3 Summation of flow visualisation behaviour ............................................................................. 58 Table 4 Difference in results for XFLR5 and TORNADO to experimental results. ................................. 83 Table 5 High-lift device comparison of the conventional T240 and the morphing wing at Re 168000. .............................................................................................................................................................. 85 Table 6 Comparison of the roll performance between the conventional T240 wing and the morphing wing at Re 337000. ............................................................................................................................... 86 Table 7 Comparison of conventional T240 and morphing wing in cruise condition at Re 337000 ...... 87 Table 8 Experimental results of similar morphing concepts in literature [34, 49, 51, 76, 86]. ............ 88 Table 9 Load results for the calibration of the JR3 load cell in the x-axis (Lift axis). .......................... 101 Table 10 Load results for the calibration of the JR3 load cell in the y-axis (Drag axis). ...................... 103 Table 11 Pure moment Yaw results for the calibration of the JR3 load cell in the y-axis (Drag axis). 104 Table 12 Pure moment Roll results for the calibration of the JR3 load cell in the x-axis (Lift axis). ... 107 Table 13 List of non-converged conditions in XFLR5 .......................................................................... 159 viii Nomenclature b Wingspan, in 𝑚 c Chord, in 𝑚 𝐶𝐿 Coefficient of Lift 𝐶𝐿𝑙𝑜𝑐𝑎𝑙 Section Lift coefficient 𝐶𝐷 Coefficient of Drag 𝐶𝐷𝑖 Induced Drag 𝐶𝐷0 Parasite Drag 𝐶𝑙 Rolling moment coefficient 𝐶𝐿𝛼 Lift-curve slope 𝐶𝑙𝛿 Aileron effectiveness 𝑐𝑝 Coefficient of Pressure 𝑐𝜏 Specific fuel consumption 𝐷 Drag, in N 𝑒 Oswald efficiency factor 𝐹 Fuel consumption, in 𝑘𝑔/ℎ 𝑓 Frequency, in ℎ𝑧 𝐼𝑥𝑥 Mass moment of Inertia about x-axis, in 𝑘𝑔/𝑚2 𝐿 Lift, in N L Roll Moment, in Nm 𝐿/𝐷 Lift to Drag ratio 𝑀 Moment, in Nm 𝑝 Roll rate, in 𝑑𝑒𝑔/𝑠 𝑝̇ Roll rate acceleration, in 𝑑𝑒𝑔/𝑠 2 𝑞 Dynamic pressure, 1/2𝜌𝑉 2 S Wing area, in 𝑚2 ix 𝜌∞ 𝑉∞ 𝑑 𝜇∞ Re Reynolds number, 𝑊 Aircraft Weight, in N 𝑉 Airspeed, in 𝑚/𝑠 𝑉𝑠𝑡𝑎𝑙𝑙 Stall Airspeed, in 𝑚/𝑠 𝛼 Angle of attack, in 𝑑𝑒𝑔 𝛼𝑠𝑡𝑎𝑙𝑙 Angle of attack at which stall occurs, in 𝑑𝑒𝑔 𝛽 Rotation angle of load cell, in 𝑑𝑒𝑔 𝛿 Deflection angle, in 𝑑𝑒𝑔 𝛿𝑚 Morphing wing deflection angle, in 𝑑𝑒𝑔 𝛿𝑓 Flap deflection angle, in 𝑑𝑒𝑔 𝛿𝑎 Aileron deflection angle, in 𝑑𝑒𝑔 𝜃 Angle, in 𝑑𝑒𝑔 𝜃𝑜𝑓𝑓𝑠𝑒𝑡 𝜃𝑟𝑒𝑐𝑜𝑟𝑑𝑒𝑑 Control arm angle at zero deflection in the morphing wing, in 𝑑𝑒𝑔 Recorded control arm angle at a deflected position for the morphing wing, in 𝑑𝑒𝑔 𝜇 Viscosity, in 𝑘𝑔/𝑚𝑠 p Density, in 𝑘𝑔/𝑚3 Γ Dihedral angle, in 𝑑𝑒𝑔 ∆ Change in (subscript) Λ Sweep angle, in 𝑑𝑒𝑔 x Subscripts 𝑚𝑎𝑥 Maximum 𝑚𝑖𝑛 Minimum 𝑥 About x-axis 𝑦 About y-axis 𝑧 About z-axis 𝑑𝑎𝑚𝑝𝑖𝑛𝑔 1/2𝑏 Due to damping Half wing 𝑖 Initial 𝑟 Recorded 𝑀𝑊 Morphing Wing 𝑏𝑜𝑑𝑦 The Aircraft body without the wings 𝑜𝑢𝑡𝑝𝑢𝑡 Recorded output of JR3 load cell 𝑖𝑛𝑝𝑢𝑡 Recorded input for the JR3 load cell 𝑝ℎ𝑎𝑛𝑡𝑜𝑚 Non-physical occurrence in load cell 𝑐𝑜𝑢𝑝𝑙𝑖𝑛𝑔 When moment/force is linked to another parameter 𝑑𝑒𝑐𝑜𝑢𝑝𝑙𝑒𝑑 When a coupled moment/force is separated from the coupled parameter xi Abstract Morphing wings were once in the common in early aviation however due to a lack of strong and lightweight materials they were abandoned in favour of conventional wings. Due to the recent advances in smart technologies, morphing wings has become of interest in aviation. This paper proposes the use of internal mechanisms to promote morphing in a wing to increase aerodynamic performance as opposed to the smart technologies. To determine the aerodynamic superiority of the morphing wing it was compared to a conventional wing of the same geometry. The remote-control (RC) aircraft Precedent T240 was used as the basis of the wing design for the morphing wing. The FishBAC (Fish Bone Active Camber) morphing concept is used in this research, to design and prototype a morphing wing for the Precedent T240 RC model aircraft. The simplicity and cost effectiveness of the internal mechanisms will allow for a wider audience to adopt the morphing wing design. The conventional wing will be compared to a morphing wing of the same geometry through simulations and wind tunnel testing. The morphing wing required a compliant morphing skin suitable to facilitate the extension of the top surface and contraction of the lower surface of the wing. A FMC (Flexible Matrix Composite) skin was developed for facilitation of extension the top surface of the wing whilst the contraction of the bottom surface was bypassed through the usage of a thin aluminium plate. The morphing wing and the conventional wing performance were simulated using the TORNADO program and the XFOIL adapted XFLR5 and validated experimentally through wind tunnel testing. The wind tunnel experiments showed that the morphing wing had superior aerodynamic performance in comparison to the conventional wing, with the exception of stall speed due to the increased weight of the morphing wing. The theoretical results accurately predicted the performance of the morphing wing for low morphing deflections and angles of attack. The results have shown that the design of the morphing wing is acceptable as a simple and an affordable option. Due to the higher performance (in most areas) while considering the weight penalty due to the more increased complexity of a morphing wing system as opposed to a conventional wing system. Hence a FishBAC morphing wing is aerodynamically superior to its conventional counterpart. Keywords: Morphing wing, FishBAC, wind tunnel testing, XFLR5, TORNADO, Flexible Matrix Composite 1 1 Introduction A morphing object is an object that undergoes a large change in form or shape [1], therefore a morphing wing is a wing which undergoes a continuous change in its wing geometry to adapt to its mission profile [2]. By altering the wing mid-flight, in theory the wing will be able to increase its aerodynamic performance with a small increase in drag. Although flaps, ailerons and other conventional control surfaces do change the wing geometry it is not considered as morphing the wing, since it causes a discontinuous profile. There is an increasing pressure for aircraft designs to become quieter and more efficient due to regulations on noise pollution and carbon emissions [3, 4]. Reducing noise pollution and carbon emissions could both be achieved by implementing morphing wings into commercial aircraft [4]. With the emergence of advanced materials, further research into morphing wing design and their aerodynamic benefits can be explored. While conventional rigid wings use hinged control surfaces which causes breaks in continuity of the curvature of the wing profile, hence increasing parasite drag. While an advantage of morphing wings are the smooth continuous gapless control surfaces which will reduce the drag [5]. Profile discontinuities, sharp edges and deflected surfaces cause the aircraft to be more prone to detection in both radar and acoustics [6]. Morphing wings provide various benefits for aircraft depending on the type of morphing wing the aircraft adopts, however there are general advantages that all morphing wings offer. Morphing wings improve the aerodynamic efficiency of the aircraft since they have smooth continuous profile [2, 7] (discontinuous profiles cause disrupted airflow) and can increase the lift coefficient for the same altitude through changes in wingspan, chord length, camber, and sweep. Implementing morphing wings can also lead to a reduction in noise due to the lack of control surfaces [8]. The improved aerodynamic performance of an aircraft results in less fuel consumption and results in improved range [7]. The lift-drag ratio is also improved due to the increase in lift from morphing deflection [7]. This is shown in equation 1 where fuel consumption (𝐹) is dependent on the specific fuel consumption, weight of the aircraft and the lift-drag ratio, 𝐿/𝐷 [9]. In general, the largest amount of time for a flight profile is spent in the cruise. During cruise at the recommended setting, fuel consumption is dependent on the specific fuel consumption 𝑐𝑇 , lift-to drag ratio 𝐿/𝐷 and the weight of the aircraft, 𝑊. 𝐶 𝐹 = 𝑐𝑇 𝐶𝐷 𝑊 𝐿 (1) Decreasing the fuel consumption would increase the endurance of an aircraft. Decreasing 𝐹 could be done by decreasing engine specific fuel consumption, increasing the lift-drag ratio and/or decreasing the weight of the aircraft. A solution to increase the lift-drag ratio is to change the camber line of the wing by altering the shape of the airfoil [9]. 2 Figure 1 Principles of aircraft drag polar affected by airfoil camber variation in steady cruise flight [10]. In Figure 1, the linear dashed line from the origin represents the maximum 𝐿/𝐷 written as 𝐶𝐿 /𝐶𝐷 . For a fixed airfoil section wing, as angle of attack, 𝛼 increases the 𝐿/𝐷 decreases. The lift coefficient, 𝐶𝐿 is not exclusively determined by 𝛼, by altering the airfoil profile through camber morphing 𝐶𝐿 can be increased. The objective of morphing the airfoil wing section is to follow the optimal 𝐶𝐿 /𝐶𝐷 for any given 𝐶𝐿 . Typically, the cruise phase of flight for an aircraft is the longest phase of flight for an aircraft. For unmanned aerial vehicles, UAVs this is emphasised since there is not a human component thereby the operation of the UAV is not limited to human stamina [9]. Hence morphing to maximise the liftdrag ratio during the entire cruise of the aircraft is the ideal outcome. Due to their complexity, morphing wings are more difficult to design and are generally heavier than conventional wings [11, 12]. The objective of the research was to confirm the increase in aerodynamic performance that comes with camber morphing in comparison to the aerodynamic performance of a fixed airfoil wing. The specific aerodynamic performance examined in this study were lift coefficient, lift-drag ratio, rolling moment coefficient and the initial roll rate. A remote control, RC aircraft was used as the base of the research. The conventional fixed airfoil wing’s performance was compared to the morphing wing’s performance. The RC aircraft used was the Precedent T240 which is a scale model of the Cessna 180 aircraft, the design of the T240 wing illustrated in Figure 2. To compare the conventional T240 wing to the morphing configuration, the morphing wing must have geometry to the conventional wing so that it can be retrofitted to the T240 aircraft. As such the morphing wing was designed for the T240 aircraft, which is seen in Figure 3. In the morphing wing configuration, the ailerons and flaps were removed because the entire morphing section of the wing (aft of the spar) acts as the control surface as shown in Figure 3. The spar of the T240 ends at 30% chord. Hence 70% of the chord aft of the spar 3 was used for morphing. The T240 features two struts, to support the wing. The morphing wing removes the rear strut of the wing to allow for further morphing. Figure 2 Precedent T240 aircraft and its wing’s dimensions in plan view. Previous research was conducted on the camber morphing wing, seen in section 3 which concluded in a skeleton design of the morphing wing and an actuation system. To achieve the objective of aerodynamic analysis of the morphing wing, a suitable skin and a prototype of the morphing wing was required. Previous research did not achieve a skin that maintained a zero Poisson’s ratio and provided spanwise structural support. Hence in this study a morphing skin was designed and manufactured for the purpose of wind tunnel testing. The morphing skin design underwent many iterations before satisfying the spanwise support and achieving a zero Poisson’s ratio. Which was accomplished via a “dual” skin, where the upper and lower surfaces of the wing used different materials as a skin. The upper surface skin was a Fibre Matrix Composite, FMC where the matrix material was silicone and the fibre material was carbon fibre. The lower surface skin was a thin aluminium plate that bends when the morphing wing was deflected. UAVs are the ideal test bed for morphing technologies, due to their small size, ability for autonomous navigation and control, are cheaper to build than full scale aircraft and lack of pilot making it safer than manned aircraft. 4 Figure 3 Morphing wing dimensions in plan view. 2 Literature Review The Morphing Aircraft Structures (MAS) program by Defense Advanced Research Projects Agency (DARPA) defines morphing aircraft as a multi-role platform that changes its state to adapt to changing environments providing superior system capability not possible without reconfiguration [13, 14]. Additionally, morphing aircraft uses designs that integrates innovative combinations of advanced materials, actuators, flow controllers and mechanisms to achieve the state change [13, 14]. The morphing wing concept was introduced before the first powered flight in 1903 [15]. However due to the technological limits at the time that is materials available during that time were not strong and flexible enough, the concept was abandoned in favour of rigid wings i.e. conventional wings. Birds inspired early aviators to pursue flight which led to the pursuit of morphing vehicles. The smooth and continuous shape-changing capability that birds possess however was beyond what was technological capable at the time. Aviators turned to variable geometry designs using conventional hinges and pivots both of which were used for many years. Since the recent advances in aerodynamics, controls, materials and structures the interest in morphing vehicles have been reignited and bird-like flight that is smooth and continuous shape change for aircraft is now once again pursued [16]. Valasek mentioned that that the connection between bio-inspiration and aeronautical engineering is an important one [16]. As without birds (or bats) the concept of flight may have never occurred to early aviators. Otto Lilienthal a Prussian aviator who lived in the nineteenth century, was fascinated by bird flight which led him to become a designer. He insisted on using flapping wing tips instead of the conventional propeller due to his fascination of bird flight. From his observations of bird flight particularly their twist and camber distributions led to the development of his air-pressure tables and airfoil data. Several early pioneers recognized the value in morphing as a control effect [17]. The Wright brothers used wing warping for lateral control. The warping was accomplished by attaching wires to the pilot’s belt and controlled by the shifting body position. The Etrich Taube design series were completely bio-inspired except for the omission of flapping wings [16]. The Wright and Taube designs demonstrated that warping controls can be effective on aircraft with thin and flexible 5 wings. However, conventional hinged controls; ailerons and rudders, were more appropriate for aircraft with rigid structures. The technological state of materials at the time was not advanced enough to allow usable warping for high performance aircraft hence the conventional control surfaces were used. However, morphing was still achieved as the geometry of wings camber was actively altered via conventional hinges, pivots and rails [8, 16]. The design by the Wright brothers showed that warping controls can be effective on aircraft with thin and flexible wings. One of the first successful modern morphing flight was due to Ivan Makhonine, the aim was to improve cruise performance by reducing induced drag due to lift. Makhonine used in-flight wing planform area morphing to reduce the landing speed while providing a smaller wing for highspeed flight. He developed a telescoping wing planform which was used on the MAK-10 seen in Figure 4 [18, 19]. Figure 4 Makhonine Mak-10 aircraft [4]. In the 1950s variable geometry research sponsored by NASA led to experimental transonic designs such as the Bell X-5. The X-5 was the first full scale aircraft that was capable of wing sweep during flight, seen in Figure 5 at different sweep settings. Take-off and landing were improved when the wings were fully extended and at low speeds whilst high speed performance and drag was reduced when the wings were swept backwards. The wing could be swept to 20°, 45° and 60° during flight and were tested at both subsonic and transonic flight [18, 19, 20]. Figure 5 Examples of variable sweep wings [4]. 6 The AFTI F-111 Mission Adaptive Wing (MAW), was intended to minimize the penalties for off-design flight conditions through smooth-skin variable camber and variable wing sweep angle. Since the MAW has variable camber surfaces it does not suffer from discontinuous surfaces or exposed mechanisms that conventional aircraft experience. Because of the smooth flexible upper surfaces and fully enclosed lower surfaces that can be actuated during flight to provide the desired camber. Due to the success of the program advancements to a fully morphing aircraft were made. The variable geometry concept found its way into commercial air transport, it was considered for various conceptual designs such as the Boeing 2707 Supersonic Transport in the 1960s. Due to the success of the variable geometry concept, bio-inspiration was overlooked or it was not considered promising enough during the period [8, 16]. Recent discoveries in bird flight mechanics and new insights of bio-inspired research resulted in the re-ignition of using flying animals as a design base for morphing aircraft. And the recent advances in materials, where materials are strong, lightweight and flexible also contribute to the re-ignition in morphing wing design research. Research programs have appeared in the recent years bringing in most of the early morphing concepts including bio-inspiration, warping, shape changing, variable geometry, structures, materials, controls and aerodynamics. The NASA morphing aircraft project developed from the Langley Research Centre (LaRC), was program conducted from 1994-2004 [8].The program sponsored research across a wide range of technologies that included biotechnology, nanotechnology, biomaterials, adaptive structures, micro-flow control, biomimetic concepts, optimization and controls. The focus of this project was to bring together the NASA morphing unmanned air vehicle. The aircraft concept was made up of the various morphing concepts which include bio-inspiration, warping, shape changing, variable geometry [8, 16, 18, 19]. Due to advances in technology, modern morphing systems use shape memory alloys, piezoelectric, magnetostrictive materials, magnetorheological fluids and electrorheological fluids into compliant structures activated by electric fields, temperature or magnetic fields [8]. Where a compliant structure could be a structure that is flexible and changes its shape through elastic deformation. Smart material based morphing wings will be covered in section 2.2.2 and 2.2.3. 2.1 Types of Morphing Wings Early aircraft like the Wright flyer were bio-inspired from observing birds, for their wing warping capabilities [21]. Nature provides a rich source of inspiration for the new generation of morphing wings. During flight, animals perform active changes in wing shape that are associated with stability and manoeuvre control and those that are associated with the wingbeat cycle [7]. Biological wings i.e. wings of birds, bats and insects are of morphing designs with continuous variable planform, camber or twist. It can be said that morphing wings are the norm of small scale flying in nature whilst for engineers’ rigid wings have been the norm for all aircraft. The limitation to rigid wing design is due to a lack of material strength and flexibility. Due to recent developments of new materials, bio-inspired morphing wings are once again of interest to engineers [22]. Birds have been the main driving force for bio-inspiration among morphing structures for engineers. The fascination of birds led to the Wright brothers’ developing the wing-warping control system which eventually led them to undertake the first powered, manned and controlled flights. It should be noted that birds have a total of three morphing structures; two being the wings and the third being their 7 horizontal tail, the tail changes its shape similarly to the changes in the wings [16]. Birds can rapidly morph between different planforms [7]. The changes in wing area are possible due to the degree of overlap between feathers changes as the bird flexes, spreads its wings and tail. The feathers create the lifting surfaces of the wing, which comes from the follicles within the skin and which in the case of the flight feathers is attached by ligaments to wing bones. The flight feathers are large feathers that are responsible for most lift and thrust during flight. They are flexible structures, and the slight roughness may generate turbulence even when they lie flush to the wing surface [8, 16, 18, 19]. As for mammals only bats can fly (mammals that are capable of gliding such as the flying squirrel are not considered to be flying mammals). A bat’s wings must resist extensive load changes over the course of a wingbeat cycle, to accommodate this, their wings have evolved to sustain the forces associated with powered flight. Bat wings are composed of an elastic muscularized membrane that is stretched between the digits of their hands, hindlimbs and body, this enables high-order control of the wing. Bat wing bones experience torsional loads whereas bones of other mammals’ experience bending loads. The bones of bats are highly dense which correlates with strength and stiffness; therefore, the bones of the wing are relatively strong and heavy [16]. Morphing wings can be split into three major groups: planform alternation, out-of-plane transformation, and camber change. Planform alternation is when the wing is altered through a change in area or wing sweep adjustment. Out-of-plane transformation is when the wing is twisted or the chord or the span wise camber are adjusted. Airfoil adjustment is when the thickness of the airfoil is altered. Planform alternation and out-of-plane transformation both have multiple methods of morphing. Planform alternation has three general methods of alteration; wingspan adjustment, change in chord length and change in sweep angle. Out-of-plane transformation also has three general methods of alteration: chord-wise bending, span-wise bending and wing twisting [2]. All these methods of alteration can be broken down into various methods. There are various types of morphing structural arrangement some of these include: Fish Bone Active Camber (FishBAC), Compliant Spar, Zig-zag wingbox and Gear Driven Autonomous Twin Spar (GNATSpar) [6, 23, 24, 25]. Conventional control surfaces such flaps, slats and landing gears are discrete morphing [14]. The discontinuous structure caused by these control surfaces results in loss of aerodynamic efficiency. Whereas morphing structures provides continuous wing profile hence no loss in aerodynamic efficiency, due to their morphing nature the aerodynamic efficiency of the morphing wing is more efficient than the conventional wing. Since morphing wings change their wing geometry, the skin of the wing is required to morph with the wing. These skins are called morphing skins, morphing skins are generally comprised of flexible rubber like material such as silicone, this is explored further in section 2.6. 2.1.1 Planform Morphing 2.1.1.1 Wingspan Wingspan is generally adjusted by using telescopic structures seen in Figure 6, where the span of the wing is increased. Another method of span alteration is to use a scissor like mechanism [26]. The 8