Masters thesis of engineering aerodynamic performance comparison of a conventional uav wing and a fishbac morphing wing

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

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

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

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

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

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

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 T-beam 104

Figure 70 Calibration of the JR3 load cell by applying pure moments in the Yaw axis, at x= 0.3m via T-beam 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

Figure 74 Calibration of the JR3 load cell by applying pure moments in the Roll axis, at y= -0.3m via

T-beam 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

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

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 Control arm angle at zero deflection in the morphing wing, in 𝑑𝑒𝑔

𝜃𝑟𝑒𝑐𝑜𝑟𝑑𝑒𝑑 Recorded control arm angle at a deflected position for the morphing wing, in 𝑑𝑒𝑔

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

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 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,

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 lift-drag 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

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

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

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 high-speed 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]

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

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

planform can also be altered by using extendable ribs and spars which enable independent changes

in span and chord [27]

Low aspect ratio wings suffer from poor aerodynamic efficiency however they fly faster and are more manoeuvrable [24] Hence the drag of the wing is reduced therefore the range/endurance of the aircraft is increased Increasing the wingspan, leads to an increase in aspect ratio Which has a major effect on aerodynamic performance as aspect ratio is relevant in many aerodynamic properties such as: lift therefore induced drag 𝐶𝐷𝑖, lift-drag ratio, range and endurance [28, 29, 30] Hence in wingspan morphing designs, both wingspan contraction and extension should be utilized

Increasing the wingspan leads to an increase in wing root bending moment [30] Ajaj et al conducted analyses, which showed that by increasing the wingspan by 50% led to a 50% increase in wing root bending moment [6] With increased wingspan, spanwise lift distributions and induced drag are decreased for the same lift [25, 30] Neal et al found that increasing the wingspan results in a low drag for higher 𝐶𝐿 [29] In Neal’s wind tunnel experiment, at zero span change, 𝐶𝐿= 0.6 and at a 100% increase in wingspan 𝐶𝐿≈ 0.68 at both conditions 𝐶𝐷= 0.15 [29] Hence maintaining a low drag for higher 𝐶𝐿 also results in a higher lift-drag ratio Using a telescopic wing design, Blondeau et al increased the aspect ratio of the wing by up to 114% [28] Within the telescopic archetype of span morphing, there are two sub-archetypes The first, features telescopic shells which has each subsequent shell being smaller than the previous one resulting in a step between each telescopic shell Therefore, the chord progressively decreases between each shell, resulting in a slight tapering effect The step between each of the shells results in the generation of parasite drag The skin for this wing type is rigid The second, uses an extendable spar mechanism, this type is covered by a compliant skin [14, 24] Compliant skin or morphing skin will be covered in section 2.6

Rolling moment generated by asymmetric span morphing is sensitive to angle of attack [24] While conventional ailerons do not display this behaviour therefore morphing wing aircraft should not be operated in the same manner as conventional aircraft Additional inertial terms are introduced in the roll equation of motion when using span morphing Assuming the basic operating weight is kept, excess span morphing yields diminishing returns in endurance A 35% increase in wingspan resulted

in a 6.5% increase in endurance whilst a 22% increase in wingspan resulted in 6% increase in endurance [24] A 22% increase in wingspan can reduce the take-off field length and landing distance

by 28% and 10% respectively Ajaj et al determined that span morphing becomes less effective as the weight of the aircraft increases and becomes detrimental if the weight of the morphing aircraft is 12.5% heavier than the conventional counterpart [24]

Figure 6 Span morphing wing via telescopic wing [28]

2.1.1.2 Chord Length Changes

Changes in chord length are usually achieved through the extension or retraction of leading edge or the trailing edge, by usage of actuation systems Another method is to change the chord length without resizing any leading or trailing edge flaps using an interpenetrating rib mechanism through

DC motors and lead screws [31] A visualization of increasing chord is illustrated in Figure 7 By increasing the chord length, the wing area S, increases therefore increasing lift Perkin et al achieved

a chord length increase by using sliding ribs, they also considered telescopic ribs to increase chord length [32] Chord length change can also be achieved by using extendable ribs and De costa used screws that when rotated would lengthen the chord size of the rib [27]

2.1.1.3 Sweep

Sweep angle variation is usually achieved by pivoting the wings of the aircraft However, sweep angle can also be actuated by two electromechanical, lead screw actuators [29] Another wing sweep is based on bi-stable composite spars that are interconnected with a truss-rib structure [29] A change

in sweep concept is illustrated in Figure 7

Sweep changes the aerodynamic centre and Centre of Gravity (CG) position, the change in position depends on the sweep angle, which in turn changes the aircraft stability Hence affecting handling and control of the aircraft Sweeping wings at high speeds can decrease the drag the wing generates [6, 30] In their fully swept position Neal found that the both the maximum 𝐶𝐿 and minimum 𝐶𝐷 increases from the unswept case [29] From Neals experiment, sweep also increases 𝛼𝑠𝑡𝑎𝑙𝑙, i.e increases the angle at which stall occurs [29]

Sweep is generally suited for trans-sonic flight, As the wing is swept back it reduces the normal velocity component of the airflow to the leading edge Which means that the normal velocity component of the airflow is smaller than the actual airspeed hence reducing compressibility effects, wave drag and lift Siouris found that by sweeping the wings back generate an increase in lift-drag ratio by up to 80%

by sweeping the wing from 20° to 40° for the same lift conditions [3] The increase in lift-drag ratio was due to the decrease in wave drag Siouris obtained a maximum lift-drag ratio at Λ = 40° for their given conditions [3] Siouris found that sweeping the wings forwards had an adverse effect on lift-drag ratio [3]

Figure 7 Planform alteration types [2]

2.1.2 Out-of-Plane Morphing

2.1.2.1 Camber Morphing

Camber morphing is usually achieved by bending the wing at any given point/s along the chord of the wing seen in Figure 8 this method is quite common in literature Camber morphing can be conducted through multiple actuation methods such as the use of internal mechanisms, piezoelectric actuation and shape memory alloy actuation [2]

Figure 8 Camber morphing concept visualization [2]

This principle is like flaps however unlike flaps which are non-discrete and non-continuous, camber morphing features continuous surfaces and can be discrete geometrical changes in the airfoil section Since the principle of camber morphing is like the flap, it features the same aerodynamic advantages with a smaller drag penalty than conventional flaps Such as the lift is increased in comparison to the non-cambered counterpart of the wing, the zero-lift angle of attack becomes more negative [33] Morphing camber designs have become compliance based over the years [23] Camber morphing has been used in variety of applications such as helicopter rotors, ship rudders, submarines and hydrofoil boats [23] Increasing camber leads to a shift in the drag polar of the airfoil and causes an increase in

𝐶𝐿 for a minimal drag penalty [9] Which increases the lift-drag ratio of the aircraft, which can lead to

an increase in endurance and range as both are dependent on lift-drag ratio However, the increase is subject to the increased weight of the morphing wing system Variable Cambered airfoils have higher stall angles and higher lift-drag ratio than rigid cambered airfoils [34] and by extension non-cambered airfoils

Figure 9 Span-wise bending morphing concept [2]

Span-wise bending wings can be applied to effect vehicles When effect vehicles fly close to the surface, they benefit from an increase in lift and a reduction of drag Applying variable dihedral morphing to wing-in-ground-effect vehicles can benefit in reduced drag and increased lift while in ground effect [12] While at high altitude it can be used in the standard planar wing configuration i.e the non-morphed condition Therefore, span-wise bending morphing wings can benefit from the wing-in-ground-effect when flying close to the surface and transition to conventional planar wing configuration at higher altitudes

wing-in-ground-Wiggins et al saw that as wingspan undergoes wingspan bending downwards, side force coefficient increases towards the wingtip this is due to the pressure force acting normal to the wing surface [12] Wingspan bending also affects the distribution of lift and side forces generated by the angle of attack and camber of the airfoil changes Span-wise bending morphing increases the Oswald efficiency factor,

𝑒, however the total induced drag also increases [12] It should be noted that as span-wise bending morphing increases the projected wingspan of the wing decreases This is seen in Wiggins et al study where in the fully morphed case, the projected wingspan decreased by 11% [12] In the same case the drag of the fully deflected case was 10% larger than the drag of the non-deflected wing for the same lift

2.1.2.3 Wing Twist

Wing twisting (seen in Figure 10) is usually achieved by twisting the wing at any given point/s along the span and the chord of the wing This method is similar to the chord wise morphing as the camber along the span does change but it is not uniform as the span before the twist remains the same However, unlike camber morphing, the camber remains the same but the angle of attack of the wing changes instead By wing twisting can obtain low-drag and high lift [30] This morphing method is not

as common in literature as others due to the complexity of the morphing method [35] Wing twisting morphing can be used for roll control

Figure 10 Wing twisting concept seen in the 1899 Wright Kite [36]

A disadvantage of the concept is that to enable the morphing, most of the inner space of the wing must be utilized to achieve morphing Positive twist increases lift and drag however it also provides greater roll performance [35] From their experiments, Rodrigue et al found that for 𝛼 ≤ 8° with wing twisting the coefficient of lift increased with minimal drag increase [35] However, the drag had a major increase for 𝛼 > 6° and for 𝛼 = 10° the wing twisted wing had a similar coefficient of lift as the conventional configuration [35] This shows that for wing twisting has diminishing returns at higher angles of attack Rodrigue et al found that lift-drag ratio is also increased for wing twisting morphing for 𝛼 ≤ 6° [35] Wing twisting sees the most gains in lift-drag ratio for lower angles of attack, where at

𝛼 = 2°, Rodrigue et al saw an increase of approximately 13% for lift-drag ratio [35]

2.1.3 Airfoil Adjustment

Airfoil adjustment is when the airfoil shape is changed without majorly affecting the mean camber line, as shown in Figure 11 This could be done by changing the thickness distribution of the base airfoil using actuators, as shown in Figure 12 With this morphing method, the wing would be able to morph between its normal thickness to a desired thickness Hence, the wing can alternate between thin or thick body aerodynamic properties depending on flight phase Thin airfoils suffer from flow separation

at lower angles of attack and a lower section lift coefficient, 𝑐𝑙 [33] Thin airfoils however have lower drag characteristics and is much better suited for supersonic flow [33] Thick airfoils suffer from higher drag coefficients however they provide higher maximum lift coefficient [33] This means that thick airfoils also have a higher rate of climb than the thin airfoil counterparts [33] Therefore, this method

is not as popular as the other morphing methods since thin airfoil sections are better suited for supersonic flow And most applications morphing is required for subsonic flow

Figure 11 Airfoil adjustment morphing concept visualization [2]

Figure 12 Airfoil adjustment via actuators inside the wing [37]

2.2 Morphing Wing Actuation

There are three general methods to morph a wing which are: morphing through internal mechanisms, piezoelectric actuation, and shape memory alloy actuation Disadvantages of smart materials-based actuators are their limitations in achievable active strain, blocking stress or actuation frequency [38] Actuation characteristics of smart materials are dependent on the physical principle they are based

on Currently there is no solution that can satisfy all three properties: strain, stress, and frequency [38] Because of this there are not many concepts that have reached the wind tunnel testing or flight-testing phase Since the response time of shape memory actuators tend to be slow, they are not suited for fast response situations in flight

Smart material-based actuators can limit some of the structural properties of a wing, such as usage of smart material-based actuators as the part of the skin to be compliant with the morphing wing Giulio found that in some cases the smart material-based actuators negatively impacted the aerodynamic properties which in turn nullified the lightweight advantage [38]

2.2.1 Internal Mechanisms

Morphing is achieved through the alteration of the internal structure of the wing Where rib deformation via hinges is the most common [2], there are various methods to achieve this such as the segmenting the ribs or by altering the leading edge and/or trailing edge by means of actuators [2] This method can typically be used for airfoil adjustment, sweep and increase in span examples are shown in Figure 6, Figure 7 and Figure 12

Internal mechanisms include linear actuators, servos, stepper motors and pneumatic actuators [26,

34, 39, 40, 41, 42, 43, 44, 45] Hinges, joints and a combination of threaded nuts and bolts are often seen with the aforementioned internal mechanisms [26, 34, 39, 40, 41, 42, 43, 44, 45] Compliant mechanisms can be used in tandem with the hinges and actuators as seen in Vasista et al and Yang et

al [41, 46] Variable geometry truss manipulators have also been used as a mechanism to achieve morphing [40]

Linear actuators and pneumatic actuators operate in a different manner from one another, but both use horizontal translation to achieve morphing in morphing wings Where linear actuators and

pneumatic actuators use electrical and compressed air/gas respectively, to actuate Linear actuators and pneumatic actuators were used in the works of Moosavian, Monner, Joo, Yang and Poonsong [26,

34, 40, 43, 45]

Rotational forces utilized by electric servos, stepper motors and threaded nuts and bolts (which use rotation to translate the mechanism) are another method of actuating morphing wings [39, 41, 42, 44]

Internal mechanisms have been used in the follow morphing configurations: Span and chord extension morphing, spanwise bending, camber and sweep [26, 34, 39, 40, 41, 42, 43, 44, 45] Hence showing the versatility of internal mechanisms as the actuation method of a morphing wings

2.2.2 Piezoelectric Actuators

Morphing by piezoelectric actuation is achieved through the deformation through electrical current i.e when the piezoelectric material experiences an electrical current the material deflects Piezoelectric actuators are generally used in high frequency applications like rotary wing aircraft and controlling local flow, they also generally have small deflections that require high voltage [5]

2.2.3 Shape Memory Alloys

Shape memory alloys (SMA) has the property of shape memory as the name implies can return to the initial shape after being deformed by a weight load that is activated by heat caused by an electrical current [47] Morphing is achieved through the electrical current, the concept of the shape memory effect can be seen in Figure 13 The shape memory alloys can be woven with anisotropic fibres to be used as wires Since SMA require heat to be maintain the desired shape, more energy is required by the system hence the system is less energy efficient The actuation mechanism may interfere with other system components for example structural ribs [48] SMA actuators can develop permanent strain throughout life due to the heat cycles this result in actuators becoming loose and their length must be adjusted

Figure 13 A SMA spring actuator recovering its original shape after heating [47]

2.3 Examples of Morphing Structures

2.3.1 Fish Bone Active Camber (FishBAC)

The structure of a FishBAC is generally made up of a rib that consists of thin chordwise bending beam spine with stringers branching off that are connected to a pre-tensioned elastomeric matric composite (EMC) skin surface, seen in Figure 14 Structural deformation occurs though compliance hence

mechanisms, linkages or sliding skins are required The deformation of the FishBAC occurs through compliance, the bending deflections are caused by a high stiffness tendon system

The design of the core and skin feature near-zero Poisson’s ratio in the spanwise direction By tensioning, the skin in the chordwise direction the out-of-plane stiffness is increased whilst also prevents lower surface skin buckling when morphing The out-of-plane stiffness is increased by pre-tensioning the skin in the chordwise direction this also eliminates lower surface skin buckling when undertaking morphing In Woods et al design actuators are mounted in the D-spar drive, a tendon spooling pulley through a non-backdrivable mechanism i.e large loads are unable to displace the deflection of the system [23] Since the tendon system is non-backdrivable, no actuation energy is required to hold the deflection position of the structure this also allows the stiffness of the tendons

pre-to contribute pre-to load carrying (chordwise bending is experienced when under aerodynamic loads), without increasing the energy required to deflect the structure

Figure 14 FishBAC rib design [23]

FishBAC is more aerodynamic efficient than traditional trailing edge flaps since there was a 25% in drag ratio at equivalent lift conditions [23] In Woods’ comparison with the flapped airfoil the 𝐶𝐿𝑚𝑎𝑥was almost identical 𝐶𝐿𝑚𝑎𝑥 = 1.07 for the morphing condition and 𝐶𝐿𝑚𝑎𝑥 = 1.08 for the flapped condition [11] The 𝐶𝐿𝑚𝑎𝑥 however did occur earlier by an angle of attack of 1.2° Woods did not encounter a drop off in morphing benefit in their testing Woods believes this means that the trailing edge separation phenomena was delayed [11]

lift-Increasing camber deflection shifts the drag polars left for a 𝐶𝐷 vs 𝛼 plot, which means minimum drag becomes increasingly negative Woods also found due to morphing there was an increase in lift and only a small increase in drag [11] This is supported by Woods experimental data where the increase

in zero lift drag coefficient from zero deflection to maximum deflection was 0.009, which was 59% of their minimum zero lift drag coefficient [11] The FishBAC design has higher lift-drag ratio and can maintain it longer over a range of angles of attacks roughly a range of 9.05°, it does however plateau While flaps have a smaller range of maximum lift-drag ratio, a range of 3.6° [11]

Further work conducted by Woods et al shows variation in the utilization of the FishBAC, where the FishBAC is utilized for the trailing edge [49] seen in Figure 15 Woods expected to see that increasing camber shifts the lift curve up and to the left, increasing lift at a given angle of attack but also lowering the angle at which stall occurs [49] It was also noted that, the amount of additional lift ∆𝐶𝐿 generated for each deflection increment diminishes with increasing deflections [49] From the wind tunnel testing woods confirmed that the FishBAC achieves a higher 𝐿/𝐷 ratio across the entire operating

envelope (-5° ≤ α ≤ 14°) when compared to the 𝐿/𝐷 ratio of the flapped airfoil across the entire operating envelope [49] Woods saw significant increases in efficiency (𝐿/𝐷 ratio), from 160% for low

α (0° ≤ α ≤ 5°) and 27% for high α (α > 10°) The minimum efficiency improvement was 16% at 𝐶𝐿≈ 0.5 and above 200% for higher 𝐶𝐿 [49]

Woods concluded that the FishBAC is more aerodynamically efficient than the flapped configuration

at all angles of attack and lift coefficients [49]

Figure 15 FishBAC utilized as a morphing trailing edge and model parameters [49]

A possible disadvantage of this design is that it could be difficult to repair since the FishBAC rib would tend to be a single piece It would also be difficult to gain access into the internal structure of the wing due to the skin as it is likely the skin would be bonded onto the wing surface The bottom surface would require pre-tensioning to avoid buckling when undergoing deflection There would also be limited empty space in the wing due to design There are also some variations of the FishBAC design, the variations can be in the form of percentage amount of chord that utilizes the FishBAC design, the design of the spine and stringers [50, 51]

The large deflections and continuous compliant architecture of this design is applicable for fixed wing aircraft ranging from small UAV to commercial airliners The design is also suitable for rotary wing applications such as helicopters, tilt rotors, wing turbines and tidal stream turbines

2.3.2 Zig-Zag Wingbox

The zig-zag wingbox is composed of two main parts a rigid part and a morphing part The rigid part is near the wing root and is a semi-monocoque construction like the baseline wingbox of the UAV it consists of two straight spars running the spanwise direction with stressed covers (skin and stringers) and ribs are running chordwise The rigid part contains the fuel tank and transfers loads to the morphing part of the fuselage The morphing part consists of various morphing partitions where each partition consists of two spars located at the leading edge and trailing edge, each spar consists of two hinged beams that have rectangular cross sections The angle between the beams can be varied during actuation which alters the span of the morphing elements Rotation of the beams in each morphing partition with respect to the z-axis (as seen in Figure 16) of the wing allows the span or the length of the partition to be altered The spars are hinged at its two end points and are attached to the adjacent ribs

Figure 16 Top-view of the zig-zag wingbox concept [25]

The ribs transfer the loads experienced from the spars of one partition to the adjacent one which then transfers it to the next partition, until the load is transferred to the inboard rigid part which can then transfer loads to the bulkheads then to the fuselage To avoid the deformation of the flexible skin, the skin is only connected to the ribs and not the spars

The zig-zag wingbox allows 44% variation in wingspan that is a 22% in both extension and contraction, which corresponds with the ideal 22% increase in wingspan [25] The weight of the zig-zag wingbox system was found to be 34% heavier than the conventional wingbox and ribs Hence an increase in weight by approximately 5.7% compared to the conventional counterpart Approximately a 5.5% increase in endurance without factoring the weight of the flexible skin, hinges, clamps and actuation components [25]

An advantage of this concept is the design of the flexible morphing skin The flexible morphing skin is

a sandwich panel skin which is comprised of tensioned elastomeric matrix composite covers that are

reinforced by a zero Poisson’s ratio core The elastomeric matrix composite is usually made up of a silicone or polyurethane elastomer matrix and reinforced with carbon fibre [52] This is similar to the Flexible Matrix Composite (FMC) concept which is covered in section 2.6 In short, the morphing skin has low stiffness in the direction where extension is desired and high stiffness in the other Resulting

in a near zero Poisson’s ratio when undergoing extension

The zig-zag wingbox concept could be more successful on smaller UAVs since a lower number of morphing partitions and smaller structural deformation would be required Ajaj et al also suggests coupling sweep and span for further benefits in the concept [25]

2.3.3 GNAT Spar

Gear driven autonomous twin spar better known as GNATSpar is a spanwise morphing concept, seen

in Figure 17 and Figure 18 The GNATSpar does not use telescopic structures to alter the length of the span but instead uses excess spar length to alter the length of wingspan The spars are longer than the semi-span of the wing, the excess length of spar is stored in the opposite sides of the wing and in the wing-fuselage interface

Figure 17 Schematic of GNATSpar concept [24]

The design allows for a uniform cross-section along the wing semi-span The GNATSpar is a multifunctional morphing concept because it is the primary load carrying structure and it is also the actuation system to achieve span extension The actuation system is located in the wing-fuselage interface, the actuation system consists of a pinon gear placed between two racks corresponding to each of the spars which produces a symmetrical movement on both spars, spur gear (which is mounted together with the pinion gear) and a DC (Direct Current) motor that drives the spur gear via

a worm gear, the spur gear drives the pinion gear which in turn drives the racks

The GNATSpar is covered by a flexible elastomeric skin, to allow span variations whilst still being able

to maintain the aerodynamic profile of the wing Some of the ribs are bonded to the flexible skin this allows the skin to deform uniformly when the spar alters its length The GNATSpar is self-locking due

to the low lead angle of the worm gear, this results in no actuation energy required to overcome the flexible elastic skin loads to keep the spar in its desired length

GNATSpar allows up to 25% extension in wingspan, which reduces induced drag and increases flight endurance [6] The GNATSpar is structurally superior to conventional telescopic spars Due to the

additional wingspan being stored in the fuselage and other side of the wing This means that the GNATSpar can withstand the increased wing root bending moment loads more than the traditional telescopic spar wing designs

Figure 18 Rack and pinion actuation system for GNATSpar [6]

An advantage of this design is that the spar is not split for each wing resulting in a stiffer structure A disadvantage of the design is the strict manufacturing tolerances in the joint of the GNATSpar This issue was addressed, however a rotation of 5° of components could still occur at the joint [6] Another issue is the large actuation force required for the flexible skin covered wing The design also suffered from non-uniform airfoil across the wingspan when undergoing morphing Which was due to the non-uniform expansion nature of the flexible skin due to Poisson’s contraction Overall Ajaj et al, found that the design was both a simpler design and weighed less than a traditional telescopic spar design [6]

2.4 Morphing Wings in Industry

Morphing wings have yet to be introduced into commercial use due to the low maturity level of the technology Research and development projects have been conducted for example NASA’s Mission Adaptive Digital Composite Aerostructure Technologies (MADCAT) team in collaboration with various universities and Flexsys

Flexsys has been testing and developing their flexfoil compliant wing to replace conventional continuous trailing edges The flexfoil is capable of deflections from -9 to +40 degrees, span-wise twist and high response rate (50 degrees per second) The flexfoil can be retrofitted to existing aircraft, flaps and sub-flaps capable of ±10 degrees of camber deflection and span-wise twist The flexfoil has been tested on a Gulfstream III business jet [53, 54]

non-In 2015 a wing morphing project by NASA in collaboration with the Air Force Research Laboratory (AFRL) and FlexSys Inc conducted successful flight testing with morphing wings 22 test flights were conducted in six months with the experimental Adaptive Compliant Trailing Edge (ACTE) Control surfaces, seen in Figure 19

Figure 19 Flexsys' Flexfoil deflected [22]

The ACTE offers significant improvements over conventional flaps, ACTE technology can be retrofitted

or used in new designs, ACTE reduces wing structural weight, improve aerodynamic performance, improve fuel economy and generate less noise than conventional flaps [55] The testing was conducted on large scale aircraft The flight test consisted of setting control surfaces at flap angles from -2° to 30° The tests were conducted at a single fixed setting to gather incremental data and to mitigate risk even though ACTE flaps are designed to morph throughout the entire range of motion

In 2016 and ongoing, NASA’s MADCAT team have been developing a morphing wing, the wing as seen

in Figure 20 is made of building-block units made of carbon fibre composite material The building blocks are assembled into a lattice/arrangement of repeating structures; the way the blocks are assembled determines the way they flex The wing morphs by using actuators and computers The project is being funded by ARMD’s Transformative Aeronautics Concepts Program under Convergent Aeronautics Solutions project The wing has been tested and further investigation is being conducted

Figure 20 Composite Cellular Material Morphing Wing [56]

It is unlikely for morphing wings to be commonplace in the aviation industry soon, due to the low maturity level of the technology as ongoing research and development is still being conducted as well furthermore the technology must be able to pass the regulations set by the regulatory body (which is dependent on where the aircraft will be potentially manufactured and operated)

2.5 Morphing Wing Concept Selection

From the observed literature, the two most common methods of morphing were camber morphing (specifically FishBAC camber morphing) and wingspan morphing The advantages and disadvantages

of the two methods were considered and were examined in sections 2.1.1.1 and section 2.1.2.1 Both methods share similarities with the other such as, increase in lift for a small decrease in drag, both can increase endurance and range of the aircraft and both can conduct a roll moment asymmetric

morphing Although both methods share similarities, wingspan morphing does not seem to have as a large benefit on endurance and range as examined by Ajaj et al, where maximum increase of 6.5% of endurance was observed for 35% increase in wingspan [24] This increase in endurance was likely to

be less than the stated 6.5% when increased weight of the morphing system is considered As of the time of this research was conducted, the endurance increases due to FishBAC morphing method has yet to be explored Considering the studies conducted by Woods et al and Ajaj et al, the FishBAC seems

to provide better lift-drag ratio performance than the wingspan morphing method When other factors such as potential for increase in endurance, potential weight of the system and ease of manufacture and reliability, it further suggests that FishBAC morphing could be the better option

2.6 Literature Review on Morphing Skins

There are various types of compliant morphing skins found in literature, common examples include: using an elastomer sheet and a combination of structural actuation components and an elastomer skin [57, 58, 59, 60, 61]

Some examples of morphing skins, seen in literature are shown in Table 1:

Table 1 Morphing Skin Concepts

Silicone matrix with

2.6.1 Honeycomb and Honeycomb Variants

The morphing wing skin concept involves a flexible honeycomb (or variations of honeycombs) accompanied by a flexible elastomer The honeycomb allows for deformation in one direction i.e has

low in plane stress whilst providing high out of plane stress The flexible elastomer sheet assists in shear loads, keeping the aerodynamic profile and resists the strain experienced by the extension caused by morphing This morphing wing concept is suitable for one dimensional morphing whether that be span, chord or camber morphing, some examples of honeycomb or accordion style are depicted in Table 1

In general, if no constraints are placed on the honeycomb, it can be extended to experience a high Poisson’s ratio [58], showing high elasticity Olympio proposed two possible solutions a hybrid cellular honeycomb and the accordion honeycomb concepts [58] The hybrid cellular honeycomb concept essentially has alternating honeycomb cells where the first is a positive cell and the second being negative and so forth When the hybrid cellular honeycomb undergoes deformation, the positive cells will contract in y-direction whilst the negative will match the expansion but, in the x-direction Whilst for the accordion honeycomb concept it behaves exactly like an accordion when undergoing deformation When examined both concepts showed zero Poisson’s ratio meaning both are suitable for one dimensional morphing [58] The accordion style honeycomb morphing skin was also adapted into a cosine honeycomb by Liu et al [59]

2.6.2 Corrugated structures

Corrugated structures are in general structures that have a series of parallel ridges and furrows an example of this would be a corrugated panel or a corrugated cardboard box Corrugated structures can be suitable for a morphing skin due to their anisotropic behaviour which allows for high stiffness

in the transverse direction of corrugation and compliant in the corrugation direction [64].In the application of one-dimensional camber this means that the corrugated morphing skin would be stiff

in the spanwise direction and flexible in the chordwise direction The corrugated morphing skin would

be a corrugated core material with elastomer face sheets sandwiching the corrugated core material The corrugated core material would provide the spanwise stiffness and chordwise compliance while the elastomer face sheets would provide a smooth aerodynamic surface The corrugated core material could be manufactured via the use of a mould or rapid proto typing depending on the type of core material to be used Dayyani et al used the concept alongside the FishBAC ribs as seen in Figure 21 [60]

Figure 21 FishBAC and corrugated morphing trailing edge concept [60]

2.6.3 Flexible Matrix Composites (FMC)

Flexible matrix composites (FMC) like normal composites are constructed from two or more materials; one generally being an elastic material which is called the matrix material and the second being the rigid, strength providing fibre material which is simply the fibre material An FMC allows for large strains and low in-plane stiffness in the matrix dominated direction whilst the fibre matrix dominated has high stiffness and improves out of plane load carrying capability

Hence FMCs combine properties of two materials, therefore the strength of materials and rules of mixtures for composite can be used to determine the mechanical properties Then elastic modulus for the FMC in the fibre dominated direction is,

𝐸𝑦= 𝐸𝑓𝑉𝑓+ 𝐸𝑚(1 − 𝑉𝑓) The elastic modulus for the FMC in the matrix dominated direction is,

(1 − 𝑉𝑓)𝐸𝑓+ 𝑉𝑓𝐸𝑚And the Poisson’s ratio is,

𝑣𝑦𝑥= 𝑣𝑓𝑉𝑓+ 𝑣𝑚(1 − 𝑉𝑓) Note that 𝑣𝑚 is also another composite material in an aspect since the (fibre is generally hardened using epoxy) hence the Poisson’s ratio needs to be interpolated from the materials used

Where, 𝐸𝑓 and 𝐸𝑚 is the elastic modulus of the fibre and matrix dominated direction respectively Similarly, 𝑣𝑓 and 𝑣𝑚 is the Poisson’s ratio of the fibre and matrix material And the fibre volume ratio

is 𝑉𝑓

This visualization of this concept can be seen in Figure 22Error! Reference source not found In

comparison to regular composites FMC have a higher ratio of fibre/matrix however they use the same formulae for various mechanical properties for example elastic modulus and Poisson’s ratio [52]

Figure 22 FMC fibre orientation for a) span morphing and b) for camber morphing [52]

Matrix materials for FMC tend to typically be silicone, rubber and thermoplastic whilst fibre materials are typically fibreglass, carbon fibre and Kevlar [52, 66]

be true for the honeycomb accordion or cellular honeycomb as the appropriate pattern and positive

& negative cells would need to be determined as well as they are both similar concepts Considering that part of the scope of the research is to have a simple design that can be utilized by a wider aerospace community that may lack resources to implement said design A limitation of morphing skins is that they cannot withstand large aerodynamic loads because of the flexibility required for structural deformations [25] The FMC skin concept was selected over the other concepts as it the simplest concept of the three yet still effective The variables involved in manufacturing a FMC skin are the matrix material, fibre material and the fibre orientation, however it should be noted that the FMC skin was not required to be perfect as its purpose is to provide a smooth and streamline profile for the morphing wing Due to the advancements in fibre materials, the FMC should allow for morphing skin to withstand aerodynamic loads without buckling

2.6.5 Further Investigation into Flexible Matrix Composites (FMC)

Like regular composites the mechanical properties of the FMC are determined by the mechanical properties of all the materials which make up the composite In Kirn’s paper explores the feasibility of FMC through manufacturing and testing of many combinations and variations of the matrix material and fibre material seen in Table 2 [66]

Kirn found that pressure moulding is suitable manufacturing method for FMC however issues regarding misorientation of the fibres are apparent unless preventative measures are taken Kirn secured the fibre orientation through clamping down the fibres whilst laying up the FMC to fix the orientation the carbon fibre as a result few fibres were disorientated Kirn manufactured test specimens with fibre orientations of 0°, ±45° and 90° in the fibre direction related to the applied load and conducted tensile tests The tensile tests determined that a 10% strain can be achieved and sustained for both ±45° and 90° specimens without failure The failure modes seen for ±45° and 90° are delamination of the edges and cracks in the coupons, respectively

Table 2 Material combinations tested by Kirn [66]

3 Motivations and Past Research

The objective of this project is to compare the aerodynamic performance of a morphing wing as a direct replacement of a conventional wing of similar shape For this study, the wing of a T240 Precedent RC model aircraft was used as a case study This is a conventional, non-swept, constant chord wing with a single flap and ailerons the wing design is shown in Figure 2 The flap and ailerons are replaced by full-span morphing using asymmetric morphing for roll control

With regards to the aerodynamic performance comparison, three aspects were compared:

3.1 Wing Concept and Conceptual Design

The morphing wing design was proposed by Vivian et al [69] with further analysis by Scopelliti and Gou [18, 19] Given the requirements that the morphing wing must be of simple design and low cost, the FishBAC design was selected (see Figure 23) and a simple mechanical torque-based actuation system was used The complete morphing wing concept can be seen in Figure 24

Figure 23 Three-view of the initial rib design that connects to the trailing edge [69]

The inherited morphing mechanism was manufactured by using affordable and simple materials such

as balsa wood, aluminium, and polymers through rapid prototyping Due to the simple design and