314 JAXA Special Publication JAXA-SP E 2. WING TRACKING EXPERIMENTS The aim of the experiments is to isolate the wing geometry and kinematics wi

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1 First International Symposium on Flutter and its Application, ANALYSIS OF KINEMATICS OF FLAPPING WING MAV USING OPTITRACK SYSTEMS Matthew NG Rongfa 1, Teppatat PANTUPHAG 2, Sutthiphong SRIGRAROM 3 and Chinnapat THIPYOPAS 4 1, 3 University of Glasgow Singapore, Singapore 2, 4 Kasertsart University, Bangkok, Thailand This paper presents the kinematics of the wing of the ornithopter-like MAV by means of motion-capturing technique (Optitrack). The positions of the marker(s) of one complete oscillation are presented with respect to time in a two-dimensional plane and understand the wing dynamic behaviour of an ornithopter through these graphs. Specifically the wing geometry and kinematics with time in three dimensional space is analysed on the kinematic data of the wing tip path, leading edge bending and trailing edge flap. With the wing geometry obtained, fluid-structure interaction of this flapping wing MAV, by means of CFD. The commercial software ANSYS Fluent is used. Keyword: Motion-capturing 1, Optitrack 2, Kinematics 3, Fluid-Structure Interaction 4, Ornithopter MAV INTRODUCTION Flapping wing flight is one of the most successful and widely used forms of locomotion in the natural world. Approximately ten thousand scientifically described species of birds and nearly a million known insects rely on powered flight as a form of aerial locomotion 1). Unlike conventional aerial vehicles which rely on a rigid wing and propeller to generate lift and thrust respectively, or a rotary wing in the case of a rotorcraft to generate lift and thrust, birds generate lift and thrust by moving their wings relative to their body in an oscillatory (flapping) motion. Aerial vehicles that imitate this oscillatory motion for the purpose of flight are known as ornithopters. Ornithopters offer several potential advantageous performance benefits which includes an increase of propulsive efficiency and manoeuvrability compared to a fixed-wing aircraft. Recent theoretical work concerning minimum induced loss suggests that ornithopters may be able to reach a propulsive efficiency of 85% for micro air vehicles (MAV) 2) and for large ornithopters with wingspans up to 3m, the wing s propulsive efficiency may reach 77% 3). Based on observations of birds, researchers claim that ornithopters are capable of operating with better manoeuvrability compared to fixed-wing vehicles and can be made to hover more easily than conventional aircraft. In theory, this signifies a promising alternative to conventional aerial vehicles. While it is unlikely that humans can engineer ornithopters that perform as well as nature s flyers in the near term, the propulsive efficiency of flapping flight has been shown to meet and possibly even exceed that of more traditional means of propulsion. The goal of this study is to provide an insight to ornithopter flight by analyzing the flapping wing kinematics and flexible wing membrane shapes of a 1.3m wingspan ornithopter by means of motion capture. 1 Matthew.Ng_2014@SIT.SingaporeTech.edu.sg, 2 teppatat06@gmail.com, 3 spot.srigrarom@glasgow.ac.uk, 4 fengcpt@ku.ac.th

2 314 JAXA Special Publication JAXA-SP E 2. WING TRACKING EXPERIMENTS The aim of the experiments is to isolate the wing geometry and kinematics with time in three dimensional space and perform analysis on the kinematic data of the wing tip path, leading edge bending and trailing edge flap. (1) Description of the ornithopter platform The ornithopter studied in this experiment, see Figure 1, is a commercially available 1.3 metre wingspan, rib-stop fabric remote controlled ornithopter kit. Figure 1 illustrates the axes orientation, with respect to the ornithopter, which will be used when describing the kinematic results in the subsequent subsections. The origin for the axes of the graphs will be set at the pivot joint of the wing. Figure 1: Axes orientation with respect to ornithopter (2) Experiment setup and procedure Figure 2: Reflective marker layout on the ornithopter To prepare for the experiments, forty-two 3mm retroreflective hemispherical markers were fitted onto the left wing of the ornithopter. The wing was split up into six sections: the leading edge and five blade elements along the wing membrane. Markers were placed at a 5cm interval along both the leading edge spar and each blade elements. Each blade elements were spaced at 10cm intervals. Lastly two markers were

3 First International Symposium on Flutter and its Application, placed at the wing tip along the membrane. The markers were labelled from w1 to w42 as shown in Figure 2. Blade elements are defined as follows: blade element 1 is made up of markers w12 to w18, blade element 2 is made up of markers w19 to w25, blade element 3 is made up of markers w26 to w31, blade element 4 is made up of markers w32 to w36 and blade element 5 is made up of markers w37 to w40. (3) Kinematic Results a) Wing Tip Path The left wing leading edge support rod is made from carbon fibre. Due to the possibility of the wing spar flexing, the wing tip path (the tip of the rod furthest from the wing root) was expected to travel in an elliptical or figure-of-eight motion along the z-y axis with minimal flex in the z-direction, similar to the wingtip strokes of larger birds. While the plot of the wing tip path on the x-y axis would follow a predictable crescent movement due to the wing flexing in the y-direction. Figure 3 shows the trajectories of the wingtip and its deflection from the zero point of the Y-axis (when the wing is horizontal) as captured by Optitrack motion tracking system. For this motion capture take, the wing oscillated at an approximate 3.93Hz when throttle is at 50%. The upward and downward stroke path had been identified by analysing the translational position of the wing tip marker from the data exported from motive. Figure 3: Graph of trajectories of wingtip for X-Y and Z-Y views The experiment confirmed predictions of the wing bending along the y-axis during one complete cycle of an upward and downward stroke. A single stroke phase can be subdivided into three sub-phases: (1) initial acceleration, (2) mid swing and (3) final deceleration. For deflections along the z-axis, while the wing tip took a figure-of-eight path, there is additional flexing between 0.2m to 0.3m of the y-axis as the wing decelerated at the end of an upstroke to transit into a downstroke. This additional bending by observation of a

4 316 JAXA Special Publication JAXA-SP E frame-by-frame video of the wing flapping at 50% throttle, can be attributed to the elastic energy stored in the wing spar as it travels up and down a stroke, this energy is released when the pivoting arm stops at the end of an upstroke and downstroke causing the wing tip to travel a few additional centremeters. It was also noted that some elastic energy could have been transferred from the stand to the wing when the stand flexed as it resisted the forward thrust of the ornithopter, adding to the z-axis motion. b) Leading edge bending The motivation for analysing the kinematic results of the leading edge bending is due in part to the observation of a large difference in leading edge flex when comparing a single upstroke to a single downstroke (see Figure 4). Figure 4: Side-by-side views of a single downstroke (left) and upstroke (right) of an ornithopter The trajectories of markers w1 to w11 which represented the leading edge of the wing were plotted for the purpose of this experiment. Figure 5 shows the X-Y axis (front view) of the ornithopter at flapping frequency 3.93Hz. Figure 5: Graph of trajectories of leading edge plot for a downstroke (left) and an upstroke (right)

5 First International Symposium on Flutter and its Application, In subsection 3a, the wingtip path showed up to 7cm rearward deflection and 2cm forward deflection along the z-axis due to passive leading edge bending in the downstroke phase. The downstroke phase theoretically being the power phase is the period which most of lift and thrust is generated. In addition to the body force acting on the leading edge due to its acceleration through the air, there is a combined vectored force of lift and thrust also acting on the leading edge. As a result the leading edge experiences high inertial loads causing it to flex. This flex is greatest at mid swing when the acceleration reaches its peak, see Figure 5. A response to high inertial loading which is especially strong near the wing tip increases the local stroke angle. A lag then forms between the wing root and wing tip stroke angle. c) Wing membrane shape Unlike conventional aerial vehicles where the wing profile is generally constant throughout flight, the wing profile of an ornithopter in a single wing-beat cycle is in constant change as it reacts to the changes in airflow and forces acting on the wing. This subsection examines the nominal wing profile by examining blade element 3 which is located at the centre of the wing, at various frames for a single downstroke and upstroke cycle mid-flight. Figure 6: Graph of nominal wing membrane shape along the Z and Y axes of a downstroke (left) and an upstroke (right) Frame 1 to 3 as seen in Figure 6 shows the transition from an upstroke of the previous wing-beat cycle to the start of a downstroke of the current wing-beat cycle. Similarly Frame 11 to 13 shows the transition from the downstroke to an upstroke. These transition phases demonstrate the lead-lag behaviour of the wing between the leading edge and the trailing edge. This lead-lag behaviour contributes to the twisting motion component of the wing. The Movie of the entire experiments can be viewed at this Youtube link: /watch?v=83rvx7pw0ew&feature=youtu.be

6 318 JAXA Special Publication JAXA-SP E 3. AERODYNAMICS OF FLAPPING WINGS With the geometry information obtained in previous section, we do create the 3D CAD model of this flapping wing model and analyze the flow and aerodynamics around this wing models, corresponds to each of the flapping mode. The geometry is created by SolidWorks, and analyse using commercial finite element analysis software ANSYS Fluent Release 13. Figure 7 (left): Ornithopter wing reconstructed from Optitrack systems, Figure 8 (right): Ornithopter wing positions during downwards (powered) flaps, rebuilt in SolidWorks. We truncate the body of the flapping wing model and consider only the flexible wing. The undulating texture of the wing is due to the flexibility of the wing, obtained from tracing point in previous section. We focus attention to the middle of the upward (recovery) stroke and the middle of the downward (power) stroke. Figures 9 and 10 show the pressure contours on the flapping wing, while the wing was in the middle of the upward (recovery) stroke. On the lower surface (figure 9), there is mostly suction (negative) pressure. On the upper surface (figure 10), there pressure is positive, as anticipated. We notice that most force applied mostly in the inboard part of the wing (toward straight edge -which are the truncated body). Figure 12 show the streamline plot, where the trailing edge is obvious, following the motion of the wing. Figures 9 (left) and 10 (right): Pressure contours on the lower (suction) side (figure 9), and the upper (pressure) side (figure 10) of flapping wing, while the wing is in the middle of recovery stroke.

7 First International Symposium on Flutter and its Application, Figure 11 (left): Pressure contours on the lower (suction) side while the wing is in recovery stroke. Figure 12 (right): Streamline around the wing, indicating the strong trailing edge vortex. Figures 13 and 14 show the pressure contours on the flapping wing, while the wing was in the middle of the downward (power) stroke. Likewise, the undulating texture of the wing is due to the flexibility of the wing, obtained from tracing point in previous section. On the lower surface (figure 13), there is mostly positive pressure, indicating the lift force is generated. Also, figure 13 show that the lift is mostly from the inboard part of the wing, where the pressure is more smooth. The outboard part (to the right of the figure), there is more separated and highly unsteady. This shows that the outboard part of the wing is mostly for direction control. Also, it is flexed upwards following the downwards motion of the wing. On the upper surface (figure 14), there pressure is negative (suction), as anticipated. Again, the contour shows that most force applied mostly in the inboard part of the wing, whereas the outboard part of the wing (to the left of the figure), the flow is simply separated with small scales eddies. Figure 15 shows the 3D view of the wing. The wing is under highly twisted position in this power stroke. Figure 16 shows the streamline plot. The extent of the wake behind the wing (area around the upper right part) is obvious. Figures 13 (left) and 14 (right): Pressure contours on the lower (suction) side (figure 9), and the upper (pressure) side (figure 10) of flapping wing, while the wing is in the middle of power stroke. From these figures 9 to 16, we find that the flexibility of wing allows the wing appear to produce more pressure (lift and thrust forces subsequently) at the inboard section of the wing. Whereas in the area at the outboard part of the wing, the wing is more deflected following the flapping motion, and that, the flow is mostly separated and highly unsteady with lesser pressure force applied.

8 320 JAXA Special Publication JAXA-SP E Figure 15 (left): 3D view of this highly twisted flapping wing, while the wing is in recovery stroke. Figure 16 (right): Streamline around the wing, showing the extend of the wake. 4. CONCLUSIONS This the kinematics of the wing of the ornithopter-like MAV by means of motion-capturing technique (Optitrack). We could capture the full motion of the flapping ornithopter at various position and obtain the geometry (texture) of the deformed wing. This gives us opportunity to analyse the fluid-structure interaction behaviour of this flapping wing. The passive deformation of the wing surface follows the motion of the wing and gives more lift and propulsive force. We analyse the flow around this deformed wing, using information of the geometry obtained. The flexibility of wing allows the wing appear to produce more force at the inboard section of the wing. REFERENCES 1) Alexander, D.D.E. and S.L. Taliaferro, On the Wing: Insects, Pterosaurs, Birds, Bats and the Evolution of Animal Flight. 2015: Oxford University Press Incorporated. 2) Hall, K.C. and S.R. Hall, Minimum induced power requirements for flapping flight. Journal of Fluid Mechanics, : p ) DeLaurier, J.D., An ornithopter wing design. Canadian aeronautics and space journal, (1): p