Design of a Butterfly Ornithopter

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1 Journal of Applied Science and Engineering, Vol. 19, No. 1, pp (2016) DOI: /jase Design of a Butterfly Ornithopter Bo-Hsun Chen, Li-Shu Chen, Yueh Lu, Zih-Jie Wang and Pei-Chun Lin* Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan 106, R.O.C. Abstract Research on ornithopters is receiving more attention because they exhibit good controllability, maneuverability, and robustness in the natural environment. Here, we report on the design, fabrication, and experimental validation of the mid-size butterfly ornithopter, which mimics the morphology of a dead leaf butterfly. The wing flapping mechanism can support wings with a span of 565 mm and can flap the wings in a 120-degree range and at a frequency of 5 Hz. A lift force measurement and a particle image velocimetry experiment were performed to validate the performance of the ornithopter. Key Words: Ornithopter, Butterfly, Bio-inspiration, PIV, Robot, Lift Force Measurement, Wing Flapping Stroke 1. Introduction The unmanned aerial vehicle (UAV) has become one of the most prevalent robotics fields in the last ten years. The UAVs mainly include three types: fixed-wing, rotarywing, and flapping-wing (also known as ornithopter). A system with a fixed-wing morphology has better stability and endurance, but it also must be operated in a well-defined environment. A system with a rotary-wing morphology has less constraints for take-off and landing, but it cannot fly in narrow or turbulent environments. In contrast, the flap-wing system, which has a morphology adapted from birds and flying insects, has been proven to be the most robust form and has an unmatchable maneuverability, which motivated the researchers to develop an ornithopter. Biomimetics and scaling effects are two important issues in the research of ornithopters. The scale has a strong relationship with the Reynolds number. The small-size micro aerial vehicles (MAVs) (span less 25 cm, weight less *Corresponding author. peichunlin@ntu.edu.tw This paper is the extension from the authors technical abstract presented in the 1 st International Conference on Biomimetics and Ornithopters (ICBAO-2015), held by Tamkang University, Tamsui, Taiwan, during June 28 30, than 50 g [1]) suitable for insect flying models are expected to perform monitoring or exploring. The Reynolds number of the models is generally lower than 20000, and an unsteady aerodynamic model is often deployed [2]. The MAVs are usually fabricated using MEMS manufacturing technology. Some of the MAVs use piezoelectric materials as actuators to generate a wing-flapping motion, reaching about 100 Hz. The Robotic Fly [3] and Robotic Bee [4] are two examples. On the other hand, some of the MAVs use micro motors to flap the small wings, such as the Delfly [5]. Mid-size ornithopters (a span larger than 30 cm but less than 2 m) have been researched, such as the Smart- Bird [6] and emotionbutterflies [7]. The aerodynamics and system dynamics of the ornithopters on this scale have been studied as well [8], along with the movement control and moving target tracking of ornithopters with image processing [9]. The Reynolds number of middle size ornithopters is about 75,000 to 200,000, and a quasi-steady aerodynamic model is often utilized. The designed ornithopters usually mimic the flying method of birds with a flapping frequency of about 5 to 10 Hz and with a small flapping angle range (lower than 30 ). These ornithopters have a good payload, and they have a high cruise speed and a wider flying area compared to MAVs.

2 8 Bo-Hsun Chen et al. Though the developed mid-size ornithopters have the ability to cruise, they cannot perform hovering in the air due to their quasi-steady aerodynamics. In contrast, the MAVs can perform hovering, but they have a limited payload. This motivated us to investigate the possibility of developing a mid-size ornithopter that can potentially hover and carry the payload. Following our initial presentation at [10], we report on our design of the ornithopter and on our initial performance validation of the fabricated ornithopter. More specifically, the butterfly served as our biomimetic target because its characteristics include a simple wing structure and a low wing flapping frequency, which are easier to implement in the ornithopter. Note that a small-size butterfly ornithopter [11] and a study on the aerodynamic force between various butterfly wing types [12] have been reported; however, these ornithopters are not power autonomous. Section 2 describes some of the flying characteristics of butterflies observed from nature, which can offer a foundation for a new flying aerodynamic model. Section 3 presents the design concepts of the ornithopter as well as the mechanism design, mechatronic system, and fabrication of the ornithopter. Sections 4 and 5 discuss the setup and results of the lift force experiment and particle image velocimetry (PIV) experiment on the designed ornithopter. Section 4 concludes the work. 2. Learning from Nature The flying model of butterflies, by observation, is neither similar to bird models nor to small flying insect models. The morphology of butterflies is different from other flying insects, which have relatively small wings adapted to a high flapping frequency in the range of Hz. In contrast, the flapping frequency of butterflies is about 10 to 12 Hz, which is lower than insects but higher than birds. Compared to other flying insects, butterflies have a relatively large wing surface, which weighs 1/10 of the total weight [13]. The large wing flapping angle range (about 120 ) compared to flying birds (about 60 ) helps to generate strong vortex rings for flying. In addition, the front wing and back wing overlap and confine the degree of freedom (DOF) of flapping movement [13,14]. The large wings are anisotropic and have good inflexibility, which increases the lift force by increasing the fluid speed during flight [15]. Moreover, the clap and fling movement caused by the wing phase difference from the leading edge to the trailing edge also increases the aerodynamic force [16]. Furthermore, the attached leading edge vortex (LEV) helps butterflies reach the delayed stall [17]. These special characteristics enable hovering in a high angle of attack (AOA) and flying at a low speed (1.44 m/s) [18]. The detailed flight research data of the dead leaf butterfly (Kallima Inachus) reveals that the fling effect and clap effect are important in the beginning of the downstroke and the ending of the upstroke, respectively, to help butterflies fly forward [18]. Research conducted by Fei and Yang suggests that in the downstroke process, the large wings of butterflies shed a series of inner rotation vortex rings composed of the leading edge vortex, the trailing edge vortex (TEV) and the tip vortex, as shown in Figure 1 [19], to generate the main lift and force. In the upstroke process, the butterflies produce outer rotation vortex rings to generate impulse force and transform the negative lift force to positive by swinging their bodies appropriately. This is also why the dead leaf butterfly flies unevenly and slowly in a high AOA. The computational fluid dynamics (CFD) simulation indicates that the butterflies can generate a lift force of 60% weight if the cruise speed is zero [19], which further suggests that the butterfly robot with a similar morphology may be able to hover in the air. Instead of mimicking other birds or flying insects, we designed a middle-size ornithopter based on the morphology of butterflies for several reasons. First, the insectlike high flapping frequency of the wings is difficult to achieve in artificial systems due to the limited motor power density. Thus, butterflies with a relatively low flapping Figure 1. The vortex ring of butterflies.

3 Design of a Butterfly Ornithopter 9 frequency (i.e., 10 Hz) were more feasible. Next, the wing morphology of butterflies is simpler than that of birds or insects, and the wing flapping motion of butterflies is more straightforward than those of birds. Third, butterflies are the only species whose wing span is large enough to create a middle-size ornithopter that hovers in air. Lastly, the flying mechanism of butterflies (i.e., a 10 Hz flapping frequency, a high flapping angle, and a large wing surface) has rarely been studied in the research of ornithopters thus far. These observations prompted the exploration of the concept of a bio-inspired robot butterfly. 3. Design and Fabrication of the Butterfly Ornithopter The dead leaf butterfly was used as the biomimetic goal due to the availability of the associated research data. Furthermore, the morphology of the butterfly wing resembles a piece of a leaf, which is easier to fabricate. Similar to a butterfly, the wing flapping range was set to around 120, which is considerably large compared to that of other common ornithopters available in the market (about 60 range). The flapping mechanism was expected to endure a huge force variance when the butterfly flaps in a high frequency range (5~10 Hz) compared to other middle-size ornithopters. The mechanism is shown in Figure 2, which is modified from a mechanism reported in [20]. Because it is only composed of slender bars and ball joints, it is very light. By changing the length ratios among the linkages, the range and neutral point of the flapping angle can be adequately adjusted. The ball joint at the upper end of the red bar in the original design was replaced with two perpendicular rotational joints because the ball joint does not have a sufficient angle range as desired. The linkages were made of Aluminum-alloy or Teflon (Polytetrafluoroethene). The brushless DC motor (Hobbymate HB KV) was used as the actuator to drive the wing flapping mechanism. The maximum power of this motor is 220 W, and it has 3900 rpm/volt under no-load condition. The input voltage to the motor is within 12 V. The reduction ratio of the gearbox was set to 5/196 based on the characteristics of the flapping frequency and the torque. The plastic gears were used to reduce the inertia and the weight. The housing was acrylic. The gearbox is shown in Figure 3. The membrane and veins of the wing determine the dynamic interactions between the butterfly and the environment. The wings of the dead leaf butterfly are large compared to its body, and it is anisotropic because it is composed of a thin membrane and many veins with different thicknesses. The wing membrane and the veins of the robot butterfly are made of kite clothes and celluloid sheets, respectively. The vein is stiffer than the membrane as they are in real butterfly, and the pattern of the veins of the ornithopter are arranged according to those of the real butterfly. The overall appearance resembles that of the butterfly, as shown in Figure 4. The electronic system was mounted on the fuselage. Figure 2. The 5-bar wing flapping mechanism: (a) Illustrative diagram. Different colors indicate different linkages. The rotation directions of the revolute joints marked in grey are along the green and orange linkages, respectively. Those marked in white are perpendicular to the paper. (b) Photo of the wing flapping mechanism. The same parts shown in (a) and (b) are linked by the gray lines for clear presentation. Figure 3. The motor and the gearbox transmission system.

4 10 Bo-Hsun Chen et al. Figure 4. The wing of the real dead leaf butterfly [21] (a) and the butterfly ornithopter (b). A customized circuit board was designed, which contains a microprocessor (Sparkfun Teensy3.1), an inertia measurement unit (IMU GY80), and the associated wire ports. In addition to controlling and driving the motor, the microprocessor is also utilized to record the wing configuration by measuring the voltage data of the potentiometer mounted on the joint of the flapping wing bar as shown in Figure 2(a). In order to fit the geometric constraint, the original shaft of the potentiometer were removed and replaced by the shaft of the revolute joint which connects with the flapping wing bar. The output of the load cell utilized in the experiments was also recorded. The fuselage was made of ABS by a 3D printer for its complex spatial configuration. The body houses the wing flapping mechanism, the power transmission system, and the electronic devices. The photo of the entire ornithopter is shown in Figure 5. The wing span is 565 mm, the wing chord length is 430 mm, the fuselage length is 160 mm, and the weight is 384 gw. (Transducer TechniquesÒ, DPM-3 Load Cell Display and MLP-50 Load Cell) was mounted at the top of a ladder, and the butterfly ornithopter was connected to the load cell through a long rod and a revolute joint, which provided enough space for wing flapping and enough freedom for the body wobbling. This setup can accurately simulate the hovering configuration of the dead leaf butterfly. If the ornithopter is rigidly mounted to the rod: (i) the COM of the ornithopter should be carefully mounted below the end of the rod, or the moment created by the gravity force would initiate uncertain dynamics to the system; (ii) the lateral force (i.e., thrust force) would trigger lateral vibration of the rod, which further deteriorates the overall dynamic behavior of the ornithopter. In contrast, when a revolute joint is mounted in between as shown in Figure 6(c), the COM of the ornithopter would self-align to the configuration directly below the end of the rod, and the lateral vibration would be transformed to the rotational motion of the ornithopter. Some small lateral motion may remain, but the load cell is insensitive 4. Experiment Method 4.1 The Lift Force Measurement It is important to analyze the aerodynamic forces of the ornithopter. A direct measurement method usually requires attaching the fuselage of the ornithopter to a load cell attached to a fixed end and then using a wind tunnel to produce incident flow for creating the forward flying condition [22]. An indirect measurement method is commonly used to calculate the acceleration of the ornithopters by analyzing sequential images taken from a high speed camera [11]. To accurately record the lift force of the ornithopter while its body wobbles, a new measurement setup was designed, as shown in Figure 6(a). A 1-DOF load cell Figure 5. The side view (a) and bottom view (b) of the robot ornithopter.

5 Design of a Butterfly Ornithopter 11 to the force in this direction, and the motion would be damped by the tape mounted around the middle of the bar shown in Figure 6(b) as well. Because the weight is larger than the thrust force, the rotational motion can be neglected in the analysis. In this case, the force equilibrium in the vertical direction reveals that the vertical force measured by the load cell is equal to the lift force generated by the ornithopter. The reading of the load cell was connected to the microprocessor on the ornithopter so that the force data and the wing configuration data could be synchronized and the data could further be exported to a computer through a USB port. When the ornithopter was at rest, the load cell was calibrated to zero, so when the ornithopter was, flapped its wings, the reading of the load cell directly represented the lift force. Four sets of experiments were conducted. In each experiment, the ornithopter flapped its wings with a flapping frequency of 5 Hz for 5 seconds and then stopped for 5 seconds. 4.2 The PIV Experiment The PIV experiment setup is shown in Figure 7. This experiment was conducted to observe the fluid field, and the flap frequency of the ornithopter was set at 3 Hz. A high speed camera system (Phantom v7.3, Lighthouse Photonics Sprout-G-12W Laser) was utilized to record the trajectory of the aluminum powder with a diameter of 3 m, spreading in a 90 cm height, 90 cm length, and 45 cm width acrylic chamber for observation. The butterfly ornithopter was fixed at one side of the chamber. During the experiment, only one wing of the ornithopter was mounted to eliminate the effect caused by the other wing. To observe the 3D vortex ring, both the top view and side view of the ornithopter were recorded in different sets of experiments. The wing flapping period was used Figure 6. The lift force measurement: (a) (b) The experiment setup and (c) The simplified free-body-diagram. Figure 7. The PIV experiment setup: photo (a) and the illustrative drawing (b), which shows locations of the light pages (yellow planes) and the direction of the laser illumination (black half circles).

6 12 Bo-Hsun Chen et al. to synchronize the images captured from the different sets of experiments. The image processing software (Insight3G) and the post-processing software (Tecplot) were then utilized to generate vector files and to plot the streamlines, respectively. 5. Results and Discussion 5.1 The Lift Force Measurement The experiment results of the lift force measurement are shown in Figure 8. The 0 degree of the wing angle was set at the direction orthogonal to the fuselage, and the angle increased when the wing flapped down. As shown in the figure, at the downstroke stage, the lift force was positive, as expected. At the upstroke stage, the lift force decreased to negative. The results reveal that the wing flapping mechanism of the dead leaf butterfly applied to the ornithopter can produce a positive and considerable average lift force of gw. Although the ornithopter did not wobble much during the experiments because the weight of the wings (7% of the total weight) is much smaller than that of the fuselage, the correct lift force pattern suggests that the reported design of the ornithopter can be a realistic approach in fabricating a middle-size ornithopter. The improvement lies in the weight reduction and the flap frequency increase. A characteristic comparison between the real butterfly and the ornithopter was also conducted. Figure 9 shows the comparison of the flapping angles, and the figure reveals that the mechanism can successfully reproduce the profile of the flapping wing. The real butterfly has a slightly slower reversal of wing rotation at the bottom of the flapping motion because the wing of the real butterfly has considerable flexibility and more time is required to invert its rotation direction. Note that the potentiometer is a resistance-based sensor which has no dynamic properties, so it can respond to real wing configuration without any delay. The wing configuration may also be captured by external measurement device such as a high-speed camera. However, the measurement is also quite challenge. For example, when the ornithopter flaps its wings, the body vibrates (wobbles), so the wing configuration measured by the camera is actually the projected wing configuration. Only the multi-camera system such as VICON or Motion Analysis can reconstruct the true wing configuration. Figure 10 shows the comparison of the lift force between the simulation of a butterfly flying forward and the ornithopter. Although in the simulation the butterfly cruises at a low speed, some observation can be made. In the downstroke process, the lift forces in both cases were positive, and they increased to their maximum values at about 0.25T. Then, they decreased to zero. The largest discrepancy lies in the fact that the ornithopter is unable to generate a large lift force during its downstroke phase, which is likely due to the insufficient wing stiffness. In Figure 8. The lift force measurement: (a) The lift force and flapping angle versus a normalized period. (b) The lift force versus the flapping angle. Figure 9. The flapping angle of the real dead leaf butterfly (a) and the ornithopter (b). The subplot (a) is redrawn from the data reported in [18].

7 Design of a Butterfly Ornithopter 13 Figure 10. The lift force of the simulated butterfly (a) and the ornithopter (b). The subplot (a) is redrawn from the data reported in [19]. the middle of the upstroke, the simulated lift force becomes positive and then decreases because the real butterfly wobbles its body backward and transfers the upward force to the downward force, while the ornithopter barely wobbles. Thus, the lift force becomes negative and reaches the minimum. The comparison shown in Figure 10 also involves the scaling effect. The Reynolds number of the butterfly ornithopter was computed as (1) where is the air density (1.293 kg/m 3 ), L is the half span length (about 0.3 m), U is the wing tip velocity (U = r =(2 f)*0.3 = 9.42 m/s), and is the air viscosity ( kg/m-s). Thus, the Reynolds number is about , which is one-order larger than that of the real butterfly (10 3 ~10 4 ). The maximum lift force of the ornithopter is 251 gw, and the average lift force is gw. In contrast, that of the simulated butterfly with a weight of 0.4 gw is 2 gw, and the average is 0.42 gw [19]. The span of the ornithopter is about 56.5 cm, and that of the real butterfly is 6.67 cm [18]. The vortex ring theory model reveals (2) where V is the fluid velocity around the vortices, is the circulation around the vortices, D is the distance between the two centers of vortices, A v is the average area of the vortices, is the density of air, M is the momentum, T is the period time, and F is the average lift force or propulsion force [18,23]. The aerodynamic force is proportional to the characteristic length to the fourth power. Because the ornithopter is 8.5 times the characteristic length of the butterfly, it would become (8.5) 4 = 5220 times the force strength of the real butterfly. The experiment results reveal that the maximum and average lift forces of the ornithopter are times and 104 times than those of the real butterfly, respectively. The value is much less than estimated. The discrepancy is likely due to the slow flapping frequency of the ornithopter (i.e., 5 Hz, half of that of the butterfly) and to the different range of the Reynolds numbers. Thus, the aerodynamic model may not be suitable for the mid-size ornithopter. 5.2 The PIV Measurement The aerodynamics model and the results from the PIV experiments were utilized to explain the mechanism of the lift force generated in the ornithopter while it flapped the wings. For convenience, six images were extracted for PIV analysis per motion period located at time stamps t = 0 T, t = 0.2 T, t = 0.4 T, t = 0.6 T, t = 0.73 T and t = 0.86 T, respectively. Figure 11 shows the results of PIV experiments. At t = 0 T, the side-view image reveals that a clockwise TEV is generated from the upstroke motion at the end of the previous period, and a counterclockwise vortex behind the ornithopter is the TEV generated in the previous period from the downstroke motion. The front view image reveals that a large clockwise vortex under the ornithopter is generated, so there is an inner rotation vortex ring generated from the TEV in the previous period as well. A jet flow backward along the back of the fuselage caused by the fling effect can also be observed. At this stage, the rotation of the vortex ring attached to the wing is not consistent, so the measured lift force is small and even negative because of the oscillation of the fuselage. At t = 0.2 T, the side-view image reveals that the TEV is still in a clockwise direction because of the large phase delay between the wing leading edge and trailing edge, and the vortex behind the ornithopter is further left behind. The front-view image shows an obvious vortex under the ornithopter, implying that an inner rotation vortex ring is swung down from the wing. Thus, the lift force becomes positive and increases gradually in this instance. The LEV is generated but not obvious. At t = 0.4 T, the TEV begins rotating in a counterclockwise di-

8 14 Bo-Hsun Chen et al. Figure 11. The results of the PIV experiments. The symbols (i) (ii) (iii) represents the side view, front view, and side view, respectively. The green solid, blue rectangle, and the brown line represent the wing, the fuselage, and the wing flapping bar, respectively. The small vectors and the light orange lines are the fluid velocities and the stream lines. The red lines represent the flow and vortices.

9 Design of a Butterfly Ornithopter 15 rection, the LEV becomes obvious, and the induced flow appears. The front view image indicates that the tip vortex appears, and the counterclockwise vortices are generated from the tip vortex and the TEV under the ornithopter. An inner rotation vortex ring is swung down from the wing edge. In this instance, the vortices become very obvious and strong, so the lift force reaches its maximum value. At t = 0.6 T, the downstroke process ends, and the wing bar stops and begins the up process, so the LEV diverges and becomes larger and weaker. The large vortex behind the ornithopter is strengthened, which is caused by the induced flow flowing back along the fuselage. The front view image reveals that the tip vortex is still obvious but begins to diverge, and the vortex ring under the ornithopter diverges. At t = 0.73 T, the ornithopter is in the upstroke process, and the TEV disappears. The characteristic of the upstroke is not obvious. A counterclockwise vortex is generated by the tip and swung down in the downstroke process. At t = 0.86 T, a small counterclockwise LEV appears. A pair of inner rotation vortices are generated on the bottom side of the ornithopter. The vortex ring is swung forward from the back-end of the wing because of the phase delay. The lift force becomes negative because the vortex ring disappears, and the pair of wings rise. Next, the upstroke ends, and the process enters the next period, beginning from t = 0 T again. The clap effect does not clearly appear in the process because the rotational acceleration of the wing bar is not large enough at the end of the period and because the phase delay of the ornithopter is much larger than that of a real butterfly. Thus, the PIV results of the ornithopter do not resemble those of a real butterfly. In addition, the characteristic of the upstroke is not obvious because the light pages are blocked. Note that the PIV experiments only caught the 2D movement of the particles, so though the information is extremely valuable, the information is not sufficient to reconstruct the 3D fluid field. Thus, the PIV results were analyzed in a qualitative manner but not in a quantitative manner. 6. Conclusions and Future Work The observation from nature and the literature review motivated us to use butterflies as a bio-inspired target to make a middle-size hovering ornithopter. By mimicking the movement and morphology of the dead leaf butterfly, the butterfly ornithopter composed of wing flapping mechanism, a power transmission system, fuselage, an integrated electronic board, and a wing membrane was designed and fabricated. The flapping mechanism of the ornithopter can generate a wing motion resembling that of real butterflies. The results of the lift force experiment confirms that the ornithopter can indeed generate a lift force while the wings flap, and the results of the PIV experiment further provide an explanation of the lift force generation. The current ornithopter can only generate about 1/8 the lift force of its weight; however, this result indeed serves as an important and adequate first step toward the design and fabrication of a butterfly ornithopter capable of hovering. We are in the process of reducing the weight and revising the wing material and composition to make the ornithopter much lighter with a stronger wing efficiency. In the future, we plan to increase the DOFs of the ornithopter to incorporate more functions, such as flying forward, ascending, and turning. Acknowledgements The authors would like to thank Y. H. Fe and Dr. J. T. Yang for several useful discussions and their help on PIV experiments. This work is supported by the Ministry of Science and Technology (MOST), Taiwan, under contract: MOST E MY3 and MOST C M. References [1] Shyy, W., Lian, Y., Tang, J., Dragos, V. and Liu, H., Aerodynamics of Low Reynolds Number Flyers, New York: Cambridge University Press (2007). doi: / CBO [2] Mueller, T. J., Fixed and Flapping Wing Aerodynamics for Micro Air Vehicle Applications, AIAA. Progress in Astronautics and Aeronautics (Book 195), Reston, VA2001 (2001). [3] Ma, K. Y., Chirarattananon, P., Fuller, S. B. and Wood, R. J., Controlled Flight of a Biologically Inspired,

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