An Experimental Study on Effect of Wing Geometry of Hummingbird-like Flapping Wing in the Hover Yanghai Nan1*, MatČj Karásek2, Mohamed Lalami1, Hussein Altartouri1 1 Active Structure Laboratory, Université Libre de Bruxelles, Belgium 2 Faculty of Aerospace Engineering, Delft University of Technology, Netherlands ABSTRACT Wing, as one of important components of a flapping wing Micro Air Vehicle (MAV), is directly related to aerodynamic performance such as lift force and torque around body. Therefore, design of efficient flapping wing has become an interesting research topic recently. In this paper, the aerodynamic performance of flexible micromembrane flapping wing under the condition of hovering was investigated to ascertain the best combination of wing geometric parameters. For this purpose, a flapping wing mechanism and an experiment setup were built to measure the aerodynamic forces and power usage of the flapping motion. Then four different families of wings were fabricated. The lift and power usage was measured at different flapping frequencies with different shapes of wings. In order to investigate the effects of wing geometry, only one parameter of wing was varied at a time. The results indicate that the aspect ratio and wing surface have a critical impact on the force production and wing efficiency. The best performance was obtained with a trapezoidal wing with a straight leading edge; both its shape and aspect ratio ( ܴܣൎ ͻǤ͵) are similar to a typical hummingbird wing. 1 INTRODUCTION 1 MAV is a class of unmanned aerial vehicle (UAV) with restricted size and remote or autonomous characteristic, which can be utilized for civilian and military application such as terrain reconnaissance, video surveillance or search and rescue missions after earthquakes, etc. The bio-inspired MAVs, like hummingbird and insect robots, are dramatically developing during recent years. Many researchers are exploring and researching them, for instance Berkeley micro mechanical flying insect [1], Harvard robotics insect based on Diptera [2], Beetle-like flapping wing MAV [3] and Hummingbird-like robot [4, 5]. All these MAVs can perform hovering flight; the necessary lift force is produced by a pair of small כEmail: Yanghai.Nan@ulb.ac.be flapping wings. Besides, another important advantage of flapping wing MAV is that they have high maneuverability and capability to sustain hover flight in constrained space. This is because flapping wing with small scales may offer some unique aerodynamic advantages compared with traditional fixed and rotary wing modes of propulsion [6, 7]. Flapping wing MAV consists of different components such as a flapping mechanism, a pair of flexible wings, a micro control system, a motor and a battery. The flexible wing is one of the most important subsystem of flapping wing MAV since it directly relates to the aerodynamic forces and torques around the body. In nature, we can find surprisingly many different wing materials, wing shapes, sizes, configurations, and components, even kinematics of the wings. Therefore, wing design becomes a significant challenge because it influences the unsteady aerodynamics. In other words, how to design the wing and how to keep the wing geometry during flapping motion are considerably important. Flapping wing is required to be light, strong and fatigue resistant, to be able to properly flap during flight. Thus, the wing material and wing geometry plays an important role. Numerous studies were done on the mechanism and aerodynamics of flapping wings, employing both unsteady flow simulation methods as well as experiments. These works were developed based on the natural species such as dragonfly [8, 9], manduca sexta [10, 11] and other flying insect like beetle [3]. They analyzed the flapping wing kinematics. The effect of wing geometries was studied as well [12]. The optimization of flapping wing based on simulation model was reported by two categories wings: symmetric wings and asymmetric wings. The result shows that best performance for the wing shape has nearly straight leading edges with more surfaces outboard [12]. However, we note that these works focus on rigid wing rather than flexible wing. This is because that the aeroelastic interaction between the wing and surrounding fluid can be neglected and the overall complexity of the problem is reduced for the rigid wing [13]. Moreover, it is difficult to accurately calculate the aerodynamic loads generated by a flapping wing since the 3D transient computational fluid dynamics (CFD) solution with low Reynolds numbers is necessary [14]. Although some results from numerical studies can capture experimental behaviors, any conclusion cannot be extrapolated by these numerical studies [15, 16]. Many researchers have begun to pay attention to the flexible wings due to its important role in the aerodynamics of flapping flight, but there exist few studies to research optimization of flexible wing, especially for the hover situation. Therefore, experimental study of the flexible membrane flapping wing under condition of hovering is necessary. The designed membrane flapping wing has bending and torsional flexibility. It can be deformed by varying wing speed, time-dependent pressure, and local acceleration of the wing surface. These dynamic quantities decide the instantaneous wing deformation. That is, lift force is determined by the dynamic coupling between the wings and surrounding air. Changing wing deformation will influence the aerodynamic and subsequently the wing performance. Therefore, the study of flexible membrane wing geometry is considerably meaningful. In this paper, the objective is to optimize the wing geometry such that the wing produces maximal lift per power delivered to it. The effects of wing geometry, for example, wing length, aspect ratio, wing surface, wing chord, are investigated by comparing mean lift force over one wingbeat while only one parameter is changed at a time. Each mean lift force is achieved by three measurements. Four different families of wings are fabricated. The aerodynamic performance of these wings for hovering situation without wind was experimentally studied by mean lift force and electric power. The generated mean lift is measured at different geometry wings, different flapping frequencies and powers. The frame and links, which are made of material Digital ABS, are built by an Objet 3D printer. The links are connected by aluminum and steel rivet. Wing bars are made of carbon material. The flapping frequency is controlled by the input voltage of the DC drive. The maximum flapping frequency is approximately 26 Hz; the maximum amplitude of flapping angle is about up to 180° at the tips, including the leading edge bar deformation. Fig.1: The flapping wing mechanism. 2.2 Wing parameters Here the aerodynamic performance of a flapping wing is studied to optimize wing geometry. In this paper, mean lift force is used to estimate the wing performance. Therefore, according to quasi-steady aerodynamics, the mean lift force is function of flapping frequency (݂), air density (ߩ), wing surface ഥ ). (ܵ) and mean wing speed of center pressure (ܷ The geometric parameters considered in our investigation were wing aspect ratio ሺܴܣሻ , wing length ሺܴሻ , wing surface, wing tapper ratio (see Figure 2). Wing length is the distance from wing root to its tip (i.e., length of single wing). Wing surface refers to the planform area of each wing. ܴܣis computed from wing length and wing area as ܴܣൌ ʹܴଶ Τܵ. Taper ratio refers to the ratio between wing tip chord ሺ ்ܥሻ and wing root chordሺܥோ ሻ. Last, wing planform shape refers to the actual shape of the wing (i.e. rectangular and right-angled trapezoidal). 2 FLAPPING MECHANISM, WING PARAMETERS AND WING DESIGN 2.1 Flapping mechanism In this section, flapping mechanism, aerodynamic parameters, wing geometry parameters and flexible wing design will be studied. The flapping wing mechanism was firstly designed based on a slider crank and four-bar linkage which was presented in [5]. Slider crank is used to generate a low amplitude harmonic motion, while four-bar linkage is used to amplify the motion to the desired amplitude. The flapping wing MAV prototype, shown in Figure 1, is composed of three components: the driving motor (Faulhaber 0824), flapping mechanism and wings. Fig.2: Flexible membrane wing structure. The mean lift force of a pair of flapping wings can be written as: ଵ ଶ തതത ሺʹܵሻܷ ഥ (1) ܨഥ ൌ ߩܥ ଶ With the assumption of flat and rigid wing, the center of pressure is placed at the mid-length of wingܴ ൌ ܴΤʹ. Therefore, the mean wing velocity of center pressure can be expressed as ഥ ൌ ʹȰ݂ܴ ൌ Ȱ݂ܴ ܷ (2) Based on the definition of the wing ܴܣ, the cycle average lift force of a pair of wing can be rewritten as ଵ തതത ܴܣሺܵȰ݂ሻଶ ܨഥ ൌ ߩܥ (3) ଶ തതത is the mean lift coefficient. whereܥ The formula (3) indicates that the mean lift force depends on wing lift coefficient,ܴܣ, wing surface, wing flapping amplitude angle Ȱ and flapping frequency. In this work, we just consider the flapping frequency, wing surface, ܴܣand power to estimate the wing performance. The flapping amplitude is determined by the flapping mechanism. The average lift coefficient cannot be constant because real wing deforms more with condition of higher aerodynamic loads and higher frequencies, etc. wing root edge. The sleeves can rotate freely around the two bars: leading edge bar and wing root bar. The leading edge bar is connected to the output link of flapping mechanism, while the root bar is inserted into the frame which is aligned with the shoulder. The carbon-fiber-reinforced polymer (CFRP) bands are used as stiffeners to keep wing geometry when flapping. Since the angle between the sleeves is greater than the angle between the two bars, the wing becomes cambered and twisted after the assembly. In order to improve the durability of the two sleeves, the Icarex material is glued on the wing root top corner and wing root sleeve marked by red color shown in Figure 3. Therefore, the strength of wing is increased significantly, making it more suitable for large amplitude flapping and high flapping frequency. 2.3Wing design As we know, natural birds have many degrees of freedom to change wing motion and wing geometry as they maneuver or transition from one flight mode to another, which is, for now, impossible in the mechanical flappers. The only simple way is to change wing kinematics via four-bar or other mechanisms which is driven by rotary DC motor. Therefore, changing the structure of wing may affect the aeroelastic performance of the wing. This is because the generation, growth rate and shedding of the unsteaday leading edge vortex are completely changed over the whole wing [13]. Thus, the optimum wing structure is possible to find by testing different wing designs. Here the aeroelastic wings are designed to understand how wing flexibility affects the aerodynamic load. In this study, the wing motion is constrained between the wing leading edge and wing root edge. In other words, the desirable configuration wing is obtained by passive elastic property. Therefore, the desired lift force can be achieved by changing elastic performance of wing such as wing geometry and stiffener. In this paper, the designed wing is inspired by the Nano-hummingbird [4] and natural hummingbird. The components of the wing include a carbon fiber frame, a carbon fiber stiffeners, and membrane (polyester membrane and Icarex membrane), shown in Figure 2. The wing is made of a 15-ߤ݉ -thick polyester membrane. Polyester membrane was observed to have desired properties such as light weight, flexibility, strength, fatigue resistance. It has two sleeves, one is connected to the leading edge and another is close to the robot body which is called Fig.3: Assembled flexible membrane wing structure. Four families of wings were designed and tested which are presented in Table 1(see Appendix). The wing is developed from rectangular wing to trapezoidal wing through increasing the surface of wing root. The wings of family 1 are rectangular wings which are inspired by the Nano-hummingbird [4]. The family 2 and 3 wings are right-angled trapezoidal shapes to mimic natural hummingbird wings. Firstly, the first three families’ planforms are considered. Each of these wings planforms is investigated depending on the geometry parameters noted earlier. Only one parameter is varied at a time to identify the trends of wing performance. The effect of ܴܣwas investigated by varying wing length with constant wing surface. Therefore, the result can be comparable. These wings have lengths of 70-100 ݉݉ and ܴܣof 5.6-11.4. These parameters lie in the scope of natural hummingbird shown in Table 2(see Appendix). After that, the last family wing was designed based on the best performance wing. The effect of wing area was studied in last family wings with constantܴܣ. 3 EXPERIMENTAL SETUP In order to investigate the wing’s aeroelastic characteristics, an experimental setup was constructed, illustrated in Figure 4. The experimental setup was used to measure the motor voltage, motor current, flapping frequency and mean lift force. Measuring the effort of a flapping wing robot is a challenging task because the measured force is relatively small to order of 0.01N. A force balance is designed by our team since there are no suitable commercial sensors. The working principle of force balance was already presented in [5]. The force balance signals are processed by a dSpace 1103 digital signal processor with the voltage and current reading of the DC motor. The flapping frequency can be measured by the motor speed ݊ with relation ݂ ൌ ܩ ݊ ( ܩ is gear ratio). Lift force is measured three times for each pair of wings separately, and the average lift force is used in this study. The other parameters (voltage, current, frequency and power) are averaged in the same way. Fig. 4: Diagram of experimental setup (input voltage, current, system lift, and flapping frequency are all recorded). 4 RESULTS AND DISCUSSION Here, we present the results of our experimental investigation of the mean lift force of hovering flexible flapping wings. Some important factors will influence overall system performance such as motor choice, wing kinematics, wing geometry and gear ratio. In this study, the wing kinematics, the driving motor and the gear ratio are kept constant in all the experiment, so that the tested results are comparable. The wing surface (1750݉݉ଶ ) is kept constant as well for the first three families. The rectangular wing with 70 ݉݉ wing length and 25 ݉݉ wing chord serves as reference wing. 4.1 Varying aspect ratio 4.1.1 Rectangular wing The rectangular wing is firstly studied in this subsection. The wing length is increased from 70 to 100݉݉, and two wing chords (wing root chord and wing tip chord) become narrow from 25 to 17.5݉݉. Therefore, the ܴܣis varied from 5.6 to 11.4. Figure 5 displays the effect of the wingܴܣ. Figure 5(a) shows that mean lift generally increases within ܴܣof 5.6 to 9.3 for the same frequency, then decreases from 9.3 to 11.4. That is, the wing efficiency increases with increasing AR, until a value of ܴܣൌ ͻǤ͵ is reached. This behavior can be explained that as wing length ܴ increases, the mean chord ܿҧ decreases, therefore, the wing becomes narrow. It might be that the ܥ coefficient reaches maximum at ܴܣof 9.3. Therefore, the mean lift force increases by increasing ( ܴܣ5.6-9.3). In addition, by a quasi-steady perspective, the formula 3 shows linear relationship between mean lift force and ܴܣ. This has been captured the experiment results. However, ܴܣfrom 9.3 to 11.4, it is observed that the lift force significantly reduces. It might be that the angle of attack rises up with narrow wing after certainܴܣ. Therefore, the lift coefficient becomes smaller. The same behavior is displayed in Figure 5(b) from power perspective. 4.1.2 Trapezoidal wing with constant wing root chord This family of wings mimics the hummingbird wings. The wing length was being increased from 70 to 100݉݉, while decreasing the wing tip chord. The wing root chord was kept constant (25݉݉) so that the wing shape transformed from a rectangular wing to a right-angled trapezoidal wing. The experiment results are shown in Figure 6. The Figure 6(a) presents that more lift force can be generated with higher ܴܣat the same frequency. However, from the power viewpoint, it is observed that the lift force increases with certain ( ܴܣfrom 5.6 to 9.3) then slightly decreases from 9.3 to 11.4 for constant power shown in Figure 6(b). It indicates that there is not much benefit by increasing ܴܣbeyond 9.3. In other words, the wing of ܴܣ9.3 is a critical value in this family. Comparing between rectangular wing and trapezoidal wing of family 2, it is indicated that the ܴܣ9.3 is a critical value in both families. This is close from the ܴܣof giant hummingbird ( ܴܣൎ ͻǤͳ). Furthermore, it is observed that the aerodynamic performance of trapezoidal wing of family 2 is better than the rectangular wing by increasingܴܣ. Despite the performance of trapezoidal wing of family 2 decreases from power perspective after ܴܣ9.3, it’s still better than the rectangular wing. 4.2 Constant aspect ratio 4.2.1 Trapezoidal wing with constant wing length In this subsection, the performance of rightangled trapezoidal wing was investigated as well. However, the wing length is kept constant (70 mm) in this family, therefore, ܴܣis constant. Wing root chord is increased, while the wing tip chord is decreased so that the taper ratio is smaller. In quasisteady model, the mean lift force is proportional to theܴܣ. Hence, the mean lift force should be constant at the same ܴܣand the same flapping frequency. However, the experimental results show that the lift force goes up by increasing ܥோ Τ ்ܥthen gradually decreases as shown in Figure 7(a). We assume that the mean angle of attack increases by increasing wing root chord, which affects the lift coefficient. Therefore, the lift force firstly rises up and then goes down by increasing wing root surface. At the same time, however, the lift force remains nearly constant for a constant power, as is shown in Figure 7(b). Therefore, increasing wing root surface does not affect the wing efficiency. 4.2.2 Wing area In this subsection, in order to investigate the effect of wing area on wing performance, ܴܣwas kept constant by preserving the overall wing shape. Based on the previous results, the best wing of ܴܣ 9.3 is selected. The wing surface is varied from 1059 to 2160݉݉ଶ . In quasi-steady models, the mean lift force over flapping cycle is approximated quadratically with wing surface. It is observed that the lift slightly varies quadratically in the interval of wing surface from 1059 to 1750݉݉ଶ . After that, the lift tendency alters from 1750 to 2160݉݉ଶ , as is shown in Figure 8(a). A possible explanation is that by increasing the wing surface, the area close to wing root chord grows significantly, and the angle of attack may increase, affecting the mean lift coefficient. Therefore, the parabolic relation changes after wing surface 1750݉݉ଶ . Figure 8(b) displays that the lift force increases at certain wing surfaces then gradually decreases from 1750 to 2160 ݉݉ଶ for a constant power. It means that there is no benefit of increasing wing surface beyond 1750 ݉݉ଶ . Therefore, we can conclude that the wing with surface of 1750݉݉ଶ and ܴܣ9.3 is the best wing in our study. Since the goal of the project is to mimic the natural hummingbird, their characteristics are summarized in Table 2. Comparing with the natural hummingbird performance, it is demonstrated that the designed wing in this study has a reasonable performance as shown in Figure 9. Eventually, wing of 90݉݉, 9.3 ܴܣand 1750 ݉݉ଶ wing surface is chosen to serve our robot. Consequently, a 17.2 g robot take-off test was achieved as demonstrated in Figure10. 16 18 10Hz 14Hz 16Hz 18Hz 20Hz 22Hz 24Hz 14 12 14 12 Lift [g] Lift [g] 10 8 10 6 8 4 6 2 4 0 5 6 0.3W 0.5W 1W 1.5W 2W 2.5W 3w 16 7 8 9 10 11 2 5 12 6 7 8 AR 9 10 11 12 AR (a) (b) Fig.5: Relationship between the lift force andܴܣ. Lift vs. AR at different flapping frequencies (a). Lift vs. AR at different electrical power (b). All the measurements taken from family 1 (rectangular wing with constant wing surface). 18 16 14 16 14 12 10 Lift [g] Lift [g] 12 18 10Hz 14Hz 16Hz 18Hz 20Hz 22Hz 24Hz 8 10 8 6 6 4 4 2 2 0 5 6 7 8 9 AR 10 11 12 0.3W 0.5W 1W 1.5W 2W 2.5W 3w 0 5 6 7 8 9 10 11 12 AR (a) (b) Fig.6: Relationship between the lift force andܴܣ. Lift vs. AR at different flapping frequencies (a). Lift vs. AR at different electrical power (b). All the measurements taken from family 2 (right-angled trapezoidal wing with constant wing surface). 13 13 13Hz 17Hz 21Hz 24Hz 27Hz 12 11 11 10 9 9 8 8 Lift [g] Lift [g] 10 0.3W 0.6W 1W 1.5W 2W 12 7 7 6 6 5 5 4 4 3 3 2 1 1.5 2 2.5 C /C R 3 3.5 2 1 4 1.5 2 2.5 C /C T R 3 3.5 4 T (a) (b) Fig.7: Relationship between the lift force and ratio ܥோ Τ ்ܥ. Lift vs. Ratioܥோ Τ ்ܥat different flapping frequencies (a). Lift vs. Ratioܥோ Τ ்ܥat different electrical power (b). All the measurements taken from wing of family 3. 20 18 10Hz 14Hz 16Hz 18Hz 20Hz 22Hz 24Hz 18 16 14 14 12 Lift [g] Lift [g] 12 16 10 10 0.25W 0.5W 1W 1.5W 2W 2.5W 3w 8 8 6 6 4 4 2 2 0 1000 1200 1400 1600 1800 2000 2200 2 wing surface [mm ] 0 1000 1200 1400 1600 1800 2000 2200 2 Wing surface[mm ] (a) (b) Fig.8: Relationship between the lift force and wing surface. Lift vs. wing surface at different flapping frequencies (a). Lift vs. wing surface at different electrical power (b). All the measurements taken for the same ܴܣof family 4. (a) (b) Fig.9: Comparison wing characteristics with natural hummingbird wing. ܴܣvs. wing length (a) and single wing surface vs. wing length (b). Wing of ASL is marked by black square. 5 CONCLUSION The effect of flexible wing geometry on the aerodynamics performance in hovering was studied for four different families of wings. The experimental results indicate that the mean lift force increases by increasingܴܣ, wing length, and wing surface within certain range. The value of ܴܣ9.3 was found out to be optimal for our system and is also close to hummingbird wings. The best wing in this study is right-angled trapezoidal wing of 90݉݉ length, 9.3 ܴܣand 1750 ݉݉ଶ wing surface. The planform has straight leading edge and the chord length decreases towards the wingtip. The constraints of size, and wing loading, power, even weight were considered when designing wing. Therefore, the final wing design delivers maximal lift for a given amount of power. Finally, the wing performance was demonstrated by a take-off a 17.2 g flapping wing robot. The study presented here focused mostly on hummingbird-like MAV in the hover. Nevertheless, the current study provides a useful dataset for the future study of flapping wing MAV. species for the application of biomimetic flapping wing micro air vehicles. 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Wing name Length () Root Chord ࡾ () Tip Chord ࢀ () ࡾ Wing surface ࡿሺ ሻ Normal wing Family 1 MLF 46 MLF 47 MLF 48 MLF 49 MLF 50 MLF 51 MLF 52 MLF 53 MLF 54 MLF 55 MLF 56 MLF 57 MLF 58 MLF 59 MLF 60 MLF 61 MLF 62 MLF 63 MLF 64 MLF 65 MLF 66 MLF 67 MLF 68 MLF 69 Family 2 Family 3 Family 4 70 75 80 85 90 95 100 75 80 85 90 95 100 70 70 70 70 70 75 80 85 90 95 100 25 23.3 22 20.6 19.4 18.4 17.5 25 25 25 25 25 25 30 32.5 35 40 19.4 20.2 22.2 23.6 25 26.4 27.8 25 23.3 22 20.6 19.4 18.4 17.5 21.7 18.8 16.2 14 11.8 10 20 17.5 15 10 10.8 11.6 12.4 13.1 13.8 14.7 15.4 چWing name: wing material + serial number of design. 5.6 6.4 7.3 8.3 9.3 10.3 11.4 6.4 7.3 8.3 9.3 10.3 11.4 5.6 5.6 5.6 5.6 9.3 9.3 9.3 9.3 9.3 9.3 9.3 1750 1059 1215 1383 1561 1750 1950 2161 Tab. 2: Summary of wing parameters of natural hummingbird. Mass (g) ࡸ (mm) ࢌ (HZ) S( ) ࢝ ሺࡺΤ ሻ Blue-throated [18] 8.4 85 23.3 1762.95 23.5 8.2 Magnificent [18] 7.4 79 24 1485.95 24.7 8.4 Hummingbird Species AR Black-chinned [18] 3 47 51.2 622.25 23.5 7.1 Rufous [18] 3.3 42 51.7 476.75 33.6 7.4 [19] 3.2 46.1 - 599.2 36.7 7.1 [20] 4.24 45 53.25 494 42.11 8.2 4.24 48 49.1 584 35.61 7.89 Anna (male) Broad-tailed 4.1 51 47.3 668.5 30.08 7.78 4.54 52 42.57 662.5 33.6 8.16 [21] 4.52 54.5 45.9 713.95 - - [22] 5.6 50 - 588.2 - 8.5 9.2 [20] Ruby- throated(male) [23] Ruby- throated(female) Amaziliafimbriata [23] 5 50 - 543.5 - 4.7 59 - 838.8 - 8.3 4.22 55 41.32 780.5 26.52 7.75 3.46 54 36.38 736.5 23.05 7.92 3.66 55 38.71 748 24.01 8.09 5.16 57 39.25 799.5 31.67 8.13 3.6 52 39.53 680.5 25.96 7.95 3.61 56 37.17 817.5 21.66 7.76 3.93 55 38.1 860.5 22.4 7.03 7.76 4.1 57 37.94 837.5 24.02 3.58 41 - 460 38.4 7.34 3.67 41 - 485 37.3 6.96 4.01 40 - 445 44.1 7.17 4.16 43 - 455 44.9 8.13 4.36 49 - 635 33.6 7.55 4.36 48 - 640 33.3 7.18 8 4.18 49 - 600 34.2 [24] 3.1 44.5 52 - - - [25] 5.1 58.5 35 850 29.4 8.05 22.7 - 15.3 - - - Giant hummingbird(male) [26] (female) (meal) (female) [27] 20.4 - - - - - 22.6 143 13.9 4508.35 24 9.1 19.6 139.7 13 4307.8 21.5 8.3 چThe empty data cells are due to data being unavailable. ࢝ is wing load. 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