Experimental optimization of wing shape for a hummingbird-like flapping wing micro air vehicle

Effect of incorporating wing veins on soft wings for flapping micro air vehicles

Small insects with flapping wings, such as bees and flies, have flexible wings with veins, and their compliant motion enhances flight efficiency and robustness. This study investigated the effects of integrating wing veins into soft wings for micro-flapping aerial vehicles. Prototypes of soft wings, featuring various wing areas and vein patterns in both the wing-chord and wing-span directions, were fabricated and evaluated to determine the force generated through flapping. The results indicated that the force is not solely dependent upon the wing area and is influenced by the wing vein pattern. Wings incorporating wing-chord veins produced more force compared to those with wing-span veins. In contrast, when the wing area was the specific wing area, wings with crossed wing veins, comprising both wing-span veins and wing-chord veins, produced more force. Although wing-chord veins tended to exert more influence on the force generated than the wing-span veins, the findings suggested that a combination of wing-span and wing-chord veins may be requisite, depending upon the wing area.

First lift-off and flight performance of a tailless flapping-wing aerial robot in high-altitude environments

Flapping flight of animals has captured the interest of researchers due to their impressive flight capabilities across diverse environments including mountains, oceans, forests, and urban areas. Despite the significant progress made in understanding flapping flight, high-altitude flight as showcased by many migrating animals remains underexplored. At high-altitudes, air density is low, and it is challenging to produce lift. Here we demonstrate a first lift-off of a flapping wing robot in a low-density environment through wing size and motion scaling. Force measurements showed that the lift remained high at 0.14 N despite a 66% reduction of air density from the sea-level condition. The flapping amplitude increased from 148 to 233 degrees, while the pitch amplitude remained nearly constant at 38.2 degrees. The combined effect is that the flapping-wing robot benefited from the angle of attack that is characteristic of flying animals. Our results suggest that it is not a simple increase in the flapping frequency, but a coordinated increase in the wing size and reduction in flapping frequency enables the flight in lower density condition. The key mechanism is to preserve the passive rotations due to wing deformation, confirmed by a bioinspired scaling relationship. Our results highlight the feasibility of flight under a low-density, high-altitude environment due to leveraging unsteady aerodynamic mechanisms unique to flapping wings. We anticipate our experimental demonstration to be a starting point for more sophisticated flapping wing models and robots for autonomous multi-altitude sensing. Furthermore, it is a preliminary step towards flapping wing flight in the ultra-low density Martian atmosphere.

Development of flapping wing robot and vision-based obstacle avoidance strategy

Due to the flight characteristics such as small size, low noise, and high efficiency, studies on flapping wing robots are being actively conducted. In particular, the flapping wing robot is in the spotlight in the field of search and reconnaissance. Most of the research focuses on the development of flapping wing robots rather than autonomous flight. However, because of the unique characteristics of flapping wings, it is essential to consider the development of flapping wing robots and autonomous flight simultaneously. In this article, we describe the development of the flapping wing robot and computationally efficient vision-based obstacle avoidance algorithm suitable for the lightweight robot. We developed a 27 cm and 45 g flapping wing robot named CNUX Mini that features an X-type wing and tailed configuration to attenuate oscillation caused by flapping motion. The flight experiment showed that the robot is capable of stable flight for 1.5 min and changing its direction with a small turn radius in a slow forward flight condition. For the obstacle detection algorithm, the appearance variation cue is used with the optical flow-based algorithm to cope robustly with the motion-blurred and feature-less images obtained during flight. If the obstacle is detected during straight flight, the avoidance maneuver is conducted for a certain period, depending on the state machine logic. The proposed obstacle avoidance algorithm was validated in ground tests using a testbed. The experiment shows that the CNUX Mini performs a suitable evasive maneuver with 90.2% success rate in 50 incoming obstacle situations.

Experiments and numerical simulations on hovering three-dimensional flexible flapping wings

In this paper, the applicability and accuracy of high-fidelity experimental and numerical approaches in the analysis of three-dimensional flapping (revolving and pitching) wings operating under hovering flight conditions, i.e. where unsteady and three-dimensional rotational effects are strong, are assessed. Numerical simulations are then used to explore the role of mass and frequency ratios on aerodynamic performance, wing dynamics and flow physics. It is shown that time-averaged lift increases with frequency ratio, up to a certain limit that depends on mass ratio and beyond which upward wing bending and flexibility induced phase lag between revolving an pitching motions at stroke reversal become strong and contribute to phases of negative lift that counterbalances the initial lift increase. This wing dynamics, which is dominated by spanwise bending, also affects wing-wake interactions and, in turn, leading edge vortex formation.

Effect of Wing Membrane Material on the Aerodynamic Performance of Flexible Flapping Wing

Flexible deformation of the insect wing has been proven to be beneficial to lift generation and power consumption. There is great potential for shared research between natural insects and bio-inspired Flapping wing Micro Aerial Vehicles (FWMAVs) for performance enhancement. However, the aerodynamic characteristics and deformation process of the flexible flapping wing, especially influenced by wing membrane material, are still lacking in-depth understanding. In this study, the flexible flapping wings with different membrane materials have been experimentally investigated. Power input and lift force were measured to evaluate the influence of membrane material. The rotation angles at different wing sections were extracted to analyze the deformation process. It was found that wings with higher elastic modulus membrane could generate more lift but at the cost of more power. A lower elastic modulus means the wing is more flexible and shows an advantage in power loading. Twisting deformation is more obvious for the wing with higher flexibility. Additionally, flexibility is also beneficial to attenuate the rotation angle fluctuation, which in turn enhances the aerodynamic efficiency. The research in this paper is helpful to further understand the aerodynamic characteristics of flexible flapping wing and to design bio-inspired FWMAVs with higher performance.

The Flight Mechanism of a Bird-like Flapping Wing Robot at a Low Reynolds Number

The flight mechanism of a bird-like flapping wing robot at a low Reynolds number was studied in this study for improving the robot performances. Both the physical model and the kinematic model were first established. The dynamic model of the robot at a low Reynolds number was built with the RANS (Reynolds-averaged Navier-Stokes) equations and the Spalart-Allmaras turbulence model. The flight experiments were carried out and the results were discussed. Lift and drag coefficient curves show that it generates upward lift and forward thrust in the phase that the wing flaps downwards, the rate of the coefficient curves is the biggest when the flapping direction changes. Pressure contours indicate that small vortexes with high pressure values appear at the wing edges. There are four velocity vortex groups in total at the front and back of the wing in the velocity contours. Some methods for improving the robot flight efficiency and the robot strength as well as the stitching position of the robot skin have been obtained from the above results. The methods provide the important guidance for the stable flights of the flapping wing robot with the high efficiency.

Longitudinal Mode System Identification of an Insect-like Tailless Flapping-Wing Micro Air Vehicle Using Onboard Sensors

In this paper, model parameter identification results are presented for a longitudinal mode dynamic model of an insect-like tailless flapping-wing micro air vehicle (FWMAV) using angle and angular rate data from onboard sensors only. A gray box model approach with indirect method was utilized with adaptive Gauss–Newton, Levenberg–Marquardt, and gradient search identification methods. Regular and low-frequency reference commands were mainly used for identification since they gave higher fit percentages than irregular and high-frequency reference commands. Dynamic parameters obtained using three identification methods with two different datasets were similar to each other, indicating that the obtained dynamic model was sufficiently reliable. Most of the identified dynamic model parameters had similar values to the computationally obtained ones, except stability derivatives for pitching moment with forward velocity and pitching rate variations. Differences were mainly due to certain neglected body, nonlinear dynamics, and the shift of the center of gravity. Fit percentage of the identified dynamic model (~49%) was more than two-fold higher than that of the computationally obtained one (~22%). Frequency domain analysis showed that the identified model was much different from that of the computationally obtained one in the frequency range of 0.3 rad/s to 5 rad/s, which affected transient responses. Both dynamic models showed that the phase margin was very low, and that it should be increased by a feedback controller to have a robustly stable system. The stable dominant pole of the identified model had a higher magnitude which resulted in faster responses. The identified dynamic model exhibited much closer responses to experimental flight data in pitching motion than the computationally obtained dynamic model, demonstrating that the identified dynamic model could be used for the design of more effective pitch angle-stabilizing controllers.

Liquid-amplified zipping actuators for micro-air vehicles with transmission-free flapping

Flapping micro-air vehicles (MAVs) can access a wide range of locations, including confined spaces such as the inside of industrial plants and collapsed buildings, and offer high maneuverability and tolerance to disturbances. However, current flapping MAVs require transmission systems between their actuators and wings, which introduce energetic losses and additional mass, hindering performance. Here, we introduce a high-performance electrostatic flapping actuation system, the liquid-amplified zipping actuator (LAZA), which induces wing movement by direct application of liquid-amplified electrostatic forces at the wing root, eliminating the requirement of any transmission system and their associated downsides. The LAZA allows for accurate control of flapping frequency and amplitude, exhibits no variation in performance over more than 1 million actuation cycles, and delivers peak and average specific powers of 200 and 124 watts per kilogram, respectively, exceeding mammalian and insect flight muscle and on par with modern flapping MAV actuation systems. The inclusion of 50-millimeter-long passively pitching wings in a dragonfly-sized LAZA flapping system allowed the rectification of net directional thrust up to 5.73 millinewtons. This thrust was achieved while consuming only 243 milliwatts of electrical power, implying a thrust-to-power ratio of 23.6 newtons per kilowatt, similar to state-of-the-art flapping MAVs, helicopter rotors, and commercial drone motors. Last, a horizontally moving LAZA flapping system supported by a taut nylon wire was able to accelerate from at-rest and travel at speeds up to 0.71 meters per second. The LAZA enables lightweight, high-performance transmission-free flapping MAVs for long-term remote exploration and search-and-rescue missions.

Elastodynamic model for flapping-wing micro aerial vehicle

Lightweight design is key to high efficiency and long durability of micro air vehicle (MAV), while it will inevitably reduce the stiffness of the structures and affect the motion of the mechanism. In this study, an elastodynamic model for flapping-wing MAV (FMAV) is established to unveil the effect of elastic deformation of transmission mechanism on the flapping motion. Based on kineto-elastostatic analysis, an elastodynamic model of the transmission mechanism is built, which reveals that the inertial force of the transmission mechanism for typical FMAV is much smaller than the force transmitted. Thus, the inertial force can be ignored, and analytical formula between the deformation of transmission mechanism and the flapping angle is derived. Finite element method (FEM) simulations are conducted to validate the analytical formula, and the results show that the flapping angle obtained from the analytical formula matches well with FEM simulations. The proposed elastodynamic model and analytical formula will provide theoretical guidance for designing and optimizing FMAV with desired transmission mechanism and flapping motion.

Aerodynamic analysis of hummingbird-like hovering flight

Flapping wing micro aerial vehicles are studied as the substitute for fixed and rotary wing micro aerial vehicles because of the advantages such as agility, maneuverability, and employability in confined environments. Hummingbird's sustainable hovering capability inspires many researchers to develop micro aerial vehicles with similar dynamics. In this research, a wing of a ruby-throated hummingbird is modeled as an insect wing using membrane and stiffeners. The effect of flexibility on the aerodynamic performance of a wing in hovering flight has been studied numerically by using a fluid-structure interaction scheme at a Reynolds number of 3000. Different wings have been developed by using different positions and thicknesses of the stiffeners. The chordwise and spanwise flexural stiffnesses of all the wings modeled in this work are comparable to insects of similar span and chord length. When the position of the stiffener is varied, the best-performing wing has an average lift coefficient of 0.51. Subsequently, the average lift coefficient is increased to 0.56 when the appropriate thickness of the stiffeners is chosen. The best flexible wing outperforms its rigid counterpart and produces lift and power economy comparable to a real hummingbird's wing. That is, the average lift coefficient and power economy of 0.56 and 0.88 for the best flexible wing as compared to 0.61 and 1.07 for the hummingbird's wing. It can be concluded that a simple manufacturable flexible wing design based on appropriate positioning and thickness of stiffeners can serve as a potential candidate for bio-inspired flapping-wing micro aerial vehicles.

Generative design of bioinspired wings based on deployable hindwings of Anomala Corpulenta Motschulsky

In view of the application prospect of the hindwing of Anomala Corpulenta Motschulsky in the field of foldable Micro Aerial Vehicles (MAVs), this paper investigated the morphology, macro/microstructure of the hindwing, and the nanomechanical properties of the wing veins and the wing membrane. It revealed the variation of nanohardness and elastic modulus between different veins and different positions of the same wing veins. This paper established a 3D coupling model of the hindwing based on the principle of coupling bionics. This paper presents a simulation analysis of the structural statics (uniform load distribution) and aerodynamics (under different attack angles, flight velocities, and flapping frequencies). Two 3D coupling models (HW-I and HW-II) of the hindwing were discussed the deformation and flight aerodynamic performance of Workbenches and Fluent. On that basis, the bionic wing was generatively designed, and a 3D bionic wing (BioW) model was established using the generative design method. Simulation analyses were performed through structural statics and aerodynamics. The results showed that the stress distribution was relatively uniform and that the overall displacement deformation was minimal for the BioW model. Moreover, the BioW model had better flight efficiency and aerodynamic performance.

Wing shape optimization design inspired by beetle hindwings in wind tunnel experiments

Flighted beetles have deployable hindwings, which enable them to directly reduce their body size, and thus are excellent bioinspired prototypes for microair vehicles (MAVs). The wing shape of MAVs has an important influence on their aerodynamics. In this paper, wing shapes, inspired from three beetle species' hindwings and designed in terms of the wing camber angle, geometry (including wing length, aspect ratio (AR), and taper ratio (TR)) and wing area, were selected and varied to optimize lift together with the efficiency of wing. All the wings were fabricated by a Tyvek membrane and tested in a wind tunnel. The camber angle and AR were found to have a critical role in force production. The best performance was obtained by a wing with a camber angle of 10°, wing length of 125 mm, AR of 7.06, TR of 0.40 and wing area of 4115 mm².

Flexibility Effects of a Flapping Mechanism Inspired by Insect Musculoskeletal System on Flight Performance

Flying animals such as insects display great flight performances with high stability and maneuverability even under unpredictable disturbances in natural and man-made environments. Unlike man-made mechanical systems like a drone, insects can achieve various flapping motions through their flexible musculoskeletal systems. However, it remains poorly understood whether flexibility affects flight performances or not. Here, we conducted an experimental study on the effects of the flexibility associated with the flapping mechanisms on aerodynamic performance with a flexible flapping mechanism (FFM) inspired by the flexible musculoskeletal system of insects. Based on wing kinematic and force measurements, we found an appropriate combination of the flexible components could improve the aerodynamic efficiency by increasing the wingbeat amplitude. Results of the wind tunnel experiments suggested that, through some passive adjustment of the wing kinematics in concert with the flexible mechanism, the disturbance-induced effects could be suppressed. Therefore, the flight stability under the disturbances is improved. While the FFM with the most rigid spring was least efficient in the static experiments, the model was most robust against the wind within the range of the study. Our results, particularly regarding the trade-off between the efficiency and the robustness, point out the importance of the passive response of the flapping mechanisms, which may provide a functional biomimetic design for the flapping micro air vehicles (MAVs) capable of achieving high efficiency and stability.

Phenomenology and scaling of optimal flapping wing kinematics

Biological flapping wing fliers operate efficiently and robustly in a wide range of flight conditions and are a great source of inspiration to engineers. The unsteady aerodynamics of flapping wing flight are dominated by large-scale vortical structures that augment the aerodynamic performance but are sensitive to minor changes in the wing actuation. We experimentally optimise the pitch angle kinematics of a flapping wing system in hover to maximise the stroke average lift and hovering efficiency with the help of an evolutionary algorithm andin situforce and torque measurements at the wing root. Additional flow field measurements are conducted to link the vortical flow structures to the aerodynamic performance for the Pareto-optimal kinematics. The optimised pitch angle profiles yielding maximum stroke-average lift coefficients have trapezoidal shapes and high average angles of attack. These kinematics create strong leading-edge vortices early in the cycle which enhance the force production on the wing. The most efficient pitch angle kinematics resemble sinusoidal evolutions and have lower average angles of attack. The leading-edge vortex grows slower and stays close-bound to the wing throughout the majority of the stroke-cycle. This requires less aerodynamic power and increases the hovering efficiency by 93% but sacrifices 43% of the maximum lift in the process. In all cases, a leading-edge vortex is fed by vorticity through the leading edge shear layer which makes the shear layer velocity a good indicator for the growth of the vortex and its impact on the aerodynamic forces. We estimate the shear layer velocity at the leading edge solely from the input kinematics and use it to scale the average and the time-resolved evolution of the circulation and the aerodynamic forces. The experimental data agree well with the shear layer velocity prediction, making it a promising metric to quantify and predict the aerodynamic performance of the flapping wing hovering motion.

Mimicking nature's flyers: a review of insect-inspired flying robots

Insects have attracted much interest from scientists and engineers as they offer an endless source of inspiration for creating innovative engineering designs. By mimicking flying insects, it may be possible to create highly efficient biomimetic drones. In this paper, we provide an overview on how the principles of insect flight, including large stroke amplitudes and wing rotations, the clap-and-fling effect and flight control have been implemented to successfully demonstrate untethered, controlled free-flight in the insect-inspired flying robots. Despite the lack of insect-like muscles, various electro-mechanical systems have been invented to actuate insect robots. Achieving controlled free-flight is a cornerstone of next-generation insect-inspired robots which in addition to flight will be equipped with multiple modes of transportation, similar to real flying insects.

Batch fabrication process of biomimetic wing with high flexibility of stiffness design for flapping-wing micro aerial vehicles

A lamination-based batch-fabrication process of biomimetic wing for flapping-wing micro aerial vehicles is presented. The key benefits of this process are:•

One of the advantages of the process is high productivity; eight wings were successfully fabricated simultaneously in our experiment.

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The wing fabricated with the reported process is made of soft polyimide and partially reinforced by a titanium layer. This configuration enables the flexible design of the bending stiffness distribution on the wing, which is the key specification for generating lift force.

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The reinforcing material can be replaced with other metals or heat-resistant polymers, and the number of layers and layer thicknesses are also variable. This indicates that the reported process can be customized considerably to suit individual needs.

Aerodynamic performance of flexible flapping wings deformed by slack angle

Wing flexibility is unavoidable for flapping wing flyers to ensure a lightweight body and for higher payload allowances on board. It also effectively minimizes the inertia force from high-frequency wingbeat motion. However, related studies that attempt to clarify the essence of wing flexibility remain insufficient. Here, a parametric study of a flexible wing was conducted as part of the effort to build an aerodynamic model and analyze its aerodynamic performance. The quasi-steady modeling was adopted with experimentally determined translational forces. These forces were determined from 84 flexible wing cases while varying the angle of attack at the wing rootα_rand the flexibility parameter, slack angleθ_S, with 19 additional rigid wing cases. This study foundα_rfor optimum lift generation to exceed 45° irrespective ofθ_S. The coefficient curves were well-fitted with a cubed-sine function. The model was rigorously validated with various wing kinematics, giving a good estimation of the experimental results. The estimated error was less than 5%, 6%, and 8% for the lift, drag, and moment, respectively, considering fast to moderate wing kinematics. The study was extended to analyze the pure aerodynamic performance of the flexible wing. The most suitable wing for a flapping-wing micro-aerial vehicle wing design with a simple vein structure was found to be the 5° slack-angled wing. The inference from this study further shows that a small amount of deformation is needed to increase the lift, as observed in natural flyers. Thus, wing deformation could allow living flyers to undertake less pitching motion in order to reduce the mechanical power and increase the efficiency of their wings.

Efficient flapping wing drone arrests high-speed flight using post-stall soaring

The aerobatic maneuvers of swifts could be very useful for micro aerial vehicle missions. Rapid arrests and turns would allow flight in cluttered and unstructured spaces. However, these decelerating aerobatic maneuvers have been difficult to demonstrate in flapping wing craft to date because of limited thrust and control authority. Here, we report a 26-gram X-wing ornithopter of 200-millimeter fuselage length capable of multimodal flight. Using tail elevation and high thrust, the ornithopter was piloted to hover, fly fast forward (dart), turn aerobatically, and dive with smooth transitions. The aerobatic turn was achieved within a 32-millimeter radius by stopping a dart with a maximum deceleration of 31.4 meters per second squared. In this soaring maneuver, braking was possible by rapid body pitch and dynamic stall of wings at relatively high air speed. This ornithopter can recover to glide stability without tumbling after a 90-degree body flip. We showed that the tail presented a strong stabilizing moment under high thrust, whereas the wing membrane flexibility alleviated the destabilizing effect of the forewings. To achieve these demands for high thrust, we developed a low-loss anti-whirl transmission that maximized thrust output by the flapping wings to 40 grams in excess of body weight. By reducing the reactive load and whirl, this indirect drive consumed 40% less maximum electrical power for the same thrust generation than direct drive of a propeller. The triple roles of flapping wings for propulsion, lift, and drag enable the performance of aggressive flight by simple tail control.

Analysis on Hover Control Performance of T- and Cross-Shaped Tail Fin of X-Wing Single-Bar Biplane Flapping Wing

The current flapping wing adopts T-shaped or cross-shaped tail fin to adjust its flight posture. However, how the tail fin will affect the hover control is not very clear. So, the effects of the two types of tail on flight will be analyzed and compared by actual flight tests in this paper. Firstly, we proposed a new X-wing single-bar biplane flapping-wing mechanism with two pairs of wings. Thereafter, the overall structure, gearbox structure, tail, frame, and control system of the flapping wing were designed and analyzed. Secondly, the control mechanism of hover is analyzed to describe the effect of two-tail fin on posture control. Thirdly, the Beetle was used as the control unit to achieve a controllable flight of flapping wing. The MPU6050 electronic gyroscope was used to monitor the drone’s posture in real time, and the Bluetooth BLE4.0 wireless communication module was used to receive remote control instructions. At last, to verify the flight effect, two actual flapping wings were fabricated and flight experiments were conducted. The experiments show that the cross-shaped tail fin has a better controllable performance than the T-shaped tail fin. The flapping wing has a high lift-to-mass ratio and good maneuverability. The designed control system can achieve the controllable flight of the flapping wing.

Wing Design, Fabrication, and Analysis for an X-Wing Flapping-Wing Micro Air Vehicle

Flapping-wing Micro Air Vehicles (FW-MAVs), inspired by small insects, have limitless potential to be capable of performing tasks in urban and indoor environments. Through the process of mimicking insect flight, however, there are a lot of challenges for successful flight of these vehicles, which include their design, fabrication, control, and propulsion. To this end, this paper investigates the wing design and fabrication of an X-wing FW-MAV and analyzes its performance in terms of thrust generation. It was designed and developed using a systematic approach. Two pairs of wings were fabricated with a traditional cut-and-glue method and an advanced vacuum mold method. The FW-MAV is equipped with inexpensive and tiny avionics, such as the smallest Arduino controller board, a remote-control receiver, standard sensors, servos, a motor, and a 1-cell battery. Thrust measurement was conducted to compare the performance of different wings at full throttle. Overall, this FW-MAV produces maximum vertical thrust at a pitch angle of 10 degrees. The wing having stiffeners and manufactured using the vacuum mold produces the highest thrust among the tested wings.

Wing inertia as a cause of aerodynamically uneconomical flight with high angles of attack in hovering insects

Flying insects can maintain maneuverability in the air by flapping their wings, and, to save energy, the wings should operate following optimal kinematics. However, unlike conventional rotary wings, insects operate their wings at aerodynamically uneconomical and high angles of attack (AoA). Although insects have continuously received attention from biologists and aerodynamicists, the high AoA operation in insect flight has not been clearly explained. Here, we used a theoretical blade-element model to examine the impact of wing inertia on the power requirement and flapping AoA, based on 3D free-hovering flight wing kinematics of a horned beetle, Allomyrina dichotoma The relative simplicity of the model allowed us to search for the best AoA distributed along the wingspan, which generate the highest vertical force per unit power. We show that, although elastic elements may be involved in flight muscles to store and save energy, the insect still has to use substantial power to accelerate its wings, because inertial energy stores should be used to overcome aerodynamic drag before being stored elastically. At the same flapping speed, a wing operating at a higher AoA requires lower inertial torque, and therefore lower inertial power output, at stroke reversals than a wing operating at an aerodynamically optimal low AoA. An interactive aerodynamic-inertial effect thereby enables the wing to flap at sufficiently high AoA, which causes an aerodynamically uneconomical flight in an effort to minimize the net flight energy.