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Ornithopters are bird‐like flapping‐wing robots. Only small ornithopters can hover, with long endurance at hummingbird size. Could larger ornithopters be improved further to hover longer? This paper reviews and examines the drive and power of hovering ornithopters, and elastic means of energy or thrust boosters. While the rotation of flexible wings enhance the thrust generation, two‐winged ornithopters did not scale up well because of higher disk loading. In comparison, the X‐winged or multiple‐V‐winged ornithopters enjoy a lower disk loading by beating multiple wings slower, at a smaller stroke angle or a longer span. Further, the clap‐and‐fling interaction of V and X‐wings boosts the thrust generation. Future works can explore the wing flexibility and morphology change to improve the hoverability and flight agility of ornithopters. Only small ornithopters can hover, longest at the hummingbird size. This article reviews the drive and power of these hovering machines, focusing on elastic energy and thrust boosters. Unlike two‐winged designs, X‐winged and multiple‐V‐winged ornithopters benefit from lower disk loading and slower, smaller wingbeats, and the clap‐and‐fling effect.

This paper describes the results of a six-year project aiming at designing and constructing a flapping twin-wing robot of the size of hummingbird (Colibri in French) capable of hovering. Our prototype has a total mass of 22 g, a wing span of 21 cm and a flapping frequency of 22 Hz; it is actively stabilized in pitch and roll by changing the wing camber with a mechanism known as wing twist modulation. The proposed design of wing twist modulation effectively alters the mean lift vector with respect to the center of gravity by reorganization of the airflow. This mechanism is modulated by an onboard control board which calculates the corrective feedback control signals through a closed-loop PD controller in order to stabilize the robot. Currently, there is no control on the yaw axis which is passively stable, and the vertical position is controlled manually by tuning the flapping frequency. The paper describes the recent evolution of the various sub-systems: the wings, the flapping mechanism, the generation of control torques, the avionics and the PD control. The robot has demonstrated successful hovering flights with an on-board battery for the flight autonomy of 15–20 s.

This paper describes the design and development of the Dove, a flapping-wing micro air vehicle (FWMAV), which was developed in Northwestern Polytechnical University. FWMAVs have attracted international attentions since the past two decades. Since some achievements have been obtained, such as the capability of supporting an air vehicle to fly, our research goal was to design an FWMAV that has the ability to accomplish a task. Main investigations were presented in this paper, including the flexible wing design, the flapping mechanism design, and the on-board avionics development. The current Dove has a mass of 220 g, a wingspan of 50 cm, and the ability of operating fully autonomously, flying lasts half an hour, and transmitting live stabilized color video to a ground station over 4 km away.

Micro aerial vehicles (MAVs) have flexibility and maneuverability, which can offer vast potential for applications in both civilian and military domains. Compared to Fixed-wing/Rotor-wing MAVs, Flapping Wing Micro Robots (FWMRs) have garnered widespread attention among scientists due to their superior miniaturized aerodynamic theory, reduced noise, and enhanced resistance to disturbances in complex and diverse environments. Flying insects, it not only has remarkable flapping flight ability (wings), but also takeoff and landing habitat ability (legs). If the various functions of flying insects can be imitated, efficient biomimetic FWMRs can be produced. This paper provides a review of the flight kinematics, aerodynamics, and wing structural parameters of insects. Then, the traditional wings and folding wings of insect-inspired FWMRs were compared. The research progress in takeoff and landing of FWMRs was also summarized, and the future developments and challenges for insect-inspired FWMRs were discussed.

This paper is based on a previously developed bio-inspired Flapping Wing Aerial Vehicle (FWAV), RoboFalcon, which can fly with a morphing-coupled flapping pattern. In this paper, a simple flapping stroke control system based on Hall effect sensors is designed and applied, which is capable of assigning different up- and down-stroke speeds for the RoboFalcon platform to achieve an adjustable downstroke ratio. The aerodynamic and power characteristics of the morphing-coupled flapping pattern and the conventional flapping pattern with varying downstroke ratios are measured through a wind tunnel experiment, and the corresponding aerodynamic models are developed and analyzed by the nonlinear least squares method. The relatively low power consumption of the slow-downstroke mode of this vehicle is verified through outdoor flight tests. The results of wind tunnel experiments and flight tests indicate that increased downstroke duration can improve aerodynamic and power performance for the RoboFalcon platform.

This study focuses on the flapping mechanisms found in recently developed biometric flapping-wing air vehicles (FWAVs). FWAVs mimic the flight characteristics of flying animals, providing advantages such as maneuverability, inconspicuousness, and excellent flight efficiency in the low Reynolds number region. The flapping mechanism is a critical part of determining the aerodynamic performance of an FWAV since it is directly related to the wing motion. In this study, the flight characteristics of birds and bats are introduced, the incorporation of these flight characteristics into the development of FWAVs is elucidated, and the utilization of these flight characteristics in the development of FWAVs is explained. Next, the classification and analysis of flapping mechanisms are conducted based on wing motion and the strategy for improving aerodynamic performance. Lastly, the current research gap is elucidated, and potential future directions for further research are proposed. This review can serve as a guide during the early development stage of FWAVs.

PurposeBionic flapping-wing aerial vehicles (FWAVs) mimic natural flyers to generate the lift and thrust, such as birds, bats and insects. As an important component of the FWAVs, the flapping wings are crucial for the flight performance. The aim of this paper is to study the effects of different wings on aerodynamic performance.Design/methodology/approachInspired by the wings structure of birds, the authors design four cambered wings to analyze the effect of airfoils on the FWAVs aerodynamic performance. The authors design the motor-driven mechanism of flapping wings, and realize the control of flapping frequency. Combined with the wind tunnel equipment, the authors build the FWAVs force test platform to test the static and dynamic aerodynamic performance of different flapping wings under the state variables of flapping frequency, wind speed and inclined angle.FindingsThe results show that the aerodynamic performance of flapping wing with a camber of 20 mm is the best. Compared with flat wing, the average lift can be improved by 59.5%.Originality/valueDifferent from the traditional flat wing design of FWAVs, different cambered flapping wings are given in this paper. The influence of airfoils on aerodynamic performance of FWAVs is analyzed and the optimal flapping wing is obtained.

The aerodynamic damping induced by the body motion is significant in flapping wing aircraft due to the low Reynolds number flight and the flapping motion. It can influence the flight stability and is coupled with active control. This study proposes a position control strategy for a self-designed underactuated flapping wing aircraft by considering the aerodynamic damping. Inspired by the pendulum-like dynamics, the dominant aerodynamic damping is theoretically modelled under linear damping assumption and the damping parameters are identified by two specially designed experimental platforms. It is experimentally found that the flapping motion does increase the damping compared with the non-flapping case. The damping influence is thus non-negligible in flapping wing aircraft. The outer position controller and the inner attitude controller are designed by including the modelled linear damping terms, and the unmodelled nonlinear little damping terms and other nonlinearities are estimated via the radial based function neural network. The kinematic solve is used to deal with the under-actuated characteristics which can solve the expected roll and pitch angles based on the position controller. The algorithm is compared with traditional methods under two different cases. It is proved that by using human prior knowledge of the aerodynamic damping model to compensate the damping influence, the desired angles computed by the outer position controller are more targeted. The attitude tracking control accuracy is also improved. These two points contribute to the speed and accuracy of the position tracking control. This research proves the necessity of including the aerodynamic damping in the active control of flapping wing aircraft as well.

For an insect-like tailless flying robot, flapping wings should be able to produce control force as well as flight force to keep the robot staying airborne. This capability requires an active control mechanism, which should be integrated with lightweight microcontrol actuators that can produce sufficient control torques to stabilize the robot due to its inherent instability. In this work, we propose a control mechanism integrated in a hover-capable, two-winged, flapping-wing, 16.4 g flying robot (KUBeetle-S) that can simultaneously change the wing stroke-plane and wing twist. Tilting the stroke plane causes changes in the direction of average thrust and the wing twist distribution to produce control torques for pitch and roll. For yaw (heading change), root spars of left and right wings are adjusted asymmetrically to change the wing twist during flapping motion, resulting in yaw torque generation. Changes in wing kinematics were validated by measuring wing kinematics using three synchronized high-speed cameras. We then performed a series of experiments using a six-axis force/torque load cell to evaluate the effectiveness of the control mechanism via torque generation. We prototyped the robot by integrating the control mechanism with sub-micro servos as control actuators and flight control board. Free flight tests were finally conducted to verify the possibility of attitude control.

Many flying and swimming creatures have morphing pectoral propulsors (wings or fins) for propulsion, typically with flapping, rowing, and pitching motions; flapping and rowing motions are responsible for the stroke plane angle that is important for a broader performance space of the propulsor, while the stroke plane angle has been less characterized and implemented by artificial propulsors of biomimetic vehicles and thus has lack of stroke plane angle control. In this paper, we consider robotic pectoral propulsors with combined flapping and rowing motions for a stroke plane angle that can be generally specified. We consider two possible rotation axes configurations (i.e., the dependence of the rotation axes for flapping and rowing). For each rotation axes configuration, we propose the kinematic relations between the flapping and rowing motions for a generally specified stroke plane angle and provide the general flapping (or rowing) kinematics as a function of the rowing (or flapping) kinematics, which have not been characterized previously. These results serve as the reference trajectories of the propulsor for specified stroke plane angles and have implications for stroke plane angle control and thus have implications to achieve a broader performance space for biomimetic propulsors.