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Abstract Since the limitation on the onboard equipment, the sparse stereo vision is becoming a suitable choice for the deployment of micro air vehicles (MAV) and small robots. However, for the point feature‐based sparse stereo, most of the current stereo algorithms ignore the similarity between feature points, so it is hard to achieve high accuracy. In addition, the problem of clustered feature distribution will still affect the performance of point feature‐based algorithms in the application. To make up for these deficiencies, the authors propose an improved features from accelerated segment test (FAST) feature detector to suppress the point detection in complex texture regions. Most importantly, the authors present a novel census transform (CT)‐based algorithm that contains two encoders ‘texture orientation’ and ‘texture gradient’ to get a more efficient census bit string for the feature point. Instead of randomly selecting pixels to calculate the bit string, we combine the texture characteristics of the census windows where feature points are located. Compared with the original CT, the processing speed of our method is improved, and the average error of our method is reduced by 18.05%. The evaluation results show the presented improved point feature‐based sparse stereo algorithm has a great value in engineering applications.

Bird wings have split primary feathers that extend out from the wing surface. This structure is called the wingtip slot, which is recognized as a product of bird evolution to improve flight performance. In this paper, numerical simulations based on RANS (Reynolds-averaged Navier–Stokes) equations are conducted to examine and understand the influence of wingtip slots on six wings at Re = 100 000. The overlapping grid method, driven by an in-house UDF (User Defined Function), is used to model the motion of the bionic slotted wings. The motion law of the winglets is improved based on the law extracted from a level-flying bald eagle. Then the aerodynamic force, pressure distribution, vorticity contours, wake stream, and other flow structures of the slotted wings with different layouts were compared and analyzed. The results show a significant increase in aerodynamic force when the slotted wingtips are employed. The maximum lift-to-drag ratio is also improved in our designed wing model with a non-planar wingtip by a maximum of 34% from the base wing. Each winglet works as a single wing due to the existence of slots, with a chordwise pressure distribution similar to that of the main wing. The vortex structures of slotted wings show expressive changes in the tip vortex as compared with the base wing. Additionally, an innovative bionic slotted wing is proposed with a dynamic wingtip that forms varying gaps between winglets. Due to the collective mechanism of aerodynamic interaction among multiple winglets for the innovative wing, it acquires the optimal time-averaged force during a flapping period. As expected, the slotted wingtip reduces the main wingtip vortex intensity and creates weaker vortices. The non-planarity and relative motion of the wingtip strengthen its weakening effect on the wingtip vortex and wake.

An innovative morphing scheme, i.e., dynamical chordwise retracting and stretching, has been proposed for flapping wings. Its aerodynamic effects have been investigated based on a plunging two-dimensional flat wing. The conclusive results suggest that the novel morphing scheme can help to enhance the power utilization efficiency of flapping wings in forward motions. Plunging wings equipped with the novel morphing scheme can generate more thrust for a given power consumption or consume less power for a given thrust generation compared with their non-morphing counterparts. Moreover, the morphing scheme tends to function more positively as the defined morphing amplitude increases. In the currently covered parameter spaces, the largest peak propulsive efficiency improvements induced by the morphing scheme are 12.3% at Re = 15 × 104 and 10.8% at Re = 5 × 104, respectively. Examination of the flow structures around the plunging wings reveals that dynamical chordwise retracting and stretching of the wing can ameliorate the vortex distribution, which could account for the propulsive property changes caused by the morphing scheme. The current study may have yielded an inspiring and prospective scheme for enhancing the propulsive capacity of micro-flapping-wing aerial and underwater robots.

End-to-end deep neural network (DNN)-based motion planners have shown great potential in high-speed autonomous UAV flight. Yet, most existing methods only employ a single high-capacity DNN, which typically lacks generalization ability and suffers from high sample complexity. We propose a novel event-triggered hierarchical planner (ETHP), which exploits the bi-level optimization nature of the navigation task to achieve both efficient training and improved optimality. Specifically, we learn a depth-image-based end-to-end motion planner in a hierarchical reinforcement learning framework, where the high-level DNN is a reactive collision avoidance rerouter triggered by the clearance distance, and the low-level DNN is a goal-chaser that generates the heading and velocity references in real time. Our training considers the field-of-view constraint and explores the bi-level structural flexibility to promote the spatio–temporal optimality of planning. Moreover, we design simple yet effective rules to collect hindsight experience replay buffers, yielding more high-quality samples and faster convergence. The experiments show that, compared with a single-DNN baseline planner, ETHP significantly improves the success rate and generalizes better to the unseen environment.

People have raised their expectations for UAV performance due to the widespread use of UAVs in both military and non-military settings. One of the most significant fields of research right now is how to enhance UAVs’ endurance capabilities. Many birds in the wild have the ability to fly for extended periods of time or great distances using the wind, which is called energy harvesting. A seabird called an albatross can use the wind’s horizontal gradient on the water’s surface to generate energy. By gliding, they may fly for a very long time without flapping their wings, thus lowering their own energy consumption. Due to the albatrosses’ nostrils sensitive sensory nerves and sensitivity to environmental information, such as airflow, the albatrosses can modify its flight attitude. Similar to this, real-time dynamic planning of the trajectory can only be done for UAVs in order to realize energy-capturing flying if correct and real-time wind field information is obtained. As a result, developing wind field sensing technology is crucial to the realization of energy-capturing gliding. In this study, we built a 3D wind field sensing system with wind vane sensor and pitot tube. Wind tunnel tests were used to calibrate and alter it. The system’s operation is initially validated by real flight, which may give environmental information, enabling UAVs to utilize the wind field as a reference for planning their flight paths.

The bionic flapping-wing aircraft has the advantages of high flexibility and strong concealment; however, in the existing flapping-wing aircraft, the platform performance is influenced by the payload capacity, endurance, and durability; additionally, the mission capability is constrained, making it challenging to put into use in real-world scenarios. In response to this issue, this article offers a thorough design approach for a large-span flapping-wing aircraft, focusing on effective flapping wings, effective flapping mechanism design, and enhancement of flapping mechanism reliability, and ultimately realizing the design and verification of a new bionic flapping-wing aircraft with a large wingspan, called “Cloud Owl”. It has a wingspan of 1.82 m and weighs 980 g. The aircraft is capable of autonomous flight and remote control, and it can carry a range of mission-specific equipment. More than 200 flights have been made by “Cloud Owl” so far in Xi’an, Beijing, Tianjin, Tibet, Ganzi, and other places. It has evolved into a flapping-wing aircraft platform with exceptional stability, payload capacity, and long endurance.

Due to the complexity of tailoring the wing flexibility and selecting favorable kinematics, the design of flapping wings is a considerably challenging problem. Therefore, there is an urgent need to investigate methods that can be used to design wings with high energy efficiency. In this study, an optimization model was developed to improve energy efficiency by optimizing wing geometric and kinematic parameters. Then, surrogate optimization was used to solve the design optimization model. Finally, the optimal design parameters and the associated sensitivity were provided. The optimized flapping wing, inspired by hummingbirds, features large geometrical parameters, a moderate amplitude of the flapping angle, and low frequency. With the spanwise twisting deformation considered in the parameterization model, the optimization solver gave an optimized wing with a pitching amplitude of approximately 39 deg at the root and 76 deg at the tip. According to the sensitivity analysis, the length of the wing, flapping frequency, and flapping amplitude are the three critical parameters that determine both force generation and power consumption. The amplitude of the pitching motion at the wing root contributes to lowering power consumption. These results provide some guidance for the optimal design of flapping wings.

Dynamic soaring helps albatross achieve long-distance migration. From a bionic view, dynamic soaring has great potential to enhance unmanned aerial vehicles (“UAVs”) flight range and endurance. The previous application studies focus on flight strategies to guide UAV soaring. However, the energy harvesting efficiency problem emerges. The lack of clear dynamic soaring influencing factors hinders dynamic soaring UAV design and flight strategy design from a theoretical perspective. Hence this paper aims to analyze the influence law of different UAV mass, initial airspeed, and entering angle. Trajectories and flight data in different factors are obtained through trajectory optimization. The results show that UAV mass has a positive influence on energy harvesting. The initial airspeed and entering angle affect both energy efficiency and trajectory. For UAV design, weight balance needs to be considered rather than a pursuit of the lightest. For flight strategy design, finding an optimal initial state will improve energy efficiency.

To deepen our understanding of the aerodynamics by which hummingbirds use flexible wings to hover efficiently during flapping flight, a three-dimensional wing model with dynamic morphing was developed according to the morphological and kinematic data of a hovering hummingbird’s wing. Navier-Stokes equations were solved on a dynamically deforming grid to study wing aerodynamics. In numerical simulations, boundary-based smoothing and overset methodologies were used in combination to update the interior nodes of cells in the computational domain, allowing those nodes to accommodate the motion of the flexible wall. This study showed that the leading edge vortex (LEV) attached to the wing was stable during the downstroke but extremely unstable and shed continuously during the upstroke. In the results of the downward stroke, the different vortices separated from the surface and formed a vortex ring. The difference is that the leading edge vortex induced a vortex ring near the root and a smaller and weaker vortex ring near the wingtip during the upstroke. A significant enhancement in aerodynamic forces was found during the downstroke, along with a large number of power consumptions. We found that the asymmetry in the time-averaged vertical force between the two half strokes was 3.5, and the value was higher than that reported earlier. Aerodynamic force coefficients and efficiency matched well with those of another hummingbird wing (the ruby-throated hummingbird). It is worth noting that hummingbirds can maintain a similar wingtip speed by flapping their wings, but different strategies are adapted to hover efficiently due to the differences in size and body weight. The results of this study help to better understand the aerodynamics of the hovering hummingbird.

The wing planform and flapping kinematics are critical for the hovering flight of flapping wing micro air vehicles (FWMAVs). The degree of influence of wing geometry and kinematic parameters on aerodynamic performance still lacks in-depth analysis. In this study, a sensitivity analysis was conducted based on the quasi-steady aerodynamic model. Each parameter was investigated independently by using the control variable method. The degree of each variable’s influence on lift, power, and power loading is evaluated and compared. Furthermore, detailed exponential relationships were established between the parameters and the corresponding aerodynamic properties. It is found that, for the geometric parameters, wing area has the greatest influence on lift, and the distribution of area has the most visible effect on aerodynamic power. All geometric parameters are negatively correlated with power loading. For the kinematic parameters, flapping frequency, compared with sweeping amplitude, results in faster lift growth and slower drop in power loading, while their influence on aerodynamic power is nearly comparable. A moderate pitching amplitude with advanced rotation will maximize the lift. For the flapping trajectory, lift and power loading are primarily affected by the shape of the pitching motion rather than the sweeping motion. But the sweeping motion seems to dominate the power consumption. The research in this paper is helpful to understand the effect of each parameter and provide theoretical guidance for the development of FWMAVs.

Variation modes of avian wings offer large-scale morphing aircraft an effective approach for solving problems such as the mass center shift in longitudinal trim and control system design during the morphing process. In this paper, a numerical method is established for fully understanding the influence of combined morphing on roll efficiency of a biomimetic wing unmanned aerial vehicle (UAV) from the perspective of aerodynamic change, system convergence time and aerodynamic energy consumption. Analysis and simulation results show that the biomimetic wing provides effective measures to improve the mission execution efficiency of UAV. System oscillation can obviously be reduced with asymmetric sweep angle change as the supervisory control surface for pure roll maneuver, and actuators require higher output power than that with the single deflection of the flexible trailing edge (Flex-TE). In addition, for aircraft design, a larger mass ratio of the inner wing is beneficial to enhance system stability. Energy consumption of wing and Flex-TE show laws of first increasing and then decreasing along the spanwise direction during the morphing process. For actuators of the Flex-TE, the output power of units 1 to 15 should higher than other spanwise units. For the sweep angle generalized control surface, the maximum value of energy consumption per unit time is located near the 20-th spanwise unit.

To solve the attitude control problem of bird-like flapping wing micro air vehicles (FWMAVs) during automatic landing, an active disturbance rejection control (ADRC) architecture is proposed in this paper. This control scheme takes into account the attitude control of flapping, transition and gliding modes in the process of automatic landing. To verify the control effect, the aerodynamic estimation method of the flapping wing based on quasi-steady theory and the dynamics of an FWMAV in Lagrangian form are applied in the simulation. The proposed control architecture consists of two independent ADRC controllers to stabilize the attitude of the pitch and roll channels. The system disturbance and the coupling effects between channels are estimated by an extended state observer (ESO) and compensated in real time in the control output. The convergence of the ESO and ADRC is proven. Simulation results show that even if the aircraft is in different flight modes, the ADRC controller can track the target trajectory quickly and accurately. Then, to realize automatic landing in a real environment, a simplified two-stage landing trajectory is designed. A landing test is carried out on this basis. The test results show that ADRC can not only stabilize the flight attitude in flapping mode but also obtain a satisfactory control accuracy and convergence speed when the aircraft is in the transition and gliding modes, confirming its usefulness in automatic landing.

With the rapid development of high energy laser weapons (HELWs), the integration of HELW on unmanned combat aerial vehicles (UCAVs) has become a hot research topic. To study the impact of HELW on UCAV mission effectiveness, this paper proposes a 4-level design framework based on the system-of-system (SoS) oriented design. To validate the framework, the impact of HELW is analyzed in four aspects: the strike capability, the stealth performance, the vulnerability, and the defense capability, where the UCAV design constrains are noted. Simulation experiments of penetration scenario are carried out using agent-based modeling and simulation. The simulation results show that the integration of HELW can increase the survivability and mission effectiveness rate (MER) of the UCAV, especially for UCAVs with high speed and stealth performance. But the MER of the UCAV does not increase with the increase of HELW output power in most cases, indicating the importance of balancing the HELW and UCAV performances in concept design.

Aiming at the obstacle avoidance trajectory planning problem in the landing process of the micro drone, this paper proposes a swarm optimization algorithm that combines the dragonfly optimization method and the differential evolution method. The orthogonal learning mechanism is adopted to realize the adaptive switch between the two algorithms. In the process of landing route planning, the planning plane is first obtained by making the gliding plane tangent to the obstacle. In the planning plane, the projection of obstacle is transformed into multiple unreachable line segments. By designing an optimization model, the 3D landing route planning problem is transformed into a 2D obstacle avoidance route optimization problem. Taking the shortest route as the optimization objective, the penalty factor is introduced into the cost function to avoid the intersection of the landing route and obstacle. During the optimization process, through the orthogonal learning of the intermediate iterative results, the hybrid algorithm can adaptively select the next iterative algorithm, so it can give full play to the respective advantages of the two algorithms. The optimization results show that, compared with the single optimization algorithm, the hybrid optimization algorithm proposed in this paper can better solve the problem of landing route planning for micro-small UAVs.

The input constraints of control surfaces may lead to saturations, which could limit the achievable performance or even cause instability of vertical take-off and landing (VTOL) tail-sitter unmanned aerial vehicle (UAV). To improve flight safety and attitude tracking performance, a novel L1 adaptive control architecture for attitude tracking of a tail sitter subjected to input constraints is proposed in this study. The imprecise mathematical model, low weight, and small size all present different challenges when designing a control system. In this work, a feedforward compensator is first used to narrow down the uncertain bounds with the consideration of finite accuracy from prior knowledge. Second, according to the linearized model, a baseline controller is designed to offer basic performance for a nominal system without uncertainty. Finally, the L1 adaptive controller is developed to compensate for unmatched uncertainties based on the control system developed before. The stability and performance bounds of the closed-loop system are analyzed to illustrate the impact of input constraints. The numerical simulations and flight tests are carried out to verify the improved attitude tracking performance when input saturation exists.

In this paper, we propose an error importance measure (EIM) method to quantify the relative contribution of error sources to the system output error. In order that the contribution of both variation and mean shift of error sources are taken into account, we use second-order moment to characterize the degree of system output deviation from ideal or target value. First, the second-order moment is decomposed into a series of terms through the Taylor series expansion, and the individual effects, the interaction effects, and the total effects for one or a group of error sources are defined accordingly. Second, we define the EIM indices as the value of total effect divided by second-order moment of output error. Third, three test models and an application case are introduced to demonstrate the effectiveness and engineering significance of the proposed EIM indices. The results show that the EIM indices can reflect both the impacts of variation and mean the shift in error sources.

In reliability analysis for the practical engineering problems with the time-consuming model, it has become an important challenge that how to obtain accurate reliability assessment with a minimum number of calls. In order to reduce the computational cost, this paper develops a new sequential surrogate method combining adaptive kriging and Markov chain Monte Carlo simulation with a novel learning strategy for reliability analysis. The proposed method is named AK-MCMC, which takes full advantage of the classification feature of reliability analysis based on the surrogate models, and it can efficiently approximate the classification boundary of the performance function. First, the learning strategy is developed to sequentially pick out the informative samples for updating the experimental design samples. Then, a new stopping criterion is adopted to guarantee the classification accuracy of the constructed kriging model. In this way, the proposed method skillfully makes reliability evaluation independent of an adaptive iterative process, which greatly improves the efficiency of model refinement. Finally, the proposed method is applied to several examples, which contain small failure probability problem, non-linearity problem, and engineering problem with an implicit performance function. In particular, the efficiency of the proposed AK-MCMC method is proved for the problems with small failure probability.

With the purpose to improve the performance of the flexible membrane flapping wing, a kind of flapping wing with an attached airfoil to the root is designed and tested in a wind tunnel. In order to study, the actual performance improvement of the flapping wing, the flight performance is calculated and compared instead of the aerodynamic performance. A method to establish a semi-empirical cycle-averaged mathematical model for the flapping wing with high precision based on the wind tunnel experiment is proposed to calculate the flight performance. The established continuous mathematical model can solve the problem of how to obtain the trimmed state by using discrete experimental data. By attaching the airfoil EPPLER 378 with a thickness of 4.07% chord length to the root, the cruise velocity envelope is expanded, especially with a small cruise velocity. Although the attached airfoil to the root can cause a slight decrease in endurance and range of the flapping wing, it will greatly enhance the climbing performance and effectively reduce the demand for the takeoff sites. Besides, the flapping wing with an attached airfoil to the root will result in a significant reduction in the radius of steady turning and will improve the maneuverability. In addition, the airfoil EJ 85, which is similar to the airfoil EPPLER 378 in shape and camber but has a larger thickness of 6.5% chord length, is attached to the root of the original flapping wing. The same experiment and modeling procedure are performed on the airfoil EJ 85 to analyze the difference in performance of the flapping wing with different airfoil thickness.

Large-scaled flapping wings for high altitude environments have great potential for border patrol and biodiversity exploration due to their high flight efficiency and concealment. In this paper, wind tunnel experimental techniques, neural network models, and flight tests are implemented to optimize and validate the performance of flapping wings. Numerical simulation methods were used to give recommendations for the flight state of the vehicle at high altitudes. From sea level to 4000 m altitude, the Reynolds number was subsequently reduced by 27.98%, and the time-averaged lift, drag, and pitching moment decreased by 33.31%, 33.08%, and 33.33%, respectively. A combination of planform with an increase in the internal area of the wing, six wing ribs, and linen film material was selected for its moderate stiffness to generate at least 1300 g of lift and considerable positive thrust, making it easier to reach a trim state. For high altitude environments, the vehicle needs to increase its flight speed and frequency to compensate for the loss of lift and drag due to reduced air density, but this is at the cost of power consumption, which results in reduced endurance, as verified by flight tests. Finally, this study aims to provide guidance on the design of large-scaled flapping wings for high-altitude environments.

Albatross have a significant gliding ability called dynamic soaring. In the bionic study, dynamic soaring has a great potential to enhance Unmanned Aerial Vehicles (UAVs) flight range and endurance. The previous study on dynamic soaring mainly focused on the energy harvesting mechanism, trajectory optimization, and flight test. However, the longtime optimization became the challenge of the UAV dynamic soaring application. Similarly, due to the difficulty of the dynamic soaring test flight, both the feasibility of dynamic soaring application and the energy harvesting mechanism model’s accuracy lack validation. This paper proposes a new method to give a dynamic soaring trajectory, and a flight simulation program is built to realize the dynamic soaring flight test. The results show that the new method can guide the UAV to achieve dynamic soaring and verify the improvement of its flight performance through dynamic soaring. Meanwhile, the accuracy of the energy harvesting mechanism model is verified. The results indicate the indirect way to achieve the UAV dynamic soaring and provide a flight test foundation for a profound dynamic soaring mechanism study.

Understanding the mechanisms of trim and flight stability in birds is critical to guide the design of bionic micro air vehicles. The complex movements (plunging, sweeping, twisting) and morphing of wings always keeps the flapping flight of birds in dynamic equilibrium, which makes it difficult to determine the critical factors of trim and stability. Hence, a model has been developed that takes real complex movement and the calculation of unsteady aerodynamics into consideration. Two trim methods, including wash-out and forward-sweep, have been used to achieve equilibrium in the longitudinal direction. It is interesting to find that these two methods are both important to realize a larger take-off weight, lower power consumption, and stronger longitudinal stability. This implies that the seagull probably uses both of them to obtain the requirement of equilibrium and stability, which further inspires the design of seagull-inspired micro air vehicles.

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.

Birds in nature have many unique devices to help them acquire excellent flight abilities under various complex flight conditions. One of the unique devices is the leading-edge alula, located at the junction of the arm wing and the hand wing of most birds. It often spreads out during takeoff and landing, probably playing a similar role to high-lift devices in fixed-wing aircraft. This paper analyzed and reviewed the results of current research on leading-edge alula, finding some important factors, such as the complex flapping motions, flexibility, and the plane and section shape of the wing, that have been ignored in current research to a certain extent. These would greatly affect the conclusions obtained. Hence, for a deeper understanding of the aerodynamic mechanisms and functions of the alula, some new study predictions for future research are presented. In addition, the feasible models and methods for further research based on these predictions are discussed and proposed. For example, the higher-accuracy LES or hybrid LES/RANS method and the combinations of these methods with wind-tunnel experiments using PIV technology are recommended.

Rotor airfoil design involves multi-point and multi-objective complex constraints. How to significantly improve the maximum lift coefficient and lift-to-drag ratio of rotor airfoil is a fundamental problem, which should be solved urgently in the development of high-performance helicopter rotor blades. To address this, discrete co-flow jet (DCFJ) technology is one methods with the most potential that can be harnessed to improve the performance of the rotor airfoil. In this study, wind tunnel experiments are conducted to study the effect of DCFJ technology on lift enhancement and drag reduction of OA312 airfoil. Furthermore, the performance improvement effects of the open co-flow jet (CFJ) and DCFJ technologies are studied. In addition, the influence of fundamental parameters, such as the obstruction factor and relative unit length, are analyzed. Results demonstrate that DCFJ technology is better than CFJ technology on the performance enhancement of the OA312 airfoil. Moreover, the DCFJ rotor airfoil can significantly reduce the drag coefficient and increase the maximum lift coefficient and the stall angle of attack. The maximum lift coefficient can be increased by nearly 67.3%, and the stall angle of attack can be delayed by about 12°. The DCFJ rotor airfoil can achieve the optimal performance when the obstruction factor is 1/2 and the relative unit length is 0.025.

Making full use of wind energy can effectively alleviate the global energy shortage and environment contamination problems. Nevertheless, how to significantly improve the performance of the wind turbine airfoil and blade is a crucial issue. As the novel flow control method, the co-flow jet (CFJ) technology is one of the most potential methods to solve this problem. Thus, the effects of the CFJ technology on the performance enhancement of the S809 airfoil and Phase VI wind turbine blade are explored in this study. Furthermore, the effects of the injection location and jet momentum coefficient are studied, and an adaptive jet momentum coefficient strategy of the CFJ technology is proposed. Results demonstrate that the CFJ technology can significantly improve the maximum lift coefficient and maximum corrected lift-to-drag ratio of the S809 airfoil. Moreover, the power coefficient of the Phase VI wind turbine blade at the low tip speed ratio is greatly enhanced as well. In particular, the maximum lift coefficient and maximum corrected lift-to-drag ratio of the typical S809 CFJ airfoil with adaptive Cμ are improved by 119.7% and 36.2%, respectively. The maximum power coefficient of CFJ blade can be increased by 4.5%, and the power coefficient of CFJ blade can be boosted by 226.7% when the tip speed ratio is 1.52.

In this study, a design scheme for a high-aspect-ratio bionic multi-section variable-sweep wing unmanned aerial vehicle (UAV) that utilizes the reverse coordinated change in the sweep angle of the inner and outer wing sections is proposed, which improves the aerodynamic performance and realizes the self-trim compensation of the wing’s centroid. According to the layout characteristics of this type of UAV, a reasonable distribution design of the wingspan ratio of the inner and outer sections is explored, to reduce the impact of aerodynamic center movement and moment of inertia change. The calculation and analysis results show that the coordinated variable-sweep scheme can significantly improve the influence of sweep angle change on the longitudinal static stability margin of UAVs with a high aspect ratio. The coordinated sweep angle change in the inner and outer wing sections can not only reduce the drag during high-speed flight, but also play a significant role in improving the performance of the aircraft at different stages in the mission profile. Appropriately increasing the wingspan proportion of the inner section can reduce the trim resistance of the V-tail, reduce the thrust of the engine, and increase the range and duration of the UAV. From the perspective of stability change, the multi-section variable-sweep wing UAV with a wingspan ratio of the inner and outer sections that is between 1.41 and 1.78 has better dynamic stability performance. Among them, the UAV with a wingspan ratio of the inner and outer sections that is equal to 1.41 has better longitudinal stability performance, while the UAV with a wingspan ratio of the inner and outer sections that is equal to 1.78 has better lateral/directional stability performance.

A flexible flapping wing with a rectangular planform was designed to investigate the influence of flexible deformation. This planform is more convenient and easier to define and analyzed its deforming properties in the direction of spanwise and chordwise. The flapping wings were created from carbon fiber skeleton and polyester membrane with similar size to medium birds. Their flexibility of deformations was tested using a pair of high-speed cameras, and the 3D deformations were reconstructed using the digital image correlation technology. To obtain the relationship between the flexible deformation and aerodynamic forces, a force/torque sensor with 6 components was used to test the corresponding aerodynamic forces. Experimental results indicated that the flexible deformations demonstrate apparent cyclic features, in accordance with the flapping cyclic movements. The deformations in spanwise and chordwise are coupled together; a change of chordwise rib stiffness can cause more change in spanwise deformation. A certain lag in phase was observed between the deformation and the flapping movements. This was because the deformation was caused by both the aerodynamic force and the inertial force. The stiffness had a significant effect on the deformation, which in turn, affected the aerodynamic and power characteristics. In the scope of this study, the wing with medium stiffness consumed the least power. The purpose of this research is to explore some fundamental characteristics, as well as the experimental setup is described in detail, which is helpful to understand the basic aerodynamic characteristics of flapping wings. The results of this study can provide an inspiration to further understand and design flapping-wing micro air vehicles with better performance.

It is challenging and important to establish reliability model for mechanical systems with multiple functional performances. This article develops a model of competing failure for mechanical systems under catastrophic failure due to external shocks and multi-functional failures. Shock model is used to analyze the catastrophic failure. Functional failures are demonstrated by functional indicators which are functions of specific variables including constant variables and degradation variables. Functional failure occurs if the functional indicators are beyond a limit δ . Brownian motion with non-linear drift is used to model the process of degradation variables. Besides, there are two kinds of dependence around these failure modes. The dependence of catastrophic failure and functional failure, due to shared external shock source, is illustrated by the instantaneous increments during degradation process; the dependence between multiple functional performances themselves is modeled by copula functions. Reliability of systems is calculated based on this model. A mechanical lock system is chosen as a case to verify the validity and practicability. This model can also be applied for other mechanical systems.

Avian flight has an outstanding performance than the manmade flapping wing MAVs. Considering that the feather is light and strong, a new type of the flapping wing was designed and made, whose skeleton is carbon fiber rods and covered by goose feathers as the skin. Its aerodynamics is tested by experiments and can be compared with conventional artificial flapping wings made of carbon fiber rods as the skeleton and polyester membrane as the skin. The results showed that the feathered wing could generate more lift than the membrane wing in the same flapping kinematics because the feathered wing can have slots between feathers in an upstroke process, which can mainly reduce the negative lift. At the same time, the power consumption also decreased significantly, due to the decrease in the fluctuating range of the periodic lift curve, which reduced the offset consumption of lift. At the same time, the thrusts generated by the feather wing and the membrane wing are similar with each other, which increases with the increase of flapping frequency. In general, the aerodynamic performances of the feather wing are superior to that of the membrane wings.