Flexible Flapping Wing Research Articles

Resonance mechanism of flapping wing based on fluid structure interaction simulation

Certain insect species have been observed to exploit the resonance mechanism of their wings. In order to achieve resonance and optimize aerodynamic performance, the conventional approach is to set the flapping frequency of flexible wings based on the Traditional Structural Modal (TSM) analysis. However, there exists controversy among researchers regarding the relationship between frequency and aerodynamic performance. Recognizing that the structural response of wings can be influenced by the surrounding air vibrations, an analysis known as Acoustic Structure Interaction Modal (ASIM) is introduced to calculate the resonant frequency. In this study, Fluid Structure Interaction (FSI) simulations are employed to investigate the aerodynamic performance of flapping wings at modal frequencies derived from both TSM and ASIM analyses. The performance is evaluated for various mass ratios and frequency ratios, and the findings indicate that the deformation and changes in vortex structure exhibit similarities at mass ratios that yield the highest aerodynamic performance. Notably, the flapping frequency associated with the maximum time-averaged vertical force coefficient at each mass ratio closely aligns with the ASIM frequency, as does the frequency corresponding to maximum efficiency. Thus, the ASIM analysis can provide an effective means for predicting the optimal flapping frequency for flexible wings. Furthermore, it enables the prediction that flexible wings with varying mass ratios will exhibit similar deformation and vortex structure changes. This paper offers a fresh perspective on the ongoing debate concerning the resonance mechanism of Flexible Flapping Wings (FFWs) and proposes an effective methodology for predicting their aerodynamic performance.

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.

Nonlinear Aeroelastic Analysis for Highly Flexible Flapping Wing in Hover

Aeromechanics of highly flexible flapping wings is a complex nonlinear fluid–structure interaction problem and, therefore, cannot be analyzed using conventional linear aeroelasticity methods. This paper presents a standalone coupled aeroelastic framework for highly flexible flapping wings in hover for micro air vehicle (MAV) applications. The MAV-scale flapping wing structure is modeled using fully nonlinear beam and shell finite elements. A potential-flow-based unsteady aerodynamic model is then coupled with the structural model to generate the coupled aeroelastic framework. Both the structural and aerodynamic models are validated independently before coupling. Instantaneous lift force and wing deflection predictions from the coupled aeroelastic simulations are compared with the force and deflection measurements (using digital image correlation) obtained from in-house flapping wing experiments at both moderate (13 Hz) and high (20 Hz) flapping frequencies. Coupled trim analysis is then performed by simultaneously solving wing response equations and vehicle trim equations until trim controls, wing elastic response, inflow and circulation converge all together. The dependence of control inputs on weight and center of gravity (cg) location of the vehicle is studied for the hovering flight case.

Simulation of Aerodynamic Performances of Flexible Flapping Wing Airfoils

Compared with fixed wings, the flapping wing has a significant aerodynamic performance advantage at low speeds and low Reynolds numbers, which draws more and more attentions. But most previous studies focus on rigid flapping airfoils, the aerodynamic performances of flexible flapping airfoils are still unclear. A fluid-solid coupling model for the flexible elliptical airfoils was developed to analyze the flow field around the airfoil, the airfoil deformation and the aerodynamic characteristics of airfoils, at different wind speeds and attack angles. Compared with the rigid airfoil, the flexible airfoil can delay the shedding time of the wake vortex and reduce the oscillation frequency of the disturbance on the lift force. The flexible airfoil significantly suppresses the disturbance of the wake flow and reduces the oscillation amplitude of disturbance. Even, the airfoil disturbance oscillation can be completely eliminated at an appropriate Young’s modulus of the airfoil. These results provide a theoretical guidance for the design of soft aircraft.

Simulation For Aerodynamic Performance of flexible Flapping Wing Airfoil

Simulation For Aerodynamic Performance of flexible Flapping Wing Airfoil

Coupled Aeroelastic Study of a Flexible Micro Air Vehicle-Scale Flapping Wing in Hovering Flight

Instantaneous force and structural deformation experiments were performed on a flexible, structurally characterized, low-aspect-ratio representative flapping wing. A six-component force balance was used to measure aerodynamic-force time history over a flap cycle. A VICON motion capture setup tracked the wing deformations while flapping. Measured aerodynamic forces and wing deformations were compared against coupled computational fluid dynamics/computational structural dynamics (CFD/CSD) aeroelastic analysis results. The CFD solver is an unsteady Reynolds-averaged Navier–Stokes solver. The CSD solver is a general-purpose multibody dynamics solver capable of modeling geometrically nonlinear beam and shell elements. The CFD/CSD results were able to capture the overall trend in aerodynamic forces and wing deformations. Although predicted and measured variations in lift were similar, the drag-force magnitudes tended to be underpredicted by the coupled aeroelastic solver. The coupled CFD/CSD solver was used to evaluate the influence of wing flexibility on wing performance. Results showed that, in particular instances, decreasing wing stiffness increased the time-averaged aerodynamic lift and lift-to-power ratio. Decreased wing stiffness led to reduced leading-edge vortex circulation strength. This suggests that the increased wing performance is mainly due to a redirection of the resultant force vector allowed by the increased wing compliance.

A Wind Tunnel Experimental Study on the Flexible Flapping Wing With an Attached Airfoil to the Root

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.

Bio-Inspired Flexible Flapping Wings with Elastic Deformation

Over the last decades, there has been great interest in understanding the aerodynamics of flapping flight and development of flapping wing Micro Air Vehicles (FWMAVs). The camber deformation and twisting has been demonstrated quantitatively in a number of insects, but making artificial wings that mimic those features is a challenge. This paper reports the development and characterization of artificial wings that can reproduce camber and twisting deformations. By replacing the elastic material at the wing root vein, the root vein would bend upward and inward generating an angle of attack, camber, and twisting deformations while the wing was flapping due to the aerodynamic forces acting on the wing. The flapping wing apparatus was employed to study the flexible wing kinematics and aerodynamics of real scale insect wings. Multidisciplinary experiments were conducted to provide the natural frequency, the force production, three-dimensional wing kinematics, and the effects of wing flexibility experienced by the flexible wings. The results have shown that the present artificial wing was able to mimic the two important features of insect wings: twisting and camber generation. From the force measurement, it is found that the wing with the uniform deformation showed the higher lift/power generation in the flapping wing system. The present developed artificial wing suggests a new guideline for the bio-inspired wing of the FWMAV.

The Aerodynamic Improvement of a Flexible Flapping Wing

The Aerodynamic Improvement of a Flexible Flapping Wing

Aeroelastic Analysis of Avian-Based Flexible Flapping Wings for Micro Air Vehicles

Aeroelastic Analysis of Avian-Based Flexible Flapping Wings for Micro Air Vehicles

Aerodynamic Performance of Micro Flexible Flapping Wing by Numerical Simulation

The aerodynamic performance of flexible flapping wing is investigated through numerical simulation based on fluid-structure coupling method, which is developed on the Navier-Stokes equations and structural dynamic equations. The interface data exchange method is based on radial basis function and the moving mesh generation method is based on infinite interpolation. The flexible flapping wings we investigated are constructed of a carbon fiber skeleton and a thin extensible membrane in a bird like scale with the chord Re of 5e4. This research focused on the influence of different flexibilities with same wing structure topology. The flexibility is controlled by diameters of carbon fiber skeleton and five structures are analyzed. The simulation results proved that the structural deformation has a significant effect on the aerodynamic force, especially on the thrust. A wing with maximum rigidity has the best lift performance but the worst thrust performance, and vice versa. A wing with medium rigidity gets better performance in both lift and thrust. According to the results, it is necessary to balance the relations between structure flexibility and aerodynamic performance. Meantime, the characteristics of different structures are consistent with the variety of attack angle and flapping frequency.

Optimization of Flexible Flapping-Wing Kinematics in Hover

Hover-capable flapping-wing micro air vehicles are well suited for missions in confined spaces. The best design practices for flapping wings and their kinematics are largely unknown, especially for flexible wings. To address this issue, numerical optimization is applied to the design of the kinematics and structural sizing of a flapping wing using a surrogate-based approach. The surrogates are generated using kriging interpolation of the time-averaged thrust generated and power required by the wings. The thrust and power data are computed using a nonlinear approximate aeroelastic model developed in previous studies by the authors. A numerical optimization algorithm is used to identify designs that produce the desired combination of thrust and power. The design variables consist of parameters describing the flap–pitch kinematics and the stiffness of the flexible wings. A trend study of thrust and power indicate that the phase angle between flap and pitch motions significantly affects the wing performance when the stroke amplitudes and the frequency are fixed. Smaller amounts of pitch actuation produced peak thrust in flexible wings when compared to the rigid wings. Several flexible configurations produce higher thrust when compared to the best thrust-producing rigid configuration. However, rigid wings have higher propulsive efficiency when compared to flexible wings for the same amount of generated thrust. Thus, the actual design of a flapping wing will depend on the relative importance given to thrust production and propulsive efficiency.

Experimental Investigation on the Aerodynamics of a Bio-Inspired Flexible Flapping Wing Micro Air Vehicle

An experimental investigation on a 10 cm bio-inspired flexible Flapping-Wing Micro Air Vehicle (FWMAV) was conducted in both hovering and forward-flight conditions with the objective to characterize its aerodynamic performance. The measurements in hovering conditions were performed with the particular objective to explore the effect of different wing configurations (i.e. different aspect ratios and wing flexibilities), whereas forward flight tests in a wind tunnel were carried out to assess the aerodynamic performance of the FWMAV as a function of flow speed, flapping frequency and body angle. The cyclic variation of forces (lift and thrust) generated as a result of the wing flapping was captured by means of a high-resolution force sensor, in combination with high-speed imaging to track the wing motion. Results of measurements in hover show that the flapping frequency, aspect ratio and wing flexibility have a crucial impact on the efficiency and the force generation during the flapping cycle. An estimated flight envelop for the MAV's operation is defined from the data obtained in the wind tunnel measurements. Furthermore, additional tests on several brushless DC motors provide a feasible option in future engine selection and design.

Dynamics and Control of a Flexible Flapping Wing Aircraft

To investigate the aerodynamic performance of a flexible flapping wing aircraft, a flapping-wing system were design and an experiment were set up to measure the unsteady aerodynamic forces of the flapping motion. The thrust formula and resistance formula described aerodynamic forces. The lift and thrust of this mechanism were measured for different angles of attack and wind velocities. Results indicate that the thrust increases with the flapping frequency and the lift increase with the wind velocity, while the lift coefficient decreases while the velocity increases. It is realized that the wing’s transformation which imitated birds leads less resistance when flapping upward which impacts the aerodynamic lift generation and the bionic winglet leads to a change in the leading edge vortex and span-wise flow structures, which decrease the airflow’s backward pull. Models were introduced which were used in the design process and show its aerodynamic performance. The flexible flapping wing vehicle is still an open research area.

Simulation and Experiment for Rigid and Flexible Wings of Flapping-Wings Microrobots

In this paper, an insect-based flapping-wing flying microrobot was built which can successfully fly in the sky. The unsteady aerodynamics associated with this microrobot was studied by using the method of computational fluid dynamics (CFD). On the basis of numerical simulation, the Fluid-Structure coupling mechanics for flexible flapping-wings were studied and discussed. According to the practically developed flapping-wing microrobot, a 2-D simulation model for flexible flapping-wings was established. Fluid-Structure coupling deformation and the effects of this model on the aero dynamic performance were analyzed, which have offered a theoretical basis for design of the aircraft with flexible flapping-wing. In order to verify the results of numerical simulation, aerodynamic performance tests have been conducted for the rigid and flexible flapping-wings in a low turbulence and low Reynolds number wind tunnel.

Force Measurements and Flow Visualization for a Flexible Flapping Wing Mechanism

This paper describes a series of experiments using a flapping wing mechanism. Force measurements were combined with high speed photography for two sets of flexible wings. The aerodynamic forces generated by flexible membrane type wings were measured using a two degree of freedom force balance constructed during the course of these experiments which measured the aerodynamic forces of lift and thrust. Lift and thrust measurements were acquired as the mechanism flapped the flexible wings for multiple cases, and the two most interesting conditions were explored in more detail. These two conditions consisted of a zero velocity free stream condition and a forward flight condition of 5 m/s. For these two conditions, high-speed video of the flapping wing was taken. The images from the video were also correlated with cycle averaged aerodynamic forces produced by the mechanism. Several observations were made regarding the behavior of flexible flapping wings that should aid in the design of future flexible flapping wing vehicles. In addition, flow visualization images were taken of the flapping wing under forward flight condition and flow field velocity vectors were calculated.

COMBINATION OF MESH DEFORMATION AND OVERSET GRID FOR SIMULATING UNSTEADY FLOW OF INSECT FLIGHT

As numerical simulation of unsteady flows due to moving boundaries such as flexible flapping-wings is difficult by conventional approaches, an effective strategy which combines mesh deformation based on Delaunay graph mapping and unstructured overset grids is proposed in this paper. A Delaunay graph is generated for each body-fitted grid cluster which overlaps or is embedded within an off-body background grid cluster. At each time step, the graph moves according to the wing's motion and deformation, and the grids move to new positions according to a one-to-one mapping between the graph and the grid. Then, intergrid-boundary definition is implemented automatically for computation.