Flapping Wing Aircraft Research Articles

A wind tunnel investigation of the unsteady aerodynamic characteristics of bionic flapping wings with three-dimensional complex motion

The bionic flapping wing aircraft realizes flight by imitating the structure and flapping mode of birds. In this paper, a three-dimensional composite motion aerodynamic analysis model of a bionic flapping wing is established. The diverting field and flapping wing grid are divided up using dynamic hybrid grid technology. The flow field of the flapping wing is analyzed by solving the Navier–Stokes (N–S) equation combined with the Boussinesq hypothesis. The lift of the flapping wing under different flutter frequencies and incoming wind speeds is studied under the asymmetric flutter condition. In order to verify the accuracy of the aerodynamic simulation results, a flapping-wing shrinking-ratio model prototype is made, and low-speed wind tunnel experiments are carried out to test the changes in the flight lift of the wing at different flutter frequencies and angles of attack. A comparative analysis of the wind tunnel experiment and the aerodynamic simulation results shows that when the flapping frequency (1–5 Hz) and incoming wind speed (1–5 m/s) increase, the lift force generated by the wing flapping increases. Due to the deviation between the experimental sample airfoil area and the simulated airfoil area, as well as the wing-driven fuselage vibration during the experiment, a sensor error is produced, resulting in a deviation of about 1 N between the experimental result curve and the simulation result curve. However, the aerodynamic characteristics obtained from the aerodynamic simulation analysis are basically consistent with the aerodynamic change law measured in the wind tunnel experiment.

Dynamic CFD/CSD Coupling Research for Flexible Micro Aircraft

Bio-inspired flapping wing aircraft have made progress in recent years. Flying birds can fly in various low Reynolds numbers (103-105) with high flight stability and maneuvering ability by use of the flexible deformation of skeletal muscle structures or thin-film structure wings. Early studies mainly focused on the aerodynamics and kinematics of the flapping wings, with relatively little research on the flexible structure, and the aerodynamic structure coupling characteristics of micro flappings are still unclear. At present, the focus of research is to evaluate the impact of structural deformation on the aerodynamic characteristics of flexible micro-flapping wing. A CFD/CSD coupling solver are developed and several technical challenges related to the CFD/CSD interface are resolved in this study (CFD/CSD data exchange method and moving mesh generation). The aerodynamic characteristics of five flexible wings with different structural parameters are calculated and compared. The results show that structural parameters play an important role in aerodynamic performance, which also indicates that the potential aerodynamic performance can be improved by the optimal structural design.

Numerical study on the aerodynamic performance of the three-dimensional wing of a jellyfish-like flyer

The jellyfish-like flying machine is a new development direction of the future bionic flapping-wing aircraft besides the insect-mimic and bird-mimic micro air vehicles (MAVs). To better understand the underlying fluid mechanisms of the jellyfish-like flyer, we numerically simulated the aerodynamic forces of the three-dimensional flapping wings under different control parameters. The effects of flapping amplitude, vortex wake, up-flight speed, and wing–wing interaction on aerodynamic performance were investigated. The results show that, at hovering, the mean lift rises rapidly at first and then tends to be stable with the increase in flapping amplitude. The vortex wake can improve the lift at large flapping amplitudes, while it reduces the lift at very small flapping amplitudes. With the increase in up-flight speed, the lift decreases. However, the sources of lift reduction are different for different flapping amplitudes. When the two wings flap together and the distance between the wings is small enough, the wing–wing interaction can improve the lift by about 15% compared with that of a single wing, but much higher power is required, resulting in lower efficiency. The results of this study provide new insights into the flight mechanism of the jellyfish-like aircraft and have important guiding significance for the design and optimization of the jellyfish-like flying machine.

Difficulty of Controlling a Flapping-wing Aircraft Compared to a Rotary-wing Aircraft

Difficulty of Controlling a Flapping-wing Aircraft Compared to a Rotary-wing Aircraft

Utilizing Dynamic Scattering for Learning Radar Cross-Section of a Flapping-Wing Aircraft

To study the radar cross-section (RCS) of a flapping-wing aircraft, a method of flapping-wing dynamic scattering is presented. The aircraft has two rigid wings and a fuselage similar to a bird’s body, where the plane shape of the wing is U-type. The aircraft model is established and the pitching and flapping actions of the wing are simulated. The electromagnetic scattering characteristics of aircraft under two flight modes, different radar wave frequencies and observation angles are investigated. The results show that in the given gliding mode, the fuselage provides the main contribution to the lateral peak value. For the given forward azimuth range in the flapping mode, increasing the azimuth can reduce the mean and peak of the aircraft dynamic RCS curve, while increasing the elevation angle will increase the peak and mean of the aircraft dynamic RCS curve. In the flapping mode, the mean RCS of the aircraft in the lateral backward azimuth is significantly lower than that in the lateral forward azimuth. For the given tail azimuth range, the increase of azimuth will increase the mean and peak level of aircraft dynamic RCS curve. The presented method is effective to study the dynamic RCS characteristics of the flapping-wing aircraft.

Design of an autonomous flight control system for imitation falcon flapping-wing aircraft

Bionic flapping-wing aircraft is a new type of aircraft with broad application prospects in the civil and military fields due to their low noise, good concealment, strong maneuverability, and high energy efficiency. The autonomous flying ability of bionic flapping-wing aircraft is the key to improving their flight efficiency. Although autonomous flight research on aircraft at home and abroad have made some achievements, few researchers have conducted corresponding studies on bionic flapping-wing aircraft. The unique driving structure of the bionic flapping-wing aircraft introduces great challenges to research on its autonomous flight control. In this paper, an autonomous flight control system is designed on the research platform of falcon flapping-wing aircraft. In consideration of the load problem of falcon flapping-wing aircraft, the hardware system is designed based on an STM32 microchip. A linear/nonlinear switching guidance algorithm is proposed considering the guidance accuracy and operation speed, due to the limited computing power of microcomputing platforms. Simulations prove that the proposed algorithm is more suitable for imitation falcon flapping-wing aircraft than linear guidance and nonlinear guidance algorithms. A cascade PID controller for roll angle and height is designed in consideration of the mechanism lag of falcon flapping-wing aircraft. By using this approach combined with ground station software for the imitation falcon flapping-wing aircraft, the task of autonomous fixed-height arc trajectory tracking based on imitation falcon flapping-wing aircraft is finally realized.

Design and Research on Flapping Mechanism of Biomimetic Albatross

In order to solve the problems of low flying efficiency, poor aerodynamic performance of wing and short flying time distance existing in the research of flapping wing aircraft, a kind of albatron-like flapping wing structure with higher flying efficiency is proposed in this paper. The functions of bird wing flutter, folding and gliding are realized by two degrees of freedom control respectively. First, the flying characteristics of albatross are analyzed and the flying characteristics suitable for albatross are summarized. Propose design requirements for bionic design objectives; The bionic structure design is carried out by referring to the physical structure of wing and the flight motion law, and the key parameters such as the size and angle of connecting rod mechanism are analyzed, calculated and optimized. A clamping mechanism is designed to achieve gear stuck in order to achieve gliding kinetic energy. The three-dimensional model of the flapping wing mechanism is built by solid works, and the motion analysis is carried out by using Motion plug-in. The analysis results of key-point parameters are output, which verifies the movement law and requirements of the mechanism to achieve the design objectives.

Adaptive Finite-Time Fault-Tolerant Control for Uncertain Flexible Flapping Wings Based on Rigid Finite Element Method

The bionic flapping-wing robotic aircraft is inspired by the flight of birds or insects. This article focuses on the flexible wings of the aircraft, which has great advantages, such as being lightweight, having high flexibility, and offering low energy consumption. However, flexible wings might generate the unexpected deformation and vibration during the flying process. The vibration will degrade the flight performance, even shorten the lifespan of the aircraft. Therefore, designing an effective control method for suppressing vibrations of the flexible wings is significant in practice. The main purpose of this article is to develop an adaptive fault-tolerant control scheme for the flexible wings of the aircraft. Dynamic modeling, control design, and stability verification for the aircraft system are conducted. First, the dynamic model of the flexible flapping-wing aircraft is established by an improved rigid finite element (IRFE) method. Second, a novel adaptive fault-tolerant controller based on the fuzzy neural network (FNN) and nonsingular fast terminal sliding-mode (NFTSM) control scheme are proposed for tracking control and vibration suppression of the flexible wings, while successfully addressing the issues of system uncertainties and actuator failures. Third, the stability of the closed-loop system is analyzed through Lyapunov's direct method. Finally, co-simulations through MapleSim and MATLAB/Simulink are carried out to verify the performance of the proposed controller.

The forewing of a black cicada Cryptotympana atrata (Hemiptera, Homoptera: Cicadidae): Microscopic structures and mechanical properties.

Insects in nature flap their wings to generate lift force and driving torque to adjust their attitude and control stability. An insect wing is a biomaterial composed of flexible membranes and tough veins. In this paper, we study the microscopic structures and mechanical properties of the forewing of the black cicada, Cryptotympana atrata. The thickness of the wing membranes and the diameter of veins varied from the wing root to the tip. The thickness of the wing membranes ranged from 6.0 to 29.9μm, and the diameter of the wing veins decreased in a gradient from the wing root to the tip, demonstrating that the forewing of the black cicada is a nonuniform biomaterial. The elastic modulus of the membrane near the wing root ranged from 4.45 to 5.03 GPa, which is comparable to that of some industrial membranes. The microstructure of the wing vein exhibited a hollow tubular structure with flocculent structure inside. The "fresh" sample stored more water than the "dry" sample, resulting in a significant difference in the elastic modulus between the fresh and dried veins. The different membrane thicknesses and elastic moduli of the wing veins near the root and tip resulted in varied degrees of deformation on both sides of the flexion line of the forewing during twisting. The measurements of the forewing of the cicada may serve as a guide for selecting airfoil materials for the bionic flapping-wing aircraft and promote the design and manufacture of more durable bionic wings in the future. RESEARCH HIGHLIGHTS: The distribution of the wing vein diameter and the wing membrane thickness indicated that the forewing of Cryptotympana atrata is composed of heterogeneous materials. The wing membrane and the outer wall of the wing vein are the layered structure with multilayer fibers, which has a great significance for improving the ability of the forewing to sustain aerodynamic loads. The elastic modulus of the wing membrane near the wing root is in the range of 4.45-5.03GPa, which is comparable to that of membranes manufactured by industries. This is a suitable reference for selecting materials for making bionic aircraft wings. We proved that the elastic moduli of the "fresh" and "dry" wing veins differ greatly compared with those of the wing membrane. Because the wing vein microstructure exhibits an internal hollow tubular structure with flocculent structure inside, the "fresh" sample stores more water than the "dry" sample. The wing membrane near the wing root is thicker and reinforced by the main wing vein with a high elastic modulus. This renders the region near the wing root difficult to deform. The membrane far from the wing root is thinner and the elastic modulus of the nearby wing veins is smaller, making them more flexible.

Design and aerodynamic characteristic analysis of unfolding-bending bionic flapping-wing aircraft

Design and aerodynamic characteristic analysis of unfolding-bending bionic flapping-wing aircraft

Experimental Study on the Wing Parameter Optimization of Flapping-Wing Aircraft Based on the Clap-and-Fling Mechanism

Experimental Study on the Wing Parameter Optimization of Flapping-Wing Aircraft Based on the Clap-and-Fling Mechanism

Limit cycle oscillation suppression controller design and stability analysis of the periodically time-varying flapping flight dynamics in hover

The periodically time-varying forces make the equilibrium state of Beihawk, an X-shaped flapping-wing aircraft, to be a periodic limit cycle oscillation. However, traditional controllers based on averaging theory fail to suppress this oscillation and the derived stability result may be inaccurate. In this study, a period-based method is proposed to design the oscillation suppression controller, locate the corresponding cycle and analyze its stability. A periodically time-varying wing-tail interaction model is built and Discrete Fourier Transform is applied to adapt the model for controller design. The harmonics less than quintuple flapping frequency account for more than 96 percent of the total harmonics and are reserved to present a concise model. Based on this model, active disturbance rejection controller (ADRC) is designed and its Extended State Observer can observe the disturbance to suppress the oscillation. Poincaré map is introduced to convert the stability analysis of the cycle to a fixed point. A multiple shooting method is adopted to locate several points on the cycle and the map is obtained by calculating the submaps between the adjacent points with the Floquet theory. The located points are proved to be accurate compared with the numerical solved cycle and the stability analysis result of the cycle is verified by the dynamic evolution. Compared with the State Feedback Controller, the ADRC performs better in suppressing the limit cycle oscillation and eliminating the attitude control error. The oscillation suppression is meaningful in maintaining a stable flight and capturing high quality images.

Implementation and Simulation of Hovering Control Model for Flapping Wing Aircraft

In recent years, bionic flapping wing aircraft is widely used in battlefield reconnaissance and flight patrol, making it a popular research in the field of UAV. However, because the wing and body forces are complex and conventional method calculation and modeling are difficult, so it is urgent to study the flight control of flapping aircraft. In this paper, the rigid body dynamics modeling method is used firstly to analyze the wing motion, establish the dynamic model of the wing aircraft, and then the PID control model is used to control the suspended motion state of the wing aircraft, and at last control simulation using Simulink. The results show that the mean value of the angular velocity fluctuation with PID control is 32.6% of the value without control, and the change rate is 38.2% of the value without control. The attitude adjustment process is more stable and the performance is better.

Unsteady aerodynamic characteristics of a bionic flapping wing in three-dimensional composite motion

Owing to continual advances in computational fluid dynamics, simulations of fluids have emerged as an important means of analyzing the aerodynamic characteristics of flapping-wing flight. This study establishes an aerodynamic model of bionic flapping-wing flight and uses dynamic hybrid grid technology to divide its flow field and other parts. We formulate equations to control the discretized flow field and combine the Navier–Stokes equation (N–S equation) with the theory of dynamic vortices to solve the flow field of the motion of flapping wings. We analyzed the lift and drag generated by the flapping wing at different wind speeds, amplitudes, frequencies, and chordal torsion angles. The results show the following: (1) wind speed had a significant influence on the lift resistance of the flapping wing. When the wind speed was 1 m/s, the lift force was 1.544 N and increased 4.3 times to 6.636 N when the wind speed was 5 m/s. The resistance increased from 0.39 to 0.88 N. (2) Changes in the amplitude of flutter had little effect on the average lift resistance. When the amplitude was increased from 15° to 45°, the lift force increased to only 0.757 N and drag changed by little. (3) The increase in the flapping frequency improved flight lift. When the frequency was increased from 1 to 5 Hz, the lift increased by 2.9 N (1.78 times) and resistance increased by only 0.08 N. (4) Increasing the chord torsion angle increased flight lift. The lifts at β = 5° and 15° were 6.636 and 6.654 N, respectively, 0.85 and 0.87 N greater than those at β = 0°. As the torsion angle continued to increase, the lift decreased, while the resistance increased more quickly. When β = 15°, the resistance increased by 1.53 N, 9.6 times larger than that at 0°. Increasing the flight speed and flapping frequency can increase flight lift. A small increase in the chord torsion angle increased flight lift, but an excessively large angle led to a substantial increase in drag. Increasing the amplitude of flapping can increase the instantaneous lift generated but has a smaller effect on the average lift. By revealing the influence of different parameters on the lifting resistance of a flapping wing during flight, this paper provides a theoretical foundation for the design and control of bionic flapping-wing aircraft.

Efficiency-Optimal Rigidity Configuration of Passive Torsional Flapping Wings

In this work, a simple analytical tool for the efficient, optimal design of passive torsional flapping wings, which is usually done with the complex fluid–structure interaction method, is proposed. For thin-skin-wrapped spar–rib wing structures, the propulsion efficiency was modeled based on the momentum theorem, and the wing passive torsion was modeled based on the thin-wall-tube torsional vibration theorem. A forward efficiency-optimal design method for passive torsional flapping wings was thereby developed. The proposed method was validated through numerical simulation and a wind-tunnel experiment for a 0.85-m-span flapping-wing model with multiple wing torsional rigidity configurations. The group with the torsional rigidity optimized by the presented method displayed the best efficiency performance. The proposed method has potential applications for the concept design and performance evaluation of flapping-wing aircraft.

Hover flight control of X-shaped flapping wing aircraft considering wing–tail interactions

The dynamic modelling and attitude controller design of a flapping wing aircraft, Beihawk, were investigated in this study. The aircraft features of an X-shaped flapping wing configuration, tail control surfaces installed downstream of the main flapping wings for roll and pitch control, and a tail rotor for yaw control. Unsteady effects including partial leading edge suction, induced flow, and post-stall behaviour, were considered in the aerodynamic modelling. Flapping-wing-induced flow was found to be crucial to the tail control force generation for the utilized aircraft layout. A periodically time-varying wing–tail interaction model was thus established, based on which a double-loop controller was designed to enable stable hovering of the aircraft. The stability of the nonlinear hovering-based manoeuvre was studied by manifold theory, based on the proposed wing–tail interaction model. Simulations and flight experiments on Beihawk were conducted to verify the proposed theory. It was found that the controller considering wing–tail interaction effect enabled more stable hover flight compared with traditional PD controller. In addition, it was found a wider stability region can be obtained by a higher flapping-wing-induced flow velocity. The functions between this velocity and the influencing factors, such as thrust force, wing span and tail installation position, are also presented. Besides, a visual parameter stability boundary between the vertical and pitching velocity is given. Relevant conclusions are vital in active flight envelope protection.

Attitude control of flapping wing aircraft based on energy optimization and ESO

Aiming at the problem of insufficient endurance performance of flapping wing aircraft, a stable attitude control algorithm based on energy optimization and ESO (extended state observer) is designed, which effectively reduces the energy consumption in cruise phase. Firstly, the longitudinal dynamic model of flapping wing aircraft is established, and then the uncertain part of the system and various unknown external disturbances are taken as the total disturbance. ESO module is introduced to observe and track the total disturbance in real time. Therefore, the system is transformed into a series integral system through the total disturbance feedback, and then the energy optimal control law is designed on the base of the transformed system. The numerical simulation results show that, compared with the traditional PID control method, the designed energy optimal control method reduces the average energy consumption by 35.28%.

Effects of airfoil on aerodynamic performance of flapping wing

Flapping-wing aircraft (FWA) operates in a flight mode that mimics natural flyers, such as birds, bats and insects. For large scale bird-inspired flapping wings aircraft, the design of airfoil parameters is crucial to improving the aerodynamic performance. In order to study the effect of camber on the aerodynamic characteristics, we developed four different airfoils with a chord length of 12 cm. The cambers of airfoils are respectively 30 mm, 25 mm, 20 mm and 15 mm. A numerical investigation into the effects of camber on aerodynamic performance is carried out through COMSOL Multiphysics software using Fluid–Structure Interaction (FSI) module. In this work, the fluid–structure module employs the Navier–Stokes equations coupled with a solid stress–strain physics module and a moving mesh module. The three-dimensional (3D) flapping wing computational models with different cambers are built. The results show that certain camber of airfoil can improve the aerodynamic characteristics of flapping wing.

Kinematics analysis and simulation of flapping mechanism of two-stage flapping-wing aircraft

In order to improve the flight performance of the flapping-wing aircraft, a two-stage foldable bionic flapping-wing mechanism is designed in this paper with reference to the flight motion characteristics of large birds. Firstly, the kinematic model of the flapping-wing mechanism is established by kinematic analysis, and the relationship equation between flapping angle and input angle and rod length is derived. Then, the three-dimensional model of the flapping-wing wing configuration is built in SolidWorks, and its output motion is obtained by ADAMS simulation to verify the theoretical analysis. The results show that the designed flapping-wing mechanism has a minimum transmission angle of Γ_min=48.6° and has good force transmission performance. The upper flapping limit of the inner wing is φ_max=45° and the lower flapping limit is φ_min=-5°; the upper flapping limit of the outer wing mechanism is φ_max=46° and the lower flapping limit is φ_min=-28°, which are consistent with the flight motion parameters of large birds.

Structure and aerodynamic characteristics of a coaxial quad-wing flapper

As a special type of micro ornithopter, the coaxial quad-wing flapper (CQWF) enables greater flight speed and higher stability than the paired-wing flapper. These characteristics are closely related to the unique pneumatic mechanism of the CQWF. Therefore, the aerodynamic generation mechanism of the CQWFs has been actively researched in recent years. This study verifies the reliability of flow-field simulations in a CQWF prototype with an aerodynamically optimized driving mechanism. For the selected motion parameters and shape dimensions of the flapping-wing aircraft, the vorticity fields at different elevation angles are observed in flow-field simulations. The elevation angle strongly affects the lift. Moreover, the wing movement based on the Clap–Fling mechanism significantly affected the acquisition of the lift, which explains the higher stability of the CQWF than that of the paired-wing flapper and provides a theoretical basis for the optimization of the flapping prototype. When tested on a wind-tunnel platform, the prototype yields slightly higher lift compared with those obtained in the simulation study. In addition to confirming the phenomenon revealed in flow visualization, it also showed that the unsteady mechanism of the two-pair wing is more powerful than calculated.