Flapping Wing Propulsion Research Articles

Mode transition mechanisms in semi-passive flapping-wing propulsion or energy extraction

To facilitate the smooth transition of operational modes in self-sustaining micro-thrusters, this study focuses on a device employing semi-passive flapping wings. Transient numerical methods are used to analyze the variation of flap energy acquisition and propulsion characteristics with pitching frequency, and the relationship between the dominant vortex structure and the force characteristics is analyzed by using the integral momentum theorem and the dynamic mode decomposition (DMD) method. The results indicate that, under the investigated operational conditions, with an increase in the pitching frequency, distinct wake evolution characteristics were observed. In the energy harvesting operational regime, the wake patterns manifest as 2P + mS, 2S + mS, and mS types. During the transitional phase from energy harvesting to propulsion, the wake patterns shift from 2S + mS to 2S transitional types, eventually leading to the manifestation of a reverse Karman vortex street (2S RBVK). In the propulsion operational regime, the wake patterns consist of a reverse Karman vortex street and asymmetric reverse Karman vortex street phenomena. Simultaneously, it was observed that the transition of flapping-wing performance from energy harvesting to propulsion conditions delays behind the transformation of vortex street structures. This delay is attributed to the necessity for the flapping-wing device to overcome its own resistance before generating a net effective propulsive force. The contributions of unsteady wake to thrust primarily encompass vortex thrust, thrust due to localized fluid acceleration, induced momentum force, and induced pressure force. As the pitching frequency increases, the influence of wake vortices on propulsion also intensifies. The contribution of wake vortices to flapping-wing propulsion is determined by the spatial distribution of Lamb vectors and localized fluid accelerations. The conclusions drawn from the dynamic modal analysis and reconstructed flow field analysis of wake vortices align with the findings of the investigation of wake vortices based on the integral momentum theorem.

Unsteady Aerodynamic Characteristics of The Flapping Wing Micro Air Vehicle (MAV)

Flapping wing propulsion is center of attention in the field of micro air vehicle research in recent years. Knowledge in unsteady aerodynamics is insufficient to design an efficient wing that produces the required lift and thrust force for the flapping wing micro air vehicle. The unsteady aerodynamic effects on aerodynamic wing design parameters were discussed in this paper. The effect of aspect ratio and the wing planform shape on lift and thrust force were determined experimentally. The experiments were conducted in an open jet low speed wind tunnel having test section 350 mmx350 mm. A rack and gear type new flapping mechanism was designed and fabricated and this was used as test bed for all the experiments. Also an experiment was conducted to determine the effect of torsional flexibility of the wing. The result shows that the chord wise flexible wing produces more lift and thrust than the other wing.

Asymptotic Theory of Flapping Wing Propulsion in Extreme Ground Effect

This study, dedicated to flapping wing propulsion in immediate proximity to a wall or between closely spaced flat walls, makes use of the method of matched asymptotic expansions. Its purpose is to create a simplified parametric model of such a propulsion system based on a single major assumption: immediate closeness of oscillating wing to a solid wall. In the case of a rectangular finite-aspect ratio wing, analytical expressions have been obtained for the coefficients of instantaneous and period-averaged thrust force as well as for the efficiency of the propulsor as a function of distance from the wall, Strouhal number and wing aspect ratio for selected cases of heaving, pitching and combined oscillations. It is shown that for some oscillation modes closeness to the ground results in increases in thrust and efficiency, and that optimally combined (considering ratio of the amplitudes and phase shift of contributing motions) heave–pitch oscillations allow to maximize thrust or efficiency of the flapping-wing propulsor. Increase in aspect ratio and decrease in Strouhal number (relative frequency of oscillations) in the case of heaving invariably brings about the increase in the ideal efficiency. An example is provided of a non-planar extreme ground effect application considering oscillations of a ring-wing embracing circular cylinder. A rule is derived for recalculation of the characteristics of the flapping-wing propulsor near a flat wall onto the characteristics of the same wing operating in a narrow canal between parallel walls. This rule can also be applied to evaluate propulsive properties of a ring-wing oscillating between co-axial cylindric surfaces.

An All Servo-Driven Bird-Like Flapping-Wing Aerial Robot Capable of Autonomous Flight

Almost all flying creatures, from centimeter-scale insects to meter-scale birds, maneuver through flapping-wing propulsion. Development of flapping-wing robots capable of long-term autonomous flight remains an ongoing challenge owing to manufacturing, actuation, and control considerations. In this article, we report the design and the control system of an all servo-driven bird-like flapping-wing aerial robot called USTBird. Different from most common flapping mechanisms combining gear transmission and motor, USTBird is driven by two independently programmable servos, which makes the wingbeat kinematics of USTBird diverse and enables it to do some agile aerobatic maneuvers. The enhanced controllability brought by the servo-driven method allows USTBird to produce both flight force and control force relying solely on the flapping wings, and exert decoupled control torques about all three body axes without a controllable tail. The flapping gait optimization problem is considered for USTBird, and three different kinds of drive signals are compared for larger aerodynamic forces generation. Toward practical applications, an outdoor flight task of autonomous patrol around a building, which can be subdivided into a coordinated takeoff and a low-altitude patrol and reconnaissance, is conducted on the robot with a good performance.

Biomimetic Soft Wings for Soft Robot Science

Flight and swimming in nature can inspire the design of highly adaptive robots capable of working in complex environments. In this letter, we reviewed our work on robotic propulsion in the air and water, with a specific focus on the crucial functions of elastic components involved in the driving mechanism and flapping wings. Elasticity in the driving mechanism inspired by birds and insects can enhance both the aerodynamic efficiency of flapping wings and robustness against disturbances with appropriate design. A flapping wing surface with a stiffness distribution inspired by hummingbirds was fabricated by combining tapered spars and ribs with a thin film. The biomimetic flexible wing could generate more lift than the nontapered wing with a similar amount of power consumption. Underwater flapping-wing propulsion inspired by penguins was investigated by combining the 3-degree-of-freedom (DoF) flapping mechanism and hydrodynamic calculation, which indicates that wing bending increases the propulsion efficiency. This work demonstrates the importance of passive deformation of both wing surfaces and driving mechanisms for improving the fluid dynamic efficiency and robustness in flight and swimming, as well as providing biological insight from an engineering perspective.

Flapping wing propulsion: Comparison between discrete vortex method and other models

Cetacean propulsion by a periodic flapping motion of their fluke is considered and studied on a benchmark flexible straight wing. The aim of the study was to validate low-order models for this configuration. First, the two-dimensional rigid case is investigated, comparing the aerodynamic performance of the airfoil periodic motion vs the reduced frequency, with published data and unsteady Reynolds-averaged numerical simulation results. It appears that viscous drag modeling must be added to the discrete vortex method, in order to obtain sensible thrust results, for Garrick frequencies below 2. All high- and low-order models agree at the remarkable Garrick frequency of 1.82, although the experiment shows a lower efficiency of about 25%. The positions of the shed vortices match comparing the unsteady Reynolds-averaged numerical simulation and the discrete vortex method. Then, the three-dimensional leading-edge-suction-parameter modulated discrete vortex method is extended, by means of a lifting line theory. A modification of the method is proposed in order to consider wing dihedral, resulting from the spanwise flexibility. The configuration considers a reduced frequency of 1.82. Three types of spanwise wing flexibility are examined. For the inflexible and flexible cases, a reasonable agreement is observed between the different methods for each coefficient. The intermediate flexible wing provides a better thrust coefficient, while excessive flexibility proves to be detrimental. Vorticity fields are compared with previously published data for the three wings. For the highly flexible wing and the right choice of deformation parameters, the discrete vortex method produces reliable results.

Optimized flapping flight in Venus surface atmospheric conditions

Three recently approved space missions are headed towards Venus, to help answer major questions about Venus atmosphere and geology. However, many existing questions cannot be properly addressed without direct in situ measurements from Venus surface or within the atmosphere. To this end, flapping wing vehicle concept is selected, optimized for Venus atmospheric flight, and evaluated using energy efficiency as performance criteria. Flapping wing vehicle computational model is derived based on discrete variational mechanics and quasi-steady aerodynamics, with all relevant aerodynamic phenomena included. Flapping wing vehicle computational model is then embedded within optimization algorithm, which is utilized to obtain energy efficient flapping patterns for forward flight in Venus surface atmospheric conditions. Numerical optimization is performed for different neutrally buoyant configurations, with wingspan ranging from 10 mm to 1 m. Different forward velocities are used as well, where maximum velocity is limited by an advance ratio of 0.5. Bumblebee and hummingbird-sized vehicles, with a wingspan of 30 mm and 30 cm, are selected as the most representative test cases and thoroughly studied. It is proved that flapping wing propulsion is a feasible and effective concept for Venus exploration purposes. Finally, based on a comparison of the selected test cases, general conclusions are drawn on the flapping wing dynamics and flight mechanics in the Venus atmosphere. Significant difference in the propulsion mechanism has been observed, based on the aerial vehicle size. In order to maximize propulsive efficiency, the smaller vehicle mostly exploited aerodynamic forces related to the leading edge vortex, while the larger vehicle relied more on added mass and rotational forces.

Scalability of resonant motor-driven flapping wing propulsion systems

This work aims to develop an integrated conceptual design process to assess the scalability and performance of propulsion systems of resonant motor-driven flapping wing vehicles. The developed process allows designers to explore the interaction between electrical, mechanical and aerodynamic domains in a single transparent design environment. Wings are modelled based on a quasi-steady treatment that evaluates aerodynamics from geometry and kinematic information. System mechanics is modelled as a damped second-order dynamic system operating at resonance with nonlinear aerodynamic damping. Motors are modelled using standard equations that relate operational parameters and AC voltage input. Design scaling laws are developed using available data based on current levels of technology. The design method provides insights into the effects of changing core design variables such as the actuator size, actuator mass fraction and pitching kinematics on the overall design solution. It is shown that system efficiency achieves peak values of 30–36% at motor masses of 0.5–1 g when a constant angle of attack kinematics is employed. While sinusoidal angle of attack kinematics demands more aerodynamic and electric powers compared with the constant angle of attack case, sinusoidal angle of attack kinematics can lead to a maximum difference of around 15% in peak system efficiency.

Delaying leading edge vortex detachment by plasma flow control at topologically critical locations

Flapping wing propulsion offers unrivalled manoeuvrability and efficiency at low flight speeds and in hover. These advantages are attributed to the leading edge vortex developing on an unsteady wing, which induces additional lift. We propose and validate a manipulation hypothesis that allows prolongation of the leading edge vortex growth phase, by delaying its detachment with the aid of flow control. This approach targets an overall lift increase on unsteady airfoils. A dielectric barrier discharge plasma actuator is successfully used to compress secondary structures upstream of the main vortex on a pitching and plunging flat plate. To determine flow control timing and location, the tangential velocity on the airfoil surface is used, which is also used to quantify topological effects of flow control. This flow control is then tested for different motion kinematics and on a NACA 0012 airfoil. Significant increase of the peak circulation of the leading edge vortex of about 20% for all cases with flow control indicates that this approach is applicable for various kinematics, dynamics and airfoil types.

Investigation of the Propulsion Properties of an M-shaped Bending-surface Feather Fan under Different Wind Velocities

Flight speed is an important factor that affects flapping-wing propulsion. This study conducted tests on a self-designed rotatory force-measuring platform and simulated different inflow conditions by changing the rotatory angular velocity. An M-shaped bending-surface feather fan was selected as the test model, and the rotatory platform was rotated at different speeds under different propulsion so that the flapping-wing propulsion at different inflow velocities could be measured. Moreover, the wind speed corresponding to the maximum propulsion was determined. The results show that as wind velocity increases within a certain range, the propulsion first increases, then decreases, and finally increases until reaching a maximum wind velocity value of 7.3 m/s.

Generic Analytical Thrust-Force Model for Flapping Wings

The purpose of this paper is to propose a generic analytical flapping-wing cycle-average thrust-force estimation method by implementing elongated body theory into actuator disk theory. Compared with previous a flapping-wing analysis based on actuator disk theory, the proposed method relates wing geometry and kinematics directly to induced velocity, and the periodical pressure pulsation correction factor measurement for every different flapping-wing design is no longer necessary. The proposed method is much simpler than unsteady aerodynamics or computational implementations, and thus the complex multiparameter optimization can be avoided at the early design stage of flapping-wing vehicles. Theoretical deviation of the proposed thrust model is evaluated by a numerical simulation method, and a correction term is proposed. The corrected method is then validated by wind-tunnel experiments on an 850-cm-wing-span active-twisting flapping-wing model and experimental data from previous studies that were over large parameter ranges. The results suggest that the proposed method is valid for the flapping-wing cycle-average thrust estimation over a large range of variables. Theoretical deductions about the effects of dominant factors of flapping-wing propulsion behavior and application of the present method in both passive- and active-twisting flapping-wing designs are also discussed.

Spatial network analysis to construct simplified wing structural models for Biomimetic Micro Air Vehicles

A procedure for designing a simplified, dragonfly-like wing model that is suitable for use in a Biomimetic Micro Air Vehicle (BMAV) is presented. BMAV are a relatively new class of micro-scaled unmanned air systems that mimic the flapping wing propulsion system of flying biological organisms (like insects). Many insects (e.g. dragonflies) have complex wing vein and membrane patterns that are too small to fabricate using many types of machine cutting tools (e.g. micro laser cutting). Structural dynamic modification using the spatial network analysis approach is used to create a simplified model. Our objective was to minimize the wing vein patterns so that they were within our fabrication tolerances. Simulations were performed for both the detailed and simplified models. The natural frequency and corresponding mode shapes, modal assurance criterion (MAC) and static bend-twist coupling results were very similar. This analysis shows that a simplified model can be designed and fabricated to closely biomimic a real dragonfly wing.

Intertwined vorticity and elastodynamics in flapping wing propulsion

We performed numerical experiments on a one-dimensional elastic solid oscillating in a two-dimensional viscous incompressible fluid with the intent of discerning the interplay of vorticity and elastodynamics in flapping wing propulsion. Perhaps for the first time, we have established the role of foil deflection topology and its influence on vorticity generation, through spatially and temporally evolving foil slope and curvature. Though the frequency of oscillation of the foil has a definite role, it is the phase relation between foil slope and pressure that determines thrust or drag. Similarly, the phase difference between flapping velocity, and pressure and inertial forces, determine the power input to the foil, and in turn drives propulsive efficiency. At low frequencies of oscillation, the sympathetic slope and curvature of deformation of the foil allow generation of leading-edge vortices that do not separate; they cause substantial rise in pressure between the leading edge and mid-chord. The circulatory component of pressure is determined primarily by the leading-edge vortex and therefore thrust too is predominantly circulatory in origin at low frequencies. In the intermediate and high-frequency range, thrust and drag on the foil spatially alternate and non-circulatory forces dominate over circulatory and viscous forces. For the mass ratios we simulated, thrust due to flapping varies quadratically as a function of Strouhal number or trailing-edge flapping velocity; further, the trailing edge flapping velocities peak at the same set of frequencies where the thrust is also a maximum. Propulsive efficiency, on the other hand, is roughly a mirror image of the thrust variation with respect to Strouhal number. Given that most instances of flapping propulsion in nature are primarily through distributed muscular actuation that enables precise control of deformation shape, leading to high thrust and efficiency, the results presented here are pointers towards understanding some of the mechanisms that drive thrust and propulsive efficiency.

Biplane and Tail Effects in Flapping Flight

A numerical investigation is performed on the interference effects in single or biplane flapping airfoil propulsion in the presence of a stationary downstream tail. At a Reynolds number of 1000, this corresponds to the regime of small micro aerial vehicles. The objective of this study is to provide insight into the complex wing–tail interaction effects occurring in flapping-wing propulsion configurations. The effect of the relative distance between the airfoils, as well as the positioning and incidence angle of the tail, is investigated. Adding a tail behind a single flapping airfoil increases the efficiency and average thrust by up to 10 and 25%, respectively. For the biplane flapping airfoils without tail, overall efficiency and average thrust per airfoil increase up to 17 and 126%, respectively, with respect to the single airfoil due to the formation of a strong momentum jet. The effect of adding a tail behind the biplane flapping airfoils depends on the tail’s orientation and location. Increasing the incidence angle of the tail generates higher lift, although at the expense of decreased efficiency and thrust. Lastly, shifting the vertical position of the tail to have it coincide with the middle heaving position of the leading top airfoil gives the best overall performance.

Flow control for VATT by fixed and oscillating flap

The present study is aimed to explore the potential to improve Vertical Axis Tidal Turbine (VATT) energy harnessing efficiency by using modified blades with fixed and oscillating flap. The fixed flap concept is borrowed from its application in aerodynamics area for reaching a high lift force at low flying speed. Oscillating flap is motivated by our relevant biomimetic studies on the flapping wing propulsion or energy extraction. Present investigation is concentrated on a VATT with NACA 0018 profile blade as its baseline turbine. Numerical simulations are carried out by solving incompressible Unsteady Navier–Stokes equations at turbulence flow condition. Computed results show that under certain optimal flap geometry and flow conditions, turbine power coefficient reaches 28% enhancement as compared to the conventional blade turbine. Detailed analysis on the flow structure demonstrates that this is related to the effective flow separation suppression and vortex control by applying a fixed and oscillating flap.

Evaluation of flapping wing propulsion based on a new experimentally validated aeroelastic model

To evaluate the propulsion system capabilities of a Flapping Micro Air Vehicle (FMAV), a new aeroelastic model of a typical flexible FMAV is developed, utilizing the Euler–Bernoulli torsion beam and quasi steady aerodynamic model. The new model accounts for all existing complex interactions between the mass, inertia, elastic properties, aerodynamic loading, flapping amplitude and frequency of the FMAV, as well as the effects of several geometric and design parameters. To validate the proposed theoretical model, a typical FMAV, as well as an instrumented test stand for the online measurement of forces, flapping angle and power consumption, has been constructed. The experimental results are initially utilized to validate the flight dynamic model, and several appropriate conclusions are drawn. The model is subsequently used to demonstrate the flapping propulsion characteristics of the FMAV via simulation. Using dimensionless parameters, a set of new aeroelastic coordinates are introduced. In this reduced design space, new generalized performance curves have been deduced. The results indicate that by proper adjustment of the wing stiffness parameter, as a function of reduced frequency, the FMAV will attain its optimum propulsive efficiency. This fact raises additional ideas of utilizing intelligent variable stiffness materials and/or an active morphing technology for the sustained flight of FMAVs.

Design of Bionic Flapping-Wing Propulsion Device

The underwater vehicle has never been able to shake off the mode of pushing forward by the propeller and changing motion with underwater fin rudder. However, this traditional propeller propulsion has many shortcomings, such as a low utilization rate of energy and poor flexibility. In this passage, through the analysis of the movement of sea turtles, a brand-new propulsion project is proposed, which is Flapping-Wing propulsion. In such a limited way to proceed forward, the design can be realized like turtles campaign.

Performance Analysis of the Flapping Wing Propulsion Based on a New Experimentally Validated Aeroelastic Model

Flapping micro air vehicle (FMAV) is considered to exhibit much better performance at low speeds and small sizes compared to fixed-wing MAVs. To maximize the potential and capabilities of FMAVs also to produce adequate design implications, a new aeroelastic model of a typical flexible FMAV is being developed utilizing Euler-Bernoulli torsion beam and quasi steady aerodynamic model. The new model accounts for all natural existing complex interactions between the mass, inertia, elastic properties, aerodynamic loading, flapping amplitude and frequency of the FMAV as well as the effects of several geometric and design parameters. To validate the proposed theoretical model, a typical FMAV as well as instrumented test stand for the online measurement of forces, flapping angle and power consumption have been constructed. The experimental results are initially utilized to validate the flight dynamic model, and several appropriate conclusions are drawn. The model is subsequently used to demonstrate the flapping propulsion characteristics of the FMAV via simulation. Using dimensionless parameters, a set of new generalized curves have been deduced. The results indicate that by proper adjustment of the wing stiffness parameter as a function of the reduced frequency, the FMAV will attain its optimum propulsive efficiency. This fact raises additional new ideas for further research in this area by utilizing intelligent variable stiffness materials and/or or active morphing technology for the sustained, high-performance flight of FMAVs. The generalized model can also be used to conduct a performance and stability analysis of FMAVs and to design and optimize flapping-wing structures.

A comparison of the flow around a flexible and a non‐flexible airfoil for flapping wing propulsion

AbstractThe flow around flapping airfoils is evaluated using the Stereoscopic Particle Image Velocimetry (PIV) technique. Resulting from the measurements is the distribution of the turbulent shear stress. This serves as an indicator for the position of transition from laminar to turbulent flow. The distributions were compared for both a flapping non–flexible airfoil and a flexible airfoil. It was determined that there are differences between the flow around the flexible and the non–flexible airfoils. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fluid mechanics of flapping wings

Based on the results of two-dimensional flapping wing studies, the selection of Strouhal number for steady cruising is explored. The hypothesis that Strouhal number selection in natural flapping wing propulsion is a limit cycle process is proposed and the implications of this hypothesis for the design, control and computational/experimental modeling of freely flying systems are explored. It is suggested that the steady-state propulsive efficiency of man-made systems that use flapping propulsion must be designed-in, rather than actively controlled and that sensory feedback may be more important for control during maneuvering/acceleration and play only a minor role in the regulation of a flying/swimming system’s steady forward motion. A brief literature review of the structure of the flow around three-dimensional translating and revolving wings is also presented. In the case of translating wings the flow structure consists of a chain of interconnected vortex loops and has a strong Strouhal number dependence. For revolving wings the leading edge vortex is especially important for the development of lift and thrust. The stability of this leading edge vortex is strongly dependent on the development of a spanwise flow that creates a balance between the creation of vorticity at the leading edge of the wing and the transport of vorticity into the wake.