Hovering Wing Research Articles

Leading-edge curvature effect on aerodynamic performance of flapping wings in hover and forward flight

This study investigates the role of leading-edge (LE) curvature in flapping wing aerodynamics considering hovering and forward flight conditions. A scaled-up robotic model is towed along its longitudinal axis by a rack gear carriage system. The forward velocity of the robotic model is changed by varying the advance ratio J from 0 (hovering) to 1.0. The study reveals that the LE curvature has insignificant influence on the cycle-average aerodynamic lift and drag. However, the time-history lift coefficient shows that the curvature can enhance the lift around the middle of downstroke. This enhanced lift is reduced from 5% to 1.2% as J changed from 0 to 1.0. Further flow examinations reveal that the LE curvature is beneficial by enhancing circulation only at the outboard wing sections. The enhanced outboard circulation is found to emanate from the less stretched leading-edge vortices (LEVs), weakened trailing-edge vortices (TEVs), and the coherent merging of the tip vortices (TVs) with the minor LEVs as observed from the phase-lock planar digital particle image velocimetry measurements. The far-wake observation shows that the LE curvature enhances the vorticity within the TV, helping to reduce the overall flow fluctuations in the far field. These findings can be extended to explain the predominantly straight LE wing shape with a small amount of curvature only observed near the wing tip for flapping fliers with Re from 103 to 104.

Role of the deviation motion on the aerodynamic performance of a mosquito wing in hover

The deviation angles during wing flapping motions are generally small for many insects and are known to have insignificant or even detrimental effects on the flight performance. Unlike many other insects, a mosquito hovers with a small sweeping amplitude but relatively large deviation angles. Thus, we investigate the effect of a deviation motion (‘figure-eight’ motion measured by Bomphrey et al. (2017)) on the aerodynamic performance of a mosquito wing in hover by comparing it with that without deviation motion (called in-plane motion). During the stroke, the deviation motion greatly changes the angle of attack and wing moving direction with respect to the stroke plane, but little changes the wing speed. These changes by the deviation motion significantly influence the instantaneous force and power generation. In particular, with the deviation motion, a large drag force is generated at early phase of the downstroke due to the increased angle of attack, and contributes to the vertical force generation. As results, the time-averaged vertical force is increased by the deviation motion, while the time-averaged power required is only slightly increased. Therefore, the mosquito wing generates a larger vertical force more efficiently with the deviation motion, suggesting that a deviation motion may be beneficial for an insect flight with a small sweeping amplitude.

Effect of marginal variation of aspect ratio and frequency with kinematic asymmetry on flow field around flapping rigid wing in hover

Phase-locked two dimensional particle image velocimetry (PIV) measurements are used to capture flow field around rigid flat plate wings undergoing main flapping motion (hover) with water as fluid medium in quiescent condition. Experiments are conducted using asymmetric upper-lower stroke single degree of freedom main flapping motion. Two different aspect ratio (AR) 1.5 and 1.0 rectangular wings at 1.5 Hz and 2.0 Hz flapping frequency and chord based Reynolds number of the order of 104 are studied. To achieve the desired aspect ratio, wing span is varied while wing chord remains fixed. Flow features for downstroke and upstroke are comparable for all four cases investigated. This includes vortex shedding from wingtip during wing stroke, vorticity intensification caused due to vortex stretching which occurs more dominantly for AR = 1.0, occurrence of KH instability in shear layer formed due to wing sweep and added mass effect due to the flow dragged by flapping wing. Vortex filamentation and fragmentation phenomena are observed during downstroke and upstroke respectively due to faster downstroke and slower upstroke. Rossby number, which is the ratio of inertial to Coriolis force is significant for rotational flapping wing. With increase in Rossby number there is a reduction in the spanwise flow over wing which influences strength of wingtip vortex at beginning of a stroke. It is found that spanwise flow for AR = 1.5 is stronger than AR = 1.0 and consequently the strength of wingtip vortex is higher for higher aspect ratio. However, peak vorticity is higher for AR = 1.0 due to vortex stretching effect which is expected to be more dominant at lower AR due to three dimensional effects. Effect of marginal variation in wing aspect ratio, flapping frequency with kinematic asymmetry are studied using velocity field, vorticity (ωz) contours, λ2 criterion, peak vorticity variation and its spatial distribution as well as kinetic energy variation of the flow field. Kinetic energy distribution and its connection with wake capture is explored.

Multibody-Dynamics Approach to Study the Deformation and Aerodynamics of an Insect Wing

In this study, a multibody-dynamics simulation approach was developed for a hawkmoth flexible wing. The wing structure is modeled as a chain of rigid bodies connected by elastic springs, and the aerodynamic force is measured by the extended unsteady vortex-lattice method. The multibody-dynamics and aerodynamic solvers are combined by an implicit coupling approach, and the quasi-Newtonian method is adopted to solve the system of nonlinear differential equations of motion. For validation, numerical results were compared with measurement data from a robotic wing and a living insect. A parametric analysis was conducted to study the effects of several kinematic parameters on the deformation and aerodynamic performance of the wing in hover. In most cases, using a flexible wing is far more efficient in terms of force production in comparison with its rigid counterpart. In general, wing deformation may cause considerable differences between the wing-tip and wing-base kinematic parameters. In particular, elevation motion as observed in living insects may be due to the passive oscillations of elastic elements, as opposed to a deliberate motion.

Ground Effect of a Two-Dimensional Flapping Wing Hovering in Inclined Stroke Plane

Abstract The ground effect aerodynamics and flow physics of a 2D dragonfly wing hovering (the Reynolds number is 157) in an inclined stroke plane are investigated via solving 2D unsteady incompressible laminar flow Navier–Stokes equations. An analysis road map is proposed to explain the influence of the ground on the flow field, pressure distribution on the wing surface, and the aerodynamic force. In the analysis road map, the flow relative to the wing surface induced by the wing motion and vortex is classified into vertical and parallel wing surface flows. The vertical flow impinges on the wing surface to form a positive pressure zone. In contrast, the parallel flow generates the boundary layer and further concentrated vortex and secondary vortex, which induce negative pressure on the wing surface. The ground impacts the flow relative to the wing in three ways: changing the trajectory of the shed vortex by the mirror effect, promoting the deformation and fusion of the vortices, and causing the cushion effect at extremely small ground clearance.

Aerodynamic characteristics of flexible flapping wings depending on aspect ratio and slack angle

Experimental investigations are made for the combined effects of aspect ratio (AR), slack (βS), and pitch angles on the aerodynamic characteristics of flexible flapping wings in hover. βS is introduced as a way to indirectly alter the flexibility of the wing. An optimum AR range of 3–5 based on the lift coefficient is observed depending on the flexibility. For a constant AR, the intensity of the leading-edge vortex (LEV) with corresponding circulatory-based lift mitigates as βS increases beyond 2.5°. The variation of βS affects the magnitude of the shed trailing-edge vortices (TEVs) but the vorticity core is maintained. We found the shed TEVs to be the key vortical feature of twistable flexible wings in comparison with the rigid (untwisted) cases. More intriguingly, the negative wing twist played a significant role in sustaining the circulatory lift at the outboard section for even high AR cases. The primary LEV trace is found to be an indicator for the effective spanwise limit of the LEV. Although an increase in AR reduces the effective spanwise limit, it is found that wing flexibility further decreases the radial distance. Again, the study reveals that lift enhancement in the rigid wing requires a wider effective downwash area induced by the outward movement of the LEV traces to merge with the tip vortex. Contrarily, the flexible wing requires an elongated downwash area induced by the wing twist to enhance the aerodynamic 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.

Numerical study on aerodynamic characteristics of two-dimensional propulsive wing in cruise and hover

The propulsive wing vehicle is a new concept vehicle, which is driven by a cross-flow fan (CFF) embedded in the trailing edge of the wing. The propulsive wing vehicle is capable of cruising and hovering at high angles of attack with very high aerodynamic force coefficients, and has the potential to become a new type of vertical take-off and landing (VTOL) vehicle. The cruise and hover states of the propulsive wing vehicle are defined, and a numerical model of the two-dimensional propulsive wing is established. Based on the computational fluid dynamics (CFD) method, the rotation of the CFF is simulated by using the sliding mesh technique. The effects of cruise speed, angle of attack and CFF rotation speed on the aerodynamics of the two-dimensional propulsive wing are evaluated, and the mechanism of the propulsive wing flow field changes is revealed. The results show that the propulsive wing has a very high lift coefficient of up to 60 at low speed and high angle of attack cruise, and a high thrust coefficient of up to 40 at low speed and small angle of attack cruise. The aerodynamic force of the propulsive wing fluctuates periodically with the rotation of the CFF, and the amplitude of the fluctuation is related to the vorticity of the CFF blade shedding vortex. In hover, the flow field of the propulsive wing is affected by the geometry and wake deflection into an asymmetric distribution, forming a vortex on each side of the airfoil, and the vortex diameter varies with the CFF rotation speed, which in turn has an impact on the hovering performance of the propulsive wing.

Phenomenology and scaling of optimal flapping wing kinematics

Biological flapping wing fliers operate efficiently and robustly in a wide range of flight conditions and are a great source of inspiration to engineers. The unsteady aerodynamics of flapping wing flight are dominated by large-scale vortical structures that augment the aerodynamic performance but are sensitive to minor changes in the wing actuation. We experimentally optimise the pitch angle kinematics of a flapping wing system in hover to maximise the stroke average lift and hovering efficiency with the help of an evolutionary algorithm and in situ force and torque measurements at the wing root. Additional flow field measurements are conducted to link the vortical flow structures to the aerodynamic performance for the Pareto-optimal kinematics. The optimised pitch angle profiles yielding maximum stroke-average lift coefficients have trapezoidal shapes and high average angles of attack. These kinematics create strong leading-edge vortices early in the cycle which enhance the force production on the wing. The most efficient pitch angle kinematics resemble sinusoidal evolutions and have lower average angles of attack. The leading-edge vortex grows slower and stays close-bound to the wing throughout the majority of the stroke-cycle. This requires less aerodynamic power and increases the hovering efficiency by 93% but sacrifices 43% of the maximum lift in the process. In all cases, a leading-edge vortex is fed by vorticity through the leading edge shear layer which makes the shear layer velocity a good indicator for the growth of the vortex and its impact on the aerodynamic forces. We estimate the shear layer velocity at the leading edge solely from the input kinematics and use it to scale the average and the time-resolved evolution of the circulation and the aerodynamic forces. The experimental data agree well with the shear layer velocity prediction, making it a promising metric to quantify and predict the aerodynamic performance of the flapping wing hovering motion.

The modeling and numerical solution for flapping wing hovering wingbeat dynamics

The production of wingbeat motion of flapping wing hovering flight are determined by the actuating, aerodynamic and inertia forces/moments, which influence the dynamic unsteadiness and controllability of flapping wing flying. This paper presents the feasible solution for cracking the problem of two degrees of freedom (two DoFs, namely, flapping and pitch motion, respectively) highly coupled nonlinear hovering wingbeat dynamics. Firstly, two DoFs nonlinear hovering wingbeat dynamic ordinary differential equations (ODEs) are derived on basis of the extended quasi-steady aerodynamic and inertial forces/moments model. Then, we perform their numerical solution by using tractable ODEs numerical algorithm, boundary value problem-solving format, and least square method. The numerical results have a good consistency with those measured by Dr. Muijres. Moreover, the adjustable rule of phase offset of wing pitch angle relative to the flapping angle is quantificationally studied by introducing frequency ratio between pitch frequency and flapping frequency. We find that the phase offset can be directly regulated by wing pitch hinge stiffness or indirectly modulated by frequency ratio, and the peak value of wing pitch angle monotonously decreases with the increase of wing pitch hinge stiffness, opposite to the angle of attack (AoA). This adjustable rule paves a useful way for the bio-inspired flapping wing micro aerial vehicle (FWMAV) featuring passive or semi-passive pitch flexible hinge to maintain high variable AoA.

Investigation of High Lift Force Generation of Dragonfly Wing by a Novel Advanced Mode in Hover

In the paper, a novel flapping mode is presented that can generate high lift force by a dragonfly wing in hover. The new mode, named partial advanced mode (PAM), starts pitching earlier than symmetric rotation during the downstroke cycle of the hovering motion. As a result, high lift force can be generated due to rapid pitching coupling with high flapping velocity in the stroke plane. Aerodynamic performance of the new mode is investigated thoroughly using numerical simulation. The results obtained show that the period-averaged lift coefficient, CL, increases up to 16% compared with that of the traditional symmetrical mode when an earlier pitching time is set to 8% of the flapping period. The reason for the high lift force generation mechanism is explained in detail using not only force investigation, but also by analyzing vortices produced around the wing. The proposed PAM is believed to lengthen the dynamic stall mechanism and enhance the LEV generated during the downstroke. The improvement of lift force could be considered as a result of a combination of the dynamic stall mechanism and rapid pitch mechanism. Finally, the energy expenditure of the new mode is also analyzed.

Computational Analysis of Rotor-Blown-Wing for Electric Rotorcraft Applications

This Paper examines the performance of a rotor-blown-wing in hover and forward flight conditions. Flow solutions were generated using Helios, with a delayed detached eddy simulation model. The configuration is based on one set of rotors/wing of a quadrotor biplane tail-sitter aircraft. Simulations were conducted to assess the effect of varying angle of attack, rotor revolutions per minute with rotor speed, and rotor diameter. Two configurations were examined, the rotor-blown-wing baseline with two 60.96 cm diameter rotors and the configuration with four 30.48 cm diameter rotors. The smaller rotors were analyzed at the revolutions per minute corresponding to producing half the thrust of the rotor-blown-wing baseline rotor. The presence of the wing in hover resulted in the rotor-blown-wing baseline rotor operating at 1.9% lower power loading than the same rotor in isolation. In comparison, the configuration reduced power loading by 13.6% compared to the isolated rotor. In forward flight (airplane mode), an increasing angle of attack was shown to increase thrust produced on the downstroke side of the rotor. Furthermore, the presence of the wing increases the thrust produced on the bottom half of the disk. Both the blown wings increase the ratio from 3.2 for the isolated wing to 3.6 and 4.3 for the rotor-blown-wing baseline and configurations, respectively.

Influence of aspect ratio on wing–wake interaction for flapping wing in hover

The interaction between an insect wing and its wake during hovering flight is inevitable. The effect of this wing–wake interaction (WWI) during stroke reversal is an intricate phenomenon that is difficult to forecast. Previous studies have mostly concentrated on the effect of aspect ratio (AR) on the leading-edge vortex (LEV) at the middle of stroke motion. However, the effect of AR on LEV during stroke reversal, which is where the intricate WWI phenomenon occurs, has not been fully exploited. This study aimed at revealing how AR affects WWI during stroke reversal at Re of ~ 104, and the role LEV plays. The experimental time-course measurement of force showed that the WWI at stroke reversal was strengthened by decreasing AR irrespective of pitching duration. A time-varying digital particle image velocimetry (DPIV) measurement revealed a jet-like flow induced by a pair of counter-rotating trailing edge vortex (TEV) as the primitive source of the interaction. This study found a WWI that appeared at the wing root contrary to the conventional type in literature, which appeared at the wing tip. The size of the LEV at the wing root played a key role by facilitating the effectiveness of the wing rotation. AR = 2 wing benefited much from this form of WWI by generating strong coherent shear layer at the end of stroke, and relatively weak LEV during stroke reversal. Thus, this type of WWI might be mostly beneficial for low AR wings (AR < 3).

Finite obstacle effect on the aerodynamic performance of a hovering wing

The finite obstacle effect on the aerodynamic performance of a normal hovering wing is studied using the immersed boundary method. Phenomena of a two-dimensional wing hovering above, under, or on the side of a circular obstacle are presented. Parameters including obstacle size, distance, location, and flapping angle are investigated to study how the aerodynamic force and flow field are affected. The diameter of the obstacle ranges from 0.5c to 12c and the distance between the centroid of the wing and obstacle surface from 0.5c to 6c (c is the wing chord length). Previous observations of ground effects including force enhancement, reduction, and recovery occur similarly when the wing hovers above the obstacle of diameter greater than 2c. However, finite obstacles affect the aerodynamic performance differently when the size shrinks to a critical value. Force drops when the wing moves close and rises when moving away, opposite to the ground effect. As flapping angle amplitude increases, the force change tends to be consistent for different-sized obstacles. The top or side effect shows a different influence on the force change. Force monotonically increases as the distance decreases when the wing hovers under the obstacle. The side effect places a less important factor on the aerodynamic performance. All force changes under such circumstance are less than 13% referring to nonobstacle result. The gap between the leading or trailing edge of the wing and obstacle surface plays a significant role in the leading and trailing edge vortices generating, shedding, and pairing, which greatly affects the force change.

A Neural-network-based Approach to Study the Energy-optimal Hovering Wing Kinematics of a Bionic Hawkmoth Model

This paper presents the application of an artificial neural network to develop an approach to determine and study the energy-optimal wing kinematics of a hovering bionic hawkmoth model. A three-layered artificial neural network is used for the rapid prediction of the unsteady aerodynamic force acting on the wings and the required power. When this artificial network is integrated into genetic and simplex algorithms, the running time of the optimization process is reduced considerably. The validity of this new approach is confirmed in a comparison with a conventional method using an aerodynamic model based on an extended unsteady vortex-lattice method for a sinusoidal wing kinematics problem. When studying the obtained results, it is found that actual hawkmoths do not hover under an energyoptimal condition. Instead, by tilting the stroke plane and lowering the wing positions, they can compromise and expend some energy to enhance their maneuverability and the stability of their flight.

Aerodynamics of a two-dimensional flapping wing hovering in proximity of ground

The difference in aerodynamic forces of a two-dimensional flapping wing hovering in unbounded flow field and ground effect is studied. The unsteady laminar Navier–Stokes equations are solved by the finite volume method to simulate the flow field around the wing. In the unbounded flow field, the correspondence between the aerodynamic force, pressure distribution on wing, and typical vortex structures is established, and then the high-lift mechanism of the flapping wing is further explained. In the ground effect, based on the lift variation, the dimensionless height H/ C ( H is the height of the wing above ground and C is the chord length of the wing) can be divided into transition and ground effect regimes. In the transition regime ( H/ C &gt; 2.5), the lift decreases with the decreasing height, and the ground indirectly impacts the vortices near wing by changing the shed vortices in space. In the ground effect regime ( H/ C &lt; 2.5), the lift increases with the decreasing height, and the ground directly impacts the vortices near the wing.

Effect of pitch on the flow behavior around a hovering wing

The effect of pitch on the evolution of flow and aerodynamic forces around a hovering flat-plate wing has been experimentally investigated in this study using particle image velocimetry and direct force measurements. The measurements are conducted on a wing at a reduced frequency of $$k=0.32$$ and Reynolds number of $$Re=220$$ . The Lagrangian finite-time Lyapunov exponent method is used to analyze the unsteady flowfields by identifying dynamically relevant flow features and their evolution. First, the effect of a change in the duration of pitch for a symmetric pitch is discussed. The flow stages based on the LEV emergence, growth, lift-off, and decay remain the same for the compared cases whereas the duration of flow stages varies. Second, we introduce a phase lead and lag with respect to the stroke timing and detailed flow development is discussed for these cases. This is further corroborated with the measured aerodynamic forces to highlight the effect of varying the phase-shift on the different characteristics of the hovering wing. Changing the pitching phase results in distinct flow changes that correlate with a higher lift production when the pitch precedes the stroke reversal and lower lift production when the pitch succeeds the stroke reversal. A schematic of differences in flow development based on the vorticity during advanced, symmetric, and delayed wing pitch. Blue features are indicative of the LEV and orange are indicative of the TEV. Maroon arrows indicate the magnitude and timing of the maximum lift in the half-stroke. Black arrows indicate the magnitude and timing of the maximum drag coefficients for three representative cases.

Interactions of the wakes of two flapping wings in hover

This paper deals with interactions of the wakes of two flapping wings on aerodynamic characteristics in hover. With dynamically scaled-up robotic wings installed in a water tank, the effects of both the shape of motion profiles and the mean flapping angle of the wings (the average location of the wings with respect to pivots) were investigated. For the zero mean flapping angle case, the interaction commonly reduced aerodynamic force production regardless of the shape of motion profiles. A leading-edge vortex (LEV) near the wing base was slightly deconcentrated, and the effective angle of attack was less than that in the single wing cases. Further investigation on the mean flapping angle revealed that regardless of the shape of motion profiles, the interaction of the two wakes gradually intensified at the dorsal part where the wings came closer, resulting in a periodic imbalanced force production for every half wingbeat cycle. A particle image velocimetry showed a deconcentrated LEV during the upstroke. Deeply inclined downward fluxes with a fair wind brought on the lower effective angle of attack on the wings in the upstroke. During the downstroke, on the other hand, the primary vortex of LEV clearly intensified with a massive headwind. This was clearly observed near each wing base. The minor vortex along the leading edge was noticeably more concentrated than that in the single wing case. These periodically appeared with a combination of back-and-forth wingbeat motions, preserving such imbalanced production while keeping hovering flight.

Correction: Aeromechanics Analysis of a Hummingbird-Like Flapping Wing in Hover

Correction: Aeromechanics Analysis of a Hummingbird-Like Flapping Wing in Hover

Aeromechanics Analysis of a Hummingbird-Like Flapping Wing in Hover

This paper presents an in-depth investigation into the instantaneous forces, flowfield, and wing deflections of a flapping wing used on a hover-capable robotic hummingbird. The goal was to understand the relationship between instantaneous lift and wing shape. An experimental rig was constructed, which flapped the wing at 20 Hz and measured instantaneous loads normal to the stroke plane. To separate inertial and aerodynamic loads, a novel approach was developed in which the wing spatial and temporal displacement was obtained using digital image correlation. From this, the instantaneous inertial force normal to the stroke plane was calculated based on mass distribution. The flowfield on the wing was resolved using particle image velocimetry, which revealed attached flow at steep angles of attack and several strong vortices. It was found that lift was sensitive to both unsteady aerodynamics and deflection. This study marks the first time that these quantities have been measured for a flapping wing used on a hover-capable vehicle, using a procedure developed for extracting pure aerodynamic force on a flexible structure. This effort has resulted in experimental data useful for validating high-fidelity flapping wing aeroelastic computational models.