Membrane Wing Research Articles

Impact of flexibility on the aerodynamics of an aspect ratio two membrane wing

Development of an aeroelastic solver with application to flexible membrane wings for micro air vehicles is presented. A high-order (up to sixth order) Navier–Stokes solver is coupled with a geometrically nonlinear p-version Reissner–Mindlin finite element plate model to simulate the highly flexible elastic membrane. An implicit LES approach is employed to compute the mixed laminar/transitional/turbulent flowfields present for the low Reynolds number flows associated with micro air vehicles. Computations are performed for an aspect ratio two membrane wing at angles of attack α=10°, 16° and 23° for a Reynolds number, Re=24300. Comparisons of the computational results with experimental PIV and surface deflection measurements demonstrated reasonable agreement. Reduced separation and enhanced lift are obtained due to favorable interactions between the flexible membrane wing and the unsteady flow over the wing. The impact of flexibility on the aerodynamic performance comes primarily from the development of mean camber with some further effects arising from the interaction between the dynamic motion of the membrane and the unsteady flowfield above. At lower angles of attack this lift enhancement comes at the cost of reduced L/D. The nose-down pitching moment increases with flexibility at the lowest angle of attack but is reduced for the higher two angles of attack. These results suggest that membrane flexibility might provide a means to reduce the impact of a strong gust encounter by maintaining lift and reducing the effect of the gust on pitching moment.

2A1-X05 羽ばたき機構の検討と試作機の評価(バイオロボティクス)

Two four-flapping-wing type robot prototypes using crank mechanisms were produced for this study. Their flapping aerodynamic characteristics were assessed. Their aerodynamics were measured and flapping motion mechanisms were evaluated. Based on those results fundamental design consideration was performed. We produced polyethylene film membranous wings of two kinds whose trailing edge were made free experimentally. These membrane wings performed a natural feathering motion with a flapping motion. We visualize and observed the air flow around the wing circumference using wind tunnel experiments with steam while changing the robot posture angle. As described herein, we present some results of our study.

Force estimation from incompressible flow field data using a momentum balance approach

This study investigates how aerodynamic forces on a body can be evaluated using only instantaneous or averaged velocity field data through a control volume approach in an incompressible flow. The method allows for velocity field data to be missing inside the control volume, but requires that the product of the acceleration, the volume of the solid body, and the fluid density are negligible compared to the forces of interest. Validation of the method for accuracy has been conducted using three-dimensional data from a numerical simulation of unsteady flow around a static airfoil and has shown good agreement between actual and estimated force. Additionally, the method is tested using phase-averaged PIV data from flexible flapping membrane wings to calculate flapping cycle averaged forces that are compared with directly measured time-averaged forces. The numerical validation suggests that the method is accurate, while it is only able to give a rough estimate for the noisier experimental data. The lower accuracy of the forces estimated from the PIV data can be explained through experimental constraints, such as too large grid spacing in one of the three dimensions (in one of the cases), having to interpolate data on the control volume surface especially where the PIV laser light is shaded by the wing, and not covering the entire span of the wing.

Passive Roll Compensation on Micro Air Vehicles with Perimeter Reinforced Membrane Wings

Micro air vehicles with thin membrane wings have the ability to passively adapt to flight conditions improving aircraft stability. This work is concerned with the prop-wash induced asymmetrical loading of a perimeter reinforced wing. A perimeter reinforced membrane wing is evaluated versus a relatively rigid graphite/epoxy wing to determine if this variation improves lateral-directional flight characteristics, mainly adverse roll due to propeller torque. Digital image correlation, in conjunction with loads measurements in a wind tunnel investigation, are implemented to quantify the wing deflections and stability coefficients. Results indicate that the perimeter reinforced wing does display potential benefits for low speed and high angle of attack flight that could be further tailored to enhance roll compensation for future micro air vehicles.

Controlling pretension of silicone membranes on micro air vehicle wings

This study is concerned with a new method to apply consistent and known pretension to silicone rubber membranes intended for micro air vehicles. Pretension has a marked effect on the static and dynamic responses of membrane wings and controls the overall deflections, as such control and measurement of the membrane pretension are important. The technique developed employs thermal expansion to set the pretension and digital image correlation to assess the resulting spatial strain field. Two different frame geometries were fabricated to evaluate the technique. For open-cell frames, the pretension was not uniform, whereas it was for closed-cell frames. Results show developed full-field stress and strain fields as a function of membrane attachment temperature and frame geometry along with experimental iterations to prove repeatability. The membranes can be stretched to a specific pretension according to the temperature at which it adheres to frames.

Proper Orthogonal Decomposition of Flexible Clap and Fling Elastic Motions via High-Speed Deformation Measurements

Many complex unsteady mechanisms are thought to facilitate the high efficiency and agility commonly observed in small biological flyers. One of these, the flexible clap and fling maneuver, has not been extensively studied; an experimental characterization is the focus of this work. The clap–fling mechanism is approximated with a single flexible membrane flapping wing, replacing the symmetry plane between two wings with a splitter plate simulating the pair wing. This produces a complex vibro-impact aeroelastic problem, the deformation resulting from which is measured with a high-speed visual image correlation system. A low-dimensional representation of the ensuing large data set is obtained with proper orthogonal decomposition. The POD modes, and the relative importance of each, can help elucidate crucial mechanisms and relationships within the flapping system, and are computed for various membrane wing structures and flapping frequencies, with or without the presence of the splitter plate.

Simultaneous Topology Optimization of Membrane Wings and Their Compliant Flapping Mechanisms

This work explores the interdependencies between the topological design of a compliant flapping mechanism and that of a flight-loaded elastic membrane-wing skeleton. This is done via a monolithic aeroelastic framework that encompasses the compliant-mechanism deformation, the motion of a flexible wing, and the transmission of aerodynamic and inertial forces back into the mechanism. A cellular-based evolutionary topology optimization scheme is used for mechanism design, wing design, or both simultaneously. The latter approach is shown to provide superior performance as this allows the tightly coupled nature of the two structures to be fully exploited. The results presented here also demonstrate potential issues that arise during wing design if the flapping kinematics are assumed to be prescribed. Finally, the mechanism topology is further improved via a gradient-based sizing/shape optimization to decrease actuator requirements as well as the time-dependent elastic stresses.

Passive flow control by membrane wings for aerodynamic benefit

The coupling of passive structural response of flexible membranes with the flow over them can significantly alter the aerodynamic characteristic of simple flat-plate wings. The use of flexible wings is common throughout biological flying systems inspiring many engineers to incorporate them into small engineering flying systems. In many of these systems, the motion of the membrane serves to passively alter the flow over the wing potentially resulting in an aerodynamic benefit. In this study, the aerodynamic loads and the flow field for a rigid flat-plate wing are compared to free trailing-edge membrane wings with two different pre-tensions at a chord-based Reynolds number of approximately 50,000. The membrane was silicon rubber with a scalloped free trailing edge. The analysis presented includes load measurements from a sting balance along with velocity fields and membrane deflections from synchronized, time-resolved particle image velocimetry and digital image correlation. The load measurements demonstrate increased aerodynamic efficiency and lift, while the synchronized flow and membrane measurements show how the membrane motion serves to force the flow. This passive flow control introduced by the membranes motion alters the flows development over the wing and into the wake region demonstrating how, at least for lower angles of attack, the membranes motion drives the flow as opposed to the flow driving the membrane motion.

Aerodynamic control of micro air vehicle wings using electroactive membranes

Dielectric elastomer materials are ideal candidates for developing high-agility micro air vehicles due to their electric field–induced deformation. Consequently, the aero-structural response and control authority of the dielectric elastomer material, VHB 4910, are characterized on an elliptical membrane wing. An experimental membrane wing platform was constructed by stretching VHB 4910 over a rigid elliptical wing-frame. The low Reynolds number (chord Reynolds number < 106) and aerodynamics of the elliptical wing were characterized when different electrostatic fields were applied to the membrane. We observe an overall increase in lift with maximum gains of 20% at an applied voltage of 4.5 kV and demonstrate the ability to delay stall. The time-averaged aerodynamic surface pressure is also investigated by comparing sting balance data and membrane deformation measured using visual image correlation. The experimental results are compared to a nonlinear finite element membrane model to further understand the effects of aerodynamic load and electric fields on membrane displacements. Model predictions of surface pressure provide insight into how the electrostrictive constitutive relations influence the fluid–structure interactions of the membrane. This is validated by comparing lift predictions from the model with time-averaged wind tunnel lift measurements near stall.

Aerodynamics of permeable membrane wings. Part 2: Seepage drag

This study addresses a thin wing section, built with an impermeable forward part and a permeable membrane aft part, in a steady flow of incompressible fluid. Assuming that seepage velocity through the membrane is related with the pressure difference across it by Darcy’s law, seepage drag of the wing is found in a closed analytical form. It vanishes as the membrane seals up or thins out, and, under most circumstances, increases with increasing camber. It is shown that the seepage drag can be contained by keeping the width of the aft part less than half the wing’s chord.

Unsteady fluid–structure interactions of a pitching membrane wing

Experiments were conducted on rectangular wings with an aspect ratio of two, at a chord Reynolds number Rec=46,000, in order to understand how membrane wings behave in a prescribed transient motion. As a reference, high-speed Digital Image Correlation (DIC) and Particle Image Velocimetry (PIV) measurements were performed on stationary membrane wings in the range 0°<α<25°. Four distinct incidence regions were identified based on the mode shape, frequency and amplitude of the surface vibrations. Flow-field measurements indicated that the different regions were caused by the interactions of the leading-edge separated shear layer with the membrane. Then the membrane wing was subjected to transient sinusoidal pitching manoeuvres at starting incidences in each of these regions. The membrane deformation characteristics were seen to vary considerably with starting angle; both in terms of time-averaged and instantaneous quantities. In some cases there was significant time-lag in the response of the membrane. The PIV flow field measurements for this case showed signs of hysteresis between the pitch-up and pitch-down parts of the wing motion, which was not the case for a rigid wing with identical planform.

Aerodynamic Investigations of a Morphing Membrane Wing

This paper presents the aerodynamic analysis of a morphing wing configuration using a compliant membrane as lifting surface to enable large variations of the planform geometry (aspect ratio and sweep angle) and airfoil shape. Comprehensive tests of a specifically designed wind-tunnel model including force measurements, deformation measurements, surface flow visualizations, and wake flow measurements are used to assess the aerodynamic behavior and the performances of this morphing wing. The results of the force measurements performed on five different wing configurations ranging from a high-aspect-ratio, straight wing to a highly swept back, low-aspectratio configuration show that the active variation of the wing planform effectively alters the lift and drag characteristics in such a way that relatively high lift-to-drag ratios can be maintained over a broad range of flight conditions.Inaddition,duetoitsmaterialproperties,thewingsurfacepassivelydeflectsunderaerodynamicloading, whichaffectstheairfoilshapeaccordingly.Itresultsin anadditional dependencyoftheaerodynamiccharacteristics on the flow conditions, which can be used for passive flow control.

Unsteady aerodynamics and flow-induced vibrations of a low aspect ratio rectangular membrane wing with excess length

In this study, an experimental investigation of a low aspect ratio rectangular membrane wing with excess length was presented at angles of attack ranging from 0° to 25° at Reynolds number of 48,700. Time-accurate measurements of membrane deformation using Digital Image Correlation system, and velocity field measurements with Digital Particle Image Velocimetry system were carried out while normal forces of the wing were measured by helping a load-cell system. The experimental results showed that the camber of membrane wing with excess length excited the separated shear layer and this situation increased the lift coefficient relatively more as compared to membrane wings without excess length. At the angles of attack of α=6–10° and α=19–22°, dominant vibrational modes were obtained as a spanwise mode of two, whereas dominant spanwise modes were three at α=11–18° and α=23–25°. Moreover, the membrane wings with excess length exhibited small separated regions since the shear layer was closer to the membrane wing surface although camber of the wing increased. Consequently, it was shown that in membrane wings with low aspect ratio, unsteadiness included tip vortices and vortex shedding, and the combination of tip vortices and vortex shedding caused complex unsteady deformations of these membrane wings.

Experimental Characterization of Aerodynamic Behavior of Membrane Wings in Low-Reynolds-Number Flow

An experimental study of the aerodynamic characteristics of a flat-plate membrane wing was conducted. Three different values of membrane prestrain (5, 7, and 10%) were investigated, along with a rigid flat plate, at Reynolds numbers of 13,700, 22,600 and 36,300 and angles of attack up to 27 deg. It was found that 1) the dependence of the prestall mean lift on model prestrain is negligible, 2) prestall mean lift at a given level of prestrain is a strong function of Reynolds number, and 3) the mean drag increased with a decrease in model prestrain. In addition, the stall angle of attack is weakly dependent on prestrain and increases with increasing Reynolds number. With the exception of the highest Reynolds number, the flexible-model stall angles were found to be similar to the rigid flat-plate results. At the largest Reynolds number, dynamic results for the flexible models revealed large rms values of lift and drag, which generally decrease with increasing angle of attack. In comparison to the flexible models, the rms values for the rigid flat plate were insignificant. An aeroelastic flutter instability is postulated to be the cause of the large dynamic response for the flexible models. This hypothesis is supported by results that were generated using a potential flow- based computational aeroelastic model. These aeroelastic instability-induced vibrations are also proposed as the mechanism by which the flexible models showed better stall characteristics at the highest-tested Reynolds number.

High-order simulations of low Reynolds number membrane airfoils under prescribed motion

The aerodynamics and aeroelastic response of a membrane wing under prescribed motion are investigated using a high-order, two-dimensional Navier–Stokes solver coupled to a geometrically nonlinear membrane model. The impact of increasing Reynolds number on the vortex dynamics and unsteady aerodynamic loads is examined for moderate-amplitude plunge and combined pitch–plunge motions at low frequency. Simulation results are compared with classical thin airfoil theory and highlight the differences between rigid and flexible membrane airfoils undergoing small and moderate amplitude motions. The present study demonstrates the ability of lifting membrane surface flexibility to enhance thrust production and propulsive efficiency, which may inform the design of flapping wing membrane fliers.

Cell Geometry and Material Property Effects on Membrane and Flow Response

TO OPTIMIZE micro aerial vehicle (MAV) designs for size and speed regimes characteristic of small birds and insects, a growing subset of low-Reynolds-number research groups practice biomimetics to take advantage of nature-optimized designs proven effective within the MAV flight regime [1]. One common feature of many natural flyers is wing flexibility. Engineering models that mimic low-Reynolds-number fliers with fixed-wing flexibility have demonstrated improved aerodynamics compared with conventional rigid airfoils. This improvement is attributed to a combination of passive static and dynamic deformations of the flexible-membrane wings supported by semirigid, fixed-wing frames. Static deformation, of bothmembrane and frame, can advantageously affect airfoil camber and tip washout through inflation and alteration of aerodynamic and geometric twist [2–4]. Dynamic membrane deformations can improvemomentum transfer between the leeside free shear layer and recirculation region of models with rigid leading and trailing edges and a membrane attached in between, thereby, decreasing the extent of flow separation and corresponding wake size, as well as advantageously affecting the aerodynamic forces [5,6]. Similar vibrations also exist for membrane wings with an unattached trailing edge. Hubner and Hicks [7] showed aerodynamic efficiency improvement by scalloping the trailing edge and developed scaling trends with respect to cell geometry. Johnston et al. [8] demonstrated via high-speed imagery that membrane prestrain increased flutter onset velocity while reducing the magnitude of the postflutter limit cycle oscillations. A free trailing-edge wing structure typically includes a flat or cambered frame of moderate or low aspect ratio that is significantly less compliant than the attached membrane. The membrane is attached along the leading edge of the frame and along battens (or ribs) extending from the leading edge. In this Note, it is documented that the initial flutter onset frequency of zero-applied-pretension membranes originates from small-amplitude membrane vibrations at the natural frequency of the membrane. Although applied pretensioning will affect the frequency, cell geometry and material properties are shown to play an important role as well. Spectral data from the membrane and the leeside shear layer via simultaneously sampled hot-wire anemometry and laser vibrometry are presented and discussed in this Note.

Modal Analysis of a Flexible Membrane Wing of Micro Air Vehicles

Modal Analysis of a Flexible Membrane Wing of Micro Air Vehicles

Flow-induced vibrations of low aspect ratio rectangular membrane wings

An experimental study of a low aspect ratio rectangular membrane wing in a wind tunnel was conducted for a Reynolds number range of 2.4×10 4–4.8×10 4. Time-accurate measurements of membrane deformation were combined with the flow field measurements. Analysis of the fluctuating deformation reveals chordwise and spanwise modes, which are due to the shedding of leading-edge vortices as well as tip vortices. At higher angles of attack, the second mode in the chordwise direction becomes dominant as the vortex shedding takes place. The dominant frequencies of the membrane vibrations are similar to those of two-dimensional membrane airfoils. Measured frequency of vortex shedding from the low aspect ratio rigid wing suggests that membrane vibrations occur at the natural frequencies close to the harmonics of the wake instabilities. Vortex shedding frequency from rigid wings shows remarkably small effect of aspect ratio even when it is as low as unity.

Aerodynamics of permeable membrane wings

This study addresses a thin wing section, built with a solid impermeable forward part and permeable membrane aft part, in a steady flow of incompressible fluid. Closed-form expressions are found for the lift and pitching-moment coefficients of the wing, relating them with the width of its forward part and permeability of its aft part. It is shown that the lift slope coefficient uniformly decreases with increasing permeability, changing from the familiar value of 2 π to 2 π times the relative width of the forward part. Concurrently, the aerodynamic center moves from its familiar position at the quarter-chord point of the wing to the quarter-chord point of the forward part, but its movement is not uniform. When the width of the forward part is less than half of the chord, the aerodynamic center initially moves backwards, and only then forwards.

Aeroelastic Analysis of Membrane Microair Vehicles—Part I: Flutter and Limit Cycle Analysis for Fixed-Wing Configurations

This is the first of two papers concerning the fluid and structural dynamic characteristics of membrane wing microair vehicles. In this paper, a (three) batten-reinforced fixed-wing membrane microair vehicle is used to determine the effect of membrane prestrain on flutter and limit cycle behavior of fixed-wing membrane microair vehicles. For each configuration tested, flutter and subsequent limit cycle oscillations are measured in wind tunnel tests and predicted using an aeroelastic computational model consisting of a nonlinear finite element model coupled to a vortex lattice solution of the Laplace equation and boundary conditions. Agreement between the predicted and measured onset of limit cycle oscillation is good as is the prediction of the amplitude of the limit cycle at the trailing edge of the lower membrane. A direct correlation between levels of strain and the phase of the membranes during the limit cycle is found in the computation and thought to also occur in the experiment.