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The paper discusses the development of a nonlinear aeroelastic coupled trim model of a twin cyclocopter in forward flight. The twin cyclocopter consists of two cycloidal rotors as main thrusters an...

A large variety of gusts that develop in the atmospheric boundary layer affect aerial vehicles. This study, performed in water tow tanks, compares the response of a flat plate wing to transverse ve...

Investigated and revealed are new insights into the individual, decoupled effects of induced camber and spanwise bending on the lift, drag, and endurance for an optimum flexibility membrane-and-fra...

An enhanced nonlinear aeroelastic-coupled-flight dynamics framework that enables the investigation of propeller aerodynamics and inertial effects on the response of a very flexible aircraft is pres...

At low Reynolds numbers, revolving wings become less efficient in generating lift for hovering flight due to the increasing adverse viscous effects. Flying insects use reciprocating revolving wings...

Flapping rotary wing is a novel aerodynamic configuration proposed in recent years for micro air vehicles. To understand the aerodynamic characteristics of this wing layout, a computational fluid d...

An optical tracking facility was used to record the free flight of the Monarch butterfly for a large number of sequential flaps. The system automatically tracked reflective markers, which were modi...

Flapping-wing micro air vehicles are not confined to undergoing the normal-hovering motion implemented by flying insects. In this study, the water-treading motion, which originates from aquatic pro...

Flapping rotary wing, rotating wing, and flapping wing are feasible wing layouts applicable to micro-air-vehicles capable of hovering flight. A numerical study in this paper presents which wing layout can be more efficient in terms of aerodynamic power for the given kinematic and geometric parameters with or without the constraint of vertical force. In the cases under typical conditions, rotating wing layout is the most efficient one when a small vertical force is needed. However, if a much larger vertical force is required, flapping rotary wing is the only wing layout that fulfills the requirements of the two aspects due to its coupling effect. At relatively high Reynolds number (Re>2000), flapping amplitude (>70°), and aspect ratio (=6), comparative relationships from the cases under typical conditions among three wing layouts in terms of vertical force and aerodynamic power efficiency are kept unchanged. Nevertheless, at relatively low Re(

The vortical flow structures generated by the flapping wings of the DelFly II micro air vehicle in hovering flight configuration are investigated using particle image velocimetry. Synchronous force measurements are carried out to establish the relation between the unsteady forces and force generation mechanisms: particularly, the leading-edge vortex and the clap-and-peel motion. The formation of conical leading-edge vortices on both wings is revealed, which occurs rapidly at the start of the outstroke as a result of the wing–wing interaction. The leading-edge vortices of the outstroke interact with those of the instroke, which are shed and, by mutual induction, advect upstream as a vortex pair at the end of previous instroke. The leading-edge vortex pairs induce a strong inflow into the region formed between the upper and lower wings during the peeling phase, resulting in the formation of a low-pressure region. This, together with the leading-edge vortices and a momentum increase formed by the clap, accou...

Three-dimensional unsteady flow fields of a flapping, low-aspect-ratio wing have been investigated by means of highly resolved tomographic particle image velocimetry (Tomo-PIV) measurements and computational fluid dynamics (CFD). Furthermore, force measurements have been carried out. Tomo-PIV was applied to the flow above a flat plate wing during the downstroke. High spatial resolution and large volume thickness could be achieved by using sensitive sCMOS cameras and a traversing setup. The CFD calculations covered the complete period of motion. The analysis of the vortex-dominated flow fields provides a deeper understanding of vortex interaction and three-dimensionality of low Reynolds number (Re=18,000 and Re=36,000) flows. Two different Strouhal numbers (St=0.06 and St=0.13) are considered and their effects on the development of a leading edge and tip vortex are discussed. The PIV results show instantaneous flow fields after a leading edge separation that are dominated by small-scale turbulent vortex st...

This study explores the aerodynamic characteristics of a flapping-wing micro aerial vehicle (MAV) in hovering configuration by means of force and flowfield measurements. The effects of flapping frequency and wing geometry on force generation were examined using a miniature six-component force sensor. Additional high-speed imaging allowed identification of the notable different deformation characteristics of the flexible wings under vacuum condition in comparison to their behavior in air, illustrating the relevance of aeroelastic effects. Flow visualization around the flapping wing by means of planar particle image velocimetry (PIV) measurements revealed the formation, development, and shedding of the vortical structures by the wings during flapping motion, with particular emphasis on the clap-and-fling phase. Further stereoscopic PIV measurements performed in the wake showed a momentum surplus wake induced by the clap-and-fling, indicative of thrust generation. The vortical structures in the wake formed d...

The aerodynamic performance of flapping micro air vehicles in hover conditions is dependent on many parameters, including the wing design. With the goal of optimizing the wing for hover performance, the initial focus was to reduce the uncertainty in the thrust measurements. This is because lower uncertainty in this metric enables better resolution in comparing the performance of different designs. Aerodynamic performance variability was deemed to be the fault of an imprecise manufacturing technique. Therefore, adjustments were made to the fabrication process until a permissible level of uncertainty was attained for optimization; the goal was less than 5%. This paper chronicles the progression of the wing fabrication process and details how the uncertainty was evaluated. Four fabrication methods and two different wing designs are included in this study: a carbon fiber hand layup technique, carbon fiber cured in a machined mold, and two variations of a machined plastic skeleton reinforced with a carbon fibe...

Micro-air-vehicle wing designs often incorporate flexible structures that mimic the skeletal and membrane attributes found in natural flyers. Accurate performance predictions for these wing types require coupling of aerodynamic and structural simulations. Such fluid–structure interaction simulations are often performed using high-fidelity, numerically expensive techniques such as computational fluid dynamics coupled to nonlinear structural finite element analysis. Although the computational cost of conducting many conceptual design trade studies with these methods is prohibitive, simplified approaches may lack sufficient fidelity to provide conceptual design insights. This paper summarizes the development, comparison, and application of an efficient fluid–structure interaction method to simulate flexible-wing performance for rapid conceptual design of micro air vehicles. An advanced potential flow model computes aerodynamic performance, whereas a corotational frame and shell finite element structural mode...

A low-order phenomenological model relating dynamic wing motion to thrust production of micro flapping wings with different stiffness was examined. The motivation is toward closed-loop flight control of flapping micro air vehicles. The low-order model was based on a “black box” state-space description that used the intraperiod flapping-wing motion as input to determine modeling coefficients that minimized the error between the predicted and actual intracycle thrust. The data time series used in the modeling were provided by a companion experiment with three flapping wings of different designs and stiffness. The wings were fabricated from laminated carbon-fiber strips that were covered by a thin polyvinylidene fluoride film sheet. The wings had a span of 12.5 cm and were attached to a commercial Cybird gearbox that could be operated at different flapping frequencies. Digital high-speed video was used to determine the dynamic displacement of the wing structure. The intraperiod wing displacement was quantifi...

Hover-capable flapping-wing micro air vehicles are well suited for missions in confined spaces. The best design practices for flapping wings and their kinematics are largely unknown, especially for flexible wings. To address this issue, numerical optimization is applied to the design of the kinematics and structural sizing of a flapping wing using a surrogate-based approach. The surrogates are generated using kriging interpolation of the time-averaged thrust generated and power required by the wings. The thrust and power data are computed using a nonlinear approximate aeroelastic model developed in previous studies by the authors. A numerical optimization algorithm is used to identify designs that produce the desired combination of thrust and power. The design variables consist of parameters describing the flap–pitch kinematics and the stiffness of the flexible wings. A trend study of thrust and power indicate that the phase angle between flap and pitch motions significantly affects the wing performance w...

At Reynolds numbers approaching those of micro air vehicles (both engineered and natural), the Eppler 387 airfoil (in common with many other smooth profiles) can have multiple lift and drag states at a single wing-incidence angle. Prestall hysteresis and abrupt switching between stable states result from sudden flow reattachment and the appearance of a large separation bubble. It is shown that control of the dynamics can be achieved using external acoustic forcing. Separation control, hysteresis elimination, and more than 70% increase in lift–drag ratio are obtained at certain excitation frequencies and sound pressure levels. The global flow around the wing is effectively modified, and large, stable vortical structures appear in the separated shear layer. Correlation between the effects of acoustic excitation and wind-tunnel resonance shows that the antiresonances in an enclosed chamber correspond to the largest improvement in wing performance. Implications for control and stabilization of small aircraft ...

T HE flapping flight mechanism can provide revolutionary operation capabilities for tomorrow’s micro air vehicles (MAV) [1]. The unsteady aerodynamics of the flapping flight is vastly different from traditional fixed-wing flyers and allows for high maneuverability in cluttered environments [2]. To provide this maneuverability, high rates of thrust and propulsive efficiency beyond normal cruise flight conditions are necessary. Observations of flying birds in nature [3] revealed that birds are not only using the classic combined pitch/plunge motion for flapping flight but also incorporate an additional gliding motion of their wings in the direction of the free-stream velocity U1. The present note demonstrates that these gliding motions increase significantly both thrust and efficiency of flapping flight. For this purpose, unsteady Reynoldsaveraged Navier–Stokes (URANS) computations are used to estimate the flow around a flapping, birdlike airfoil SG04 [4] at a Reynolds number of 10 with fixed transition.

Micro air vehicles are typically small in size and are remotely controlled or autonomous aircraft that fly slowly (50; 000 < Reynolds number < 200; 000) and very close to the Earth's surface. Due to the combined influences of low Reynolds number effects and the high levels of freestream turbulence, present within the atmospheric boundary layer, micro air vehicles are very difficult to control/fly outdoors.

The aeroelastic response of a biologically inspired flappingmembrane wing to unsteady inertial and aerodynamic forces is strongly predicated upon the topological distribution of the underlying skeletal reinforcement (venation patterns). Proper exploitation of this relationship may enable the development of effective flapping wing micro air vehicles. The skeletal topology is parameterized into a developmental programwhich, when compiled and executed, evolves the wing topology in stages. A genetic algorithm can optimize the details of this program, rather than explicitly considering the topology itself, thus removing the link between design variables and topological geometry resolution. This cellular division tool is coupled to an unsteady aeroelastic representation of a flapping wing in forward flight in order to obtain the Pareto tradeoff curves between thrust generation, lift generation, and input power requirements. The topologies obtained along the front provide an understanding of the key relationships between skeletal topology, aeroelastic behavior, and performance metrics during flapping flight.

This paper describes the aeroelastic model to predict the blade loads and the average thrust of amicro-air-vehiclescale cycloidal rotor. The analysis was performed using two approaches: one using a second-order nonlinear beam finite element method analysis for moderately flexible blades and a second using a multibody-based largedeformation analysis (especially applicable for extremely flexible blades) incorporating a geometrically exact beam model. An unsteady aerodynamic model is included in the analysis with two different inflow models: single streamtube and double-multiple streamtube inflowmodels. For the cycloidal rotors usingmoderately flexible blades, the aeroelastic analysis was able to predict the average thrust with sufficient accuracy over awide range of rotational speeds, pitching amplitudes, and number of blades. However, for the extremely flexible blades, the thrust was underpredicted at higher rotational speeds, and thismaybebecause of the overprediction of blade deformations. The analysis clearly showed that the reason for the reduction in the thrust-producing capability of the cycloidal rotorwith blade flexibility may be attributed to the large nosedown elastic twisting of the blades in the upper half cylindrical section, which is not compensated by a noseup pitching in the lower half-section. The inclusion of the actual blade pitch kinematics, unsteady aerodynamics, and flow curvature effects was found crucial in the accurate lateral force prediction.

included. Simulations are compared with previous modeling efforts, which neglected the wings’ mass and the associated inertial coupling effects on the body. Simulations show a qualitative consistency for the nonlinear model with wing effects when different aerodynamic models are chosen as inputs. Simulation results show a significant differenceinthemodelbehaviorwhenthemassofthewings,initiallysetat5.7%ofthebodymass,isincludedversus whenthemassisneglected.Asthemassofthewingsisdecreased,thesimulationresultsofthemodelwithwingeffects approachtheresultswhenthestandardaircraftmodelisused.Simulationsleadtotheconclusionthatthemasseffects of the wings are important for dynamics, stability, and control analyses.

This paper presents theoretical and experimental analyses of insect flapping mechanics and aerodynamics, with the aim of developing flapping-wing micro air vehicles. A model of an insect thorax flapping mechanism is developed that includes a quasi-steady aerodynamic model. Computer simulations of the thorax model show insectlike wing kinematics, including passive wing rotation at the end of the stroke. These kinematics are perturbed in a sequence of experiments using a robotic flapping-wing device to determine optimal hovering kinematics. Experiments are supported with numerical optimization based on the aerodynamic model. Apart from optimal kinematics, this study shows negative aerodynamic power required during wing rotation, which explains passive wing rotation at the end of the stroke.

This experimental study investigates the relationship between flapping wing structure and the production of aerodynamic forces for micro air vehicle hovering flight by measuring full-field structural deformation and thrust generation. Results from four flexible micromembrane wings with different skeletal reinforcement demonstrate that wing compliance is crucial in thrust production: only certain modes of passive aeroelastic deformation allow the wing to effectively produce thrust. The experimental setup consists of a flapping mechanism with a single-degree-of- freedom rotary actuation up to 45 Hz at 70 deg stoke amplitude and with power measurement, a force and torque sensor that measures the lift and thrust, and a digital image correlation system that consists of four cameras capable of capturing the complete stroke kinematics and structural deformation. Several technical challenges related to the experimental testing of microflapping wings are resolved in this study: primarily, flapping wings less than 3 in. in length produce loads and deformations that are difficult to measure in an accurate and nonintrusive manner. Furthermore, the synchronization of the load measurement system, the vision-based wing deformation measurement system, and the flapping mechanism is demonstrated. Intensive data analyses are performed to extract useful information from the measurements in both air and vacuum.

This paper presents the work done in designing a morphing wing concept for a small experimental unmanned aerial vehicle to improve the vehicle's performance over its intended speed range. The wing is designed with a multidisciplinary design optimization tool, in which an aerodynamic shape optimization code coupled with a structural morphing model is used to obtain a set of optimal wing shapes for minimum drag at different flight speeds. The optimization procedure is described as well as the structural model. The aerodynamic shape optimization code, that uses a viscous two-dimensional panel method formulation coupled with a nonlinear lifting-line algorithm and a sequential quadratic programming optimization algorithm, is suitable for preliminary wing design optimization tasks. The morphing concept, based on changes in wing-planform shape and wing-section shape achieved by extending spars and telescopic ribs, is explained in detail. Comparisons between optimized fixed wing performance, optimal morphing wing performance, and the performance of the wing obtained from the coupled aerodynamic-structural solution are presented. Estimates for the performance enhancements achieved by the unmanned aerial vehicles when fitted with this new morphing wing are also presented. Some conclusions on this concept are addressed with comments on the benefits and drawbacks of the morphing mechanism design.

Living creatures such as insects, birds, and fish generate lift and thrust most often by executing large-amplitude wing flapping, possibly with substantial shape deformation from root to tip. The flow for these motions is unsteady, and conventional steady-state aerodynamics is unable to correctly compute the time history of their flapping-force generation. Three-dimensional unsteady computations of flapping about the deforming wing or fin surface are necessary to correctly predict the lift and thrust throughout the flapping cycle. It is only by executing such computations for creatures or vehicles with moving and deforming surfaces that we can gain insights into the time-varying pressure distribution on all surfaces and how that results in flapping-force generation. This can be coupled with visualization of the origination and evolution of body, wing, and wake vorticity. Three-dimensional unsteady flow computations of the flapping flights of the fruit fly, a pectoral-fin swimmer (the bird wrasse), and a variety of unmanned air vehicles were carried out in pursuit of this information. The performance of these flapping wings under gust conditions was also investigated. The effect of fin deformation on the force production was studied. Novel biomimetic vehicles, incorporating information gained from these computations, were designed and built and their performance is described.

It is the objective of this paper to review recent developments in the understanding and prediction of flapping-wing aerodynamics. To this end, several flapping-wing configurations are considered. First, the problem of single flapping wings is treated with special emphasis on the dependence of thrust, lift, and propulsive efficiency on flapping mode, amplitude, frequency, and wing shape. Second, the problem of hovering flight is studied for single flapping wings. Third, the aerodynamic phenomena and benefits produced by the flapping-wing interactions on tandem wings or biplane configurations are discussed. Such interactions occur on dragonflies or on a recently developed micro air vehicle. The currently available two- and three-dimensional inviscid and viscous flapping-wing flow solutions are presented. It is shown that the results are strongly dependent on flapping frequency, amplitude, and Reynolds number. These findings are substantiated by comparison with the available experimental data.

This paper addresses the aerodynamics of insect-based, biomimetic, flapping wings in hover. An experimental apparatus, with a biomimetic flapping mechanism, was used to measure the thrust generated by a number of wing designs at different wing pitch settings. To quantify the large inertial loads acting on the wings, vacuum chamber tests were conducted. Results were obtained for several high-frequency tests conducted on lightweight aluminum and composite wings. The wing mass was found to have a significant influence on the maximum frequency of the mechanism because of a high inertial power requirement. All the wings tested showed a decrease in thrust at high frequencies. In contrast, for a wing held at 90-deg pitch angle, flapping in a horizontal stroke plane with passive pitching caused by aerodynamic and inertial forces, the thrust was found to be larger. To study the effect of passive pitching, the biomimetic flapping mechanism was modified with a passive torsion spring on the flapping shaft. Results of some tests conducted with different wings and different torsion spring stiffnesses are shown. A soft torsion spring led to a greater range of pitch variation and produced more thrust at slightly lower power than with the stiff torsion spring. The lightweight and highly flexible wings used in this study had significant aeroelastic effects which need to be investigated. A finite element based structural analysis of the wing is described, along with an unsteady aerodynamic analysis based on indicial functions. The analysis was validated with experimental data available in literature, and also with experimental tests conducted on the biomimetic flapping-pitching mechanism. Results for both elastic and rigid wing analyses are compared with the thrust measured on the biomimetic flapping-pitching mechanism.

Computational fluid dynamics simulation results for a thin cambered plate airfoil, and flow visualization and force measurements from experiments conducted in water on a flapping-and-pitching thin flat-plate wing of semi-elliptic planform at low Reynolds numbers are reported. Time-varying force data, measured using a force transducer, provide a means to understand the mechanisms that lead to enhanced performance observed in insect flight compared with fixed-wing aerodynamics. Experimental uncertainties associated with low-level (∼1 N) fluid dynamic force measurements on flapping-and-pitching wings, including inertia effects, are addressed. A previously proposed pitching mode in which the leading-edge and trailing-edge switch roles to allow using cambered airfoils is shown to be feasible. A vortex-trapping model is proposed to explain the aerodynamic advantages of switching. The present results also support the authors' proposed idea that an optimum reduced flapping frequency might exist. The study has applications in micro air vehicle development.

The flapping motion of a wing based on the hind wing of the Aeschna juncea dragonfly is simulated using a three-dimensional incompressible Navier-Stokes solver. The performance of the wing is investigated by variation of a number of kinematic parameters. Flapping amplitudes of between 10 and 60 deg (half-angle) and frequencies of 1 to 300 Hz are considered, resulting in a Reynolds number range of 100 to 50,000. The flapping amplitude observed for Aeschna juncea is shown to maximize the ratio of mean vertical force produced to power required.

A low-aspect-ratio, low-Reynolds-number membrane wing has been identified as a viable platform for micro air vehicle applications. Desirable flying qualities include high lift and larger stability margins. Several challenges are associated with the numerical modeling of such a wing, including highly three-dimensional flows, separation bubbles, and nonlinear membrane behavior. A thorough model validation and system identification effort is therefore required. A novel experimental setup integrates a wind tunnel with a visual image correlation system for simultaneous measurement of wing displacements, strains, and aerodynamic loads. These three metrics are used for a direct comparison of numerical and experimental data for both pre- and poststall angles of attack. Suitable correspondence is demonstrated for moderate angles of attack; methods for increasing the model fidelity can be made for angles with poor predictive capability. Computed flow structures reveal further information concerning the aeroelastic behavior of membrane wings.

A trajectory control problem for simple micro air vehicles (MAVs) is introduced and studied. The MAVs are endowed with the ability to estimate their position and velocity and can control their drag coefficient. The objective of this work is to design a control law for the drag coefficient in order to land the MAV close to a desired streamwise location, after being dropped from an airborne carrier at low altitude. We use a simple model-predictive control law, relying on a two-dimensional time-averaged atmospheric velocity field, which is validated on an unsteady three-dimensional representation of an unstratified atmospheric boundary layer. The simulation results show that the proposed control law results in a significant (10-fold) improvement in the accuracy of the streamwise landing point location, with respect to the uncontrolled case. Similar results are obtained when errors are considered in the location of the release point and in the estimate of the mean flowfield.

§† Micro air vehicles with a maximal dimension of 15cm require original design concept due to their low Reynolds number flight regime. It has been empirically observed that a flexible membrane wing can improve the aerodynamic performance of the vehicles. To help advance our knowledge in this area, we investigate shape optimization of a membrane wing. A direct membrane wing optimization employing a coupled Navier-Stokes and flexible structure analysis is computationally expensive. Therefore, we use a rigid wing as a surrogate model. We employ a moving grid technique to facilitate automatic grid generation. Our objective is to maximize the lift-to-drag ratio under aerodynamic and geometry constraints. The optimized design exhibits reduced camber from root to tip. Overall, the aerodynamic improvement is primarily derived from the inner 70% of the wing. The optimized platform is checked for performance of the membrane wings and found to improve the performance by about the same margin. Furthermore, both optimized membrane and rigid wings improve the liftto-drag ratios mainly by reducing the form drag. However, the membrane wing demonstrates less variation in lift-to-drag ratio as the angles of attack vary.