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Instantaneous force and structural deformation experiments were performed on a flexible, structurally characterized, low-aspect-ratio representative flapping wing. A six-component force balance was...

A computational study is conducted on thin flat plates to simulate flows of Reynolds numbers at 104 to provide understanding and guidance for micro air vehicles and other low-Reynolds-number airfoi...

In this paper, aiming to drive the unmanned aerial vehicle (UAV) to the optimal position of the vertical plane in the close formation flight, a control method that is based on extended state observ...

Experimental and computational study of flapping-wing insect at low Reynolds number is conducted in this work. The paper is broadly divided into two sections: experiments and computational fluid dy...

The objective of this paper is to understand roles of abdominal motion in the pitch stability of flapping flights of butterflies numerically, and a two-dimensional butterfly model has thoracic pitc...

The performance and effectiveness of serpentine dielectric barrier discharge plasma actuators as hingeless high-lift devices on a three-dimensional airplane model were investigated. Attempts were m...

A computational study has been conducted on various airfoils to simulate flows at Reynolds numbers (Re) primarily between 104 and 105 to provide understanding and guidance for MAV and other low-Rey...

The increasing usage of low-Reynolds-number (10,000–100,000 tip Reynolds number) scale quadrotors for civilian and military applications provides the impetus for the development of reliable design ...

The main objective of this paper is to provide a rigorous coupling between body flight dynamics and flow dynamics (unsteady aerodynamics) in hovering of insects and flapping-wing micro air vehicles...

An approach to modeling longitudinal airplane aerodynamics during unsteady maneuvers was developed for a micro air vehicle at angles of attack well past stall under unsteady conditions, including dynamic stall as might be experienced in perching maneuvers. To gather unsteady micro air vehicle flight data, an offboard motion tracking system was used to capture free-flight trajectories of a micro air vehicle with a weight of 14.44 g (0.0594 oz) and a wingspan of 37.47 cm (14.75 in.), operating at a nominal Reynolds number of 25,000. The measured trajectories included nominal gliding flight as well as mild-to-aggressive stalls. For the most aggressive stall case, the maximum lift coefficient reached a value near 2.5. The new model derived from the test data relied on a so-called separation parameter that modeled the aerodynamic lag during rapid changes in the angle of attack, and it thereby captured the effects of dynamic stall seen in the lift, drag, and moment coefficient data. Results from the model were ...

The main focus of this research is the generation of a computational aeroelastic model for a macrofiber composite actuated micro air vehicle. Two M8528-P1 actuators were used to deform the surface of a forward-swept thin, compliant composite wing. The general approach, which included MATLAB, ABAQUS, and Athena Vortex Lattice, is described. The planform design and initial flight testing are discussed. The computational model was validated first with static tests, and then quasi-static wind-tunnel tests were used to validate the aeroelastic model. Digital image correlation was used to compare the expected and measured displacements of the wing. Flight loads were also compared for various angles of attack. The results indicated good agreement for angles of attack well within the standard flight envelope, despite small discrepancies in geometry.

Using a quasi-steady, blade-element analysis, we investigated the role of asymmetrical flapping mechanisms in hovering flight, for insect inspired micro air vehicles. The current analysis was applied to a 30 cm half-span wing, beating not more than 6 Hz. An implementation of asymmetrical flapping exhibited significantly greater lift generation, which can be attributed to the increase in angular velocity squared form for lift that occurs with increasing asymmetry. Significant improvements in the lift-to-power ratio were observed, for a house-fly-like mode of flapping, when the wing-beat frequency was below the natural frequency. At a frequency ratio of 0.3, a 75% increase in performance was observed with the use of asymmetrical flapping. At flapping frequencies above the natural frequency, however, asymmetry was found to be detrimental to performance, due to an increase in inertial forces. In a low inertia, an inclined stroke plane system, characteristic of dragonflies, we see that, in its most efficient flapping condition, asymmetrical flapping is detrimental to performance. However, in compliant systems in which elastic forces are significant, we see that asymmetry can improve the aerodynamic efficiency of the wing-actuation system Refereed/Peer-reviewed

Micro air vehicles promise to play an important part in public, private, and military arenas in the coming years. Small, lightweight, agile, and inexpensive, these flying machines are of interest in domestic search-and-rescue operations, law enforcement, reconnaissance, mapping, and urban combat operations. Nature demonstrates that flapping wings are an extremely effective solution to the problem of hovering flight with good agility in close quarters. However, the highly unsteady, largely separated character of the flow complicates aerodynamic analysis of these vehicles, and these challenges are magnified in a design setting when the sensitivities to many geometric and kinematic parameters must be assessed. In this paper, computational fluid dynamics simulation is applied as a means to evaluate kinematic parameters of flapping wings for a four-wing, dragonfly-like micro air vehicle. It is shown that high-frequency, moderate-amplitude flapping is preferred for wings flapping in a vertical stroke plane, and...

Recent interest in high-angle-of-attack flight, aerobatic maneuvering, vertical/short takeoff and landing, etc., of small unmanned aerial vehicles necessitates more detailed modeling of the complex aerodynamics associated with these flight regimes. This includes modeling the effect of the propeller slipstream, also called prop wash, which is the main source of airflow that helps maintain lift and control during near-zero forward-speed flight like that encountered during vertical/short takeoff and landing, as well as during high-angle-of-attack flight/aerobatic maneuvering like hovering. Propeller slipstream models based on conventional theories, such as the momentum theory, have been used extensively in the literature to predict the induced air velocity within the slipstream. However, because these conventional theories consider only acceleration of air within the slipstream and not diffusion, their applicability in regions far downstream of the propeller where diffusion is dominant, is questionable. This...

The nexus of advanced manufacturing methods, computer-aided design tools, and modern structural-analysis software has enabled the design and fabrication of structurally complex wing structures with unique features. This is particularly true for small unmanned aircraft, in which discrete structural hinges can easily be integrated into the overall vehicle design. This paper examines the use of discrete structural hinges for tailoring the low-frequency flight dynamics of the vehicle. For sufficiently soft discrete structural hinges, substantial coupling between flexible and rigid modes occurs, leading to the potential to modify the flight dynamic behavior through structural flexibility. Using a multibody flight dynamics simulation tool with a nonlinear lifting-line aerodynamic representation, different structural-hinge elastic properties, orientation, and location on the aircraft are examined. The results for a small unmanned aircraft indicate that flexibility mostly affects the longitudinal modes and associ...

Typical missions of micro aerial vehicles include remote sensing tasks in confined spaces, which limit wingspans and restrict maneuvering. Under these conditions, loiter may be limited by reduced endurance brought about by the increased power needs that may accompany wingspan and turning radius restrictions. A theoretical study was therefore performed with the primary purpose to explore aircraft configurations that are more suitable for such constrained flight domains. The theoretical performance model used for this investigation is based on a higher-order potential flow method that uses a table–lookup routine for profile drag prediction and section-lift adjustments for stall prediction. The model considers the increased lift needs during banked flight. Based on this flight performance model, parametric sweeps and multi-parameter gradient-based optimizations were applied to explore those micro aerial vehicle configurations that have minimum power requirements. Typical midspan chord Reynolds numbers ranged...

Fixed-wing micro aerial vehicles experience attitude control difficulties as they operate in highly turbulent environments. Previous research has identified pressure-based control as a potential approach for augmenting the performance of, or replacing, autopilots reliant on inertial sensors. However, implementation requires an in-depth understanding of the correlation that exists between oncoming gusts and wing surface-pressure variations. This paper investigates the variation of correlation along a representative micro aerial vehicle wing chord and wingspan between upstream flow pitch angle variation and wing surface-pressure variation. Atmospheric turbulence was replicated within the controlled environment of a wind tunnel using planar grids that generated a turbulence intensity of 12.6%. Despite the unsteady nature of the pressure field, it was discovered that high correlation is evident in the vicinity of the leading edge. Thus, a few optimally placed sensors can be used for a pressure-based attitude ...

This paper discusses the control methodology, flight dynamics identification, and disturbance rejection analysis in hover of a revolutionary horizontal-axis rotary-wing concept: the twin cyclocopter. The vehicle has a gross weight of 500 g (1.25 by 1.67 by 1 ft in dimensions) and comprises two highly optimized cyclorotors along with a tail rotor for pitch control. Stable hover flight required fast control of the rpm and thrust vectoring of the rotors through onboard feedback regulation. A six-degree-of-freedom flight dynamics model of the vehicle was extracted through input excitation and time-domain identification. The longitudinal and heave degrees of freedom were decoupled and independent from the rest of the dynamics. Longitudinal translation damping was higher than in the heave mode, indicating differences in restoring forces at different blade azimuth positions due to the different pitch angles. Strong gyroscopic coupling was observed between lateral and yaw degrees of freedom because the rotors spi...

This paper describes the experimental investigation of a quad-shrouded rotor micro air vehicle and focuses on the hover performance improvements over a conventional unshrouded micro quad rotor. The effects of number of rotor blades, blade root pitch angle, and shroud diffuser length on aerodynamic performance were studied. The rotor diameter was 6.6 cm, it had a tip Re of 20,000, and the gross weight of the vehicle was 90 g, with a shroud weight fraction of 13%. With the optimized design, the power loading of the quad-shrouded rotor was about 15% greater than the unshrouded configuration. To completely evaluate the configuration, the performance of the vehicle in edgewise flow was investigated. The drag and pitching moment for the shrouded rotor was about 2.5 times greater than the unshrouded vehicle. However, control-moment measurements suggested that the edgewise gust tolerance was at least 4 m/s. The vehicle prototype was also successfully flight tested in hover using onboard feedback regulation. Base...

It has been observed and experimentally validated that birds and bats can obtain airflow information with mechanoreceptors on their wing and body surfaces to achieve ultrastable, highly maneuverable, and energy efficient flights. Inspired by this observation, an attitude control method using the real-time flow (pressure and shear) information is studied. Previous research has demonstrated the effectiveness of using pressure information only for a micro air vehicle’s pitching control in both simulation and wind tunnel tests. In this paper, the new concept is extended to three-axis attitude controls. A new three-axis attitude motion model is developed, relating the pressure and shear information to attitude states. A nonlinear controller, which is robust with respect to bounded uncertainties, is implemented to track the attitude commands. A delta wing configuration with 20 sensor couples on the wing surface and one sensor couple on the rudder surface is proposed, designed, and tested in a simulated environm...

The design, fabrication, and testing of a hybrid micro air vehicle capable of morphing between a high-hover-endurance rotary-wing mode and a high-cruise-speed fixed-wing mode is described. The micro air vehicle can efficiently perform a mission comprising outdoor flight to a target, followed by indoor surveillance and a return to the launch point. In addition, the micro air vehicle can function as a multirole platform, capable of flying exclusively as a fixed-wing or a rotary-wing vehicle. The key design drivers were 1) minimizing power required to hover, 2) ensuring static longitudinal stability in fixed-wing mode, and 3) minimizing the change in geometry between flight modes. Configuration trade studies were performed using blade element momentum theory for the rotary-wing mode and modified lifting-line theory for the fixed-wing mode. Design drivers for the rotary-wing mode and fixed-wing mode were often conflicting, and the selection of the final vehicle planform was primarily based on longitudinal sta...

The design of a biomimetic micro air vehicle with flapping wings is an essential challenge in the military/civilian field to conduct various missions. The success of a micro-air-vehicle flight is strongly related to the maneuverability and sustainability of an unsteady aerodynamic performance of the flapping motion. Appropriate flapping kinematics need to be established that are amenable to various flight purposes under a fluctuating environment. In this research, kinematics of flapping motion are determined by the study of aerodynamic performance of a flapping airfoil for appropriate maneuverability and sustainability. The flapping motion of an airfoil is formulated by a combined sinusoidal plunging and pitching motion in various angles of the stroke plane. The optimization process is carried out to determine the efficient motions based on a well-defined surrogate model that is made from the results of two-dimensional computational-fluid-dynamics analysis. The kriging method and genetic algorithm are use...

The weight and power constraints imposed on flapping-wing micro air vehicles necessitate optimal design of the flapping kinematics. To date, the approach adopted for kinematics optimization has been to assume specific functions for the Euler angles describing the wing motion with respect to the body. Then, optimization is performed on the parameters of these functions. In another approach, a number of instants over the flapping cycle are selected, and optimization is performed on the magnitude of the Euler angles at these instants. This latter approach provides more freedom for the variations of the Euler angles rather than confining them to certain patterns. Yet, in both approaches, finite-dimensional optimization is adopted and, as such, additional constraints are imposed. Considering that the problem is an infinite-dimensional optimization problem, we use in this work the calculus of variations to obtain true optimality. The combination of the quasi-steady aerodynamics and the calculus of variations ap...

Maintaining stable flight of micro aerial vehicles is challenging, especially in complex, low-Reynolds-number flight environments while considering wind gust disturbance, flow separation, and flow reattachment. To date, most micro aerial vehicles use vision, inertial measurement units, and/or global positioning systems as their primary sensing and navigation devices; however, actual flow conditions over the aircraft wing surfaces cannot be captured directly. In this paper, a biologically inspired micro aerial vehicle pitch control system is designed using distributed pressure information. The pressure information on the wing surfaces of a micro aerial vehicle is directly measured by a array of digital barometric micropressure sensors and is then used to calculate the aerodynamic forces, center of pressure, pitching moment, etc. A new pitch motion model that can capture the pressure information is derived from the control perspective. A nonlinear controller is also designed to achieve accurate pitch contro...

Experimental studieswere conducted to study the response of a shrouded rotormicro air vehicle to edgewise gusts. In edgewise flow, the thrust, drag, and pitching moment produced by three platforms were compared: elliptic inlet, circular inlet shrouded rotor, and an unshrouded rotor. The elliptic inlet shrouded rotor wasmore efficient in hover but had ahigher penalty in drag andpitchingmoment in edgewiseflow.Cyclic pitch variation of a hingeless rotorwas used to counter these adverse pitching moments. The control authority of the shrouded rotors was at least 80–100% higher than the unshrouded rotor, with the elliptic inlet shrouded rotor producing the highest control moments. By optimizing rotor collective settings, it was possible to reduce the deteriorating effect of edgewise flow on control moments. To increase the control authority and gust tolerance of the shrouded rotor, the cyclic pitch travel and blade planform modifications were made. With a careful selection of rotor solidity, planform, operating revolutions per minute, and cyclic pitch travel, it was possible to achieve a gust tolerance of about 3 m=s for the circular inlet shrouded rotor. Free flight tests were then conducted to study the ability of the vehicle to hover in a given position in the presence of gusts generated from pedestal fans with honeycomb flow straighteners. A combination of VICON and an onboard sensor were used for feedback control. The vehicle was satisfactorily able tomaintain hover position in edgewise gusts of up to 3 m=s.

The aim of this work is to present a method to find and analyze maximum propulsive efficiency kinematics for a birdlike flapping-wing unmanned aerial vehicle using multiobjective evolutionary optimization and data-mining tools. For the sake of clarity and simplicity, simple geometry (rectangular wings with the same profile along the span) and simple kinematics (symmetrical harmonic dihedral motion) are used. In addition, it is assumed that the birdlike aerial vehicle (for which the span and surface area are, respectively, 1 m and 0.15 m 2 ) is in horizontal motion at low cruise speed (6 m/s). The aerodynamic performances of the flapping-wing vehicle are evaluated with a semiempirical flight physics model and the problem is solved using an efficient multiobjective evolutionary algorithm called ∈-MOEA. Groups of attractive solutions are defined on the Pareto surface, and the most efficient solutions within these groups are characterized. Given the high dimensionality of the Pareto surface in the kinematic parameters space, data-mining techniques are used to conduct the study. First, it is shown that these groups can be qualified versus the whole Pareto surface by accurate mathematical relations on the kinematic parameters. Second, the inner structure of each group is studied and highly accurate mathematical relations are found on the optimized parameters describing the most efficient solutions.

The idea of using small-scale vehicles, often termed micro air vehicles, for various surveillance applications has become increasingly popular in recent years. A micro air vehicle design of particular interest is the membrane wing micro air vehicles, in which the structural skeleton is covered with a thin membrane instead of conventional wing skin materials, developed in particular for its lightweight nature, static stability, and passive gust rejection. In the current work, membrane wing micro air vehicles are developed and tested experimentally in order to determine the structural response of batten-reinforced membrane wing micro air vehicles to varying conditions: small angles of attack, number of battens, and membrane pretension. A self-excited instability (flutter) was noted for each model with limit cycle oscillations occurring at postflutter flow velocities. Small angles of attack had little effect on the flutter velocity, frequency, and mode for a given configuration, while increasing the membrane pretension delayed flutter and reduced the magnitude of limit cycle oscillation experienced by the model at a given flow velocity. Increasing the number of structural battens for the membrane wing micro air vehicle models also delayed the flutter velocity and reduced the magnitude of limit cycle oscillation at a given flow velocity while altering the flutter mode.

M ICRO aerial vehicles (MAVs) are small aerial vehicles, generallywith dimensions on the order of 1 ft (or 0.3m). Their small sizes often imply that their flight speeds are also small, roughly on the order of 20–60 km=h. MAVs come in a wide variety of forms, such as fixed wing, flapping wing, and rotary wing [1]. They are finding increasingly more applications in such areas as military missions, reconnaissance of hazardous or remote areas, and monitoring of indoor areas. Because of their small sizes and light weights, MAV flight trajectories are significantly susceptible to winds in general. For example, it is possible for the wind to be so strong that the MAV is unable to advance in certain directions. (This problem can be formulated in terms of the controllability of a point and its associated reachable set, see [2].) It is therefore necessary to develop a systematic approach toMAV trajectory generation that addresses the characteristic issues of MAV flights in winds. Specifically, the following three points must be considered: 1) Target points may not always be feasible due to winds. 2) Wind profiles may have significant effects on fuel costs (both desirable and undesirable). 3) Trajectory generation must always yield a feasible solution due to the absence of human operators. The main goal of this note is to study meaningful trajectory generation problem formulations for MAV trajectories in winds, to address all three of the issues discussed above that characterize MAV flights. Specifically, MAV flights in winds are formulated as nonlinear optimal control problems, with proper constraints on states and controls. In particular, the reaching of a target point is enforced via a penalty term in the performance index. Thus, while a target point is not necessarily reached, a flyable trajectory that is optimal under prevailing conditions is obtained. Both constant and position dependent wind is considered.

This paper presents an application of the potential field panel method commonly used in aerodynamics analysis to obtain streamlinelike trajectories and use them for path planning and collaborative target tracking for swarming micro air vehicles in an urban environment filled with complex shaped buildings and other architectural structures. In addition, we introduce a performance matching technique that relates the fluid velocities, which are obtained as a partof the panel method solution, to vehicle velocities along each trajectory. The approach is further extended to track moving targets yet avoid obstacles and collision between the vehicles. Because of the inherent nature of streamlines, obstacle avoidance is automatically guaranteed. To make the micro air vehicles follow and track a moving target, dynamically changing streamline patterns are calculated for each and every one of the micro air vehicles within a swarm. To prevent vehicle-to-vehicle collisions, each micro air vehicle is represented using a point source singularity element within the potential field. The simulation results are quite encouraging, in the sense that micro air vehicle swarms quickly locate and track the assigned targetin an environment filled with complex-shaped structures while avoiding obstacles and collisions among themselves. One benefit of the method is that the trajectory computations can be relatively fast and even have the potential to be applied in real time, depending on the number and complexity of the urban structures.

A S ENGINEERS search to make micro air vehicles more stable, maneuverable, and efficient, many are turning toward biology for inspiration. Bats adapt to their environment by morphing their wings in flight. Depending on their niche, certain species soar and glide [1], whereas others perform barrel rolls in nature [2] and can pull up to 4.5g in obstacle courses [3].Morphological changes afford bats great agility at low speeds, and they are able tomaintain stability and control at low Reynolds numbers, in which viscous effects and leading edge laminar separation bubbles cause nonlinearities in lift [4–6]. Bats achieve these feats with fingerlike jointed bone structures and flexible wing membranes. These unique traits allow them to change camber and twist in flight, unlike avian span changes. Recent studies investigate replicating flapping bat flight [7–9], whereas others focus specifically on flexible, membrane wing benefits at low Reynolds numbers for improvements in gust alleviation and delayed stall characteristics [10–12]. These wings are passive elements, and it is difficult to attach control surfaces to them for flight authority. By actively controlling and morphing flexible wings, conventional controllers can be replaced. Aircraft morphing allows single vehicles to have multiple functions, ideally with continuous lifting surfaces to alleviate drag and vibration and increase efficiency. This is the focus of Garcia et al. [13], which proposes a nonflapping wing with twist capabilities. Morphing enables tailoring of wing shapes to multiple flight regimes, from takeoff through cruise to landing [14–16]. Many mechanisms have been proposed for morphing, such as the smart joint, an active rigidity composite suited to actuating a batlike membrane wing in flight [17]. This low-profile device can be embedded at joints in the fingerlike skeletal wing structure as a bimorph actuator [18]. Whereas this work focuses on static wing configurations rather than morphing behavior, results help specify actuator requirements used for a variable camber and twist wing. In this work, two key features of bat flight are studied, uniquely evolved bat wing planforms adapted to environments, and capabilities afforded through variable camber and twist, and are applied to rigid, fixed wing designs for small man-made craft. The goal of this study is not to mimic natural bat flight, but to understand how certain aspects of bat flight apply to the engineering problem of wing design for micro air vehicles. II. Background and Motivation

Electrical-discharge wire cutting is a promising technique that provides flexibility and lightness for a flapping micro aerial vehicle. Electrical-discharge wire cutting is used to fabricate the high-aspect-ratio structure of the four-bar linkage gear transmission module made of aluminum-alloy 7075. Aluminum-alloy 7075 has excellent specific strength (yield strength/density) good for durability of the transmission module in a micro aerial vehicle's tuff operation. A new flapping micro aerial vehicle of 21.6 cm wing span consequently has a minimum body mass of 5.9 g after installing the transmission module and a flexible wing frame made of carbon fibers and polyethylene terephthalate film. This micro aerial vehicle can endure a flight time of 6 min 7 s with the wingbeat frequency of 10-20 Hz. The lift and thrust coefficients of the micro aerial vehicle have been investigated through wind-tunnel testing. The proposed flapping micro aerial vehicle also exhibits the improved characteristic in the scaling law with respect to wingbeat frequency versus body mass.

S ITUATIONAL awareness is a criticality for decision-making that affects operations in urban environments. Accordingly, information about the system must be available through real-time measurements. Sensors can often be placed throughout a region using a mix of static mounts and moving vehicles; however, the ability to dynamically mount a sensor at any location in response to an event is difficult. A mission of particular interest is sensor emplacement onto vertical surfaces. Valuable information may be obtained from measurements, such as acoustic signatures or thermal images, from vantages that observe or overlookwindows and buildings along with areas of high traffic. Some obvious applications of surveillance include homeland security, fire monitoring, search and rescue, and analysis of structural integrity. In each situation, sensors must be strategically placed to provide the type of information from a specific aspect to maximize the situational awareness. A bat provides an obvious source of inspiration for this mission of sensor emplacement onto vertical surfaces within an urban environment. Its size and agility are ideal for maneuvering in small spaces. Most important, a bat is able to fly toward a wall or grating and attach itself to that surface. The bat undergoes a series of physical alterations as it transitions from flapping flight to gliding flight while approaching the wall. These alterations are mainly changes to the shape and configuration of the wings relative to the body. This concept of morphing to alter the physical configuration in flight is being incorporated into aircraft [1]. Incorporating actuators and other mechanisms can enable vehicles to vary parameters, such as sweep [2,3] and dihedral [4,5], duringflight. The resulting range of configurations will have an associated range of flight dynamics and, consequently, maneuvering. A micro air vehicle (MAV) is ideally suited for consideration of this mission and especially appropriate for consideration of a biologically inspired design given the similarity in size and airspeed to bats. Optimal design of such vehicles is generally difficult given the uncertainties associated with low Reynolds numbers [6,7]; however, adopting shapes from biological systems has generated some effective designs. Obviously, aerodynamics are an important feature of many biological systems [8], as demonstrated by testing in wind tunnels [9]. The concepts from avian systems have been studied for flight by considering pitching [10], expandable span [11], twojoint sweep [12], and even high-frequency flapping [13]. In each case, the study showed the efficiency and performance of the biological concept but were unable to realize the concept through an actual flight vehicle. ThisNote considers the design of amicro air vehicle using a bat for inspiration on morphing. The design does not consider the entire flight regime of a bat; rather, the design limits attention to the specific maneuver of sensor emplacement on a vertical surface. The effect of morphing the wings in a manner similar to bats is investigated and shown to be highly effective in achieving themaneuver. In particular, the aerodynamics are shown to alter by increasing agility in pitch and roll but remaining stable in yaw as a result of the morphing.

The wings of insectlike flapping-wing micro air vehicles experience time-periodic inertial stiffnesses during flapping motion. By modeling the wing structure as a thin beam, a linear time-periodic assumed-modes analysis is developed. The equations of motion of a flapping wing undergoing out-of-plane bending and torsion are derived. The homogeneous assumed-modes equations are nondimensionalized. It is shown that the nondimensional strain stiffness varies with the ratio of the wing's nonrotating natural frequencies to the flapping frequency, whereas the nondimensional, time-periodic inertial stiffnesses vary with the amplitudes of flapping and feathering motion. The parametric stability of a representative wing is assessed by applying Floquet analysis to the nondimensional equations of motion, and a scalable stability diagram is presented. Parametric instabilities of the wing structure, caused by time-periodic stiffnesses, are characterized and plotted in the time domain. The effects of important structural design properties on parametric stability are examined.

The aerodynamic design of a flapping-wing micro air vehicle requires a careful study of the wing design space to ascertainthebestcombinationofparameters.Anonlinearunsteadyaerodynamic modeldevelopedbytheauthorsis used to make such a study for hovering insectlike flapping wings. The work is characterized, in particular, by the insights it provides into flapping-wing flows and the use of these insights for aerodynamic design. The effects of wing geometry on the aerodynamic performance of such flapping wings are investigated by comparing the influence on a numberofsyntheticplanformshapeswhilevaryingonlyoneparameteratatime.Bestperformanceappearstobefor wingshapesthathavenearlystraight leadingedges andmoreareaoutboard,where flowvelocities arehigher.Other important trends are also identified and practical considerations are noted. When possible, comparisons are also drawn with quasi-steady expectations and discrepancies are explained.

A GILE flight inside buildings, caves, and tunnels is of significant military and civilian value and is an attractive application for micro air vehicles (MAVs), defined here as flying machines of the order of 150 mm in size. Indoor flight imposes particular design and performance requirements, including small size, low speed, hovering capability, high maneuverability at low speeds, and (for covert operations) small acoustic signature, among other things. As discussed elsewhere [1–4], insectlike flapping is a solution thatmeets these requirements and is proven in nature. Although a number of elements characterize the design of a flapping-wing MAV, the focus here is on its wing aerodynamic design. This is crucial because for a flapping-wing MAV (FMAV) the wings are not only responsible for lift, but also for propulsion and maneuvers. Although insect flapping wings offer a proven solution and are abundant in nature (there are over 170,000 species of flying insects), little is known about the optimality of their wing design. Unlike forfixed or rotarywings, the parametric space associatedwith flapping wings is largely unexplored. A study that addresses the effects of both wing kinematics and wing geometry on the aerodynamic performance of flapping wings is required, and the former forms the underlying theme of this paper. The effect of wing geometry is considered elsewhere [5]. This work also provides insights into flapping-wing flow physics and uses these insights for aerodynamic design. Although Ellington’s [6] seminal work rejuvenated interest in insect flight, it is only recently that attention has been directed toward the design of vehicles that use insectlike flapping wings, particularly at the MAV scale [1–3]. In a later study, Ellington [7] proposed design guidelines based on scaling fromnature, but this does not give physical insight or allow design optimization. Dickinson et al. [8] investigated the effect of advancing or delaying pitch rotation of the flapping wing with respect to its translational motion, using experiments on Dickinson’s Robofly: a scaled-up mechanical model of the fruit fly Drosophila. Ramamurti and Sandberg [9] used a computational fluid dynamics (CFD) method to demonstrate this effect and presented some useful flow visualization. Sun and Tang [10] also used a CFD code to investigate the effect of advancing and delaying pitch rotation on insectlike flapping flight and the effect of varying the duration of stroke reversals [11]. In an earlier study [12], they investigated the effect of Reynolds number and the duration of wing stroke reversal. They also studied the effect of advance ratio (the ratio of flight speed to wing mean tip speed) in forward flapping flight [13]. Yu and Tong [14] used an aerodynamic modeling approach [15] to study forward flapping flight at various advance ratios by varying asymmetries between upand downstrokes. However, none of the preceding studies aimed to produce an optimized wing aerodynamic design. Milano and Gharib [16] made probably the only study thus far aimed at optimizing wing kinematics. They used a genetic algorithm paired with digital particle-image velocimetry experiments on a flapping wing in a water-filled towing tank. By using insectlike kinematics, they optimized for average lift over four flapping cycles and found a number of convergent solutions in the parameter space. They noted that the optimally efficient solutions all tended to generate leading-edge vortices ofmaximum strength. However, their Received 26 October 2007; revision received 11 June 2008; accepted for publication 13 June 2008. Copyright © 2008 by SalmanA. Ansari. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0021-8669/08 $10.00 in correspondence with the CCC. ResearchOfficer, Department ofAerospace, Power and Sensors.Member AIAA. Professor of Aeromechanical Systems, Department of Aerospace, Power and Sensors. Associate Fellow AIAA. Reader in Control Engineering, Department of Aerospace, Power and Sensors. Member AIAA. JOURNAL OF AIRCRAFT Vol. 45, No. 6, November–December 2008

The research and development efforts outlined in this paper address the aerodynamic design of micro air vehicles with hovering and vertical takeoff and landing capabilities. The tilt-body configuration of the vertical takeoff and landing micro air vehicle is proposed based on a propulsion system consisting of two coaxial contrarotating motors and propellers. Values of thrust, torque, power, and efficiency of this propulsion system were measured in pusher and tractor arrangements of propellers and compared against single motor-propeller propulsion. With comparable efficiency, the developed propulsion system has very little propeller torque. Hot-wire measurements have been conducted to investigate the velocity profile in slipstream. The lower average velocity and significant decrease in velocity in the core of the slipstream found in the tractor arrangement are mostly due to the parasite drag caused by the motors. It causes the decrease of the thrust force observed for the tractor arrangement in comparison with the pusher arrangement. Wind-tunnel testing was conducted for a motor, a wing, and an arrangement of a wing with a motor. The drag force on the wing is produced by two mixing airflows: freestream and propeller-induced pulsating slipstream. The zero-lift drag coefficient increases by about 4 times with propeller-induced speed increased from 0 to 7.5 m/s. The results of this study were realized in the design of a vertical takeoff and landing micro air vehicle prototype that was successfully flight tested.

The field of micro air vehicles is relatively immature; consequently, high-fidelity simulations do not yet exist for a generic aircraft. The fidelity of flight dynamic simulations is closely correlated to the reliability of models representing the vehicle's aerodynamic and propulsion characteristics in the entire flight envelope, including the nonlinear region. This paper discusses wind-tunnel experiments performed to investigate the aerodynamic and mechanical characteristics of micro air vehicles with flexible wings in different conditions of propeller type, motor power, and elevator deflections. Visual image correlation was used to measure the deformation of the flexible wings to quantify general features such as variations in aerodynamic and geometric twist angle. Aerodynamic and propulsion results were used to formulate empirical models of the relevant coefficients in the form of multiple linear regressions and to estimate the effectors' functional dependencies and interactions. High-order nonlinear interactions were confirmed between the coefficients of lift, drag, and pitching moment with the independent variables. The rates of the dependencies with elevator deflections and angle of attack were found, to some extent, to be motor voltage and dynamic pressure dependent, evincing a strong coupling with the propeller speed.

An experimental study of a rotatable tail mechanism applied to a small unmanned aerial vehicle was performed using a six-component wind-tunnel balance in the U.S. Air Force Institute of Technology low-speed wind tunnel. Attributes of the control and stability characteristics of the original vehicle, which were documented in an earlier study, are compared with those of a unique control methodology, a tail consisting of a single surface, with controllable elevation and rotation. An advantage of this change is a reduction in the storage length of the vehicle. Because there are similarities in the rotatable tail mechanism and the tail of many birds, the rotatable tail reflects a biomimetric feature. Measured force and moment coefficient measurements for the actual vehicle at a typical flight speed indicated that a rotatable tail provides a sufficient yaw moment for turning. For example, yaw moment coefficients C n , ranging from -0.02 to +0.02, which is typical for a rudder, were achievable as long as the absolute value of the tail elevation angle was large. The dependence of the yaw moment coefficient on the elevator angle and angle of attack, in addition to the tail rotation angle, indicates that there would be significant challenges in applying a robust flight control scheme with the current actuator configuration. An additional feature of the tail design is that by deflecting the tail upward, it could also function effectively as an air brake. A more than twofold increase in drag coefficient for constant angle of attack was measured when the tail elevation angle was increased to nearly 70 deg.

The presence of streamwise vorticity in the vicinity of the wing tip contributes to lift in thin flat plate zero pitch angle flapping wings in quiescent air. In creating flapping wing micro air vehicles it is desirable to maintain only the mechanical and kinematic complexity absolutely necessary to artificially duplicate flapping wing flight. This study quantifies the lift generated from a flapping motion of absolute minimum complexity thought to be capable of generating lift Using a flapping wing micro air vehicle with wings fabricated in-house, streamwise vortices were identified along the span of wings of various aspect ratios and at numerous different points throughout the flapping cycle under a variety of operating conditions. The lift generated by the flapping mechanism was quantified experimentally using a force transducer and a high speed camera. Digital particle image velocimetry was used to determine the contributions of streamwise vorticity to the total measured lift. Further evidence was found of the importance of the relationship between wing span and flapping frequency in the nature of the formation and shedding of vortices.

In 1997 the Defense Advanced Research Projects Agency initiated a program to explore the possibility of micro air vehicles for the purpose of individually portable surveillance systems for close-range operations. The various contractors approached the problem in several ways, such as developing tiny fixed-wing airplanes, rotary-wing aircraft, and ornithopters mimicking animal flight This paper describes one such flapping-wing aircraft, which drew upon the clap-fling phenomenon that is exploited by many flying animals and insects for lift generation. Essentially this aircraft was a mechanical simulation of hummingbird flight, though with two sets of wings to eliminate the unbalanced side-to-side flapping forces. Two flying demonstration models were built, one with an internal-combustion engine and another with an electric motor. In both cases, these incorporated a drive train to reduce the high rpm rotary shaft motion to lower-frequency oscillation for flapping. Also required was a programmable logic board for stabilization. Successful hovering flight was achieved with both models, and initial studies of transition to horizontal flight were also explored.

A class of micro air vehicles uses a flexible membrane wing for weight savings and passive shape adaptation. Such a wing is not amenable to conventional aileron mechanisms for roll control, due to a lack of internal wing structure. Therefore, morphing (in the form of asymmetric twisting) is implemented through the use of a torque-actuated wing structure with thousands of discrete design permutations. A static aeroelastic model of the micro air vehicle is developed and validated to optimize the performance of the torque-actuated wing structure. Objective functions include the steady-state roll rate and the lift-to-drag ratio incurred during such a maneuver. An optimized design is obtained through the use of a genetic algorithm presenting significant improvements in both performance metrics compared with the baseline design.