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Challenges associated with flight control of agile fixed-wing Micro Air Vehicles (MAVs) operating in complex environments is significantly different to any larger scale vehicle. The micro-scale of MAVs can make them particularly sensitive to atmospheric disturbances thus limiting their operation. As described in Part 1, current conventional reactive attitude sensing systems lack the necessary response times for attitude control in high turbulence environments. This paper reviews in greater detail novel and emerging biologically inspired sensors, which can sense the disturbances before a perturbation is induced. A number of biological mechanoreceptors used by flying animals are explored for their utility in MAVs. Man-made attempts of replicating mechanoreceptors have thus been reviewed. Bio-inspired flow and pressure-based sensors were found to be the most promising for complementing or replacing current inertial-based reactive attitude sensors. Achieving practical implementations that meet the size, weight and power constraints of MAVs remains a significant challenge. Biological systems were found to rely on multiple sensors, potentially implying a number of research opportunities in the exploration of heterogeneous bio-inspired sensing solutions.

Fixed-wing Micro-Aerial Vehicles (MAVs) need effective sensors that can rapidly detect turbulence induced motion perturbations. Current MAV attitude control systems rely on inertial sensors. These systems can be described as reactive; detecting the disturbance only after the aircraft has responded to the disturbing phenomena. In this part of the paper, the current state of the art in reactive attitude sensing for fixed-wing MAVs are reviewed. A scheme for classifying the range of existing and emerging sensing techniques is presented. The features and performance of the sensing approaches are discussed in the context of their application to MAV attitude control systems in turbulent environments. It is found that the use of single sensors is insufficient for MAV control in the presence of turbulence and that potential gains can be realised from multi-sensor systems. A successive paper to be published in this journal will investigate novel attitude sensors which have the potential to improve attitude control of MAVs in Turbulence.

There are significant challenges in the control of fixed-wing Micro Air Vehicles (MAVs) in high turbulence environments. Birds can sustain stable flight in such environments by obtaining flow information through mechanoreceptors embedded in their wings. Inspired by nature's flyers, an investigation into replicating the function of mechanoreceptors with commercially available pressure sensors is presented. Implementation requires an in-depth understanding of the level of correlation that exists between pressure variations over the wing and the roll perturbation of the MAV. This paper investigates the variation in correlation and coherence along a representative wing-chord and wing-span of a MAV. Highest correlation and coherence is found to exist in the vicinity of the leading edge, with significant perturbations which are evident up to ∼35Hz for a 0.49m wingspan MAV.

The spatial and temporal variations in wind velocity which cause atmospheric turbulence are exacerbated by the topographic and temperature gradients often present at low altitudes. Such gradients exist within the atmospheric boundary layer, which is the region proximate to the ground in which the wind speed varies from near-zero at ground level to the nominal wind speed around 300 m above the ground. With few exceptions for aircraft such as medical evacuation helicopters and crop dusters, low altitude turbulence is a transient problem for manned aircraft in the brief ight phases of take-o and landing. For small unmanned aircraft, however, the entirety of the ight typically exists within an altitude band characterized by high turbulence intensity. Small xed-wing aircraft operating at slow airspeeds are particularly sensitive to turbulence which can generate large disturbances that a ect attitude and ight path stability. UAV control laws are often developed in the absence of signi cant turbulence, since ight tests are often conducted at remote test ranges and on days with benign atmospheric conditions. The environment in which these control laws are developed does not necessarily re ect the expected operating conditions. For new aircraft designs, airworthiness safety review boards may permit ight only in test areas away from structures and population centers. Flight test engineers usually schedule tests for days with calm winds to improve the quality of the aerodynamic and dynamic data measurements. The result of both the expansiveness of typical UAV test sites with the quiescence of the wind conditions is ight at low turbulence intensity which fails to provide sucient excitation to evaluate the disturbance rejection capability of control systems. The objective of the current research is to study how the design of control law parameters a ects the attitude stability and ight path tracking accuracy of xed wing aircraft operating in extreme turbulence. Turbulence is rep

DOI: 10.2514/1.46860 Air vehicle flight in turbulence is generally treated as an anomalous part of the flying environment. Aircraft geometries and flight-control systems are often designed and tested for calm atmospheric conditions, where both steady winds and gusts are minor. As a result, the flight performance of small aircraft deteriorates in the presence of atmospheric turbulence, where gust disturbances can be large relative to the flying speeds. A better approach is needed in the aircraft and control system design process that specifically accounts for the effects of turbulence and providesameansofmitigatingdisturbancestoimprovethemissioneffectivenessofmicrounmannedairvehiclesand small unmanned air vehicles. The current research considers untethered flight tests of a small unmanned air vehicle in a large wind engineering tunnel that can be configured to replicate turbulence levels expected from urban and suburban environments. Systematic changes to the configuration of the fixed-wing aircraft are made to evaluate the roleofmetrics,suchasc.g.,mass,momentofinertia,wingspan,andwingloadingtoturbulencesensitivity.Estimates of the force and moment disturbances indicate that some parameters, such as moment of inertia, have simple and expectedinfluencesontheresponsetoturbulence. Conversely,wingareaandmasshaveconflictingeffectsduetothe compounded influences on the aircraft response. The sensitivity of the various aircraft configurations to turbulence are presented as control equivalent turbulence disturbances, which equate forces and moments acting on the airframe to control deflections. This method normalizes the aircraft responses with respect to the ability to suppress disturbances with actuated controls.

Major challenges to low speed micro flight are the transient and time-averaged velocities arising from the Atmospheric Boundary Layer (ABL), particularly turbulence a few metres above the ground. In this paper, existing data from meteorologists and wind engineers are reviewed and measurements dedicated to understanding the spatial and temporal velocity fields that MAVs experience are briefly described. Data from a wide variety of terrains are analysed, with the majority of data obtained in relatively well mixed turbulent flow (i.e. away from local effects such as buildings) and for conditions of nominally neutral stability. Spectra for data well removed from local effects exhibited the expected 5/3rds Kolmogorov law. Transient flow pitch angles were investigated (obtained from four small laterally displaced probes), in order to understand the possible roll and pitch inputs to MAVs. It was noted that for all data obtained the variation with lateral separation decreased relatively slowly with reducing separation down to the closest inter-probe spacing of 14mm. This effect is thought to explain the increasing piloting difficulties experienced in maintaining good roll control for decreasing scales of craft when any appreciable atmospheric winds are present.

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.

Missions envisioned for micro air vehicles may require a high degree of autonomy to operate in unknown environments. As such, vision is a critical technology for mission capability. This paper discusses an autopilot that uses vision coupled with GPS and altitude sensors for waypoint navigation. The vision processing analyses a horizon to estimate roll and pitch information. The GPS and altitude sensors then command values to roll and pitch for navigation between waypoints. A flight test of a MAV using this autopilot demonstrates the resulting closed-loop system is able to autonomously reach several waypoints. The vehicle actually uses a telemetry link to a ground station on which all vision processing and related guidance and control is performed. Several issues, such as estimating heading to account for slow updates, are investigated to increase performance.

Biologically inspired concepts are rapidly expanding the range of aircraft technology. Consideration is given to merging two biologically-inspired concepts, morphing and micro air vehicles, and the resulting flight characteristics are investigated. Specifically, wing shaping is used to morph the membrane wings of a micro air vehicle. The micro air vehicle has poor lateral control because hinges, and consequently ailerons, are difficult to install on a membrane wing. Instead, a set of torque rods, aligned along the wings, are used to twist the membrane and shape the wing. The resulting morphing is shown to provide significant control authority for lateral dynamics. A set of flight tests are undertaken to determine the flight characteristics by commanding pulses and doublets to the control actuation. The vehicle demonstrates excellent roll performance in response to wing shaping. Futhermore, the vehicle demonstrates several types of spin behavior related to combinations of elevator deflection and the wing shaping.

Flight capability for micro air vehicles is rapidly maturing throughout the aviation community; however, mission capability has not yet matured at the same pace. Maintaining trim during a descent or in the presence of crosswinds remains challenging for fixed-wing aircraft but yet is routinely performed by birds. This paper presents an overview of designs that incorporate morphing to enhance their flight characteristics. In particular, a series of joints and structures is adopted from seagulls to alter either the dihedral or sweep of the wings and thus alter the flight characteristics. The resulting vehicles are able to trim with significantly increased angles of attack and sideslip compared to traditional fixed-wing vehicles.

This report is the research summary of flying MAV experiments in turbulent flow, utilizing prior research measuring and reproducing aspects of the outdoor environment. Turbulence was replicated in a large wind engineering tunnel and was well mixed, thus replicating atmospheric turbulence under neutrally stable conditions far removed from local effects. Current studies are focused on using increasingly small fixed-wing and flapping-wing aircraft with IMU video tracking, and upstream flow measurements to correlate measured turbulence with vehicle disturbances. Rate, acceleration, force and pressure sensors are being evaluated to determine candidates for providing phase-advanced measurement of incipient turbulence, which can allow the aircraft to preemptively move the control surfaces to suppress attitude and position disturbances.

Micro air vehicle ∞ight in turbulence is generally treated as an anomalous part of the ∞ying environment. Aircraft geometries and ∞ight control systems are often designed and tested for calm atmospheric conditions, where both steady winds and gusts are minor. As a result, the ∞ight performance of MAVs deteriorates in the presence of atmospheric turbulence, where gust disturbances can be large relative to the ∞ying speeds. A better approach is needed in the aircraft and control system design process that speciflcally accounts for the efiects of turbulence and provides a means of mitigating disturbances to improve the mission-efiectiveness of MAVs. The current research focuses on untethered ∞ight tests of a MAV in a large wind engineering tunnel which can be conflgured to replicate turbulence levels expected from urban and suburban environments. Systematic changes to the conflguration of a flxed wing aircraft are made to evaluate the role of metrics such as CG, mass, moment of inertia, wingspan, and wing loading to turbulence sensitivity. Estimates of the force and moment disturbances indicate that some parameters, such as moment of inertia, have simple and expected in∞uence on the response to turbulence. Conversely, wing area and mass have con∞icting efiects due to the compounded in∞uences on the aircraft response. The paper concludes with a brief look at equivalent disturbances, which are shown as methods to quantify the efiect of turbulence in terms of the aircraft states or control inputs.

Major challenges to low speed micro flight are the transient and time-averaged velocities arising from the Atmospheric Boundary Layer (ABL), particularly turbulence a few metres above the ground. In this paper, existing data from meteorologists and wind engineers are reviewed and measurements dedicated to understanding the spatial and temporal velocity fields that MAVs experience are briefly described. Spectra obtained for a range of terrains, flight and wind speeds followed the expected 5/3rds Kolmogorov law. Transient flow pitch angles were investigated (obtained from four small laterally displaced probes), in order to understand the possible roll and pitch inputs to MAVs. For some data sets very considerable differences in pitch angles were observed between the four probes. It was noted that the variation with lateral separation decreased relatively slowly with reducing separation. This effect is thought to explain the increasing piloting difficulties experienced in maintaining good roll control for decreasing scales of craft.

Morphing, which changes the shape and configuration of an aircraft, is being adopted to expand mission capabilities of aircraft. The introduction of biologically-inspired morphing is particularly attractive in that highly-agile birds present examples of aerodynamically-effective shapes. This paper introduces an aircraft with a multiple-joint design that allows variations in sweep to mimic some shapes observed in birds. These variations are independent on the left and right wings along with on the inboard and outboard sections. The aircraft is designed and analyzed to demonstrate the range of flight dynamics which result from the morphing. In particular, the vehicle is shown to have enhanced turning capabilities and crosswind rejection which are certainly critical metrics for the urban environments in which these aircraft are anticipated to operate.

Birds are perhaps the most versatile and graceful flying objects in the atmosphere. Even in complex urban environments, they are able to rapidly change shape to transition from efficient cruise to aggressive maneuvering and precision descents. Avian morphology permits a wide range of wing configurations, each of which may be used for a particular flight task. The shapes into which birds morph are of considerable interest for small UAVs, whose typically-fixed geometries preclude similarly-diverse maneuverability. Simple mechanical analogues to bird wing structures can be used to expand the flight envelope for conventional UAVs. Aerodynamic and dynamic metrics are used to determine optimal shapes for various missions within the configuration space afforded by the morphing wings. Computational aerodynamics are used to estimate the performance and dynamic characteristics of each wing shape. Classical mission objectives such as long range cruise, endurance loitering, and minimum radius turns are used to identify appropriate wing shapes for each mission. Other tasks such as steep descents and sensor-pointing are designed to enhance the operation of UAVs in the vicinity of urban obstacles. The optimized wing geometries are found to converge to biological solutions in several instances. A mission profile example is presented to illustrate the performance improvement resulting from morphing. Transient morphing between tasks is achieved using a simple approach where configuration trajectories move towards the shape goal while avoiding areas of undesirable dynamics. The configuration change is controlled using a set of stability criteria, which can guarantee pilotability or controllability for all morphing conditions.

A class of micro air vehicles (MAVs) developed and tested at the University of Florida use a flexible membrane wing as an effective means of gust rejection. 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 wing twisting) is implemented through the use of a torque rod structure. A static aeroelastic model of the MAV with this morphing mechanism is developed in order to optimize the torque rod structure. Objective functions include the steady state roll rate, as well as the lift to drag ratio incurred during such a maneuver. Through the use of a standard genetic algorithm, an optimized torque rod design is obtained, presenting significant improvements in both objectives.

Driving at large angles of sideslip does not necessarily indicate terminal loss of control, rather, it is the fundamental objective of the sport of drifting. Drift racing challenges drivers to navigate a course in a sustained sideslip by exploiting coupled nonlinearities in the tire force response. The current study explores some of the physical parameters affecting drift motion, both in simulation and experiment. Combined-slip tire models are used to develop nonlinear models of a drifting vehicle in order to illustrate the conditions necessary for stability. Experimental drift testing is conducted to observe the dynamics featured in the track data. An accelerometer array on the test vehicle measures the acceleration vector field in order to estimate the vehicle states throughout the drift testing. Neural networks are used to identify the patterns in the accelerations that correspond to sideslip excursions during drifts. These estimates combined with computations of angular acceleration, yaw rate, and lateral acceleration build a framework for identifying the dynamics in terms of physical parameters and stability and control derivatives. The research developments are intended to support a future study quantifying the effects of vehicle configuration changes on drift capability as related to performance potential and handling qualities.

Morphing is attractive as a control effector that can dramatically alter the flight characteristics of an aircraft. In particular, morphing can use biologically-inspired approaches that note the similarity in size and airspeed between a micro air vehicle and birds. This paper introduces several types of morphing that have been implemented and flown on vehicle. A generalized modeling framework and control synthesis are presented to enable autonomous operation of such vehicles. An example of a gull-wing aircraft is shown to demonstrate the methodology.

The practical application of UAV technology will be dependent on how well the aircraft can perform a wide range of missions. Morphing aircraft are of particular interest because the shape change can achieve a range of performance and dynamic characteristics beyond the capability of conventional aircraft. However, a significant challenge exists in the control and stabilization of such a vehicle, where the plant model is expected to change considerably throughout the flight. The approach in the current study is based on an aircraft which achieves a 2-DOF, biologically-inspired wing morphing. The morphing allows the wings to be reconfigured to achieve a set of desired dynamics. This reconfiguration is computed in the control design process by finding the morphing condition which minimizes the differences relative to the desired dynamics for a certain flight condition. H∞ model-following controllers are computed to match the output of the plant model to the desired mission dynamics, which include cruise, maneuvering, steep descent, and sensor-pointing. Simulation results are presented for each mission.

Despite the past century of innovation in aircraft technology, the versatility of modern aircraft remains far worse than airborne biological counterparts. The shape changing accomplished by birds and bats in ight stands as one of the few examples of true morphing. As such, the aircraft community is devoting considerable attention to the study of biological systems and how they might be implemented on a ight vehicle. Much of this work addresses static aerodynamic and aeroelastic effects. However, this limited focus leaves the entire areas of dynamics, control, stability, and ight performance unstudied. This paper presents a simple approach to morphing based on existing vehicle designs and actuators. The aircraft is modied to mimic basic morphing mechanisms commonly observed on bird wings, namely twisting the wingtips for roll control and varying the gull wing angle. The latter mechanism is used as a quasi-static effector, where the wing shape is changed from one position to another. The aircraft employs a thin-undercambered wing and a skeletal spar structure to achieve wing morphing. Results from a series of ight tests are presented to highlight the effect of the morphing control on the vehicle dynamics. Command and response data analysis indicates a marked change in the vehicle’s handling qualities for different levels of morphing. Additionally, the morphing has been shown to have an effect on ight performance metrics such as climb rate, glide angle, and stall characteristics.

Micro air vehicles are able to y in environments with little maneuvering room; however, such ight requires high agility and precision maneuvering. A current class of micro air vehicle is constructed with only rudder and elevator for control authority. Such surfaces are not optimal for ight control but ailerons can not be installed on the membrane used for wings on these vehicles. Morphing is used as an aeroservoelastic effector for control. Vehicles are constructed using threads to command a curl of the wings and using torque rods to command a twist of the wings. In each case, ight tests demonstrate the actuation causes sufcient deformation of the wing to result in signicant control authority. The ight dynamics show turns and spins can be repeatedly performed using this aeroservoelastic control. I. Introduction Micro air vehicles (MAV) are rapidly gaining attention in the ight test community. The small size and light weight of these vehicles make them especially attractive for some missions. The vehicles are quite portable so may be easily transported to remote locations. A MAV is inherently stealthy because of its size and quiet because of its electric propulsion. Furthermore, many types of sensors are being miniaturized and would t within the payload capabilities of a MAV. Some areas of operation, such as urban environments, require a vehicle that is small but also extremely agile. Agility is obviously needed so the vehicle can maneuver within small areas and between obstacles in a dense environment. The vehicle must also be highly responsive to reject disturbances like wind gusts which may be considerable near buildings. The concept of aeroservoelastic tailoring through morphing is being studied for control on many vehicles. The basic concept of this approach involves actively changing the shape of a vehicle to affect the aerodynamics and, consequently, the ight dynamics. Such control inherently uses the control and actuation through the structural dynamics to affect the aerodynamics as an aeroservoelastic system. The resulting system has difcult dynamics so design and analysis of aeroservoelastic control is a challenge. 1

Flight researchers are increasingly turning towards small, unmanned aircraft for achieving mission objectives. These aircraft are simple to operate and offer numerous advantages over larger manned vehicles. In addition to being light, inexpensive, and readily available, they are also more versatile in that they can be used for flight experiments that are either too risky or uncertain for a manned flight test program. One application of unmanned vehicles is in the area of increased control authority research. This paper presents the preliminary stages of one such application, where an existing UAV is modified with 16 independent wing control surfaces. These surfaces are used in place of conventional ailerons for roll control and as a supplement to rudder, elevator, and flap controls. Instrumentation and sensors on-board the aircraft allow complete characterization of the flight dynamics. A traditional control system is replaced with a microcontroller that commands each aileron segment independently. Various modes of actuation can be implemented to improve roll, pitch, and yaw response, minimize induced drag, and provide numerous levels of redundancy. The results indicate that the segmented control surfaces can be configured for a superior level of control.

Aircraft control has progressed considerably since the early experiments of the Wright brothers in wing twisting. A diverse range of control schemes has since been developed to manage various aspects of the flight environment. A modern airliner, for instance, uses dozens of different control surfaces. Such highly specialized controls prohibit high performance throughout the wide flight envelope and offer little for redundancy. The concept of a morphing aircraft, however, would address these issues by allowing complete control of the aircraft structure for both primary control and multi-role situations. This research investigates a discretized approach to morphing the trailing edge of the wing. A UAV was modified to incorporate simple morphing capability and was flight tested extensively. The resulting system identification indicated that the lateral and longitudinal dynamics could be accurately modeled using simple linear-regression techniques. Both differential and collective deflection of the wing segments showed potential for full authority control over the aircraft. Using the models generated, controllers for the aircraft are being developed to fully exploit the potential of a highly configurable control system.

A micro air vehicle is a ight system being designed for operation within urban environments. Such vehicles are small and highly agile but often have limited control authority. This paper investigates the use of morphing as an effector to provide control authority. Simple mechanisms for morphing are designed to twist the wings of a 24 in vehicle and to curl the wings of a 12 in vehicle. Flight tests show the morphing is an excellent strategy to command roll maneuvers. The resulting vehicles are relatively easy to y and consequently are suitable for autopilot design and mission deployment.

Aspects of the turbulent wind environment Micro Air Vehicles (MAVs) experience when flying outdoors were replicated in a large wind tunnel. An overview of the facility, instrumentation and initial flight tests is given. Piloting inputs and aircraft accelerations were recorded on fixed and rotary wing MAVs and for some tests, measurements of the approach flow (u,v,w sampled at 1,250Hz at four laterally disposed upstream locations) were made. The piloting aim was to hold straight and level flight in the 12m wide × 4m high × ~50m long test section, while flying in a range of turbulent conditions. The Cooper-Harper rating system showed that a rotary craft was less sensitive to the effects of turbulence compared to the fixed wing craft and that while the fixed wing aircraft was relatively easy to fly in smooth air, it became extremely difficult to fly under high turbulence conditions. The rotary craft, while more difficult to fly per. se., did not become significantly harder to fly in relatively high turbulence levels. However the rotary craft had a higher mass and MOI than the fixed wing craft and further work is planned to understand the effects of these differences.