Taking full advantage of the favorable flight properties of biologically inspired flapping-wing micro aerial vehicles requires having insight into their dynamics and providing adequate control in all flight conditions. Because of the high complexity of flapping flight and limited availability of accurate flight data, however, global models are not readily available, particularly models validated with flight data and suitable for practical applications. This paper proposes an approach for global modeling of nonlinear flapping-wing dynamics, constructing a linear parameter-varying model from a set of local linear models. The model parameters and scheduling functions are determined using system identification, from free-flight data collected on a real test platform over a significant part of the flight envelope. The resulting model allows for the dominant parts of the dynamics to be accurately represented across the considered range of conditions. With 25 parameters overall, it significantly improves on the starting point of 46 local models with 12 parameters each. Moreover, a single model that adapts to the flight condition provides flexibility and continuous coverage, highly useful for simulation and control applications. Although in the explored part of the flight envelope nonlinearity was found to be limited, the approach is scalable and expected to also cover larger variations.@en

The effects of the horizontal tail surface on the longitudinal dynamics of an or- nithopter were studied by systematically varying its surface area, aspect ratio and its longitudinal position. The objective is to improve the understanding of the tail effect on the behaviour of the ornithopter and to assess if simple models based on tail geometry can predict steady-state conditions and dynamic behaviour. A data- driven approach was adopted since no suitable theoretical models for ornithopter tail aerodynamics are available. Data was obtained through wind tunnel and free-flight experiments. Fourteen tail geometries were tested, at four positions with respect to the fl apping wings. Linearised models were used to study the effects of the tail on dynamic behaviour. The data shows that, within the tested ranges, increasing surface area or aspect ratio increases the steady-state velocity of the platform and improves pitch damping. Results also suggest that the maximum span width of the tail significantly influences the damping properties, especially when the distance between the tail and the flapping wings is large, which likely relates to the induced velocity profile of the flapping wings. Steady-state conditions can be predicted accurately based on tail geometry even when extrapolated slightly outside the original measurement range. Some trends were identified between model parameters and tail geometry, but more research is required before these trends can be applied as a design tool. @en

One of the emerging tasks for Micro Air Vehicles (MAVs) is autonomous indoor navigation. While commonly employed platforms for such tasks are micro-quadrotors, insectinspired flapping wing MAVs can offer many advantages, such
as being inherently safe due to their low inertia, reciprocating wings bouncing of objects or potentially lower noise levels compared to rotary wings. Here, we present the first flapping wing MAV to perform an autonomous multi-room exploration task. Equipped with an on-board autopilot and a 4 g stereo vision system, the DelFly Explorer succeeded in combining the two most common tasks of an autonomous indoor exploration mission: room exploration and door passage. During the room exploration, the vehicle uses stereo-vision based droplet algorithm to avoid and navigate along the walls and obstacles.
Simultaneously, it is running a newly developed monocular color based Snake-gate algorithm to locate doors. A successful detection triggers the heading-based door passage algorithm. In the real-world test, the vehicle could successfully navigate, multiple times in a row, between two rooms separated by a
corridor, demonstrating the potential of flapping wing vehicles for autonomous exploration tasks.@en

Flow visualizations have been performed on a free flying flapping-wing Micro Air Vehicle (MAV), using a large-scale particle image velocimetry (PIV) approach. The PIV method involves the use of helium filled soap bubbles (HFSB) as tracer particles. HFSB scatter light with much higher intensity than regular seeding particles and comparable to that reflected off the flexible flapping wings. This enables flow field visualization to be achieved also close to the flapping wings, in contrast to previous PIV experiments with regular seeding. Unlike previous tethered wind tunnel measurements in which the vehicle is fixed relative to the measurement setup, the MAV now flies through the measurement area. In this way, the experiment captures the actual flow field of the MAV in free flight, allowing the true asymmetric nature of the flow to be appreciated. Measurements were performed for two different orientations of the light sheet with respect to the flight direction. In the first configuration, the light sheet is parallel to the flight direction and visualizes a streamwise plane that intersects the MAV wings at a specific spanwise position. In the second configuration, the illumination plane is normal to the flight direction and visualizes the flow as the MAV passes through the light sheet.@en

Biologically-inspired flapping-wing micro aerial vehicles are characterised by nonlinear, unsteady aerodynamics and complex dynamics, both highly challenging to model. To take full advantage of the flight capabilities of such vehicles, it is necessary to obtain insight into their dynamics in the different flyable conditions, and to provide adequate control in all of these conditions. Nonetheless, the dynamics are typically only considered in a single flight regime, and controllers are frequently tuned for a particular flight condition. Due to the high complexity of flapping flight and limited availability of accurate free-flight data, global models are not yet readily available, particularly models based on free-flight data and suitable for practical applications. This paper demonstrates an approach to obtain a global dynamic model for a flapping-wing micro aerial vehicle. To allow for standard linear control and systems theory to be applied, the nonlinear dynamics are approximated using a linear parameter-varying (LPV) approach based on a set of local linear models. The scheduling parameters, and the parameters in the underlying local models, are determined using system identification methods applied to free-flight data collected on a real test platform, and covering a significant part of the flight envelope. The proposed approach allows for modelling of the vehicle and prediction of the dominant dynamic properties across the considered part of the flight envelope, using a total of 16 parameters, as opposed to the starting point of 46 local models with 12 parameters each. The use of a single model adapting to the flight condition provides flexibility and continuous coverage, and is therefore highly useful for simulation and control applications. While in the explored part of the flight envelope the nonlinearity was found to be limited, such that a weighted average model may be sufficient for some applications, the LPV model provides a higher accuracy and more consistent performance across the conditions considered. Additionally, the approach is shown to be promising and is expected to be adaptable to cover more significant variation. Improvements could be obtained through more extensive flight envelope coverage, more accurate measurement and more informative identification data. @en

During flight, natural fliers flap, twist and bend their wings to enhance flight performance. Lift and thrust benefit from flexibility as well as from both passive and active wing deformation. At the same time, the active deformations are used for flight control. In this study, we investigate strategies of control moments generation in a bio-inspired flapping-wing micro air vehicle (FWMAV). In particular, we propose a method for active control and attitude stabilization by introducing a wing deformation through adjustable wing sweep. The control method is demonstrated on a tailless FWMAV with independent wing sweep modulation on each of its four wings. The actuation mechanism consists of an arm joint at the leading edge, about which the wings are swept. Forces from the servo actuation are transferred to the leading edge of the robot through strings. The actuated strings alter the wing sweep, which affects the roll and pitch movement via different combinations of string pulls. The effectiveness of the designed mechanism is being evaluated on the basis of tethered force balance tests and free flight tests. An advantage of the proposed mechanism is its lightweight design, which is crucial for small FWMAVs with stringent weight restrictions. @en

The aim of this investigation is to provide an experimental volumetric visualization of the near wake topology of the vortex structures generated by a flapping-wing Micro Air Vehicle (MAV) by means of large-scale Robotic PIV. This novel technique implements coaxial illumination and imaging in combination with the use of Helium Filled Soap Bubbles (HFSB) as tracing particles to achieve large measurement volumes of the order of 10 liters. Information of different phases throughout the flapping cycle is obtained by means of a phase-locked averaging procedure. Experiments in both tethered and free-flight conditions have been performed, yielding an unprecedented comparison between the aerodynamics of the two conditions.@en

Robust attitude control is an essential aspect of research on autonomous flight of flapping wing Micro Air Vehicles. The mechanical solutions by which the necessary control moments are realised come at the price of extra weight and possible loss of aerodynamic efficiency. Stable flight of these vehicles has been shown by several designs using a conventional tail, but also by tailless designs that use active control of the wings. In this study a control mechanism is proposed that provides active control over the wings. The mechanism improves vehicle stability and agility by generation of control moments for roll, pitch and yaw. Its effectiveness is demonstrated by static measurements around all the three axes. Flight test results confirm that the attitude of the test vehicle, including a tail, can be successfully controlled in slow forward flight conditions. Furthermore, the flight envelope is extended with robust hovering and the ability to reverse the flight direction using a small turn space. This capability is very important for autonomous flight capabilities such as obstacle avoidance. Finally, it is demonstrated that the proposed control mechanism allows for tailless hovering flight.

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Insects are among the most agile natural flyers.Hypotheses on their flight control cannot always be validated by experiments with animals or tethered robots.To this end, we developed a programmable and agile autonomous free-flying robot controlled through bio-inspired motion changes of its flapping wings.Despite being 55 times the size of a fruit fly,the robot can accurately mimic the rapid escape maneuvers of flies,including a correcting yaw rotation toward the escape heading.Because the robot's yaw control was turned off,we showed that these yaw rotations result from passive,translation-induced aerodynamic coupling between the yaw torque and the roll and pitch torques produced throughout the maneuver.The robot enables new methods for studying animal flight,and its flight characteristics allow for real-world flight missions.2017

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Flow visualizations have been performed on a free flying, flapping-wing micro air vehicle (MAV), using a large-scale particle image velocimetry (PIV) approach. The PIV method involves the use of helium-filled soap bubbles (HFSB) as tracer particles. HFSB scatter light with much higher intensity than regular seeding particles, comparable to that reflected off the flexible flapping wings. This enables flow field visualization to be achieved close to the flapping wings, in contrast to previous PIV experiments with regular seeding. Unlike previous tethered wind tunnel measurements, in which the vehicle is fixed relative to the measurement setup, the MAV is now flown through the measurement area. In this way, the experiment captures the flow field of the MAV in free flight, allowing the true nature of the flow representative of actual flight to be appreciated. Measurements were performed for two different orientations of the light sheet with respect to the flight direction. In the first configuration, the light sheet is parallel to the flight direction, and visualizes a streamwise plane that intersects the MAV wings at a specific spanwise position. In the second configuration, the illumination plane is normal to the flight direction, and visualizes the flow as the MAV passes through the light sheet@en

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This paper describes the results of a six-year project aiming at designing and constructing a flapping twin-wing robot of the size of hummingbird (Colibri in French) capable of hovering. Our prototype has a total mass of 22 g, a wing span of 21 cm and a flapping frequency of 22 Hz; it is actively stabilized in pitch and roll by changing the wing camber with a mechanism known as wing twist modulation. The proposed design of wing twist modulation effectively alters the mean lift vector with respect to the center of gravity by reorganization of the airflow. This mechanism is modulated by an onboard control board which calculates the corrective feedback control signals through a closed-loop PD controller in order to stabilize the robot. Currently, there is no control on the yaw axis which is passively stable, and the vertical position is controlled manually by tuning the flapping frequency. The paper describes the recent evolution of the various sub-systems: the wings, the flapping mechanism, the generation of control torques, the avionics and the PD control. The robot has demonstrated successful hovering flights with an on-board battery for the flight autonomy of 15–20 s.

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Flapping wing micro air vehicles (MAVs) take inspiration from natural fliers, such as insects and hummingbirds. Existing designs manage to mimic the wing motion of natural fliers to a certain extent; nevertheless, differences will always exist due to completely different building blocks of biological and man-made systems. The same holds true for the design of the wings themselves, as biological and engineering materials differ significantly. This paper presents results of experimental optimization of wing shape of a flexible wing for a hummingbird-sized flapping wing MAV. During the experiments we varied the wing 'slackness' (defined by a camber angle), the wing shape (determined by the aspect and taper ratios) and the surface area. Apart from the generated lift, we also evaluated the overall power efficiency of the flapping wing MAV achieved with the various wing design. The results indicate that especially the camber angle and aspect ratio have a critical impact on the force production and efficiency. The best performance was obtained with a wing of trapezoidal shape with a straight leading edge and an aspect ratio of 9.3, both parameters being very similar to a typical hummingbird wing. Finally, the wing performance was demonstrated by a lift-off of a 17.2 g flapping wing robot.

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Although flapping-wing micro aerial vehicles have become a hot topic in academia, the knowledge we have of these systems, their force generation mechanisms and dynamics is still limited. Recent technological advances have allowed for the development of free flight test setups using on-board sensors and external tracking systems, for system identification purposes. Nevertheless, there is still little knowledge about the system requirements, as well as on how to perform free flight test experiments, and process the collected data. The present article presents the guidelines for flapping-wing micro aerial vehicle free flight testing. In particular, it gathers information produced by different studies and provides the best practices for the proper system dimensioning, system setup, on-board sensors, maneuver input design, error analyses and data post-processing, for the reconstruction of the forces and moments that act during free flight of a flapping-wing robot, for system identification and modeling purposes. Furthermore, this article compares the results obtained using external optical position tracking systems with on-board and external sensor fusion, and provides suitable solutions and methods for data fusion and force reconstruction.

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We present a novel design of a tailless flapping wing Micro Air Vehicle (MAV), which uses four independently driven pairs of flapping wings in order to fly and perform agile maneuvers. The wing pairs are arranged such that differential thrust generates the desired roll and pitch moments, similar to a quadrotor. Moreover, two pairs of wings are tilted clockwise and two pairs of wings anti-clockwise. This allows the MAV to generate a yaw moment. We have constructed the design and performed multiple flight tests with it, both indoors and outdoors. These tests have shown the vehicle to be capable of agile maneuvers, and able to cope with wind gusts. The main advantage is that the proposed design is relatively simple to produce, and yet has the capabilities expected of tailless flapping wing MAVs.@en

Flapping-wing MAVs represent an attractive alternative to conventional designs with rotary wings, since they promise a much higher efficiency in forward flight. However, further insight into the flapping-wing aerodynamics is still needed to get closer to the flight performance observed in natural fliers. Here we present the first step necessary to perform a flow visualization study of the air around the flapping wings of a DelFly II MAV in-flight: a precision position control of flight in a wind-tunnel. We propose a hierarchical control scheme implemented in the open-source Paparazzi UAV autopilot software. Using a decoupling, combined feed-forward and feed-back control approach as a core, we were able to achieve a precision of 2:5 cm for several seconds, which is much beyond the requirements for a time resolved stereo PIV technique. @en

Despite an intensive research on flapping flight and flapping wing MAVs in recent years, there are still no accurate models of flapping flight dynamics. This is partly due to lack of free flight data, in particular during manoeuvres. In this work, we present, for the first time, a comparison of free flight forces estimated using solely an on-board IMU with wind tunnel measurements. The IMU based estimation brings higher sampling rates and even lower variation among individual wingbeats, compared to what has been achieved with an external motion tracking system in the past. A good match was found in comparison to wind tunnel measurements; the slight differences observed are attributed to clamping effects. Further insight was gained from the on-board rpm sensor, which showed motor speed variation of ± 15% due to load variation over a wingbeat cycle. The IMU based force estimation represents an attractive solution for future studies of flapping wing MAVs as, unlike wind tunnel measurements, it allows force estimation at high temporal resolutions also during manoeuvres@en