Pin Wu

Flapping Wing Structural Deformation and Thrust Correlation Study with Flexible Membrane Wings

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

Structural dynamics and aerodynamics measurements of biologically inspired flexible flapping wings

Flapping wing flight as seen in hummingbirds and insects poses an interesting unsteady aerodynamic problem: coupling of wing kinematics, structural dynamics and aerodynamics. There have been numerous studies on the kinematics and aerodynamics in both experimental and computational cases with both natural and artificial wings. These studies tend to ignore wing flexibility; however, observation in nature affirms that passive wing deformation is predominant and may be crucial to the aerodynamic performance. This paper presents a multidisciplinary experimental endeavor in correlating a flapping micro air vehicle wings aeroelasticity and thrust production, by quantifying and comparing overall thrust, structural deformation and airflow of six pairs of hummingbird-shaped membrane wings of different properties. The results show that for a specific spatial distribution of flexibility, there is an effective frequency range in thrust production. The wing deformation at the thrust-productive frequencies indicates the importance of flexibility: both bending and twisting motion can interact with aerodynamic loads to enhance wing performance under certain conditions, such as the deformation phase and amplitude. By measuring structural deformations under the same aerodynamic conditions, beneficial effects of passive wing deformation can be observed from the visualized airflow and averaged thrust. The measurements and their presentation enable observation and understanding of the required structural properties for a thrust effective flapping wing. The intended passive responses of the different wings follow a particular pattern in correlation to their aerodynamic performance. Consequently, both the experimental technique and data analysis method can lead to further studies to determine the design principles for micro air vehicle flapping wings.

Passive Bending and Twisting Motion during the Flapping Stroke of a Micro Elastic Wing for Thrust Production

This study investigates the relationship between wing structure and the production of aerodynamic forces for flapping flight, by measuring both the wing deformation and loads during flapping. The experimental setup allows data acquisition that correlates lift and thrust generated by an artificial flapper to wing deformation. The mapping between the loads and deformation indicates the performance of flapping wings for disparate structures and materials. Several technical challenges are resolved in this study. For instance, small flapping wings (of three inches span) produce loads and deformations that are difficult to measure. Intensive data analysis is performed to extract useful information from the measurements. A novel flapping mechanism FL2D3 is created to allow actuation frequencies up to 30 Hz. Tests in both air and vacuum are performed to isolate aerodynamic loads from inertial effects. Furthermore, the synchronization of the loads measurement system, the vision-based wing deformation measurement system, and the flapping mechanism is difficult; a virtual instrument is developed with the hardware to realize the experiment.

Structural Deformation Measurements of Anisotropic Flexible Flapping Wings for Micro Air Vehicles

Anisotropic structural properties of flapping micro air vehicle wings have recently started to attract research attention. A difference in spanwise and chordwise stiffness, for various bending and twisting behaviors during flapping, seems to be the key factor in producing lift and thrust. This work presents an experimental setup and post processing techniques to characterize the behavior and response of membrane wings with different topologies and structural features. These tested wings represent typical designs used in hobby ornithopters, and are effective in lift and thrust generation for forward flight. Wing deformation measurements are of interest for the current work. The rapid motions, large out-of-plane wing displacements, localized deformation of the flexible membrane, and the small scale loads all challenge traditional measurement techniques. A customized digital image correlation system, combined with stroboscopic triggering and fine-tuned image acquisition, is developed to determine wing kinematics and surface deformation in both static air and vacuum environments. Results are given for the structural deformation along a flapping membrane wing as a function of flapping frequency, vacuum/air conditions, and anisotropic wing structure.

Micro Air Vehicle Flapping Wing Effectiveness, Efficiency and Aeroelasticity Relationships

By utilizing passive wing deformation, thrust can be produced with a single-degree-offreedom flapping motion. Studies have shown that wings of different flexibility produce effective thrust at different frequency ranges. This work focuses on the efficiency of the wings: comparing the thrust production of certain power input to wings of different structural properties. The aeroelasticity of the wings is examined with full-field deformation measurement techniques. Four pairs of flexible membrane wings of different reinforcement skeletons are tested. Their bending and twisting stiffness are characterized, with deformation plots compared and correlated with thrust production at different frequencies. The results show that the wing compliance is crucial in thrust production: thrust is effectively produced with some particular patterns of passive deformation. The experimental setup consists of a flapping mechanism that enables actuation up to 45 Hz at 70o stoke amplitude; a force and torque sensor that measures the lift and thrust; and a digital image correlation system that consists of four cameras, capable of capturing the complete stroke kinematics and structural deformation. This work solves several technical challenges: measuring low magnitude of forces, monitoring power consumption rate, measuring the aeroelastic deformation and intensive data post processing.

An Integrated Experimental and Computational Approach to Analyze Flexible Flapping Wings in Hover

Biological flyers exploit wing deformation during flapping flight. There is a substantial need to improve the understanding of the aeroelastic effects associated with the wing deformation to build flapping wing micro air vehicles. This paper presents an effort to develop an integrated approach involving both experimental and computational methods to realize this goal. As the first step, an isotropic flat plate aluminum wing is manufactured and actuated to perform a single degree-of-freedom flapping motion. The wing deformation and airflow around the wing are measured with digital image correlation (DIC) and particle image velocimetry (PIV), respectively. Computational analyses are performed on this wing configuration using a combined nonlinear structural dynamics and Navier-Stokes solution. Reasonable agreement obtained between experimental and computational data in this preliminary effort shows a potential to analyze more complicated flexible flapping wings in future.

Experimental Methodology for Flapping Wing Structure Optimization in Hovering Flight of Micro Air Vehicles

It has been observed that, in both nature and experiment, flapping wing flexibility can significantly enhance aerodynamic performance. By tailoring the structural properties, passive deformation can be utilized to generate higher lift and thrust. This work has developed a systematic methodology to experimentally optimize the flapping wing structure for maximizing thrust production under a simple one-degree-of-freedom motion. Different flapping frequencies and amplitudes are also examined along with different wing structure to study the relationship between kinematics and thrust; because these variables are closely related to the passive inertial deformation. The aerodynamic performance is measured with a force sensor recording both the lift and thrust; the net time-averaged thrust is used as a benchmark to evaluate wing performance. The wing structural property is obtained by measuring wing deformation using a digital image correlation system capable of capturing the full-field out-of-plane deformation of the whole flapping stroke up to 90o in both air and vacuum. The data are presented as contours to reveal details of structural deformation and later extracted as phase plots that show a loop of the deformation versus flap angle. A certain design scheme is formulated to generate a number of designs, all of which are tested for thrust production. The force and deformation data are then correlated to understand the structure that benefits thrust and used to predict parameters for potential optimal flapping flight. It is concluded that for flapping wing structure optimization in hovering flight on this scale, each structure must be tailored according to a certain kinematics (with a desired flapping frequency and amplitude) and optimized for performance values such as thrust/lift coefficient/efficiency.

Advanced Flapping Wing Structure Fabrication for Biologically-Inspired Hovering Flight

Insect wings have complicated vein structures that are strong enough to support large acceleration, light enough to endure high flapping frequency and flexible enough to passively deform to enhance aerodynamic performance. As hover-capable artificial flapping flight has been realized in micro/nano air vehicles (by Aerovironment Inc.), the technical challenge for creating an efficient flapping wing has attracted more research attention. This work presents the current flapping wing structure development at University of Florida, in tailoring the flexibility and mass distribution of membrane-laminated and carbon-fiber-skeletonized anisotropic flexible wings: the wing skeletal topology has been designed for enhancing passive deformation for a one-degree-of-freedom kinematics; the cross-section of each skeletal member is controlled with modern manufacturing techniques to produce the designed structure; and the final wing is carefully examined for its thrust generation efficiency and aeroelastic properties (wing deformation). Several challenging aspects are overcome: realizing a varying skeletal cross-section to achieve a controlled stiffness to weight ratio, determining the skeletal topology and mass distribution, developing a reliable and consistent manufacturing technique and examining the wings with different methods. The results show that a more energy efficient design can be achieved with the developed manufacturing techniques that allow for complicated structures.

Advanced Biologically-Inspired Flapping Wing Structure Development

This study examines the possibility to develop a manufacturing method to build a complicated composite structure comparable to insect wings. Stimulated by the research in flapping wing micro air vehicles, a wing structure that can be controlled with stiffness and mass distribution during manufacturing can enable complicated kinematics and efficient aerodynamics. Insects demonstrate superior flight performance and therefore their wings are good examples for building an artificial counterpart. Cicada wings are selected in this work for emulation. An artificial composite wing is built with computer numerical controlled tooling and manual fabrication, with similar vein pattern. The wings are compared with measurements of mass distribution in the spanwise direction. The results show that the composite reinforcement topology and cross section variation allow the two wings to have very similar property trends. Several difficulties are overcome in this work: replicating the cicada wing vein pattern, fabricating small composite structure components and measuring their weight distribution.

Active Rotation and Vibration during the Flapping Stroke of a Micro Elastic Wing for Thrust Production

The goal of this study is to understand the effect of active wing rotation in the aerodynamic performance of micro air vehicle flapping wings. Mimicking biological flight, flapping wing micro (or nano) air vehicles have been successfully developed. However, the flapping wing aerodynamic mechanisms have not been fully unveiled; especially on the topic of wing structure optimization for flapping flight. Such structure optimization is intended to enhance aerodynamic performance by passive wing deformation that results from the coupled inertial and aerodynamic loads. On the other hand, active kinematics can be used to create lift and thrust with stiff wings. Therefore, flapping wing structure can also be tailored with kinematics to achieve optimized efficiency. It has been observed that birds fly with different gaits and insects fly with different stroke patterns at different speeds. Actively controlling the wing rotation and vibration can change the interaction between the wing surface and the surrounding air. Magnetic coil actuators are combined with a one degree-offreedom flapping mechanism to realize the two degree of freedom kinematics up to 15 Hz. Besides synchronizing the rotational actuation with the flapping motion, the actuators are also actuated at much higher frequencies (from 30 to 1000 Hz) to examine the effect of vibration (multiple rotations during one flapping stroke). This would also allow the wing to exhibit dynamic properties during flapping. The results show that magnetic coil actuators can be used to effectively induce wing rotation during flapping and affect aerodynamics; the phase difference between the flapping and rotation motion can increase or decrease aerodynamic performance; and the high frequency structural vibration has insignificant impact on aerodynamics.