Peter Ifju

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.

DEVELOPMENT OF A COMPOSITE BENDABLE-WING MICRO AIR VEHICLE

This paper describes the development of a bendable-wing Micro Air Vehicle (MAV) which is capable of being packed in a minimal volume. The wing is rigid under normal flight loads but compliant when the force is reversed, allowing it to be bent into a small diameter. The bendable-wing technology was developed to accommodate packing requirements on several larger vehicles, but was explored further in the creation of an endurance aircraft for use in the 2006 International Micro Air Vehicle Competition. The design procedure entailed airframe and wing design, component selection and placement, and composite stress analysis on the bendable wing. Stress analysis led to the choice of ±45° fiber orientation on the bendable-wing. Manufacturing was aided by custom CAD software and automated machining. Wind tunnel testing and flow visualization were used to show that flow separation is significant at low angles of attack of the thin under-cambered MAV wing. Visual Image Correlation indicated that a maximum permanent deformation of less than 0.5 mm was present after the wing was removed from the storage container. Qualitative flight testing confirmed that the observed residual wing deformation did not hinder flight and verified that the aircraft could consistently complete the objectives of the endurance mission.

Wind Tunnel Testing of Load-Alleviating Membrane Wings at Low Reynolds Numbers

This work is concerned with wind tunnel testing of an elastic membrane wing intended for micro air vehicles. Erratic flow conditions are a particular problem for the smooth controllability of such vehicles, and so stiff batten structures are imbedded into the trailing edge of the membrane wing, intended to passively washout under aerodynamic loading. This passive deformation is not always effective however, particularly at higher angles of attack, and so this work is intended to provide some understanding of the complex role of wing structure and flight speed upon aerodynamic performance of membrane wings. Several disparate wing structures are fabricated, with varying batten thicknesses and spacing. Data, in terms of measured aerodynamic loads and structural deformation, is given for a wide range of relatively low Reynolds numbers. Drastic changes in lift slopes, stalling conditions, and deformation patterns are found for certain combinations of flight speed and wing structure.

Controlling Pre-tension of Silicone Membranes on Micro Air Vehicle Flexible Wings

This work is concerned with a new method to apply uniform and known pre-tension silicone membranes intended for micro air vehicles. Pre-tension has an immense effect on the static and dynamic response of membrane wings and controls the overall deflections. Two different frame geometries were fabricated to attach silicone film at various temperatures to control the pre-tension. The intent is to thermally expand the silicone membrane allowing the film to stretch, attach the frame, and cool to room temperature to develop tension. Stresses and strains are provided for membrane attachment at various temperatures. The results are plotted to predict pre-tension for the given frame geometries.

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.

Design and Optimization of Morphing Mechanisms for Highly Flexible Micro Air Vehicles

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.

Force and Deformation Measurement on Low Aspect Ratio Membrane Airfoils

This paper investigates the effects of pre-strain and cell aspect ratio on free trailing edge, aspect ratio two, membrane wings at low Reynolds number (Re ~ 50,000). At these conditions, the membrane visually vibrates. Of particular interest is the comparison of the membrane wing aerodynamic performance to rigid models fabricated in the shape of the time-averaged membrane deformation. Three pre-strain levels, 1%, 2% and 4%, were applied to flat membranous wings with one, two and three cells. Force and deformation measurements were performed in a low speed wind tunnel. From digital image correlation results, the rigid, time-averaged deformation wings were fabricated using three-dimensional printing. The aerodynamic loads for the printed wings were acquired at the same test conditions as membranous wings to extract the dynamic and static benefits of flexibility. In general, the membrane wings outperformed the printed wings at both pre-stall and stall conditions. Nomenclature AR = Aspect ratio AOA or uf061 = Angle of attack b = Membrane cell span c = Length of chord c = Membrane cell chord CL = Lift coefficient CL,α = Lift curve slope CD = Drag coefficient CD,o = Minimum drag coefficient CM = Pitching moment coefficient f = Frequency L/D = Lift-to-drag ratio q = Dynamic pressure Re = Reynolds number t = Membrane thickness uf050 = Dimensionless aeroelastic parameter 1 Graduate Research Assistant, AIAA Student Member 2 Undergraduate Research Assistant 3 Associate Professor, AIAA Associate Fellow 4 Graduate Research Assistant, AIAA Student Member 5 Associate Professor, AIAA Associate Fellow 6 Professor, AIAA Member 51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition 07 10 January 2013, Grapevine (Dallas/Ft. Worth Region), Texas AIAA 2013-0682 Copyright

Effects of Membrane Vibration on the Flow Field Surrounding Flat-Plate Membrane Airfoils

This investigation examines the fluid-structure interaction between flexible membrane airfoils and their surrounding flow field for the purpose of physically interpreting aerodynamic performance benefits attributed to certain membrane airfoil configurations in low Reynolds number (< 100,000) flight regimes. Specifically, this research effort identifies airfoil membrane vibration onset through quantifiable characteristics and presents simultaneous time-resolved hotwire anemometry and laser vibrometry to measure membrane vibration and flow field oscillation velocities, respectively. Evaluations were executed with a series of test articles consisting of rigid, flat plate airfoils containing a single, variably-sized membrane cell located at the test article’s spanwise centerline. Results demonstrate that membrane vibration onset criteria involving direct assessment of membrane oscillation velocity via laser vibrometry more closely and consistently match visual assessment of vibration onset than do onset criteria involving flow field oscillation velocity measured via hotwire anemometry. At low freestream velocities, membrane vibration does not significantly affect the surrounding flow and vibration onset cannot be consistently identified via flow field assessment. Comparison of total membrane vibration energy at varying test conditions proves to be the most consistently accurate and quantifiable indicator of vibration onset.

Modeling of Bendable MAV Wing Using Energy Method

Out of many exciting and unique micro air vehicles (MAVs) being developed at the University of Florida, one MAV design utilizes a bendable wing concept. To minimize the storage volume, the wing is rolled and the MAV is stored inside a canister. To predict the shape of the wing and consequently initial strain produced in the wing, when it is stored inside canister, energy method principal is used. An analytical model is developed which predicts the shape and initial strains in the wing by modeling the wing as a one dimensional composite beam. Results of the analytical model prediction are compared with the experimental observations.

Flow and Structure Measurements of a Passively Compliant Wing

This paper discusses an experimental effort to study the fluid-structure interactions of flexible membrane wings at low Reynolds number by synchronized acquisition of Particle Image Velocimetry and Digital Image Correlation. The compliant wings are battenreinforced flat plates with multi-cell, scalloped membrane sections similar to some micro air vehicle designs. The data analyzed in this paper consists of time-resolved, two-component flow measurements over the model/membrane surface as well as in the near wake and 3D membrane displacement measurements. The flow over a baseline rigid flat plate is included to compare time-averaged flow properties and investigate how the membrane wings alter the flow. Membrane wing models of varying pre-tension were also investigated utilizing the digital image correlation technique to better understand the frequency response and flexibility effects of the compliant wings. The time-dependent dynamics of velocity and membrane vibrations indicate that membrane fluctuations have a strong influence on the surrounding flow, affecting the shear layer emanating from the wings’ leading edge and the development of the flow in the wake of the airfoil. Spectral and correlation analysis show quantitative evidence of membrane vibrations driving the flow over the wing and into its wake.