Roberto Albertani

Static Aeroelastic Model Validation of Membrane Micro Air Vehicle Wings

A low-aspect-ratio, low-Reynolds-number membrane wing has been identified as a viable platform for micro air vehicle applications. Desirable flying qualities include high lift and larger stability margins. Several challenges are associated with the numerical modeling of such a wing, including highly three-dimensional flows, separation bubbles, and nonlinear membrane behavior. A thorough model validation and system identification effort is therefore required. A novel experimental setup integrates a wind tunnel with a visual image correlation system for simultaneous measurement of wing displacements, strains, and aerodynamic loads. These three metrics are used for a direct comparison of numerical and experimental data for both pre- and poststall angles of attack. Suitable correspondence is demonstrated for moderate angles of attack; methods for increasing the model fidelity can be made for angles with poor predictive capability. Computed flow structures reveal further information concerning the aeroelastic behavior of membrane wings.

Aerodynamic Performace of a Notional Perching MAV Design

This paper describes a mechanized wing concept for a perching micro air vehicle, and the aerodynamic behavior of such a concept at wing angles of attack up to ninety degrees. The model has wings capable of rotating in pitch at two spanwise joints to simulate the motion of a bird’s wings during a perching maneuver. The advantage of a perching landing is that it allows the vehicle to land with approximately zero vertical and horizontal velocity on a tree branch, power line, or ledge. Wind tunnel tests are conducted to measure vehicle performance and ight control parameters for further development of a free-ying model. Static measurements of lift, drag, and pitching moments are correlated to wing section angles from zero to nearly ninety degrees angle of attack. Unsteady eects related to dynamic stall are also investigated by varying the speed at which the wing sections are actuated. In order to investigate actuation and mechanization designs for this concept, additional measurements are made to determine the amount of power required to achieve the wing motions.

Effect of Tip Vortex on Wing Aerodynamics of Micro Air Vehicles

Tip vortex induces downwash movement, which reduces the effective angle of attack of a wing. For a low-aspect. ratio, low-Reynolds-number wing, such as that employed by the micro air vehicle (MAV), the induced drag by the tip vortex substantially affects its aerodynamic performance. In this paper we use the endplate concept to help probe the tip-vortex effects on the MAV aeredynamic characteristics. The investigation is facilitated by solving the Navier-Stokes equations around a rigid wing with a root-chord Reynolds number of 9 x 10 4

Validation of a Low Reynolds Number Aerodynamic Characterization Facility

The characterization of a new wind tunnel at the University of Florida’s Research and Engineering Education Facility is presented. The tunnel is specifically designed to operate in the low Reynolds number regime in which Micro Air Vehicles operate. The wind tunnel is driven by a 60 inch, 50 hp axial blower controlled by a variable frequency drive. The tunnel entrance consists of a flow conditioning section and a 8:1 area contraction ratio that results in a 42” square entrance to the open jet test section. The enclosure surrounding the test section has a volume of nearly 2000 ft with an axial length of 10 ft. The free stream velocities in the tunnel range from nominally 0 to 22 m/s by altering the frequency on the variable frequency drive. Flow uniformity studies for free stream velocities of 2 and 15 m/s were conducted demonstrating a potential core throughout the test section of at least 60% of the 1.14 m contraction exit. Experiments using hot wire anemometry were also performed and the turbulence intensities were found to be less 0.22% for free stream velocities greater than 1m/s.

An Experimental Study of Flexible Membrane Wings in Flapping Flight

An experimental study was conducted to assess the aerodynamic benefits of using flexible membrane wings for the development of flapping-wing Micro-Air-Vehicles (MAVs). The overall aerodynamic performances (i.e. time-averaged lift and thrust/drag generation) of two flexible membrane wings with different skin flexibility (i.e., a flexible nylon wing and a more flexible latex wing) were compared with those of a conventional rigid wing in order to assess the effects of skin flexibility (rigidity) of the tested wings on their aerodynamic performances for flapping flight applications. The measurement results revealed clearly that, for all the tested wings, flapping motion would bring significant aerodynamic benefits when the flapping flight is in unsteady state regime with advance ratio of the flapping flight being smaller than 1.0. The aerodynamic benefits of flapping flight were found to decay exponentially with the increasing advance ratio. Flapping motion was found to become detrimental for high speed flight applications. The skin flexibility (rigidity) of the tested wings was found to have considerable effects on their aerodynamic performances for both soaring flight and flapping flight applications: The flexible membrane wings were found to have better overall aerodynamic performance (i.e., lift-to-drag ratio) over the rigid wing for soaring flight, especially for high speed soaring flight applications. The rigid wing was found to have better lift production performance for flapping flight in general. The latex wing, which is the most flexible among the three tested wings, was found to have the best thrust generation performance for flapping flight. The less flexible nylon wing, which has the best aerodynamic performance for soaring flight applications, was found to be the worst for flapping flight applications.

Experimental Analysis of Deformation for Flexible-Wing Micro Air Vehicles

The design of Micro Air Vehicles (MAV) must consider the critical issues of aerodynamics and structures. These issues determine the MAV characteristics such as lift and drag along with the ability to withstand air loads. A class of vehicles designed by the University of Florida adopts a flexible-wing approach. A notable feature of this approach is adaptive washout which provides gust rejection and stall delay. This paper investigates the relationship between wing flexibility and performance for a member of this class of vehicle. Results in terms of aerodynamic coefficients and structural displacements from wind tunnel tests are presented to demonstrate the deformation of the wing under loading in association with the lift and drag characteristics.

Experimental and Finite Element Modal Analysis of a Pliant Elastic Membrane for Micro Air Vehicles Applications

This paper investigates the modal characteristics of a latex membrane for micro air vehicles applications. Finite element (FE) models are developed for characterizing the latex membrane at dynamic loading and validated by experimental results. The membrane at different pre-tension levels is attached to a circular steel ring, mounted on a shaker, and placed inside the vacuum chamber for modal characterization using a scanning laser Doppler vibrometer (LDV). The experimental modal analysis is conducted by imposing a structural excitation to the ring for investigating the membrane vibration characteristics at both atmospheric and reduced pressures in a vacuum chamber. FE models are developed for the natural frequencies of the membrane at different uniform and non-uniform pre-tension levels with the effect of the added mass of air. The Mooney-Rivlin hyperelastic material model is selected for the membrane. The natural frequencies of the membrane computed by experimental and FE models are correlated well, although discrepancy is expected among experimental and FE results within reasonable limits due to the variation of the thickness of the membrane. The natural frequencies increase with the mode and pre-tension level of the membrane but decrease due to an increase in ambient pressure. The damping ratios have very minimal effect on the frequencies due to low values but help to reduce the amplitude of vibration. Natural frequencies of the membrane do not change with the uniform and non-uniform nature of the pre-tension, although they increase with the pre-tension level. It is also found that the effect of added mass on the natural frequencies increases with an increase of the size of the membrane specimen.

Experimental Kinematics and Dynamics of Butterflies in Natural Flight

In this paper, we discuss the collection, post-processing and subsequent evaluation of flight data of butterflies, in various free flight scenarios. The flight data is obtained by means of a vision tracking system; from which estimates of the motion of different body parts, including the head, abdomen and wings are then determined. These estimates are subsequently post-processed with a view to examining the mathematical correlations that exist between motion of individual body parts of the butterfly with its overall flight trajectory. Preliminary results performing a comparison of different takeoff sequences, are then presented.

A Numerical and Experimental Investigation of Flexible Micro Air Vehicle Wing Deformation

A class of micro air vehicles (MAVs), developed at the University of Florida, implements a flexible-wing approach as a viable method for dealing with environmental disturbances such as wind gusts. This paper investigates the steady-state deflection field across a membrane wing. Experimental results are found using a low-speed wind tunnel in conjunction with a visual image correlation system. Numerical models are also formulated, consisting of the first iteration of a fluid-structure interaction problem. The pressure redistribution corresponding to the wing’s change in shape can be ignored for small deformations. The computational results from this single iteration are compared to the experimental deformation field, with good correlation between the two through a range of flight conditions.

Aerodynamics of Cambered Membrane Flapping Wings

The aerodynamics of cambered membrane flapping wings is the focus of this paper. A cambered airfoil was introduced into the wing by shaping metal ribs attached to the membrane skin of the 25-cm-wing-span model. Tests in still air of the flapping wings oriented horizontally and vertically with respect to the gravitational field show no effects on generated aerodynamic forces. The thrust force generated by a 9% camber wing is found to be 30% higher than that of the same size flat wing. The aerodynamic forces and pitching moment generated by flapping wings were measured in a wind tunnel with the flapping wing angle of attack varying from horizontal to vertical. Cambered wings show significantly higher lift and thrust in comparison with flat wings. Adding a dihedral angle to the wings and keeping the flapping amplitude constant improved the cambered wing’s performance even further. The aerodynamic coefficients are defined using a reference velocity as a sum of two components: a free stream velocity and a stroke-averaged wing tip flapping velocity. The lift, drag, and pitching moment coefficients obtained using this procedure collapse well for studied advance ratios, especially at lower angles of attack.