Showing papers by "James E. Hubbard published in 2012"

Passively morphing ornithopter wings constructed using a novel compliant spine: design and testing

Abstract: Ornithopters or flapping wing uncrewed aerial vehicles (UAVs) have potential applications in civil and military sectors. Amongst the UAVs, ornithopters have a unique ability to fly in low Reynolds number flight regimes and also have the agility and maneuverability of rotary wing aircraft. In nature, birds achieve such performance by exploiting various wing kinematics known as gaits. The objective of this work is to improve the steady level flight performance of an ornithopter by implementing a continuous vortex gait using a novel passive compliant spine inserted in the ornithopter's wings. This paper presents an optimal compliant spine concept for ornithopter applications. A quasi-static design optimization procedure was formulated to design the compliant spine. Finite element analysis was performed on a first generation spine and the spine was fabricated. This prototype was then tested by inserting it into an ornithopter's wing leading edge spar. The effect of inserting the compliant spine into the wings on the electric power required, the aerodynamic loads and the wing kinematics was studied. The ornithopter with the compliant spines inserted in its wings consumed 45% less power and produced an additional 16% of its weight in mean lift compared to the same ornithopter without the compliant spine. The results indicate that this passive morphing approach is promising for improved steady level flight performance.

Numerical Study and Experimental Validation of the Interaction of Multiple Synthetic Jet Actuators with Cross Flow

Abstract: Incompressible Unsteady Navier-Stokes equations are solved to study and analyze the behavior of single and multiple synthetic jet actuators in quiescent and cross-flow conditions. The effect of two different boundary conditions, a flat plate and a backwards facing step, are also studied. Combinations of operation and nondimensional parameters are simulated in order to investigate jet-cross flow performance for different operational regimes. The behavior of one, two and three jets in cross-flows is characterized based on the Velocity Ratio (R) and the Momentum Coefficient (Cμ). The influence of operation parameters and geometric parameters is also studied with particular emphasis on actuator spacing (s) and relative phase angle (φ). An array of three actuators is simulated under quiescent conditions for two different relative phase angles in symmetric configurations. CFD results predict change in momentum flux concentrations for specific array operating conditions. The results also show that the development and nature of the recirculation zone can be characterized for specific ranges of R and Cμ. Three commercially available actuators have been characterized for experimental validation of the CFD predictions of flows over plates and a backward facing step. PIV tests are performed in cross-flow as well as quiescent conditions for validation of specific CFD cases. The goal of this study is to contribute to understanding the behavior of an array of synthetic jets as function of non-dimensional, geometric and operating parameters.

Design of bend-and-sweep compliant mechanism for passive shape change

Abstract: Contact aided compliant mechanisms are a class of compliant mechanisms where parts of the mechanism come into contact with one another during motion. Such mechanisms can have nonlinear stiffness, cause stress-relief, or generate non-smooth paths. New contact aided compliant mechanisms called bend-and-sweep compliant mechanisms are presented in this paper. These bend-and-sweep mechanisms are made up of compliant joints which are alternately located in two orthogonal directions, and they also exhibit nonlinear stiffness in two orthogonal directions. The stiffness properties of these mechanisms, in each direction, can be tailored by varying the geometry of the compliant joints. One application of these mechanisms is in the passive wing morphing of flapping wing UAVs or ornithopters. A design study is conducted to understand the effect of hinge geometry on the deflections and maximum von Mises stress during upstroke and downstroke. It is shown that the bend-and-sweep compliant elements deflect as desired in both the bending and sweep directions.Copyright © 2012 by ASME

Experimentally Validated Flexible-Multi-Body Structural Dynamics Model of a Bioinspired Ornithopter

Abstract: Flapping-wing vehicles combine the ability to hover like rotary-wing aircraft while also allowing for gliding flight, much like fixed-wing aircraft. They have the potential to dive and perch, are highly maneuverable and agile, and have improved safety and reduced noise emissions when compared to rotary-wing vehicles. The dynamics of flapping-wing flight needs to be better understood in order to design efficient flapping wing vehicles. The primary objective of this paper is to model the dynamic performance of a flexible wing ornithopter considering wing kinematics. It addresses the development of flexible-multibody structural dynamic model of a bioinspired ornithopter that takes into account the inertia, gravity, and aeroelactic effects. Aerodynamic loads resulting from the blade element theory are applied on the structural dynamic model employing Finite Element Methods. The flexible wing structure is integrated in a modal based flexible-multi-body dynamics solver, which is used for the simulations of experimental ornithopter configuration. The resulting ornithopter flight simulator is validated with experimental test data revealing the time history of the wing kinematics and integrated vertical and horizontal forces on the ornithopter. The model was validated and the resulting simulated wing deflections matches well with the experimental deflections, such as the wing tip path. Integrated forces acting on the flexible-multi-body dynamics model was also compared to experimental results.

Design, Fabrication and Testing of a Passively Morphing Ornithopter Wing for Increased Lift and Agility

Abstract: : Over the last few decades, flapping wing Unmanned Aerial Vehicles (UAVs), or ornithopters, have shown the potential for advancing and revolutionizing platform performance in both the civil and military sectors. An ornithopter is unique in that it can combine the agility and maneuverability of rotary wing aircraft with excellent performance in low Reynolds number flight regimes. The objective of the proposed work was to develop methods to design novel ornithopter wings that allow passive wing morphing. Passive morphing was achieved through an optimally designed compliant spine that mimics the function of a bird s wrist. A multi-objective optimization was carried out and several designs resulting from this optimization were bench-top and free flight tested. The presence of a 1DOF compliant spine in the ornithopter wing was found to introduce an asymmetry between the upstroke and the downstroke. For any given flapping frequency or throttle setting, the ornithopter with the compliant spine consumed less electric power, produced more mean lift and did not incur any thrust penalties when compared to the ornithopter without the compliant spine. Power reduction of 44.7% was achieved at the steady level flight flapping frequency, lift gains of up to 16% of the ornithopter's weight was also realized without incurring any thrust penalties. Also during the free flight test, the ornithopter with the compliant spine inserted in its wings reduced the overall negative center of mass acceleration during one flapping cycle by 22 %. The negative acceleration reduction may translate into overall lift gains. Thus the steady level flight performance was improved due to the presence of the compliant spine. Therefore compliant mechanisms proved to be not only feasible, but also beneficial for application to general air vehicle design.