Yashwanth Tummala

Testing of Novel Compliant Spines for Passive Wing Morphing

Flapping wing Unmanned Aerial Vehicles (UAVs) or ornithopters are proliferating in both the civil and military markets. Ornithopters have the potential to combine the agility and maneuverability of rotary wing aircraft with excellent performance in low Reynolds number flight regimes. These traits promise optimized performance over multiple mission scenarios. Nature achieves this broad performance in birds using wing gaits that are optimized for a particular flight regime. The goal of this work is to improve the performance of ornithopters during steady level flight by passively implementing the Continuous Vortex Gait (CVG) found in natural avian flyers. In this paper we present new experimental results for a one degree of freedom (1DOF) compliant spine which was inserted into an experimental test ornithopter leading edge wing spar in order to achieve the desired kinematics. The lift and thrust along with electric power metrics at different flapping frequencies were measured using a six-channel load cell and a current senor, respectively. These metrics were determined for the test ornithopter both with and without the compliant spine insert. Initial results validate the ability of our compliant spine design to withstand the loads seen during flight at flapping frequencies of up to and including 5 Hz. For the ornithopter test platform used in the study, inserting the compliant spines into the wing leading edge spar accurately simulates the CVG increasing the mean lift by 16%, and reducing the power consumed by 45% without incurring any thrust penalties.Copyright

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

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 ornithopters 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 ornithopters 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.

DESIGN OF A PASSIVELY MORPHING ORNITHOPTER WING USING A NOVEL COMPLIANT SPINE

A new scheme to design morphing ornithopter wings using a passive compliant spine is presented in this paper. The objective of this work is to optimize steady level flight performance of an ornithopter by passively implementing the Continuous Vortex Gait (CVG) which requires bending, twist and sweep coupling during the upstroke. An optimization problem is formulated to design a compliant spine for pre-specified bending, sweep, and twist deflections. As a first step to achieving these 3 DOF kinematics, a 1 DOF compliant spine is considered to produce a specified bending deflection during the upstroke for drag reduction while remaining stiff during the downstroke for increased lift. The effect of the relevant geometric design parameters, namely contact gap, angle, and hinge geometry, are considered and optimized to achieve the aforementioned kinematics for both single and multiple joints, which make up a compliant spine. Results presented include the spine design optimization procedure, as well as a complete analysis for a 1DOF compliant spine to illustrate the efficacy of the methodology. This compliant spine design methodology and optimization procedure will be used, in the future, to design the 3-DOF compliant spine for the passively morphing ornithopter.© 2010 ASME

Flight testing of novel compliant spines for passive wing morphing on ornithopters

Unmanned Aerial Vehicles (UAVs) are proliferating in both the civil and military markets. Flapping wing UAVs, or ornithopters, have the potential to combine the agility and maneuverability of rotary wing aircraft with excellent performance in low Reynolds number flight regimes. The purpose of this paper is to present new free flight experimental results for an ornithopter equipped with one degree of freedom (1DOF) compliant spines that were designed and optimized in terms of mass, maximum von-Mises stress, and desired wing bending deflections. The spines were inserted in an experimental ornithopter wing spar in order to achieve a set of desired kinematics during the up and down strokes of a flapping cycle. The ornithopter was flown at Wright Patterson Air Force Base in the Air Force Research Laboratory Small Unmanned Air Systems (SUAS) indoor flight facility. Vicon motion tracking cameras were used to track the motion of the vehicle for five different wing configurations. The effect of the presence of the compliant spine on wing kinematics and leading edge spar deflection during flight is presented. Results show that the ornithopter with the compliant spine inserted in its wing reduced the body acceleration during the upstroke which translates into overall lift gains.

DESIGN OPTIMIZATION OF A COMPLIANT SPINE FOR DYNAMIC APPLICATIONS

Ornithopters or flapping wing Unmanned 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 regions and also have the agility and maneuverability of a rotary wing aircraft. In nature, birds achieve such special characteristics by morphing their wings. The compliant spine (CS) design concept presented here represents a novel method of achieving wing morphing passively. In this paper, an optimal design method is developed that incorporates dynamic finite element analysis. To solve the CS design problem a new multi-objective optimization problem is formulated with three objective functions. The first objective function seeks to minimize the mass of the compliant spine. The second objective function seeks to maximize the deflection of the compliant spine for a particular dynamic loading condition. Finally, the third objective function seeks to minimize the stress in the design observed under the dynamic loading conditions experienced during flight. The deflections and stresses in the CS design are based on measured wing loads and are calculated by applying a sinusoidal forcing function at a prescribed forcing frequency. The optimization, performed via a controlled elitist genetic algorithm which is a variant of NSGA-II, is used to design CSs operating under dynamic conditions. Modal analysis and frequency response of an optimal compliant spine during the upstroke are also shown.Copyright

Design and optimization of a bend-and-sweep compliant mechanism

A novel contact aided compliant mechanism called bend-and-sweep compliant mechanism is presented in this paper. This mechanism has nonlinear stiffness properties in two orthogonal directions. An angled compliant joint (ACJ) is the fundamental element of this mechanism. Geometric parameters of ACJs determine the stiffness of the compliant mechanism. This paper presents the design and optimization of bend-and-sweep compliant mechanism. A multi-objective optimization problem was formulated for design optimization of the bend-and-sweep compliant mechanism. The objectives of the optimization problem were to maximize or minimize the bending and sweep displacements, depending on the situation, while minimizing the von Mises stress and mass of each mechanism. This optimization problem was solved using NSGA-II (a genetic algorithm). The results of this optimization for a single ACJ during upstroke and downstroke are presented in this paper. Results of two different loading conditions used during optimization of a single ACJ for upstroke are presented. Finally, optimization results comparing the performance of compliant mechanisms with one and two ACJs are also presented. It can be inferred from these results that the number of ACJs and the design of each ACJ determines the stiffness of the bend-and-sweep compliant mechanism. These mechanisms can be used in various applications. The goal of this research is to improve the performance of ornithopters by passively morphing their wings. In order to achieve a bio-inspired wing gait called continuous vortex gait, the wings of the ornithopter need to bend, and sweep simultaneously. This can be achieved by inserting the bend-and-sweep compliant mechanism into the leading edge wing spar of the ornithopters.

Free flight testing and performance evaluation of a passively morphing ornithopter

Unmanned Aerial Vehicles (UAVs) are proliferating in both the civil and military markets. Flapping wing UAVs, or ornithopters, have the potential to combine the agility and maneuverability of rotary wing aircraft with excellent performance in the low Reynolds number flight regimes. The purpose of this paper is to present new free flight experimental results for an ornithopter equipped with single degree of freedom compliant spines. The compliant spines are designed and optimized in terms of mass, maximum von-Mises stress, and desired wing bending deflections. The spines are inserted in an experimental ornithopter wing leading edge spar, in order to achieve a set of desired kinematics during the up and down strokes of a flapping cycle. The ornithopter is flown at Wright Patterson Air Force Base in the Air Force Research Laboratory Small Unmanned Air Systems (SUAS) indoor flight facility. Vicon® motion tracking cameras are used to track the motion of the vehicle for four different wing configurations. The effect of the presence of the compliant spine on the wings and body kinematics, as well as the leading edge spar deflection during free flight is presented in this paper. Several metrics were used to evaluate the vehicle performance with various compliant spine designs inserted in the leading edge spar of the wings. Results show that passively morphing the wings, via adding compliance in the leading edge spar, does not require additional power expenditure and is beneficial to the overall vertical and horizontal propulsive force production.

Design and Optimization of a Contact-Aided Compliant Mechanism for Passive Bending

A contact-aided compliant mechanism (CCM) called a compliant spine (CS) is presented in this paper. It is flexible when bending in one direction and stiff when bending in the opposite direction, giving it a nonlinear bending stiffness. The fundamental element of this mechanism is a compliant joint (CJ), which consists of a compliant hinge (CH) and contact surfaces. The design of the compliant joint and the number of compliant joints in a compliant spine determine its stiffness. This paper presents the design and optimization of such a compliant spine. A multi-objective optimization problem with three objectives is formulated in order to perform the design optimization of the compliant spine. The goal of the optimization is to minimize the peak stress and mass while maximizing the deflection, subject to geometric and other constraints. Flapping wing unmanned air vehicles, also known as ornithopters, are used as a case study in this paper to test the accuracy of the design optimization procedure and to prove the efficacy of the compliant spine design. The optimal compliant spine designs obtained from the optimization procedure are fabricated, integrated into the ornithopters wing leading edge spar, and flight tested. Results from the flight tests prove the ability of the compliant spine to produce an asymmetry in the ornithopters wing kinematics during the up and down strokes.

Design optimization of a twist compliant mechanism with nonlinear stiffness

A contact-aided compliant mechanism called a twist compliant mechanism (TCM) is presented in this paper. This mechanism has nonlinear stiffness when it is twisted in both directions along its axis. The inner core of the mechanism is primarily responsible for its flexibility in one twisting direction. The contact surfaces of the cross-members and compliant sectors are primarily responsible for its high stiffness in the opposite direction. A desired twist angle in a given direction can be achieved by tailoring the stiffness of a TCM. The stiffness of a compliant twist mechanism can be tailored by varying thickness of its cross-members, thickness of the core and thickness of its sectors. A multi-objective optimization problem with three objective functions is proposed in this paper, and used to design an optimal TCM with desired twist angle. The objective functions are to minimize the mass and maximum von-Mises stress observed, while minimizing or maximizing the twist angles under specific loading conditions. The multi-objective optimization problem proposed in this paper is solved for an ornithopter flight research platform as a case study, with the goal of using the TCM to achieve passive twisting of the wing during upstroke, while keeping the wing fully extended and rigid during the downstroke. Prototype TCMs have been fabricated using 3D printing and tested. Testing results are also presented in this paper.

Design of Bend-and-Sweep Compliant Mechanism for Passive Shape Change

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