Performance Augmentation of Compliance-Based Morphing Wings through Optimization and Nonlinearity

Ashwin Clement Henry, Purdue University

Abstract

Conformal shape adaptation presents significant benefits in terms of increased aerodynamic efficiency for a wide range of flight conditions in addition to minimizing drag penalties via gap-less smooth shape profile change. Conformal shape adaptation in morphing wings can be achieved by distributed compliance and stiffness selectivity techniques. These morphing strategies yield light-weight monolithic structures by utilizing the multifunctionality offered by smart materials. The present thesis, therefore, attempts to enhance the performance of compliance based morphing wings by augmenting the stiffness characteristics through optimization of associated smart actuators and implementation of stiffness selectivity. These performance augmentation methods essentially attempt to improve the trade-off between compliance and load-bearing capacity. Compliance based morphing wings are considerably flexible and therefore the non-linear interactions between the structural system and aerodynamic system are non-negligible. A concurrent aeroelastic analysis method applicable to 3D morphing wings is developed to obtain realistic information regarding the aerodynamic performance and stability. A two-way coupled static aeroelastic method is devised linking the structural analysis solver, Abaqus/Standard and a low fidelity aerodynamic code, XFOIL. The principal limiting factor of the maximum achievable flight speed of a compliant wing design is the dynamic aeroelastic instability or flutter onset, therefore a flutter analysis technique is implemented. A linearized flutter analysis method based on quasi-steady aerodynamics and assumed mode shapes is employed to predict the flutter point. These aeroelastic methods can be conveniently included to seamlessly run an optimization procedure, owing to their efficient run-time. Distributed actuators employed in compliance based morphing wings are inherently multi-functional by simultaneously contributing to the actuation and load-carrying functions. These characteristics are particularly useful for increasing the performance of compliant-based morphing structures. The first section of this thesis research presents an investigation of the optimal structural parameters of distributed piezoelectric actuators and wing skin on a compliant morphing wing for maximizing the performance, measured in terms of rolling moment and dive speed. A previous design obtained following a multi-disciplinary optimization technique, yielding the ideal structural and geometrical parameters maximizing roll controllability, is utilized as the baseline individual. The global design of the morphing, however, did not optimize explicitly the size of the distributed piezoelectric actuators and thickness of the underlying skin. This investigation focuses on further extending the design space by perturbing the original optimal individual with the goal of maximizing the obtained rolling moment and flutter speed while minimizing the wing's mass. A multi-disciplinary, multi-objective optimization for the ideal actuator width and thickness active-to-substrate ratio. The results show that the performance of the baseline design can be significantly improved through width and thickness distribution optimization of the bi-morph piezoelectric actuators. The maximum increase in rolling moment achieved is 27.67% along with a 4.31% mass penalty. When constraining the mass, a potential increase of 25.17% is still possible. Furthermore, flight speed is increased by 83.4%, while maintaining sufficient roll control authority. The three optimization objectives are competing, with opposite stiffness requirements for maximizing rolling moment and flutter. Similarly, increasing flutter speed carries a linear proposal increase in mass. The results suggest that to realize significant performance enhancement without substantial mass penalty, the load carrying capability of the actuators should be exploited. The main difficulty in morphing wing design is the inherent conflicting requirement of achieving compliant structures with adequate load-bearing capability. The flight envelope of the compliance-based morphing wings is limited by virtue of this opposing requirement. The flight boundary can be possibly improved to some extent by decoupling these contradictory requirements by imparting stiffness variability characteristics to the compliant wings. The idea is to attain a flexible configuration which allows greater maneuverability and a stiff configuration when faster flight speeds have to be achieved. The second section of this thesis focuses on exploiting the selective stiffness properties obtained from bistable composite plates, by monolithically embedding them in a morphing wing. A morphing rib section previously designed utilizing a multi-disciplinary optimization procedure is selected as the foundation, employing which a 3D morphing wing is designed. The goal of this section of the thesis is to analyze the performance and validity of stiffness variability under strength constraints inherent in the 3D wing design. A passive morphing wing characterized by discrete stiffness variability, with two distinct equilibrium configurations each possessing different global stiffness response, is developed. The morphing wing layout corresponding to distributed compliance design is adopted to maximize the impact of the embedded bistable laminates. The selectively compliant retains stiffness variation characteristics with a global stiffness variability ratio of 7.2 between its stiff and flexible configurations, subjected to buckling and strength constraints. The results presented thus proves the feasibility of selectively compliant morphing strategy which decouples the conflicting stiffness requirements.

Degree

M.S.A.A.

Advisors

Yu, Purdue University.

Subject Area

Aerospace engineering|Mechanical engineering

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