36 A. Holness et al. Multifunctional structure-energy harvesting-power concepts, once completely developed, may have an enormous impact on the field of UAVs because of the current energy limitations that exist. Once energy sources are no longer limited, the field of UAVs, especially MAVs, should be able to expand exponentially. By exploiting recent advances in flexible electronics, including thin film batteries and thin film solar cells, as well as multifunctional structure, it can be possible to develop multifunctional skin materials for aerospace applications. These skins will have the capability to harvest energy and store it. The electronic circuits for performing voltage conversion will be incorporated into the skins. These skins will be capable of delivering the desired structural properties. These multi-functional skins typically need to be 1–4 mm in thickness. The envisioned skins will be useful in a large number of applications for the Air Force, particularly fixed morphing wing air vehicles. Realizing multifunctional skins is challenging due to the following reasons: • Many different underlying building blocks (e.g., flexible circuits, thin film batteries, thin film solar cells, and structural materials) need to be integrated together to realize the multifunctional compliant skins. These building blocks need to be compatible with each other and a low-cost fabrication process for integrating them into a single sheet of skin material. We will need to systematically investigate performance and compatibility of the underlying building blocks and determine the feasible combinations. We will also need to identify the right types of interfaces between building blocks for proper functioning of the skin and a feasible fabrication process for realizing them. • Multifunctional skin materials will be highly heterogeneous in nature. We will need to characterize their performance to figure out their behavior. We will be interested in knowing how these materials will deform under aerodynamic loading. We will also be interested in knowing how the loading will influence energy harvesting and storage efficiency. We will also need to characterize how thermal interactions among components during the operation will affect the performance. Multifunctional materials will be utilized in complex aerospace structures [1–7]. Spatial variation in material property can be beneficial to match the skin response to the desired behavior. Spatial variation in material properties can be achieved by figuring out the right placement of the building blocks within the skin. We will need to develop computationally-efficient synthesis tools to design skin materials with the right material property variations. 6.2 Multifunctional Materials Research Related to Flexible Wings for FWAVS 6.2.1 Integration of Power Systems into Morphing Wings Power systems for morphing wings largely consist of electrical sources. For electrical based systems, the primary consideration is the energy density of the storage material, and the replacement of energy by energy harvesting materials, such as piezoelectrics and solar cells. However, energy sources must be able to deliver energy at power levels when needed and at desired levels. Battery materials have been the desired power source for electrical systems. They can be used to drive highly efficient motors (>80% efficiency), to directly morph wing structures, or indirectly through pneumatic/hydraulic systems. However, they are largely considered independent of the morphing wing, and have not been fully integrated into them due to their stiffness. As a result, new flexible electronic concepts using thin film battery and solar cell materials are allowing for the direct integration of materials for energy systems into morphing wing structures. These require new manufacturing approaches, such as the additive manufacturing approach seen in Fig. 6.1. Recently, solar cells have been added into the highly deformable wings of a flapping wing air vehicle (FWAV) known as “Robo Raven” [8–11] (Fig. 6.1). Robo Raven is a unique FWAV with wings that are independently controlled by programmable servo motors, which allows for dynamic shape control. The deformed wing shape and aerodynamic lift/thrust loads were characterized throughout the flapping cycle using 3D Digital Image Correlation (DIC) and a novel test stand we developed in order to understand the wing mechanics. A multifunctional performance analysis was also developed to understand how integration of solar cells into the wings influences flight performance under two different operating conditions: (1) directly powering wings to increase operation time, and (2) recharging batteries to eliminate need for external charging sources. Experimental data was used in the analysis to identify a performance index for assessing benefits of multifunctional compliant wing structures. The resulting platform, Robo Raven III, was the first demonstration of a robotic bird that flew using energy harvested from solar cells (Fig. 6.2). It was also determined from experiments that residual thrust was correlated to shear deformation of the wing induced by torsional twist, while biaxial strain related to change in aerodynamic shape correlated to lift. Furthermore, it was also found that shear deformation of the solar cells induced changes in power output directly correlating to thrust generation associated with torsional deformation. Thus, it was determinedthat
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