Tech Briefs

Composite Elastic Skins for Shape-Changing Structures

Anisotropic stiffness properties can be tailored for specific applications.

Composite elastic skins having tailorable mechanical properties have been invented for covering shape-changing (“morphable”) structures. These skins are intended especially for use on advanced aircraft that change shapes in order to assume different aerodynamic properties.

Many of the proposals for aircraft that could perform large aerodynamic shape changes require flexible skins that could follow shape changes of internal structures driven by actuators. Examples of such shape changes can include growth or shrinkage of bumps, conformal changes in wing planforms, cambers, twists, and bending of integrated leading- and trailing-edge flaps. Prior to this invention, there was no way of providing smooth aerodynamic surfaces capable of large deflections while maintaining smoothness and sufficient rigidity. Although latex rubber, silicone rubber, and similar conventional materials can be made into smooth coverings, they are not suitable for this purpose because, in order to impart required stiffness against out-of-plane bending, it would be necessary to make the coverings excessively thick, thereby necessitating the use of impractically large actuation forces.

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Metal Skeletal Layers Are Patterned to obtain desired properties — in this case, to make them easily stretchable in the vertical direction but not in the horizontal direction. To complete the fabrication of a composite skin according to the invention, these metal skeletal layers would be embedded in an elastomeric sheet.
The basic idea of the invention is that of smoothly wrapping an underlying variable structure with a smooth skin that can be stretched or otherwise warped with low actuation force in one or both in-plane direction(s) and is relatively stiff against out-of-plane bending. It is envisioned that a skin according to the invention could be stretched as much as 20 percent in a desired direction. Because this basic idea admits of numerous variations, the following description is necessarily oversimplified for the sake of brevity.

A skin according to the invention can include one or more internal skeletal layer(s) made of a metal or a suitably stiff composite. By use of water-jet cutting, laser cutting, photolithography, or some other suitable technique, regular patterns of holes are cut into the skeletal layers (see figure). The skeletal layers are thereby made into planar springs. The skeletal layers are embedded in a castable elastomer. The anisotropic stiffness of the skin can be tailored through choice of the materials, the thicknesses of the skeletal and elastomeric layers, and the sizes and shapes of the cutouts. Moreover, by introducing local variations of thicknesses and/or cutout geometry, one can obtain local variations in the anisotropic stiffness. Threaded fasteners for attachment to actuators and/or the underlying structure are inserted in the internal skeleton at required locations.

In one example typical of an important class of potential applications, the internal skeleton would be made less stiff in one in-plane direction. Such a skeleton would be desirable in an application in which the skin would be part of a hinge-like structure like a flap. In another example, the internal skeleton would be equally stiff in both in-plane directions, as would be desirable in application involving a planform change or a bump.

This work was done by Christopher M. Cagle and Robin W. Schlecht of Langley Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Materials category.
LAR-16599-1

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