Design of a Flexible Skin for a Shear Morphing Wing

Abstract

This article focuses on flexskins comprising of a cellular substructure and pretensioned facesheet for shear morphing applications. The unit cell of the substructure is a strand with some strain-relief feature, and it supports a segment of pretensioned facesheet. The function of the strain-relief feature is to reduce the peak strains in the strand and consequently the actuation work during shear morphing. Central hexagonal cells, elliptical cells, and half elliptical cells in the strand provided the sought strain relief at the edges of the strand, but the sharp corners and the proximity of these central features to those from adjacent strands during shear morphing led to a tendency for the facesheet to wrinkle. Alleviating wrinkling by increasing facesheet pretension increases the morphing actuation work requirement. Facesheet wrinkling can also be reduced by increasing strand separation, but this too requires higher prestrain in the facesheet to limit out-of-plane displacement under aerodynamic loads. The best solution was found to be the use of Gaussian- or Cosine-shaped curved strands which reduced morphing actuation force requirements by around 30% compared to straight strands used on a previous demonstration aircraft, and the peak strain levels to about 1.2% (down from 3.3% for straight strands), while avoiding facesheet wrinkling. Going from a unit cell to a finite strip accounting for boundary effects it was observed that the curved strands near the rigid boundaries of the skin panel come very close to the boundaries at high morphing angles, promoting wrinkling in the facesheet. A gradient reduction in the amplitude of the curved section along the length of a strip of flexskin approaching the problematic boundary alleviates this situation. Other approaches examined prior to the adoption of the smooth curved strands, such as selective bonding of facesheet to the strands, varying the strand thickness, or offsetting the central strain relieving feature between successive strands, were unable to eliminate facesheet wrinkling at shear morphing angles.