Our work is now published in the AFOSR special issue of the Journal of Composite Materials. Check out the paper here or the preprint at Arxiv here
Figure 1 (see the full paper for more info)
Overview
Morphing wings are a fascinating problem when it comes to designing compliant structures. In general, morphing allows for a wing to optimize the aerodynamics for a certain flight profile – for instance, increasing lift during takeoff, and reducing drag during cruise. Traditional wings achieve this through control surfaces like flaps, slats, and ailerons, and real demonstrations of morphing wings have been seen in fighter jets, some of which are able to increase wing sweep in the transonic regime. But, many of these wings rely on highly complex actuation mechanisms, which aren’t as feasible when dealing with smaller wings, around the scale you might see on a bird. This constraint leads to one of the key questions of the project: how can we design a continuous structure with variable mechanical properties, tailored specifically to the types of morphing we’d need in an avian-scale wing? After deciding to target a combination of three degrees-of-freedom: twist, camber, and extension/compression, and after many iterations, this wing was our solution to that question.
We’re also very interested in increasing the interconnection between materials and sensors for new types of soft robotic systems, and this lattice not only provides the desired mechanical properties for morphing, but it also serves as an excellent testbed for integration with our stretchable fiber-optic sensors.
In short, our lattice design process relied on a few main concepts, namely (1) conformal cell maps, (2) cell map warping, and (3) functionally-graded beam thicknesses.
The conformal mapping allowed us to match the shape of the lattice to the airfoil profile, as well as maintain a smooth surface profile to support the skin. Without this, the lattice design space would have been essentially rectangular, which would have proved impossible to maintain an aerodynamic shape.
From there, cell map warping allowed us to vary the mechanical properties of the lattice across the structure via the orientation of the beams within a BCC unit cell. Here, we relied on a fairly simple concept – if you stretch a unit cell in one direction (x, y, or z), along that direction, the beams will be more aligned to that direction, thus decreasing the compliance from axial loading and increasing compliance against axial loads and bending moments in the two orthogonal directions. However, if we extend this concept via a continual variation of the unit cell dimensions across the structure, this becomes very useful, particularly with this wing. At the root, we extend the unit cell dimension along the span, increasing the compliance in camber, and we decrease this at the tip to allow for improved extension/compression and twist.
Finally, the functional grading of the beam thickness allowed for fine-tuning of the mechanical properties for the desired compliance. Our tangent-continuity thickening field primarily served to increase the beam thickness at the root and decrease it at the tip, to help support the weight of the wing itself as well as any aerodynamic loading.
For more details on the sensor integration process and wind tunnel testing, refer to the full paper at the links above. Also, check out some additional figures and supplemental videos below:
Media
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Resources
For more info on the lab, visit the ORL homepage at https://orl.mae.cornell.edu/