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Modeling and Design of a Tensegrity-based Morphing Wing

  • Author(s): Pham, Nguyen Kim
  • Advisor(s): Peraza Hernandez, Edwin A.
  • et al.
Abstract

This thesis presents a modeling and design exploration study of a novel twisting wing whose motion is enabled by a tensegrity mechanism. First, the aerodynamic characteristics of the twisting wing, which does not require control surfaces to modulate its shape, are compared with those of a conventional wing having a control surface. It is shown via computational fluid dynamics analyses that the twisting wing displays higher lift-to-drag ratio than the conventional wing and hence the twisting wing is more aerodynamically efficient. Subsequently,the torsional tensegrity mechanism, composed of multiple tensegrity cylindrical cells forming a column along the wingspan, is described. A finite element model of the wing incorporating this mechanism is developed. Using the model, a full factorial design of experiments study of the influence of the topological parameters of the torsional tensegrity mechanism on the twist angle, mass, and stress in the different components of the wing is performed. A wingspan of 142.24 cm and a chord length of 25.31 cm are assumed, corresponding to those of the Carl Goldberg Falcon 56 Mk II R/C unmanned aerial vehicle. For a wing of such dimensions, the maximum achievable twist angle from root to tip per unit mass without any component exceeding their allowable stress is 5.93◦/kg and delivered 13.5◦of twisting with-out violating any of the material failure stresses. This torsional deformation is sufficiently large to allow for effective modulation of the aerodynamic characteristics of the wing. The torsional tensegrity mechanism for this design consists of eight cylindrical cells and four setsof actuator wires along the circumference of each cell. Subsequently, a design of experiments study performed using the orthogonal array (a.k.a.Taguchi) method is conducted to evaluate the effects of varying design parameters such as the thicknesses of the skin and ribs,the wire diameters, number of cells and wire sets, spar diameter, and material types. The most favorable design with a maximum twist angle to mass ratio of 10.85◦/kg is presented and delivered up to 19.5◦of twisting without violating any of the material failure stresses.Finally, befitting for the actuation of the tensegrity mechanism due to their wire form, shape memory alloy (SMA) wire actuators are explored for the reconfiguration of the wing shape through thermally driven material actuation. The combination of lightweight and compact tensegrity mechanism and SMA wire actuators eliminate the need for bulky components,such as hydraulic and electric actuators, to enhance the flight performance. The torsional morphing capability enabled by SMA actuation is demonstrated experimentally through a tensegrity twisting wing prototype integrated with commercially available SMA wire actuators. An additional orthogonal array design of experiments study is performed on the wing model with incorporated SMA wire actuators, where results analogous to those obtained from artificial strains in the aforementioned studies are obtained. The most favorable design of this DOE delivered a substantial 15.85◦total twist angle without any material failure and had a mass of 2.02 kg. The obtained twist angle was comparable to the twist angle observed from the testing of the tensegrity twisting wing prototype with integrated SMA wire actuators.

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