Morphing aircraft structures using tendon actuated compliant cellular truss

by Ramrakhyani, Deepak Shyamlal.

iii The research described in this thesis aims to develop a structural concept capable of achieving continuous stable deformations over a large range of aircraft shapes. The basic concept underlying the approach is a compliant cellular truss, with tendons used as active elements. The members of the truss are connected through compliant joints such that only modest bending moments may be transmitted from one member to another. Actuation is achieved by pulling on one set of cables while controlling the release of another, so that the stability of the structure is maintained in any intermediate position. The tendon-actuated compliant truss can be made to behave locally, and temporarily, as a compliant mechanism, by releasing appropriate cables. As a result, in the absence of aerodynamic forces, the structure can be morphed using relatively low forces. A parallel genetic algorithm is developed for the design of unit cells using topology optimization. A “fitness” value is assigned to each candidate structural layout and it is a measure of how well the structure meets the design requirements. The fitness function was formulated to include the morphing capabilities on actuation of the cables, as well as the stability of the structure under external loads. The forces in the cables are optimized to obtain the best match between the morphed structure and the required configuration. A six-noded octahedral unit cell with diagonal tendon actuation was obtained for a bending type deformation of the NASA HECS wing. Initial cell geometry and orientation is determined by “strain matching” to the local morphing deformation required by the application. The cell size is dictated by the available space, the morphing strain, and discretization errors in approximating a smooth desired shape. A finite element analysis is performed on a wing made of these unit cells and sized for a representative vehicle weighing 3000 lbs (1360 kg). The weight of the truss wing (without the skin and actuators) was comparable to a conventional stiffened skin iv construction although its deflections are larger. Aeroelastic concerns of flutter and divergence can perhaps be addressed through the use of active control. Additional cell topologies using fewer cables are also obtained. A procedure is presented to get an estimate of the size of the compliant joint made of pseudoelastic shape memory alloy given the loads. A compliant joint was designed for a smaller vehicle weighing about a kilogram. A skin system is required that can accommodate large shape changes while carrying and transferring aerodynamic loads. The skin could be designed either using high strain-capable materials or sliding skins could be used. An analysis on the scaling of actuator requirements with aircraft size showed that, if actuation of the aircraft structure involves the morphing of the wing against the lift forces or if the actuators are in the load path, then the weight of the aircraft can be limited to something in the range of a few kilograms to a few thousands of kilograms. These analyses show that a 3D cellular truss structure can be fairly complex with a large number (18) of cables per unit cell and the area change possible using these structures is limited. Even in-plane actuation of the truss structures involves high actuation forces in the cables resulting in a high actuator weight. This can be addressed pin/compliant joint connected beams to carry the lift loads and the actuating cables then act only against the in-plane loads which are much lower. The genetic algorithm can easily be modified to design such structures. Preliminary results are shown using the algorithm and suggestions are made to improve the results. This investigation into smoothly morphing aircraft structures has yielded insight into the problem, design procedures and a powerful tool for the design of morphing aircraft structures using cables, truss elements and beams. There are still many challenges to be addressed before they become practical.