Aircraft structural morphing using tendon-actuated compliant cellular trusses

The research described in this paper 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 can 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. Highly distributed actuation also enables the achievement of global shape changes as the accumulation of local ones, while the use of compliant joints rather than true rotating joints eliminates binding as a significant concern. A six-noded octahedral unit cell with diagonal tendon actuation is developed for a bending-type deformation in the wing. Initial cell geometry 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 lb (1360 kg). The weight of the truss wing was comparable to a conventional stiffened skin construction, although its deflections are larger. Aeroelastic concerns can perhaps be addressed through the use of active control. Several ideas for a skin, required to transfer the aeroloads to the underlying structure, are presented.