Aircraft Structural Morphing using Tendon-Actuated Compliant Cellular Trusses

Abstract

Recently, smoothly-deforming aircraft structures have been investigated for their ability to adapt to varying flight conditions. Researchers aim to achieve large changes in the shape of the wings: area changes of up to 50% and aspect-ratio changes of up to 200% are being pursued. 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 truss members of the unit cell 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 releasing another set. The tendonactuated compliant truss can be made to behave locally, and temporarily, as a nearmechanism, by releasing appropriate cables. As a result, in the absence of aerodynamic forces, the structure can be morphed using relatively low forces. The cables are reeled in or released in a controlled manner while the structure is loaded, hence, the stability of the structure can be maintained in any intermediate position. Highly-distributed actuation also enables the simultaneous 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 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. A finite element analysis is performed on a wing made of these unit cells and sized for a representative UAV weighing 3000 lbs. The areas of the individual truss members are sized so that they don't fail or buckle under the air loads, while deflection at the wing tip is reduced. The octahedral unit cell is capable of achieving smooth deformations of the truss structure. The cell size is dictated by the available space and the morphing strain. The cell sizes are reasonable for strains on the order of 10% to 15% and get smaller for larger strains. Additional cell shapes are being investigated for larger area changes through a process of topology optimization using genetic algorithms. Numerous other technical challenges remain, including the details of actuation and a robust skin. Nomenclature B= kinematic matrix d = vector of displacement of joints e = vector of elongation of members l = length of truss member SR = Stress Ratio, allowable stress/actual stress,

LPV control of a tensegrity-membrane system

“…It should be pointed out that shape changes of tensegrity-membrane systems can be achieved using the tendon control technique, which alters tendon rest-lengths to change tendon forces. The tendon control technique has been used to control several structures such as tensegrity structures [14][15][16] and cellular trusses of aircraft morphing wings [57]. One way of implementing this technique in practice is to employ the motor-pulley-cable system.…”

Aircraft morphing wing concepts with radical geometry change

“…Tendons as active elements in each unit can pull the units into new configurations that will spring back when the tendon tension is released. The large shape changes of wing can be achieved by series of highly distributed shape memory alloys actuation (Ramrakhyani et al 2005).…”

Investigation on adaptive wing structure based on shape memory polymer composite hinge

“…Several morphing concepts for aircraft wings have been proposed [5][6] to change chord and span .Adaptive materials, such as shape memory materials, can be used to accomplish this purpose efficiently. Structural health monitoring of the wing can using the Extrinsic Fabry-Perot Interferometer and Fiber Bragg Grating sensors [7].…”