Design and analysis of a configuration-based lengthwise morphing structure

Abstract

The ability of changing aircraft wing shape or geometry has drawn great attention of researchers in the past decades. This paper proposes a new approach to design a lengthwise morphing structure for an airfoil morphing wing. The structure is composed of n planar mechanism units. Each unit is designed to have 3 degrees-of-freedom (DOFs) that can be fully controlled by two actuators using lockable joints. This structure is reconfigurable through dual mode switching by locking specific joints. The end-effector of the mechanism can reach any desired position and the morphing wing can change to any desired shape. When locking three or four actuators, the structure turns into a statically determinate or indeterminate truss structure to withstand required loads. Force analysis for the structure during morphing motions is conducted based on statics, and an optimization method is used to determine the appropriate number of actuators and their placements. Prototypes with or without skins are fabricated to verify the feasibility of the design.

Introduction

Morphing wings have drawn great attention of researchers in the past decades. The performance of an aircraft could be improved by adjusting the geometry of the wing in various flight conditions. The idea of changing the shapes of the wings is not new. Wright brothers adopted the wing warping, which consists of a system using pulleys and cables, to twist the wings in 1903.

Morphing concepts can be categorized into [1,2]: planform alteration (span [3], [4], [5], sweep [6,7], and chord [8,9]), out of plane transformation (twist [10], [11], [12], dihedral/gull [13], [14], [15], span-wise bending [16]), and airfoil adjustment (camber [17], [18], [19], [20], [21] and thickness [22]). The morphing structure is an essential part for a morphing wing. In [23], a structure with six morphing solutions was designed using a series of 6-degree-of-freedom (DOF) Gough-Stewart parallel mechanisms. Yu et al. [24] proposed a 6-DOF parallel mechanism enclosed by rigid sliding panels, which can be applied to morphing wings. Neal et.al [25] built a 7-DOF aircraft model whose wing can undergo span, sweep and twist motions. Rotational and linear actuators were adopted to drive the wing.

This paper focuses on the design of the structure that can change the shape of the airfoil. Morphing structures include compliant [26], [27], [28], [29], [30], [31], tensegrity [32], and rigid structures [33], [34], [35], [36]. Heo et al. [26] investigated three compliant structures, including chiral, regular and re-entrant hexagonal honeycombs, that can be applied to a morphing airfoil. Chiral structures were also adopted in [27], [28], [29] and lattice materials were utilized in [30]. Ramrakhyani [31] used a compliant cellular truss, actuated by tendons, to adjust the geometry of the morphing structure. A network of tensegrity structure was proposed to construct a morphing wing [32], whose actuation force was optimized to change to the desired shape. Zhao et al. [33] proposed a synthesis approach for rigid planar 1-DOF mechanisms that can accomplish shape-changing of the airfoil. A variable camber structure consisting of crank slider mechanism units was introduced in [17]. Moosavian et al. [34] adopted a network of four-bar linkages as the core of the morphing wing, and dimensional synthesis was carried out to obtain the parameters of the links to adapt to the desired curve. Four-bar linkages were also used in [35].

However, a morphing aircraft structure should be able to not only change shapes, but also withstand loads. Hence, a geometry variable truss is preferred. Inoyama et al. [36] constructed a morphing structure using frames, truss, telescopes, actuators, attachments and joints. In [37], an adaptive truss with internal linear actuators was also designed. However, these models either have complicated structures and need lots of actuators or cannot accomplish all the poses needed.

This paper is to propose a novel approach for the design of morphing structures that could be applied to morphing wings to change the shape of the airfoil. The lengthwise spatial structure proposed is comprised of n planar mechanism units. The unit is an under-actuated unit with 3-DOF and fully controlled by two actuators. Reconfiguration is achieved by means of switching between two modes with 2-DOF freely and the structure can accomplish different motions in each mode. In this way, the obtained morphing wing can have any desired shape. When locking three or four actuators, the structure turns into a statically determinate or indeterminate truss structure to withstand required loads. Force analysis for the structure during the morphing process is conducted, and an optimization method is used to find the appropriate number of actuators and their placements.

This paper is organized as follows: Section 2 focuses on the approach to design an under-actuated morphing structure. In Section 3, force analysis for the structure in the morphing process is conducted and the placement of actuators is determined by optimizing configurations. Section 4 provides the control strategy of the under-actuated structure. Prototypes are also fabricated. Finally, conclusions are drawn.