Design and Fabrication of a Variable Stiffness Link for Use in an Unmanned Air Vehicle

A variety of methods have been introduced in recent years to implement morphing aircraft and other shape-changing structures, including high-force actuators that can elastically deform structural members, to stiffness-changing elements that more readily accommodate the deformations. This paper introduces a structural element of the latter type that can exist in two states: a rigid state that supports structural loads and locks the structure in place, and a soft state that enables deformation of the structure. The element itself is termed the “Smart Link” and is used in the folding mechanism of a morphing aircraft. The link is a composite made up of shape memory polymer (the softening material) and a backbone of shape memor alloy, which allows high deformation but ensures that the nominal shape is recovered upon return deformation. This paper presents the design requirements for the link in terms of the morphing aircraft application, and then presents the iterative design, fabrication, and testing process used to develop the Smart Link. Finally, results of testing of the link in the aircraft during wind tunnel tests are presented to show the pros and cons of its performance. In summary, the link met the mechanical requirements by performing multiple morphing cycles through the loads and deformation ranges (103 degrees of angle change) needed. The major drawback of the design is the response time necessary to transition the heat-activated material. Suggestions are given at the end of the paper for ways to improve the time response.