Stiffness Tailoring of a Morphing Multistable Air Duct

The state-of-the-art approach to achieving large geometry changes in structural systems which are subject to high loads is to use conventional mechanisms connected to discrete actuators. The combination of conventional mechanisms, consisting of members connected by hinges, and discrete actuators can result in a heavy structure. Alternatively, morphing structures can create lighter weight solutions based on structural flexibility and distributed actuation [1, 2]. In the design of morphing structures there are conflicting requirements between a structure’s load carrying capability and shape adaptability. A structure solely designed to be load bearing will deform by a small amount under loading whereas a morphing structure requires large displacements. Due to these conflicting requirements it is not practical to generate a morphing design by simply applying external loads to a conventional structure. Instead a morphing structure must be compliant to actuation forces but stiff to external forces. The three conflicting requirements of load carrying capability, deformability, and low weight imposed on a morphing structure present a difficult design problem. This morphing concept evolved from the task of designing an air duct which could be integrated into an aircraft’s external aerodynamic surface with the ability to be both opened and closed, see Figure 1. The geometry of the air duct is based on the NACA duct, a common form of low drag air duct design. NACA ducts allow air to be drawn into the aircraft with minimal flow disturbance and avoid the flow separation and form drag that can occur with protruding air ducts [3]. In its closed state the air duct is flush with an external aerodynamic surface in order to minimise drag. The air duct is able to maintain this flush geometry without a holding force from an actuator whilst being subject to aerodynamic loads. Once actuated the air duct moves into its open shape and can remain in this ‘locked’ state without further actuation. For this application, multistable structures are good candidates to be used as a basis for the morphing design because they have ability to remain in equilibrium in at least two predefined shapes and achieve large out-of-plane deformations [4, 5]. Actuation is only required to transit, ‘snap-through,’ between these stable states. There are many ways structural multistability can be achieved. By far the most reported way has been through the use of non-symmetrically laminated composites which exhibit multistability when cooled from an elevated curing temperature to room temperature during manufacture [6, 7]. Although this is a relatively simple way to achieve multistability, the main problems with these types of laminates are that they typically have low snap-through loads, restrictive (often cylindrical) stable geometries, and they are also hygrothermally sensitive [8]. Ways to combat these problems include the use of fibre prestressing techniques in composites [9], Gaussian curvature [10], or plastic deformation in isotropic materials [11]. However, it is still thought unlikely that such structures would be able achieve the required geometric changes or withstand the aerodynamic pressures the air duct will be subject to. A novel form of multistable composite structure is presented in this paper which derives multistability through a combination of bending stiffness tailoring and prestress [12]. The mechanism by which multistability is achieved is described with both analytical and finite element models. A morphing air duct is manufactured and tested as a proof-of-concept demonstrator. The air duct is shown to have high stiffness in its open and closed states but a much lower stiffness when transiting between these stable states. The shape changing behaviour of the demonstrator is also shown to behave as a one degree-of-freedom system enabling simple lightweight actuation solutions, such as inflatable bladders, to be feasible.