## 3 Aerospace Engineering Applications
### Aplication to Wing Morphing
For a flapping wing, a novel morphing wing mechanism was proposed by [PERSON] et al., and an SMA actuator was used to achieve a smooth change of the wing configuration to prevent aerodynamic losses [93]. After fabricating a prototype wing to demonstrate the morphing mechanism, the deflection angles at the trailing edge were evaluated according to various input currents. [PERSON] used SMA actuators to deflect the plain flap and control the flap angle of the aircraft wing [94]. He fabricated SMA-controlled aerofoil structures with flap and experimental investigation under wind tunnel conditions. In this work, he used a switching circuit to control the two sets of SMA wire actuators alternatively. [PERSON] and [PERSON] proposed an adaptive exhaust nozzle for jet engines featuring SMA actuators [95]. The open area of the exhaust nozzle was actively controlled by an SMA actuator. Figure 6: Conceptual design of the morphing laminar wing [85] Reproduced with permission from Elsevier under STM Permissions Guidelines. [PERSON] et al. reported control performance evaluation of a morphing wing with SMA actuators for small-sized and medium-sized unmanned air vehicles [96]. In this work, the camber line of an airfoil section was controlled by using resistive heating and cooling in the surrounding air of the SMA actuator. To investigate the effects of the proposed morphing wing concept, experiments were conducted under wind tunnel condition with three control methodologies: the conventional PID controller, PID controller with anti-windup compensator, and PID controller with robust compensator. [PERSON] et al. proposed morphing trailing edge design with SMA to achieve controllable deflection motion under the aerodynamic load [97]. After manufacturing a prototype of the trailing edge with four sections of aluminum alloy, deflection angle test was conducted, and precise control of the morphing wing was realized. [PERSON] et al. also reported in a review paper SMA applications to morphing aircraft. They emphasized variable twist of camber, reduction of power consumption, and actuation bandwidth [98]. [PERSON] et al. reported in an in-depth review paper about SMA applications, and they clearly summarized various studies in the aerospace field conducted up to that time [99]. They classified the parts where SMA was or was likely to be applied in aerospace engineering and summarized the research literature at the time for each part. [PERSON] et al. investigated the flapping system based on the SMA thin wires [100]. The camber of trailing edge in a civil regional transportation aircraft was morphed by using an SMA actuator. The numerical and experimental results showed that the SMA actuation mechanism can perform its role adequately in loads providing morphing conditions. [PERSON] et al. proposed, designed, and fabricated an SMA spring actuators-based morphing flap, and analytical and experimental investigation on its aerodynamic characteristics were conducted [101]. Two types of morphing flaps were proposed, and they were composed of three-element and six-element configurations. The proposed conceptual design for morphing airfoil is presented in Figure 7. [PERSON] et al. introduced a new design for a soft morphing actuation of pure twisting motion by using SMA wires [102]. The proposed actuator system consists of a pair of SMA wire actuators that were embedded in a polymer matrix at constant and opposite eccentricity across the cross section in opposite directions. It was demonstrated that there was an optimal actuator thickness for both the twisting angle and the twisting force of the actuator. In addition, for the thickness of the actuator, there was a trade-off between the twisting angle and force. The aerodynamic performances of a morphing winglet for an unmanned aerial vehicle (UAV) were investigated by [PERSON] [103]. The morphing winglet was designed by mimicking gliding birds' wing tip. The morphing winglet was fabricated by using a smart soft composite, which consists of glass fibers and SMA wires within a polymer matrix.