# Review of Adaptive Shock Control Systems
## 2 Literature Review of Adaptive Concepts for Shock Control
### Shock Control Concepts Using Multiple Actuators or Adaptive Elements
#### 2.2.1 Fish-Mouth Actuator  
In 2000, [PERSON] et al. [68] proposed several possibilities to form an adaptive bump with SMAs. One of these concepts was introduced under the name fish-mouth actuator by [PERSON] et al. [69] also in 2000 and further described by [PERSON] et al. [70] in 2002, [PERSON] and [PERSON] [71] in 2003 and [PERSON] et al. [72] in 2004. The setup of one of these SMA actuators is demonstrated in Figure 5.  
The fish-mouth actuator consists of a composite spring, a carbon stiffener on top and a bundle of SMA wires in between, which are made from nickel-titanium (NiTi). These SMA wires are pre-strained and can be activated by heat (introduced by an electric current). Two copper clamps at both sides bind the spring and wires. Through their activation, the wires are contracted in longitudinal wire direction and deform the surrounding spring. The whole actuator is mounted on a metal plate to enable a deformation only in the upwards direction and consequently against an above mounted flexible skin to form the SCB. The actuators are placed next to one another in chordwise direction which is depicted in Figure 6.  
Figure 4: Variation in installation angle of the primary spoiler actuator based on [67].  
The composite spring fulfills several tasks including the conversion of the longitudinal wire contraction into a transversal movement as well as an amplification of this motion. Furthermore, the motion into the original actuator shape (before activation) is driven by the tension in the composite spring. Consequently, the SMA wires are only responsible for the formation of the bump while the spring realizes the movement back to the flat shape of the flexible spoiler skin.  
Actuator Geometry and Structural Design  
The following description is based on [PERSON] et al. [72] (2004). Over 400 fish-mouth actuators were manufactured for a wind tunnel model of which 84 have been implemented into the experimental model. With a chord length of 400 mm and a desired maximum bump height of 0.5% chord, the desired maximum stroke of one actuator is 2 mm. The stiff part of the actuator has a length of 30 mm, and with a total width of the composite spring of 50 mm this leaves 10 mm on both sides to form variable parts of the composite spring. The actuator height amounts to 8 mm (including 1 mm thickness of the laminate on top of the actuator). This geometric setup is depicted in Figure 7.  
Figure 5: Structure of a fish-mouth actuator with shape memory alloy (SMA) wires based on [71,72].  
Figure 6: Arrangement of several fish-mouth actuators with shape memory alloy (SMA) wires in a spoiler of an aircraft wing based on [71,72].  
Figure 7: Dimensions of the fish-mouth actuator spring based on [72].  
The NiTi-SMA wires feature a diameter of 0.392 mm and an austenitic start temperature of 88 \({}^{\circ}\)C. With this configuration a force of 200 N is used in the final design configuration as well as a transverse stiffness of 70 N/mm. The maximum actuator force (blocking force) amounts to 240 N [72].