# Review of Adaptive Shock Control Systems
## 1 Introduction
This means that these SCBs are optimized for one single design point and therefore fixed in their location and shape. Although such SCBs decrease drag at the corresponding design point, they can increase drag at off-design conditions. During the period of the EUROSHOCK projects, research focused on robust but fixed SCB designs that reduce drag over a wider range of flight conditions since adaptive concepts did not have the capability as adaptive systems show today. However, nowadays adaptive systems have reached a high technological readiness and the implementation in real aircraft has been demonstrated. This is why adaptive SCBs that can vary their height and position offer substantial potential, since morphing bumps can mitigate aerodynamic penalties at off-design conditions. Adaptive SCBs are one type of morphing wing technologies [32; 33; 34; 35; 36; 37; 38; 39; 40; 41] which also include, e.g., morphing leading edges (droop nose) [42; 43; 44; 45; 46; 47; 48], morphing trailing edges [30; 49; 50; 51] or morphing winglets [52; 50]. In addition to two-dimensional (2D) bumps, the potential of three-dimensional (3D) SCBs for reducing drag and/or delaying buffet onset has been investigated [6; 7; 22; 26; 27; 53; 54; 55; 56; 57]. A comparison between 2D and 3D bumps is given by [PERSON] and [PERSON] [58]. However, most of the adaptive SCB concepts presented in this review do not focus on 2D or 3D shaped bumps in particular but often the concepts could be used for both applications. These results of the EUROSHOCK projects have been a main driver for the investigation of SCBs for wave drag reduction in the past two decades. This review aims to provide an overview of several adaptive shock control concepts including approaches that mainly use the spoiler actuator, concepts that use several smaller actuated elements, passive skin deformation due to pressure differences in the vicinity of the shock, as well as using plasma for shock mitigation purposes. The goals of an adaptive shock control concept are to not only mitigate wave drag as well as overall drag but also to lower overall fuel burn. This is why, for example, the weight of the integrated adaptive system must not be excessively heavy, since otherwise the positive effect of drag reduction is compensated by the additional fuel needed for the increased weight. In general, the challenge is multidisciplinary whereby any added weight and complexity need to be offset by a sufficiently large aerodynamic benefit in order to achieve a net top-level benefit. This paper summarizes adaptive SCB concepts and compares them in terms of their advantages and challenges with respect to their structural and system integration. Focus is given to whether these systems are able to adapt the bump location and its height, and if so, how that is achieved. Although a brief review of their aerodynamic effectiveness is sometimes given, the advantages and challenges regarding their systemic realization and structural design are the main focus of this work. Section 2 provides a literature review of adaptive SCB concepts and Section 3 presents a discussion in which the advantages as well as the challenges of the respective concepts are outlined. Furthermore, a comparative table of the introduced concepts is also presented. Section 4 draws a conclusion in terms of the practical applicability of the concepts and an outlook for future investigations into SCB design is given in Section 5.