# Review of Adaptive Shock Control Systems
## 1 Introduction  
The reduction of greenhouse gas emissions is one of the most prominent challenges facing all transportation sectors today and especially so in aviation. To accomplish global objectives, such as those outlined in Flightpath 2050 [1], NASA's Environmentally Responsible Aviation (ERA) project [2], the German aviation strategy [3] and the European Green Deal [4], aircraft must be more efficient to reduce their emissions during operation. Aside from alternative fuels or weight reduction strategies, several other ideas and approaches to increase aircraft efficiency are under investigation including laminar flow wing technologies and turbulent skin friction reduction for reducing friction drag, and high aspect ratio wings for reducing lift-induced drag as summarized in [5]. Since transport aircraft usually travel at transonic speeds, wave drag generated by transonic shocks on the wing surfaces also provides a significant contribution to overall aircraft drag. Furthermore, the phenomenon of buffet can occur which can lead to structural vibrations and thus limit the flight envelop [6, 7, 8]. For a review of transonic shock buffet the reader is referred to [PERSON] et al. [9]. Research into shock control for transonic airplanes over the last few decades has been motivated by the possibility to reduce wave drag, as well as recent investigations into delaying buffet onset. The goals of shock control are to implement a device which reduces the shock strength and consequently wave drag and/or to delay the buffet onset and thus to reduce emissions, extend the flight envelop and/or reduce flow induced vibrations. While the drag reduction potential of SCBs has been investigatedover almost the past three decades [10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25], buffet control by SCBs has been investigated less extensively [6; 7; 8; 9; 26; 27; 28; 29]. For detailed research on buffet control via SCBs the reader is referred to [PERSON] [6] and [PERSON] et al. [7]. In 1992, [PERSON] et al. [10] introduced the concept of SCBs as a promising method to reduce the strength of a transonic shock, since such a local contour change in the vicinity of the shock can reduce the strength of the shock and thus has the potential to reduce the overall drag of an aircraft and therefore the overall fuel consumption and emissions. For a detailed review of aerodynamic investigations of SCBs the reader is referred to [PERSON] and [PERSON] [25]. The current review paper here is complementary to [25] in that focus is placed on adaptive concepts. Several substantial aerodynamic investigations into SCBs have been carried out among which the two European projects EUROSHOCK (1993-1995) [12] and EUROSHOCK II (1996-1999) [16] play a major role. In EUROSHOCK, passive shock control devices like perforated skins with cavities beneath them (porosities) have been investigated. Although these concepts show indeed a reduction in wave drag, they also lead to an increase in viscous drag and thus to an increase in total drag (on the investigated laminar wings) [12]. Therefore, in EUROSHOCK II adaptive systems have been analyzed like perforated strips and cavities with suction, ventilation or hybrid-control as well as physical contour bumps with and without suction. As with the results of EUROSHOCK, the results of EUROSHOCK II show a total drag increase for almost all variants of porosities. However, the physical contour bump was found to be very effective. In particular, an adaptive SCB which is variable in its height and in combination with a discrete suction upstream of the bump shows the highest potential for reducing both wave and total drag. Therefore, the recommendation for the use of adaptive SCBs to cover a wider flight envelope range is given. Another interesting finding of EUROSHOCK II regarding shock control is the positive effect of SCBs on the buffet boundary [16; 30; 31]. SCBs analyzed in EUROSHOCK II have a fixed location on the wing and thus are not variable in their positions.