## 1 Introduction  
It is undeniable that air travel makes fast long-distance transportation possible and brings significant economic growth and improvement of quality of life. However, in the meantime, the negative impacts on the environment and climate [1] have become more and more pronounced and have posed a significant challenge to the aviation industry. For economic and ecological considerations, reduction in fuel consumption and exhaust emissions is very urgent task for future air transportation. For this purpose, the aviation industry is facing stringent 'Green Aviation' goals in their future product. For example, the primary goal of Europe's Right-path 2050 is to reduce CO\({}_{2}\) emissions of aircraft by 75% relative to 2005 levels [2]. In the US, the N+3 goal proposed by NASA is to reduce Nitrogen oxides (NOx) emission by up to 80% in the landing-take-off process and reduce fuel burn by 60% for an airliner entering service in 2030-2035 [3]. To achieve these objectives, a number of technologies, such as shock control [4; 5; 6; 7], laminar flow control [8; 9; 10; 11; 12; 13; 14], turbulent drag reduction [15; 16; 17; 18], as well as novel aircraft concepts, such as BWB or hybrid wing body (HWB) [19], 'double-bubble' [20], truss-braced wing (TBW) [21] and box-wing [22], have been proposed and investigated to explore a better aerodynamic performance. However, there are significant challenges in applying these technologies mentioned above on future aircraft, especially in terms of practical application. Meanwhile, with the increasing development and maturity in computational fluid dynamics (CFD) and aerodynamic design optimization, the aerodynamic efficiency of modern swept supercritical wings has almost reached its limit with diminishing return from large research investment. Therefore, it is urgent to explore game-changing technologies for drag reduction associated with the reduction in exhaust emissions. Load control is an important topic in aerodynamics, as it can potentially provide an alternative way for drag reduction through decreasing the aircraft structure weight. Figure 1 shows the relationship among flow control, load control, aerodynamic performance and energy efficiency. It is well known that the structure mass is not determined by the cruise condition but the critical load cases such as gust and manoeuvring loads. [PERSON] et al. [23] highlighted that the gust loads can be larger than the manoeuvring loads, and generate the most critical load cases that some aircraft will experience in flight. Figure 2 demonstrates the spanwise load distributions on a typical civil transport aircraft under cruise and gust encountering conditions, respectively. Compared to the cruise condition, the spanwise loading especially on the wing experiences a significant increase when the aircraft encounters a gust. This increase will affect the riding comfort of the passengers, and sometimes can be detrimental for the aircraft structure safety if the gust load is severe enough. For the safety of large commercial aircraft, airworthiness authorities have specified typical gust models as a requirement for the certification specifications of large commercial aircraft covered by European Union Aviation Safety Agency Certification Specifications (EASA CS-25) [24]. To cope with these critical load cases, aircraft structures need to withstand the forces and stress caused by gusts with a large amount of mass penalty, since it is challenging to build the structure that is both light and robust. However, if the load can be effectively alleviated through a control means, lighter structures may be used, resulting in the reduction in lift-induced drag and fuel consumption as shown in Figure 1. In the late 1970s, flaps, spoilers and ailerons were investigated to perform the additional task for gust load alleviation. Gust load alleviation (GLA) systems were studied and tested.