## 3 Flow Control Devices for Gust Load Alleviation
### Fluidic Actuators
#### 3.4.2 Circulation Control by Jet Blowing through the Trailing-Edge Coanda Device  
Circulation Control (CC) using Coanda effect uses tangential surface jets to change the aerodynamic properties of the aerofoil or wing. The Coanda effect describes the tendency of a high-speed jet flow staying attached to a convex surface due to the balance between centrifugal forces and low static pressures created by the high-speed jet [96]. The high-speed jet flow entrains the external flow to follow it as to 'bend down' over the curved surface which generates the circulation increase, and thus results in lift augmentation. Similarly, lift reduction can be obtained through placing the jet slots on the lower surface. Conventionally, a CC device system consists of an air plenum, a rounded trailing edge and an orifice which is the slot exit of the CC jet. The initial intention for the development of the CC system was for short landing and take-off capability, especially by the US Navy, looking for ways to improve aircraft operation from carriers [97]. Many tests including a full-scale flight test and design works have been carried out on the A-6 Intruder [98]. The effectiveness and efficiency of CC for manoeuvrability control of fixed and rotary-wing aircraft have also been researchedthrough various experiments and numerical studies. After the wind tunnel test on a diamond wing tailless aircraft, [PERSON] et al. [99] pointed out that the CC device exhibited good aerodynamic performance similar to a traditional flap with an equivalent size on a fixed wing under modest blowing momentum coefficients, and the response characteristic was essentially linear. Experimental and computational work seeking for a design using trailing-edge blowing to eliminate the trailing-edge flaps, or use leading-edge blowing to eliminate the need for leading-edge slats have been carried out on a Boeing 737 aircraft [97,100]. A joint project [101,102, 32] has been carried out by University of Manchester, Cranfield University and BAE Systems to demonstrate new technologies for flapless control, and a drone has been designed named MAGMA which finished its first flight trial in 2017. Instead of traditional control surfaces, this project assessed the manoeuvrability of two novel technologies. One is to deploy CC on the wing sections and another one is to use the fluidic thrust vectoring (FTV) placed on the centre body. Engine bleed air is used for the pneumatic supply of the CC and fluidic thrust vectoring effectors. It is shown that the critical flight conditions are climb turns and descending flight where geometric and effort/power saturation limits are met, respectively [103]. To investigate the capability of CC for rolling control on the flying-wing configuration, [PERSON] [104] extended a numerical study of CC as a roll effector on the generic SACCON UCAV configuration. This work was performed with a freestream Mach number 0.145 and was concluded that CC can produce similar rolling moments to flaps at low angles of attack. The effects for providing manoeuvrability by CC on a tailless vehicle was evaluated by [PERSON] et al. [105]. The results show that CC units could provide similar three-axis control effects relative to the split flap elevons. To explore novel method for gust load alleviation to keep pace with the fast development in control system designs using CC. The feasibility and effects of gust load alleviation by means of CC is firstly numerically studied by [PERSON] and [PERSON] [106]. The NACA0012 airfoil was chosen for the study from subsonic to transonic speeds. In this study, CC via steady blowing with different momentum coefficients are firstly tested for the gust load alleviation effects in terms of lift coefficients under a 'one-minus-cosine' gust. The results demonstrated that CC can effectively suppress the maximum gust-induced lift increment, but is not feasible for suppressing the unsteady gust-induced lift perturbations.