Trimming a rigid-wing airborne wind system for coordinated circular flights

Abstract

Airborne wind energy systems (AWESs) are tethered flying devices used for electricity generation. During the power-generation phase, the aerial component usually flies in a circular or figure-of-eight pattern. This paper examines the control surface movements required for circular flights in rigid-wing AWESs. In the absence of gravity, steady trim with equilibrium solutions can be achieved if the orbit plane is normal to the wind. The radius depends on how much the aircraft leans into the turn: leaning in reduces the radius and is statically stable, while leaning out achieves a larger radius but is unstable. For the latter case, artificial stabilisation can be done by cross-feeding the pitch and roll responses to the aileron. For circular trajectories that are not normal to the wind (i.e. experiencing out-of-plane wind), energy needs to be added to the system through the periodic forcing of a control surface. The correct timing of the forcing will excite the orbit's natural frequency, enabling full control of the circle centre and orientation for navigation in the 3D space. This can be done even in the presence of gravity, which is discussed in the second half of this paper. The aileron is the most effective control effector for forcing. Although the trimming method presented in this paper is only suitable for theoretical studies, it provides insights into the flight dynamics of rigid-wing AWESs and lays the groundwork for future flight control developments.