Swinging Motion of a Kite with Suspended Control Unit Flying Turning Manoeuvres

Abstract. The flexible membrane kite employed by some airborne wind energy systems carries a suspended control unit capable of inducing a characteristic pitch and roll swinging motion during sharp turning manoeuvres. This paper assesses how accurately a two-point kite model approximates this swinging motion with two approaches: approximated as a transition through steady-rotation states and solved dynamically. The kite model comprises the rigidly linked point masses of the control unit and wing and extends a discretised tether model. The motion of the wing point mass is constrained to a figure-of-eight manoeuvre from the flight data of an existing prototype. The associated swinging motion of the kite is inferred from the attitude of the rigid link element. The computed attitude is compared against the measurements of two sensors mounted to the wing, which record varying pitch angles during the turns. The pitch and roll angles computed with the two approaches are similar during the straight sections of the figure-of-eight manoeuvre and match the measured angles within three degrees. Contrastingly, the two approaches exhibit systematic differences during the turns. Since a two-point kite model resolves the roll, the lift force may tilt along with the kite to drive turns. Hence, intricate centripetal force modelling is avoided, as seen in a single-point kite model. Furthermore, the two-point kite model complements the aerodynamic model as it allows computing the angle of attack of the wing by resolving the pitch. These characteristics improve the generalization of the kite model with little additional computational effort.