Most airborne wind energy systems use a single tether to connect the kite with the ground station. Some concepts use an additional bridle line system to transmit the aerodynamic force from the wing to the tether. The use of a bridle line system is particularly important for inflatable membrane kites, which is often complemented by a robotic control unit that is suspended below the wing and used for steering and changing the angle of attack of the wing. When flying sharp turning maneuvers during wind energy harvesting, kites with such a control unit can exhibit a characteristic pitch and roll swinging motion. This thesis investigates the effect of this motion on the steering behavior of the kite and its performance. The work uses flight test data of an implemented airborne wind energy system and several mechanistic models to identify the aerodynamic properties, steering behavior and performance of the kite.