Airborne wind energy is an evolving renewable energy technology with the potential to reduce material as well as energy investments and to harvest wind resources that have so far not been accessible. Ground generation systems use the pulling force of a kite to generate a linear traction motion driving a drum connected to a generator. To understand and quantify the aerodynamic performance of a soft kite we developed a sensor system that measures the relative flow conditions on a flying, highly flexible wing. Together with ground-based measurements of traction force, the aerodynamic efficiency of the kite can be computed. The experimental data can be used for the validation of currently available kite models. The lift-to-drag ratio is mainly affected by the power setting or trim of the kite and the steering commands. For the first time the effect of the trim control on the kite’s steering capability was quantified for a leading edge inflatable kite. We found that a trim position with higher power ratio which results in a higher angle of attack increases the steering agility of the kite. The attained yaw rate increased by 40%. For optimization of future power kites it is important to understand the mechanisms affecting a kite’s performance.