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Airborne wind energy is an emerging technology that harvests wind energy with flight systems connected via a tether to the ground. In the project “EnerGlider”, a flying wing is meant to fly fully automated by its own control units. This contribution discusses the challenges to control and trim this flying wing during vertical take-off and landing under the influence of a horizontal wind velocity. High wind velocities can lead to unstable and untrimmed states concerning the longitudinal motion of the flying wing. The paper analyzes the influence of design modifications of thrust vector and elevon area to enhance the flight envelope of the trimmed states to higher wind velocities. Besides, the tether force as additional control unit is considered for strong wind forces. It is demonstrated that a tether force acting behind the center of gravity can significantly enhance the robustness of the flight system concerning wind velocity. Moreover, the unstable flight states emerging during vertical take-off and landing can be stabilized with a flight control.

The design of controllers for the automatic operation of airborne vehicles is challenging. Especially in the emerging research field of airborne wind energy, where tethered aircraft convert energy from wind at higher altitudes into electricity, the development of robust and safe control systems will be a critical success factor. Typically, these systems consist of one or more controlled aircraft, a ground station, and one or more tethers that transfer aerodynamic forces acting on the wings to the ground. To understand these systems and control them for real-time simulations, suitable tether models must extend the classical flight mechanical models. In this paper, a hybrid approach is presented, combining an elastic spring model with a catenary model of the tether. In particular, for real-time simulations in a model-in-the-loop environment where a static modeling approach is sufficient, this hybrid model is intended to provide an alternative to the commonly used spring models. Furthermore, such a static tether model neglects high-frequency dynamic oscillations that can occur with dynamic lumped mass models. As consequence, the focus is on primary influences such as stiffness and sag due to external loads. The developed hybrid model is analyzed and compared with the spring model for a simulated tethered flight. It can be concluded that this hybrid static model, although subject to strong assumptions such as symmetrical tether sag, represents the tether more accurately. For example, it allows a more precise design of the flight controller before conducting any flight tests. This model can be used as a reasonable alternative to the simple spring model whenever a static modeling approach of the tether is adequate.