General comments

The work looks conceptually interesting and non-trivial. It can help design flight controllers for rigid-wing airborne wind energy systems. When designing a flight controller, insight into the trim conditions is very useful because it allows for designing a feed-forward controller, which then still needs to be combined with a feedback controller. However, having a feed-forward controller already in place is already half of the work. Furthermore, the efficiency of different types of control surfaces is analysed, and for the given plane, the ailerons are found to be the most effective control surface. A weak point is that it is unclear if this finding can be generalized to other wings. Modelling the trim conditions without gravity is a reasonable first step to isolate the aerodynamic and inertial effects. Modelling with gravity was included, but not Included was a simulation where the wing is always flying above the ground. Additionally, a weakness is the reliance on proprietary software to simulate the tethered wing. This makes it difficult to reproduce the results.

Specific comments

Good

- To the best knowledge of the author, this is the first paper that has a focus on the trim conditions of a tethered wing.

- A method for achieving dynamic trimming is described.

- The description of the tether model contained a small detail that was interesting: The use of cosine spacing for the point masses of the model.

- The appendix is useful to better understand the numerical approach that was chosen.

Room for improvement

- The conclusion that the aileron is the most effective control surface is supported by one specific set of numbers. But it is not clear, for example, if the areas of the aileron and the elevator are identical. So it is unclear if this conclusion is specific to the one airplane investigated or under what conditions this statement is valid. This must be improved.

- The naming of the reference frames could be improved. Instead of "primary coordinate system," I would call it a NED (north-east-down) reference frame, which is more common in the literature and more specific. Furthermore, it is not mentioned whether the "wind frame" is an inertial frame or not.

- The formatting of the reference makes them hard to read: No bold, no cursive, no indentation, no clickable links to the DOIs.

Limitations

It is not described how to fly the wing at a height larger than zero. Not even a statement, if this is possible or not using pure feed-forward control can be found.

Technical corrections

None

Review of wes-2025-193

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

by

Duc H. Nguyen, Mark H. Lowenberg, and Espen Oland.

The article addresses the dynamics and stability of tethered aircraft performing circular flights, forming a basis for the development of controllers. One of the major uses of tethered aircraft performing circular flights is in the field of Airborne Wind Energy (AWE), when fixed-wing aircraft are used. 

The study of the dynamics of tethered aircraft has had some contributions, mainly within the AWE community, e.g. Loyd, Argatov, Milanese, Schmehl, and co-authors, (which might be worth citing).  Nevertheless, many open equations still remain. Control is among the most studied topics in AWE (e.g [R1] and references therein), it remains one of the most critical aspects for a safe and reliable AWE system.

I consider that this article is a major contribution to the understanding of the dynamics and trimming in tethered aircraft performing circular flights. Therefore, it is also a very significant contribution to the field of AWE. The topic of the paper is timely and relevant for the AWES community, especially practitioners working on control strategies for the power-generation phase flight.

The study of the “Static Trim” and of the “dynamic trim” in separate dimensions not only contributes to our understanding of the behavior of the system, but also can serve as strong foundation for the development of controllers that define the circular path (circle radius and other parameters) and the required added control dependent on the position on the path, addressing the wind- and gravity-dependent effects.

The fact that just circular paths are addressed should not be seen as a limitation. The study [R2] proposes “motion primitives” for tethered aircraft that are defined to be circular paths, but can compose virtually any continuous motion on the surface of a sphere (defined by the tether length) by concatenating these primitives. 

Overall, the paper is clearly written and well structured, with the conclusions well-supported, mainly through the simulation results.

 MINOR ISSUES

• All equations in page 12 are labelled as equation (1).
• Table 1: Boxy-axis angular rates -> Body-axis …

[R1] Chris Vermillion, Mitchell Cobb, Lorenzo Fagiano, Rachel Leuthold, Moritz Diehl, Roy S. Smith, Tony A. Wood, Sebastian Rapp, Roland Schmehl, David Olinger, Michael Demetriou, Electricity in the air: Insights from two decades of advanced control research and experimental flight testing of airborne wind energy systems,Annual Reviews in Control,Volume 52, 2021,Pages 330-357,ISSN 1367-5788, https://doi.org/10.1016/j.arcontrol.2021.03.002.

[R2] Vinha, S.; Fernandes, G. M.; Fernandes, M. C.; and Fontes, F. A.. Motion Primitives on a Spherical Surface with Application to Tethered Aircraft Guidance.. In 2025 IEEE 19th International Conference on Control & Automation (ICCA), pages 186–191, June 2025. ISSN: 1948-3457. https://doi.org/10.1109/ICCA65672.2025.11129856