the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Aug 2024
Flight guidance concept for the launching and landing phase of a flying wing used in an airborne wind energy system
Abstract. Airborne wind energy is an emerging technology that utilizes tethered airborne systems in high-altitude wind fields to harvest energy. The employment of flying wings as airborne systems holds considerable promise concerning system performance, given their favorable aerodynamic characteristics. Moreover, when designed as motorized tailsitter, they can provide vertical takeoff and landing capabilities. However, the processes of launching, defined as the transition from takeoff to energy-harvesting flight, and landing, defined as the transition back, present considerable challenges for such flying wing AWES. To ensure the safe operation of the flying wing, it is essential to consider the controllability at varying wind speeds and the limitations imposed by the tether. To address this, a suitable guidance concept for the launching and landing phases of the aforementioned flying wing AWES has been devised. The concept is subjected to analysis taken into account specific system parameters, including turning radius, operating height, and wind speed. In light of these considerations, a flight regime can be identified. This is considered in the design of a guidance controller, which represents the top level of a cascaded flight controller. The lower levels of the controller comprise a translational controller and a rotational controller. The performance of the overall controller is demonstrated through a simulation of a representative wind field and corresponding system parameters. The results indicate that the control concept successfully facilitates the desired launching and landing in simulations. Future research may build upon the developed guidance and focus on identifying additional and more arbitrary flight paths for launching and landing.
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Status: open (until 18 Oct 2024)
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RC1: 'Comment on wes-2024-90', Anonymous Referee #1, 03 Sep 2024
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This paper presents the implementation of a cascaded controller to facilitate the launch and landing manoeuvres of a tail-sitting flying-wing airborne wind energy system. The study considers a 6 DoF model in tethered flight.
Major comments
1. In general, it is difficult to identify the paper's contribution, at least in its current form. For example, the first half of the abstract (up to 'at varying wind speeds and the limitations imposed by the tether') is just a general introduction to airborne wind. The second half contains somewhat generic statements about the controller and the analyses ('concept is subjected to analysis taken into account specific system parameters', 'a flight regime can be identified. This is considered in the design of a guidance controller, which represents the top level of a cascaded flight controller'). As it is now, the paper reads like a technical report that implements existing technologies on an airborne wind system. The authors should consider highlighting the novelty aspects of the work: is it in the roll-yaw manoeuvre or something else that differentiates or improves upon the existing solutions?2. Section 2, where the flight manoeuvre is described, is hard to follow. This is partly because the authors combine discussing the literature with pure technical discussions of the manoeuvre (for example, between lines 135 and 171, all of which are grouped into a single paragraph). As a suggestion, this section could be split into multiple paragraphs, first describing the existing solution (pitch transition), followed by one on the yaw-roll transition, with separate figures. Please feel free to adopt a different approach if the authors think it would improve readability.
3. The motivation of the trim study (section 3.3) needs to be clarified. Is it to determine the controllability of the system during a manoeuvre? If so, how does the existence of a trim state relate to controllability? A trimmed system can still be uncontrollable.
4. Also on the trim study, my understanding of the term 'trim' is that the system is in equilibrium. However, the authors state on lines 242-243 that 'A trimmed state is given when the equations of motion (Eq. (1) and Eq. (2)) are satisfied'. Satisfying eqs. 1 and 2 only means that numerical time-integration can be performed to generate a time history like in figures 13-15. I do not understand how this relates to a trim condition.
Minor comments
5. In section 5, it would be useful to show the control effector movements in the time histories (elevons and throttle levels), along with some discussions on whether the movement commanded by the controller is realistic.6. Not much is said about the tether model. Can the authors give a few basic descriptions, such as whether the tethers are split into multiple sections, and provide some indications on the stiffness, damping, and aerodynamic loads on the tether?
The authors are invited to consider these comments before submitting a revised manuscript.
Citation: https://doi.org/10.5194/wes-2024-90-RC1 -
RC2: 'Comment on wes-2024-90', Anonymous Referee #2, 30 Sep 2024
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General comments:
This work presents a guidance concept for a flying wing tailsitter AWES applied to the takeoff and landing maneuvers. A controller architecture is described, and the results of a trim analysis and a dynamic simulation are presented. The work presents a valuable study and solution to its topic, but lacks clarity and has inconsistencies which must be addressed.
Specific comments (major):
- The introduction reads as a rather extensive review of the different solutions developed by AWE companies and their way of operation. After reading both the abstract and the introduction, it isn’t clear what is the gap in the state-of-the-art tackled by this work, neither its contribution nor its novelty. Reading further, this seems to be more properly addressed by the second half of section 2. The authors should consider restructuring these sections to make this information clearer to the reader earlier in the paper.
- It is unclear whether the tether length ratio k >= 1.05 is chosen arbitrarily or as a conclusion of some previously cited work (line 129). Would the results change significantly if this parameter was to be altered? How do the deviations in height and radius reported in Section 5 relate to this assumption?
- The definition of trim state in line 224 seems to contradict the one given later in 243. Satisfying Eq. 1 and 2 does not imply that all forces and moments are balanced as previously stated.
- The transition ratio is presented as “the ratio of the aerodynamic force to the gravitational and tether force” on line 264. Then Fig. 8b refers to it as Fa/Fres, being Fres never named in the text. Moreover, in line 418 and Figures 14, 15 and 16 is redefined as Fa,zb/G, which seems to imply that the tether forces are actually not being considered.
Specific comments (minor):
- Line 58: "Conversely, the objective is to design a fixed-wing AWES but eliminate a rotating launch catapult mechanism and enable the airborne system to operate more independently." It is confusing whether this is stated as an objective of the authors' line of work, or as being an goal of the company mentioned right before or the field in general.
- References Fuest et al. (2021a) and (2021b) share the same DOI, journal, etc. I haven’t been able to find a paper with the title described in 2021a.
- The authors present 4 coordinate systems (cylindrical, geodetic, body-fixed and wind) in Section 2 between lines 160 and 163, then an additional fifth (aerodynamic) and sixth (tether) on Section 3.2. Some of them are not completely defined in the text and must be interpreted using Fig. 4. I would suggest to the authors providing more precise definitions of each coordinate system as well as considering restructuring the information to enhance readability.
- If feasible, the clarity of description of the Guidance Controller would improve if the intermediate signals in Fig. 12 were named, just like in Fig. 11, and the input/feedback signals were included on both Figures. The authors should also consider providing a block diagram detailing the architecture of the inner controllers.
Technical corrections
- Line 182: “As shown in Fig. 1, winglets are attached […]” Probably should be referring to Fig. 5b.
- Line 294: phase(iv) → phase (iv)
Citation: https://doi.org/10.5194/wes-2024-90-RC2
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