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.
- Preprint
(17574 KB) - Metadata XML
- BibTeX
- EndNote
Status: final response (author comments only)
-
RC1: 'Comment on wes-2024-90', Anonymous Referee #1, 03 Sep 2024
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
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 -
RC3: 'Comment on wes-2024-90', Anonymous Referee #3, 08 Oct 2024
General comments
This paper presents a guidance approach for launching and landing phases of a flying wing airborne wind energy system (AWES) with vertical takeoff and landing (VTOL). Using a demonstrator model, a comprehensive trim analysis of launching and landing is performed to identify the flight regime. The control architecture consists of a proposed guidance controller, at the top level, and rotational and transnational controllers at the lower level. Simulation studies are conducted to test the control approach.
The paper includes solid work, but it’s hard to follow the writing at some places.
Specific comments
- Highlight key (novel) contributions in the abstract (guidance concept, trim analysis, guidance controller design).
- Explain the meaning of minimum phase characteristic. Why is this a challenge to control?
- The trim analysis is comprehensive but the writing needs to be elaborated, e.g., explain the meaning of trimmed/non-trimmed states, describe the steps of trim state computation. Why 16 path points are selected in the simulation? How was the transition ratio defined?
- In Line 282-28, how to see that the results satisfy the general trim condition(s) but not the extended trim conditions? It’s also difficult (for the reviewer) to follow discussions based on the results in Fig. 10.
- The guidance controller development is a novel contribution, isn’t it? Explain why/how the architecture in Fig. 12 was proposed.
- Is the model including Equations (1) and (2) nonlinear? What was the model used in LQR design, Section 4?
- How was the decoupling obtained for the velocities and the positions, respectively?
Minor comments on writing
- Reduce general descriptions on AWE systems in the introduction. Go straight to the most relevant points on flying wing tailsitter AWES with VTOL.
- A large part of Section 2 reads like review of existing development (up to ‘… in an AWE context,’ line 148). Consider to move these writings to Introduction, that will give a stronger review with gaps and motivation/objectives naturally described.
- In Figure 4, when multiple coordinates are introduced, add arrows to each direction.
- In Line 182, ‘Fig.1’ should be ‘Fig.5.’
- In several sections, it might be helpful to put launching and landing in separate sub-headings or bullet points.
Citation: https://doi.org/10.5194/wes-2024-90-RC3 -
RC4: 'Comment on wes-2024-90', Anonymous Referee #4, 18 Oct 2024
The paper proposes a novel launching and recovery method for tail-sitter vertical take-off and landing (VTOL) Airborne Wind Energy Systems (AWES).
This suggested method is interesting as it investigates a new concept within the launching and recovery phase of AWES, an area that necessitates further research.
The authors adopt a systematic and relevant approach to analyze and evaluate this new technique.
I have minor comments on the manuscript as follow:
1- In page 1, line 24: "it exits the generation phase and enters the recovery phase":
Recovery usually used for terminating flight operation (e.g. landing), I suggest to use retraction phase instead, its most common used for pumping cycle in the literature.
2- In page2, line 30: "However, most emphasize the energy-harvesting flight, not the launching or landing":
I don't agree with this, there are papers in the literature discussing the take-off and landing for AWES, the authors need to mention them and state the difference in his research.
3- In page 2, line 41: "As soft-wing kites are prone to exhibiting markedly inferior aerodynamic performance in comparison to fixed-wing airborne systems":
Why? the author should provide reasoning to this.
4- In page 5, line 113: "the direct force control from the airborne system during vertical flight is accompanied by a pronounced minimum-phase characteristic":
More description for this is need (explain).
5- In page 5, line 116: "Makanis M600":
apostrophe is missing.
Citation: https://doi.org/10.5194/wes-2024-90-RC4
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 206 | 68 | 111 | 385 | 16 | 12 |
- HTML: 206
- PDF: 68
- XML: 111
- Total: 385
- BibTeX: 16
- EndNote: 12
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1