the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 May 2024
Experimental Evaluation of Wind Turbine Wake Turbulence Impacts on a General Aviation Aircraft
Abstract. Continued development of wind farms near populated areas has led to rising concerns about the potential risk posed to general aviation aircraft when flying through wind turbine wakes. There is an absence of experimental flight test data available with which to assess this potential risk. This paper presents the results of an instrumented flight experiment in which a general aviation aircraft was flown through the wake of a utility-scale wind turbine at an operating wind farm. Wake passes were flown at different downwind distances from the turbine, and data was collected on the orientation disturbances, altitude and speed deviations, and acceleration loads experienced by the aircraft. Videos were also collected providing qualitative information about the disturbances encountered in the wake. Results show that flight disturbances were small in all cases, with no difference observed between flight data inside and outside the wake at distances greater than six rotor diameters from the turbine. Closer than six rotor diameters, small load factor and orientation disturbances were noted, but were commensurate with those experienced in light or moderate atmospheric turbulence. Overall, the loads and disturbances experienced were far smaller than those that would risk causing loss of control or structural damage.
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RC1: 'Comment on wes-2024-51', Anonymous Referee #1, 31 May 2024
This manuscript provides a substantial, topical, and well-researched & well-documented contribution to the field of wind energy science. I am happy to recommend it for publication following the author's responses to some minor questions and suggestions:
Minor Notes:
- Line 12: I would add mention of the pilot interview when mentioning the qualitative parts of the study, e.g., "Videos and pilot statements were also collected..."
- Line 15: Nitpick: no comma needed after "noted".
- Line 134: Define acronym IMU.
- Line 188: Define a “flap configuration” and its relevance.
- Figs 7-12: Remind the reader the dashed lines indicate the time spent in the estimated wake region on the plot or in the caption (as I suspect these figures will be reused or borrowed by others in future presentations or discussions).
- Line 209: I agree that the author’s experiment represents, as he states, a “fairly worst-case scenario”. Though because wind farm wakes are generally stronger at night due to a lack of daytime-heating-induced vertical mixing diluting the wake, I would pose a nighttime wake intercept as the “ultimate” worst-case scenario. However, I assume that most general aviation aircraft are not flying during nighttime conditions, though I can’t speak to general aviation aircraft flights times myself - maybe the author could offer some commentary on typical GA flight times for clarity?
- Line 275: This interview with the pilot is fascinating and highly valuable. I’m curious if the author could comment more on how the interview was posed to the pilot and on the pilot himself: What did he know about the study beforehand? How long has he been a GA pilot?
- Line 288: There are several sentences in this manuscript that start with “This” that I think would be strengthened with a noun afterward, in this case something like “This statement matches…”
- Line 333: To give more context for the fairly worst-case scenario I suspect is being shown in video 12: How often does a GA pilot typically fly within 2D of a turbine? I would suspect infrequently but would like to know for sure.
- Line 407: Does the author have a hypothesis for why wake pass 5 is an outlier?
Citation: https://doi.org/10.5194/wes-2024-51-RC1 -
AC1: 'Reply on RC1', Jonathan Rogers, 06 Jun 2024
We thank the reviewer for the detailed comments and thorough read of the paper. In a revised manuscript we are happy to address all of the comments. Below I have provided a response to the non-editorial comments to facilitate discussion - the remainder of the comments will be addressed through straightforward edits to the paper.
6. This is a great point about nighttime wake passes generally being worse for added turbulence compared to daytime. From a general aviation standpoint, however, night passes through the wake would likely be extremely infrequent. The two most common scenarios where a general aviation aircraft may be expected to fly below the tip height, within 10 rotor diameters of a turbine, would be during aerial application missions (crop dusting), or on takeoff or landing from an airport. Aerial application is done only during the daytime. Furthermore, in North America, airports located that close to a turbine tend to be small public airstrips in rural areas (which generally have minimal traffic at night) or private airstrips (where nighttime operations are almost always impossible due to lack of runway lights). That being said, it is worth mentioning this for clarity in the manuscript so I will certainly add a brief discussion of daytime vs nighttime scenarios. Thank you for bringing this up.
7. Great questions, and it is certainly relevant to include pilot qualifications. The pilot was a commercially-rated skydiving pilot who regularly flies skydiving missions at Edmonton Skydive Centre. He has a multi-engine and instrument rating, with approximately 2 years of commercial flight experience. I will try to obtain an estimate of his total number of flying hours to add to the paper as well. He knew the purpose of the study beforehand and was asked to fly a set of test points at different distances from the turbine. He was provided with a brief set of questions to answer after the flights. I will add details about this to the paper as well.
9. Flying this close to a turbine would not be a common experience for most GA pilots, as mentioned above. In some rural areas with wind turbines, aerial applicator pilots routinely approach turbines to spray crops (aerial applicator pilots are a small fraction of the overall number of GA pilots). Also, in some places, turbines are located in the vicinity of small or private airports. So while approaching within less than 2 RD while below tip height is certainly infrequent for the vast majority of GA pilots, it is possible in some specific circumstances.
10. It is interesting to note that the roll angle deviation magnitude in Wake Pass 5 is an outlier, but the pitch angle deviation and load factor deviation in this wake pass are not outliers and are consistent with the other results. This is in contrast to, for instance, Wake Passes 12 and 13 where both the roll, pitch (for pass 12), and load factors are all higher than the closer wake passes. My hypothesis is that there was a random roll perturbation that happened to occur in this time frame during Wake Pass 5 due to atmospheric turbulence, separate from the turbine-added turbulence. For instance, a small updraft due to ground heating that is stronger on one side of the aircraft than the other can cause a roll angle disturbance of that magnitude. Thanks for bringing this up - it will be good to add mention of this in the paper.
Citation: https://doi.org/10.5194/wes-2024-51-AC1
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RC2: 'Comment on wes-2024-51', Anonymous Referee #2, 04 Jun 2024
The paper presents a very usefull experiment in an area where experimental data on the subject is scarce. With limited means an experimental evalution of GA flight through wind turbine wake has been presented. Flight data and pilot feedback have been gathered, processed and presented in a clear way. The presented results with regards to the six diameter distance beyond which no effect of a wake is to be noticed, confirms earlier findings where this distance was constructed indirectly on the basis of commercial jet pilots comments on gusts during landing.
Nevertheless hereby some minor comments on the paper
line 43 An overview of phenomena in a windturbine wake, in addition to the mentioned effects in wind turbine wakes one could also mention the velocity deficit behind the turbine. Though less for a wind turbine park in comparison with a stand-alone turbine this has shown to affect the flight path.
line 105. In figure 1 It would be illustrative to add a 6 RD measure to the drawing (as well as in figures 5 and 6).
line 190; Every wake pass was performed only once. To have a more consistent data set and to assess data quality and outliers it is advisable to repeat the flight test points.
line 275. Not much information has been provided on the pilot's background. In flight test experiments it is common to note the pilot's professional background (private/commercial/test pilot) and number of flight hours. In addition aircraft handling qualities would normally be assessed by means of a more objective, generic rating scale (Cooper-Harper or equivalent).
line 330 For better comprehension instead of noting the pass number it would be more evident to state the RD case that was flown.Citation: https://doi.org/10.5194/wes-2024-51-RC2 -
AC2: 'Reply on RC2', Jonathan Rogers, 06 Jun 2024
Again we thank the reviewer for the insightful and helpful feedback. As mentioned in the response to RC1, we are happy to address all of these comments in the a revised paper, and will provide responses to some of the non-editorial comments below to facilitate discussion. The remainder of the comments will be addressed through straightforward edits to the paper.
Comment regarding line 190: This is a good point and the test points should be better explained. The pilot was asked to fly two passes each at 15 RD, 10 RD, 5 RD, and 3 RD, with one pass at each distance flown at the 90 kts configuration and one flown at the 80 kts configuration (see Table 4). In addition, the pilot was asked to repeat the 5 RD pass at 80 kts four times to provide more data at this specific configuration (Wake Passes 7-10). So, in some cases test points were repeated albeit at different speeds and flap settings, while in some cases they were repeated identically. It should be noted that the closest distance requested for each point sometimes differed slightly from the closest distance actually flown (shown in Table 4) simply due to small errors in flight path tracking during the flights (e.g., Wake Pass 1 was intended to approach the turbine within 15 RD but approached within 13.5 RD instead).
Comment regarding line 275: Thank you for this comment, we agree that the pilot background is necessary to add and we will do so. Regarding objective rating scales such as Cooper-Harper, asking the pilot to assess handling qualities quantitatively was considered; however, although the pilot is a commercial pilot, he is not a trained test pilot and thus has no experience or training in providing quantitative handling qualities ratings for aircraft. It was determined that his qualitative descriptions of the experience flying through the wake, in combination with the flight data and videos, would be sufficient for assessing whether a hazard existed when flying through the wake.
Citation: https://doi.org/10.5194/wes-2024-51-AC2
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AC2: 'Reply on RC2', Jonathan Rogers, 06 Jun 2024
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