Wind and Flight Speed Shape Drone Spray Coverage: Lessons from 3D Deposition Mapping in Wheat

Key takeaways

Wind interacts with drone downwash to fundamentally reshape spray distribution, often overriding the expected down and back deposition pattern.
Forward-facing collectors recorded much higher deposition; retreat-facing collectors captured minimal spray, with bias increasing with flight speed.
Peak deposition occurred 1 to 5 m downwind of the flight line, corresponding with prevailing wind direction.
Flight speed reduced droplet density: 6 m/s highest but variable deposition, 10 m/s best balance of deposition and uniformity, 14 m/s lowest deposition.

This text was generated by OpenAI GPT 5 Mini

In this study, 3D sampling of drone spray applications in wheat demonstrates that coverage is strongly influenced by the interaction between drone downwash, flight speed, and wind conditions. These factors collectively determine where droplets land, how evenly they are distributed, and how reliable coverage is from pass to pass. In 2025 we characterized wheat head coverage from a DJI Agras T50. In this study, we explore the larger, faster DJI Agras T100, and relate the observations to what we’ve seen in previous studies.

Materials and Methods

Site and crop

The experiment was conducted at 45939 John Wise Line, St. Thomas, Ontario (42°43’57.0″N, 81°05’49.8″W) on June 3, 2025. Wheat was seeded at 1.8 million seeds/ac on 19 cm spacing and was at the T3 stage (~0.7 m height) at application.

Design

Twenty-one poles spaced at 1 m intervals held 3D-printed mounts with 1×3″ water-sensitive papers oriented in four directions relative to the drone flight path: advance, retreat, left, and right. A tramline behind the array preserved canopy structure while allowing access to the samplers (Figure 1).

Figure 1 – Volunteers retrieving and replacing samplers between passes.

Drone Operational Settings

The primary objective of the study was to explore the effect of flight speed on coverage. Speed was increased from 6, to 10, to 14 m/s with the following operational settings:

4 LX07550SX (sprinkler) nozzles
50 L/ha application volume
350 µm droplet size
4 m flight altitude
7 m programmed swath width
Tank volume maintained at ~50 L

The drone began spraying 50 m before and 20 m after the samplers and was flown on full auto over pole 10 and 11 (the middle of the 21 poles). The spray liquid was municipal water with 0.125% v/v of NIS (Ag-Surf II).

Weather

Weather data was collected using a Kestrel 3550AG weather meter (Kestrel Instruments) in a vane mount positioned roughly 2 m below drone altitude. Data was logged as the drone passed the samplers (table 1).

Table 1 – Weather conditions for each spray pass.

Flights were conducted under a prevailing tailwind (rather than the preferred headwind) due to field constraints. Wind conditions during application varied by treatment. The 6 m/s treatment experienced higher and more variable wind speeds (avg. 6.6 km/h, SD 3.7 km/h, 177°), predominantly from the north (tailwind). The 10 m/s treatment occurred under moderate and stable winds (avg. 4.8 km/h, SD 1.1 km/h, 136°) with a slight right-to-left crosswind component. The 14 m/s treatment experienced low and variable wind speeds (avg. 1.6 km/h, SD 2.0 km/h, 198°) including periods of calm .

Results

Deposition Magnitude and Orientation

Papers were analyzed using a DropScope™ (SprayX, São Carlos, Brazil). Deposition differed strongly by collector orientation (Table 2).

Table 2 – Average deposition by sampler orientation for each speed.

Forward-facing collectors (advance) consistently recorded the highest deposition across all speeds, followed by lateral orientations. Reverse-facing collectors (retreat) recorded substantially lower deposition. Variability was high for advance and lateral orientations, whereas retreat collectors showed consistently low variability (Table 3).

Table 3 – Average deposition by sampler orientation for all passes.

Directional Bias (Anisotropy)

Anisotropy refers to the property of having different values when measured in different directions. We can quantify this by dividing the average coverage on one plane by the opposite plane; The resulting indices show the relative direction of deposition.

For the lateral plane (left-to-right), we divide the average coverage on the left-facing orientation by the right. On the sagittal plane (advance-to-retreat), we divide the average coverage on the advance-facing orientation by the retreat (Table 4).

Table 4 – Relative coverage indices for lateral and sagittal planes.

Bias in the lateral index (left-to-right) was relatively weak, with a subtle shift from left-facing to right-facing reflecting prevailing crosswind direction. This likely underestimates the right-facing (wind-facing) orientation as some of the spray plume moved beyond the array for the 6 m/s and 14 m/s passes (see Figure 4 further in the article).

The sagittal index (advance-to-retreat) increased from a 38% bias at 6 m/s, to a 69% bias at 10 m/s to an 86% bias at 14 m/s, demonstrating strong forward bias with flight and wind direction despite the down-and-back vector created by the downwash.

Spatial Distribution

Peak deposition consistently occurred 1 to 5 m downwind of the flight line, rather than directly beneath it. A cross-tail wind shifted deposition laterally, while forward motion (inertia) and wind reinforced deposition in the advance direction. This can be illustrated by isolating the average coverage on each plane for all three speeds (Figures 2 to 5).

Figure 2- Advance Orientation (Facing tailwind). Bars = SE

Figure 3 – Retreat Orientation (facing away from tailwind). Bars = SE

Figure 4 – Right Orientation (Facing cross wind). Bars = SE

Figure 5 – Left Orientation (facing away from cross wind). Bars = SE

By combining and plotting average coverage on all planes in a top-down heatmap, we can clearly see the lateral shift to the left of the flight pass (with the light crosswind), as well as indications of bi-modal coverage (Figure 6).

Figure 6 – Coverage heatmap created by smoothing the average deposition data for each speed (σ ≈ 1.1 – 1.2 m) over a 300 x 300 grid to illustrate deposition gradients. The colour scale supports a direct comparison of deposition intensity. The 21 samplers are indicated by black dots spaced at 1 m intervals, and the drone flight path appears as a black arrow between posts 10 and 11. Average wind speed and direction appears as an inset white arrow (vector).

Effect of flight speed

Swath width was determined by averaging all deposition on each post for each speed and using our online swath width calculator. The range of flight speeds used in this study did not significantly affect swath width.

6 m/s: 8 m swath width (16.3 % C.V.).
10 m/s: 7 m swath width (22.1 % C.V.)
14 m/s: 7 m swath width (25.0 % C.V.)

These widths are 15-20% wider than the widths calculated from the 2025 study with the T50.

Calculating an average swath width is a convenient means for comparing speeds but it masks variability. Variability in swath width and position relative to the flight path has been highlighted in earlier work, but can be illustrated here by plotting the average deposition (all orientations) at each pole for each repetition (Figure 7). Note, this figure still smooths the data by averaging the four orientations.

Figure 7 – Average deposition (all orientations) from each pole plotted by speed and repoetition.

Similarly, variability in deposit intensity is revealed in Figure 8.

Figure 8 – Average deposition, all orientations. Bars = SE

From Figure 8 we see that flight speed significantly influenced deposition, where higher speeds reduced droplet density (counts) and deposit variability.

6 m/s: highest deposition but greatest variability.
10 m/s: best balance of deposition, uniformity, and swath width.
14 m/s: lowest deposition and most directional bias.

Statistical Analysis

A two-way analysis of variance (ANOVA) was conducted to evaluate the effects of flight speed and collector orientation on spray deposition. Deposition differed significantly between Advance, Left, Right, and Retreat collectors (F = 5.54, p = 0.001). Flight speed had a statistically significant effect on deposition, where deposition was reduced with speed (F = 4.66, p = 0.010). The effect of orientation did not significantly depend on speed (F = 0.17, p = 0.986), suggesting that the pattern of deposition was consistent across speeds.

Discussion

Wind direction strongly influenced deposition, overriding the down-and-back pattern seen in previous studies. Wind-driven deposition (likely occurring after the drone passed the sampling location) would explain why retreat-facing collectors captured minimal deposition, and peak deposition was accordingly displaced from the flight line. We would have expected a stronger lateral response, reflecting drift / displacement, had a portion of spray not moved beyond the collector array on the downwind side.

Overall, results confirm that wind conditions fundamentally reshape spray distribution. The implication is that wind direction must be accounted for alongside swath width when developing flight path spacing to minimize the potential for overlaps and gaps between passes.

Further, previous studies have demonstrated a direct and positive relationship between flight speed and swath width up to 8-10 m/s with no further response after ~8 m/s. This study supports the hypothesis that rotary-wing drone speed and swath width share an asymptotic relationship that inflects at ~8-10 m/s (variability makes it difficult to determine an exact value). Flight speed also has a direct and inverse impact on the degree of spray deposition and deposit variability within the swath.