Author: Jason and Tom

  • Droplet Trajectories from RPAS Application – Implications for Swath Width Measurements

    Droplet Trajectories from RPAS Application – Implications for Swath Width Measurements

    We conducted a series of drone deposition studies with three main objectives:

    We wanted to:

    • Measure the swath width of a T50 drone at two flight speeds;
    • Document the nature of the downwash along the swath width;
    • Compare different techniques for measuring and analyzing swath widths.

    The four swath width measurement methods were:

    • Horizontal bond paper (H-BP)
    • Horizontal water-sensitive paper (H-WSP)
    • Retreat-facing water-sensitive paper (R-WSP)
    • Three-dimensionally arranged water-sensitive paper (3D-WSP)

    Assuming a trapezoidal-shaped spray swath, the Effective Swath Width (ESW) can be roughly defined as the span between two points that represent 1/2 of the average maximum deposit density. The idea is to create a cumulative pattern that is as uniform as possible when adjacent flights are added (Figure 1).

    As with any application system, if we assume that the target rate provides acceptable control, any deviation from the intended target rate along the pattern is either over-dosing (waste), or under-dosing (reduced control). It is therefore imperative that the distributed dose, as received by the intended target, be as uniform as possible.

    Figure 1. A spray pattern from DJI Agras T50, as depicted by horizontally oriented bond paper and scanned by Swath Gobbler™. In this case, the effective swath width is estimated from 1/2 the maximum deposit density, spanning approximately 7.5 m.

    Materials and Methods

    The study was conducted at Ontario’s Simcoe Research Station on September 17, 2024. The site was a flat, sand/loam field with no vegetation present (Figure 2).

    Figure 2. Ground conditions at site.

    A sampling array was established perpendicular to the forecast prevailing wind direction (150º). The sampling array had 17 discrete sampling locations (0 m to 16 m at 1 m intervals).

    Two collector methods were used simultaneously, centered on and perpendicular to the flight path:

    1. A flat, horizontal, continuous bond paper strip measuring 7.5 cm wide and 16 m long (secured in a Speed Track™ and analyzed using a Swath Gobbler™, Application Insight, Lansing MI).
    2. Discrete water-sensitive paper (WSP) collectors facing in three directions (x, y, z), each clamped back to back. WSP measured 26 x 76 mm (Spraying Systems, Glendale Heights, IL) and were analyzed using a DropScope™ (SprayX, São Carlos, Brazil).

    Sampling height was 30 cm above ground to simulate a fungicide application into a soybean crop. To avoid crowding collectors on each sampler, three parallel sampler rows were established, separated by a 1 m spacing (Figure 3).

    Figure 3. Sampling array and collectors.

    The first row contained the WSP oriented in the Y direction (WSP facing upward and downward. The second row contained the WSP in the X direction (WSP facing left and right relative to flight direction), as well as Z direction (WSP facing sprayer approach and retreat (Figure 4).

    Figure 4. Examples of discrete sampler design and collector orientation.

    Drone settings

    A DJI T50 drone fitted with four rotary atomizers was used to make the spray applications. The flight controller settings were a 250 µm droplet diameter spray over a 7 m swath width, at an altitude of 3 m above ground. Flight speed was either 4 m/s or 8 m/s. Application volume was 30 L/ha. Each flight speed was replicated three times. A total of six passes were made in this trial.

    The drone tank (capacity 40 L) contained tap water water with 0.2% v/v of Rhodamine WT 20% liquid (Hoskin Scientific, Burnaby, BC), prepared in a single batch (Figure 5). The level of liquid in the RPAS tank was maintained between 20 L and 30 L throughout the trial to minimize the effect of a changing payload. A volume of spray liquid was sampled prior to each pass to serve as standards for fluorometric analysis.

    Figure 5. Preparing the dye solution.

    Trial procedure

    Collectors were placed in samplers and the drone was positioned ~75 m downwind of the array to allow it to reach the target flight speed. When wind conditions were deemed appropriate, a signal was given to initiate the flight. Upon pass completion, one minute was allowed to elapse before sampler collection to permit complete deposition and drying.

    Labelled WSP were retrieved and placed loosely in paper bags to prevent any residual moisture from ruining the collectors. Bond paper from the Swath Gobbler™ was marked with treatment information and reeled onto individual spools (one per treatment).

    Weather conditions

    Both wind speed and direction varied slightly during the study, but it was always possible to run a trial with negligible sidewinds so that the sample array captured the majority of the spray swath. Air temperature was approximately 25 °C. Wind speed was ranged from 6 to 14 km/h during the trial. All spray passes were into a headwind with maximum deviations of -10 to +30°.

    Collector analysis

    Bond paper digitization

    Bond papers (Figure 6) were scanned using a Swath Gobbler™. The software measured both deposit density and percent coverage at each scanned location, but only deposit density was used in the analysis.

    Figure 6. Bond paper secured in a Speed Track™ and sprayed with a Rhodamine WT solution.

    WSP digitzation

    WSP were removed from paper bags, sorted and sequenced into reps. WSP were scanned using a Drop Scope™ set to “Ground sprayer” and “Syngenta WSP” (Figure 7). The software reported deposit density and percent area coverage, but only deposit density was used in the analysis.

    Figure 7. In-box, Out-box procedure for scanning WSP using a Drop Scope™.

    Effective swath width calculation

    We used our Excel-based model which assumes a racetrack pattern and sums deposits from adjacent swaths. Swath width was adjusted to minimize over- and under-dosing as well as deposit coefficient of variation (CV), while maximizing swath width.

    For the WSP collectors, each of the six orientations were first evaluated separately, and then averaged to simulate a three-dimensional plant structure. Given the similar orientations, the upward-facing WSP and bond paper were used as quality-checks.

    Visualizing coverage in three dimensions

    In order to understand the direction the spray cloud moved as it imacted the collector array, we declared a dominant side to each of the three cardinal directions, x, y, and z that we captured using the WSP.

    • X-axis: Looking in the direction of travel, WSP deposits facing right were subtracted from those facing left.
    • Y-axis: WSP facing up minus papers facing down.
    • Z-axis: WSP facing the RPAS retreat minus papers facing the advance.

    This allowed us to estimate the vectors with which the spray was deposited.

    Results and Discussion

    Deposits on WSP

    We first looked at WSP data to better understand the direction that the droplets flew at the time of impact.

    X-axis: Note that the right-facing cards are depicted as being positive, whereas the left-facing cards are depicted as negative.

    Only those WSP facing the drone received deposit, with the deposit amount being larger for the faster flight speed (Figure 8). This implies that the spray moved out to either side from the centre of the flight path, carried by a laterally moving downwash.

    Figure 8. Coverage on the X-axis, with WSP faces oriented perpendicular to flight path. Note that the drone passed between the 7 and 8 metre marks.

    Y-axis: On the whole, deposition on the horozontal collectors was most variable of the three orientations, and resulted in the lowest measured droplet densities (Figure 9). Upward-facing WSP received more of the deposits than the downward-facing WSP. However, at 3 m and 12 m, the majority of deposition appeared on the downward-facing WSP. Underneath the drone rotors, downwash force would prevent re-bound. But at the edge of the rotors, a lower pressure region would permit pressurized air to escape not just laterally but also vertically. Entrained droplets would therefore gain an upward vector, and impact the downward-facing WSP. A slight wind from the right truncated the swath at the 13 m mark. That same wind may have captured any spray from the “bounce” at 3 m to become secondary deposition along the 1 m – 3 m section.

    Figure 9. Coverage on the Y-axis, with WSP faces oriented up or down. The drone passed between the 7 and 8 meter marks.

    Z-Axis: Only those WSP facing the retreat of the drone received deposits (Figure 10). As previously discussed, this is likely due to the downwash, which is vectored downward and rearward along the flight path according to the drone orientation in flight. These deposits were further reinforced by the prevailing wind direction after the drone had passed.

    This deposit pattern is opposite to that of a ground sprayer, where spray tends to deposit on the advance surfaces due to droplet inertia (assuming a low boom height and fast travel speed). A slight shift to the left is apparent in Figure 10, likely due to the headwind’s directional deviation to starboard. Note that the faster flight speed had higher deposit densities. Reasons for this are unclear, as there was no commensurate deficit in droplet numbers at other sampler orientations for the faster speed.

    The overall deposit density on the retreat-facing orientation was highest of any single collector orientation. The high deposit density and swath width is likely the result of the prevailing wind direction as well as the additional contribution of the downwash from the forward-tilted RPAS. These two factors helped transport the spray plume backwards for efficient interception by retreat-facing collectors.

    Further evidence of this dynamic was visible when examining the bond paper collector strips. In the lee of the track edge, deposits were scarce, indicating a predominant horizontal trajectory of the droplets (Figure 11).

    Figure 10. Coverage on the Z-axis, with WSP faces oriented to face drone advance and retreat. The drone passed between the 7 and 8 meter marks.
    Figure 11. A shadowed region (highlighted in light red) that sometimes appeared along the retreat-edge of the Speed Track™.

    ESW at 8 m/s flight speed

    Only the upward- and retreat-facing WSP surfaces received consistent spray coverage. As a result, only these two orientations were individually used for ESW calculations. However, deposits from all six orientations were averaged for the combined ESW measurement.

    Two analysis methods were compared. First, the ESW was calculated for each replicate run seprately, and the resulting ESW were then averaged. Second, the three replicate run deposits were first averaged, and then ESW was calculated from that average.

    When ESW from the bond paper was calculated for each replicate and then averaged, the ESW was 6.8 ± 1.4 m (Table 1). The resulting average CV of those swaths in a racetrack pattern was 14.8%.

    When ESW was calculated from the upward-facing WSP for each replicate, the ESW was 4.7 ± 0.2 m. This was narrower than the bond paper result oriented on the same plane. In addition, the average swath CV was now 34%, significantly higher than that from the bond paper collector.

    The retreat-facing WSP resulted in the highest ESW so far, 7.8 ± 0.5 m.

    To better simulate a plant’s cumulative deposit, reflecting the pesticide dose received on leaves and stems that might vary in location and orientation, all six orientations were combined for each pass. When ESW was then calculated for each replicate, it was 8.8 ± 0.2 m (CV = 20%).

    The range of swath widths onserved within each of the three reps ranged from 6 to 51% of the mean ESW. Differences between replicates could be due to automatic, instantaneous adjustments in the flight path controlled by the drone, or it may be due to changes in environmental conditions in the two hours that elspsed between consecutive replications. It may be instructive to increase the replicate sampling to obtain better estimates of variability within any given treatment.

    If reps were pooled before calculating ESW, ESW increased an average of 30% for all sampling methods (Table 1). The CV of multiple swath simulations also decreased an average of 28% with this approach. Pooling prior to analysis is, however, less accurate because it eliminates the variability one might observe between two dicrete locations, which is how product efficacy will be observed in a pest control situation.

    Table 1. Calculated ESW (m) and CV (%) for 8 m/s flight speed based on deposit density (count/cm2). Range (% of mean) calculated for the averages. Change from Average is the % change in the ESW of a pooled sample compared to the averaged ESW from each replicate. H-BP: Horizontal Bond Paper, H-WSP: Horizontal water-sensitive paper, R-WSP: Retreat-facing water-sensitive paper, 3D-WSP: Sum of all six facets of water-sensitive paper.

    ESW at 4 m/s flight speed

    ESW were significantly narrower at the slower flight speed when measured on the bond paper, but of similar widths when merasured using WSP (Table 2). The slower speed had much greater variability among replicate samples as well, ranging from 36 to 60% of the average ESW.

    The retreat orientation showed the widest ESW, with the 3D orientations resulting in slightly narrower swaths. At the high speed treatment, the 3D analysis had produced the largest ESW.

    Pooling the reps prior to analysis resulted in similar ESW for the bond paper and the upward-facing WSP, whereas the remaining orientations resulted in wider swaths when the reps were pooled.

    In general, the swath CVs at the slower flight speed were similar to the fast RPAS speed, averaging in the low to mid 20s. Pooling the reps prior to analysis reduced swath CVs for the retreat orientation and the combined orientations, but not for the upward-facing collectors.

    Table 2. Calculated ESW (m) for 4 m/s flight speed based on deposit density (count/cm2). Range (% of mean) calculated for the averages. Change from Average is the % change in the ESW of a pooled sample compared to the averaged ESW from each replicate. H-BP: Horizontal Bond Paper, H-WSP: Horizontal water-sensitive paper, R-WSP: Retreat-facing water-sensitive paper, 3D-WSP: Sum of all six facets of water-sensitive paper.

    Comparing speeds

    When ESW was calculated for each replicate, the slower flight speed resulted in ESW that were slightly smaller than the faster flight speed on average (6.8 vs 7.0 m). However, when reps were pooled, the slower flight speed resulted in significantly smaller ESW compared to the faster flight speed (7.4 vs 9 m). Pooling the reps prior to analysis also resulted in a lower coefficient of variation.

    Generally, there was less variability among replicates for the faster flying speed. Whether this was the result of the speed itself or was an artifact of the specific conditions during which the flights occurred is unclear.

    Comparing swath appearance from bond paper and optimal WSP orientations

    When the ESW from the bond paper was calculated for each replicate and then averaged, the following graph was produced (Figure 12). Note the bi-modal shape produced at the slower flight speed. This corresponds with the position of the atomizers and it’s possible the increased dwell time directed more spray in those positions compared to the faster flight speed, which increased ESW and dispersed the spray more evenly.

    This could also be responsible for greater uniformity among the three replicate flights that was observed.

    Figure 12. Average of three passes from bond paper at 4 m/s and 8 m/s speed.

    When we averaged coverage from the optimal 3D orientations (X-axis: inward-facing, Y-axis: upward-facing, and Z-axis: retreat-facing) and compared their swaths to the 2D, we are able to capture more droplets and eliminate the bi-modal pattern appearance of the lower speed, reducing CV and increasing the ESW (Figure 13). This may begin to explain why sprays that appear to have low coverage on horizontal collectors can produce better-than expected efficacy.

    Figure 13. Average of three passes from optimal-facing WSP samplers at 4 m/s and 8 m/s speed.

    Vector analysis

    The sampler array permitted the generation of spray vectors that showed the inferred direction and intensity of the downwash movement.

    To graph vectors for droplet movement at each position along the 16 m swath, we calculated net coverage as previously described (i.e. for each of the x, y, and z sampler oerientations, the deposit density on one side was subtracted from the other). The magnitude of that value represented the relative dominance of that side of the orientation for spray deposition. We assumed that droplets were primarily carried by air movement to their collectors, therefore we inverted the sign on the coverage to express it as wind direction from the from the vantage of the drone. When these data were combined for the X-Y and the X-Z direction, we were able to estimate the origin and strength of the deposit vectors, and thus infer droplet-carrying airflow.

    Plotting X by Z meant you are looking down from above (Figure 14). This created vectors that indicate lateral and rearward spray movement.

    Figure 14. Plotting X by Z coverage creates vectors that indicate lateral and rearward forces that carry spray droplets released from the drone.

    Note that the predominant direction of deposit in the X-Z plane was rearward, in the direction of the wind. The forward-tilt of the RPAS aso directed its downwash towards the rear, adding to the headwind effect. At the edge of the spray swath, the rearward vectors diminished, being solely under the influence of the headwind. The vectors were strongest at the locations corresponding to the RPAS rotors.

    Plotting X by Y for each position means looking at the RPAS from ground level as it flies away from you. The resultant vectors indicated a combination of lateral and downward spray movement for the majority of the swath (Figure 15).

    Figure 15. Plotting X by Y coverage creates vectors that indicate lateral and predominantly downward forces.

    At two locations (3 m and 13 m) the net spray deposition was on the underside of the Y-samplers. This suggested that a special region in the downwash existed, where the high pressure air generated by the rotors dissipated, allowing droplet-laden air to move upwards, essentially re-bounding from the ground. At the same time, droplets moved laterally to escape the same high pressure region.

    This region of high turbulence could be where plants in the canopy may see droplets arriving from a large number of directions, contributing to coverage that may not be similarly captured by a single flat collector.

    Summary

    ESW Measurement Method

    There was significant variability in swath width and uniformity among three replicate measurements of the same drone configuration. We observed an average of 34%, and as much as 60%, variation in swath widths within replicate passes of the same speed treatment. Spray deposition cannot be assumed to be repeatable, and understanding deposit variability is likely more important than calculating its average.

    Measurement techniques affected the observed swath widths. Generally, horizontal targets resulted in narrower swath width measurements than vertical targets facing into the wind and the direction of drone travel. A composite of various target orientations resulted in the widest and most uniform swath deposits.

    Effect of Flight Speed

    Faster drone flight speeds resulted in wider measured swath widths in all but one measurment technique. It is possible that the greater concentration of downwash energy at the lower flight prevented more of the droplets from moving laterally prior to impact with a collecting surface.

    Measuring Downwash Turbulence

    The direction of droplet movement varied with position under the drone. The majority of the droplets had a strong z-vector at deposition. This is the direction of the wind and of the retreating side of the drone. Both wind and backward-tilted downwash of the drone would contribute to this.

    As the spray droplets neared the ground, the high air pressure under the drone rotors caused the downwash to move laterally away from the centreline. Droplets entrained in the downwash therefore moved strongly to the left and right of the centreline.

    Of the dominant three collector orientations (retreat, lateral, and upwards), the lowest collection was achieved with horizontally oriented targets. Interestingly, at the rotor edge, vortices formed and these moved the droplets upwards, away from the ground. This effect was only observed in a narrow band at he outside edge of the rotors.

    These effects were somewhat variable but consistent for all spray passes, and occurred at both travel speeds.

    Overall Conclusions

    1. A drone’s downwash results in spray droplets moving in many directions which cannot be accurately sampled with a single collector orientation.
    2. If only a single plane can be sampled, it should be facing the retreat side of the spray pass.
    3. Three-dimensional sampling may be required to better simulate the spray capture of an agricultural canopy.
    4. Higher travel speeds resulted in slightly more uniform, wider, and repeatable deposition.
    5. The variability of a drone’s deposition, both in ESW and CV, is considerable and remains a barrier for consistent efficacy.
    6. Multiple replicate passes, analyzed discretely, are required to understand the variability of the drone’s spray deposit, both in ESW and in CV.

    Thanks to Drone Spray Canada and Don Murdoch (University of Guelph) for their cooperation and in kind support of this study.

  • Exploding Sprayer Myths (ep.15): Where will it land?

    Exploding Sprayer Myths (ep.15): Where will it land?

    Celebrating 10 years of “thinking we’re funny”, Exploding Sprayer Myths is back with a new opening sequence!

    Join cheesy game show host Wink Boomwobbly and his unsuspecting contestant Joe Mama as they discuss the factors that influence spray droplet fate. How do wind, travel speed, boom height and droplet size affect your pesticide application? Knowing how droplets respond to these forces makes the difference between coverage and drift. Brush up on your fundamentals in a fun way.

    Long-suffering RealAgriculture editor Dr. Jay Strove has also hidden an Easter Egg photo so you can play along! Bonus points if you know where it was taken and who it is.

    Thanks to our special guest from the UK, and be sure to stick around for the bloopers.

  • Stan and Blade’s Drone Emporium

    Stan and Blade’s Drone Emporium

    In this parody of SCTV’s Tex and Edna Boil we have some tongue-in-cheek fun while reminding people to maintain a healthy skepticism when reading RPAS marketing materials. Always be sure to ask questions and see the data before you believe what might be too good to be true.

    Special thanks to Jason Strove for his masterful post-editing magic.

  • Exploding Sprayer Myths (ep.14): Maverick’s Mojo

    Exploding Sprayer Myths (ep.14): Maverick’s Mojo

    Have you lost that loving feeling?

    Well here’s your chance to get it back while learning how to operate spray drones safely, effectively, and legally.

    Join Maverick and Goose as Viper barks out best practices when using this new application technology.

    Remember: You’re not just a pilot – you’re an applicator!

    Thanks to RealAgriculture, the Simcoe Research Station, Drone Spray Canada and special guest, Dr. Michael Reinke.

  • How to Calibrate a Drone

    How to Calibrate a Drone

    Calibration is a fundamental step in any spray application. To apply the correct product rate, we need to know how much liquid per unit land area is deposited under the sprayer.

    To conduct the calculations, either manually or through the drone software, we need to know the width of the spray swath. This task requires the operation of the sprayer under typical conditions, some kind of sampler capture the spray deposit, and a means of quantifying that deposit so the spray pattern becomes apparent. Here’s how we do it:

    1. Confirm the accuracy of the flow meter

    Drones don’t typically report the spray pressure of the spray mix. Instead, they report the flow rate using a built-in flow meter. The drone maintains the desired application rate by using the flow rate to adjust pump speed and engage nozzles over a range of travel speeds. Because everything depends on the flow meter, its accuracy needs to be verified.

    • Fill the spray tank with clean water and flush all the lines.
    • Install nozzles required for task, ensuring all nozzles are identical and in good working order.
    Nozzles installed on DJI T20 drone.
    • Select the nozzle size you installed on the spray monitor.
    • Purge the air from the system.
    • Activate the spray and wait for the flow rate to stabilize on the spray monitor. This may take a few moments.
    • With the nozzles flowing, place collectors under each nozzle and collect the spray liquid for a fixed time, say one minute.
    Capturing spray during flow meter calibration.
    • Ensure the collector catches all the spray. Buckets often create turbulence. Rotary atomizers make this more difficult.
    • When the time elapses, remove the collectors and then shut off the spray.
    • Unless the shutoff is very fast and positive, leaving the collectors in place during shutoff can introduce error as the flow diminishes.
    • Confirm that the volume collected from each nozzle was identical, and that the flow rate reported by the drone flow meter is accurate.
    • Repeat to ensure consistency.
    Use of a Spot-On digital calibrating cup ensures that all spray is captured and it also reports the volume instantly.

    2. Measure the swath width

    Spray swath width is variable. For a measurement to be relevant we must evaluate spray deposition under environmental conditions that are similar to the planned spray operation, as well as use the same operational settings such as altitude, travel speed, nozzle choice, and application volume.

    Spray samplers are positioned along the ground, perpendicular to the flight path. We use water-sensitive paper (WSP) because it’s readily available, fast and easy to use, and the deposits can be analyzed visually or using simple apps that calculate coverage. We create a sampling line of WSP positioned a 1 m intervals (or maybe 0.5 m for narrow swaths). The samplers should extend to twice the expected swath width to account for any swath displacement from sidewinds.

    • Choose a day with light, consistent winds.
    • Find an open space free of obstruction in the direction of the prevailing wind.
    • Install a weather station to document conditions during flight.
    A Kestrel 3550AG or 5550AG wind meter can record weather data and download to a phone via Bluetooth.
    • Mark an approximately 200 m long flight line into the prevailing wind direction by placing wire flags every 50 m.
    • At the 150 m mark, use wire flags to centre a sampler line perpendicular to the flight path. Sampler line length should be about twice the expected swath width.
    Swath sampling line
    • Wooden blocks with paper clips can be used to secure WSP at regular intervals along the sampler line.
    Wooden blocks attached to a 4″ tow strap allows for easy setup and movement of sampling line.
    • Fill the drone 1/2 full.
    • Manually fly the drone along the entire flight line. The spray pressure, flow rate and altitude of the drone should be stable before it reaches the sampler line. This may take 25 meters or more depending on drone model, flight speed and drone weight.
    • Fly 50 m past the sampling line without any drone maneuvering to avoid affecting the deposit.
    • Land the drone and walk along the sampler line.
    • Note the deposits in the central region. Walk along line as the deposits taper off, looking for deposits that are approximately 50% of the average central deposits.
    Water-sensitive paper following a drone application.
    • Estimate the distance between these deposits on both edges of the swath. This is the estimated swath width that can be entered for the second flight.
    • Replace the WSP with a fresh set, refill the drone to 1/2 full, and repeat the flight two more times.

    Other methods perform a more advanced assessment by analyzing the entire swath, and not just intervals. These methods use dyes and dedicated hardware to quantify the deposits along strings or paper samplers.

    The Swath Gobbler documents swaths at high resolution using lengths of 3″ bonded receipt paper, food grade dye, and a digital scanner.
    The Application Insight LLC Swath Gobbler scanner in action.

    3. Analyze the Pattern

    The nearest approximation for drone swathing is that of a manned aircraft. The spray pattern of an aircraft is tapered, meaning the highest deposition is near the centre of the swath, and the edges of the swath fade to zero deposit. In order to achieve consistent coverage, we need the edges of the spray swath to overlap so the cumulative coverage at the edges is closer to that in the centre. Too little overlap leaves gaps and too much overlap results in excessive deposit.

    Insufficient overlap creates gaps in coverage
    Excessive overlap results in over-dosing and waste
    Correct overlap is necessary for efficient and effective application.

    Deposits from drones can be highly variable. The challenge is to find an overlap distance that minimizes this variability, minimizes both over- and under-application, and maximizes swath width. Download a copy of our Excel spreadsheet to help you with this process.

    Drone-Pattern-Swath-Width-1Download

    The first step is to estimate a reasonable average deposit, called “Threshold”. Graph the deposits from each sampler, and estimate a point on the Y axis (Relative Deposition) that represents the average maximum deposit. This could be the maximum value of the plateau, or a midpoint between the maximum and a nearby dip. This is the Threshold. We then take 50% of this estimated average deposit, and find the two distances on the X axis (Sampler Locations) that intersect the curve at these points. The distance between these two points is our first estimate of the swath width. If two adjacent swaths are spaced so the edge of one overlaps 50% with the next, the overall cumulative deposit should be relatively even.

    The coverage information from each sampler location is graphed to create a deposit pattern.

    We can alter the amount of overlap to improve the apparent uniformity, but be cautious. For example, even though we can often improve the uniformity by narrowing the swath width, this can add deposit to the area under the drone and raise the overall deposit amount. Plus, the narrower swath also lowers the productivity of the drone. Use the Excel model to establish a swath width that has the lowest variability (Coefficient of Variability or CV) AND results in a balance between over- and under-dosing.

    The amount of overlap is adjusted to minimize variability (CV) and both equalize and minimize over- and under-dosing.

    4. Recognize the factors that influence swath width

    Operational use case affects swath width

    Swath width is affected by altitude, speed, water volume and spray quality. Generally, higher altitudes, lower volumes, and finer sprays will result in a wider swath. Unfortunately, the same configuration also results in greater drift. It is recommended that swath widths be determined for each spray volume and nozzle arrangement that will be used.

    Drones will be applying low water volumes and this requires a critical assessment of coverage to ensure the deposit density is sufficient to achieve the desired result. A low volume will require a finer spray for minimum coverage to be realized. Coarser sprays that reduce drift and evaporation will need higher water volumes and result in narrower swaths. Significant time may need to be invested to understand the effects of operational settings and environmental conditions on spray deposit uniformity and swath width.

    Effective Swath Width and the Agronomic Use Case

    The relatively sparse coverage at the extremes of the measured swath width may be insufficient to elicit the desired biological result. The Effective Swath Width (ESW) represents the segment of the total swath width that results in pesticide efficacy. In some use cases, the two widths can be similar, but typically the ESW is only a fraction.

    The difference is influenced by the “Agronomic Use Case” which includes factors such as:

    • Spray mix rheology (i.e. the interaction of spray mix viscosity and atomizer design on droplet size)
    • Minimum effective dose: This is a complex relationship between coverage, spray mix concentration and pesticide mode-of-action that results in an effective result while minimizing the environmental impact.
    • Target location (e.g. a pest within a dense canopy or a weed on relatively bare ground)

    Taken collectively, research has shown a 20-30% reduction in ESW for corn, wheat and soybean fungicide applications compared to swaths measured on open ground. Conversely, herbicides sprayed on bare earth or sparse vegetation can produce an efficacious response 20% wider than the measured swath width. The impact of agronomic use case on ESW must be considered during mission planning.

    Additional pointers

    Here are a few tips and tricks to help you be successful when calibrating your drone.

    • Drone patterns will have deposit peaks and valleys in the central region. Repeated runs are needed to confirm that these are real and persistent. If so, then adjustments in flying height, spray quality, or water volume may be needed to eliminate them.
    • The absence of pressure gauges on drones can be corrected by installing an analog gauge in-line with one of the spray nozzles. If may be necessary to mount an auxiliary camera on the drone to record this gauge. We have observed strong fluctuations in spray pressure, particularly on starting a spray swath, that were not reflected in the reported flow rate.
    A pressure gauge can be plumbed into a drone without affecting flight behaviour. A camera is trained on it to read pressure during a flight.
    • Many drones have the option of recording the flight screen during a mission. This will provide a record of the performance of the drone, and can be valuable should performance problems arise.
    • Although swath width calibration is done by flying into a headwind, the actual spray application should be done with a side wind. Start at the downwind edge of the field and turn into the wind. The drone is symmetrical and the tapered spray patterns should equalize the deposits. Alternately, flying into a headwind and returning with a tailwind can alter the aerodynamics of the spray deposition process, alternating between a wider and more narrow swath width, respectively.

    Drone spraying will walk a razor’s edge of sorts – there is little room for error when using scant water and fine droplets. Getting the basics right has never been more important.