PHOTO: TOM WOLF
CROP PROTECTION ▶▶▶
low volumes is to use nozzles that produce finer droplets. “ASABE fine to very fine drop- lets will have problematic effects on off-target movement and evaporation. These fine drop- lets are also more prone to the aerodynamic eccentricities of aircraft,” he adds. Entering these finer sprays into the models to assess drift from applications from conventional, manned aircraft results in buffer zones that are hundreds of times wider. “Failure to
control movement of a spray is, and should be, a problem,” he adds.
Research ongoing While research into drone spraying is progress- ing, most of this relates to coverage and depo- sition as well as spraying speeds and heights. The USDA is, however, currently conducting trials looking at drift. Wolf is part of a working group in Canada researching drone
applications, including drift analysis – see box page XX. While drones employ the latest, high- est technology for flying, control and autono- mous operation, some of the application tech- nology is quite basic, particularly compared with modern vehicle-based boom sprayers. Drone manufacturers are addressing these is- sues with more sophisticated technology, such as rotary atomising nozzles, electro-static systems and other developments.
Buffer zones required for medium and fine nozzles for aerial spraying in USA
Toxicological Endpoint (ng/L)* Spray quality (ASABE) Buffer zone (m) 2,000 medium very fine
42m
3,000 medium very fine
4,000 medium very fine
755m 14m
557m 3m
396m
*nanogram/litre Aircraft: rotary wing (Aerodyne Wasp); release height: 5m; wind speed: 8km/hr; spraydrift model:
AgDISP v 8.4; off-target area: PMRA standard water body – 100m wide x 0.3m deep
This table clearly shows the increase in the width of the buffer zone required when switching from nozzles making medium to very fine droplets, for aerial applications from a helicopter in the USA.
Computer modelling of drift shows that the wake from a UAV is randomised by the multiple rotors and their speeds.
Still a lot to learn about aerial applications by drones
Drone spraying has taken off quickly across the globe, and independent research is strug- gling to catch up with what’s happening in terms of the application efficacy, coverage, deposition and, particularly, drift. Current drift modelling (for other aerial applications) doesn’t take account of the fact drones are commonly equipped with four to eight rotors. This introduces huge complexities into the airflow around drones, because the rotors turn in opposing directions and at different speeds and angles. At the same time drift will be exacerbated by the drone’s movement through the air, its speed and any cross winds. Spray application expert Tom Wolf explains current regulatory models used for aerial drift assessment in North America – AgDISP and AgDRIFT – are not yet able to simulate drone applications. “But, by entering finer sprays into these models for their conventional manned rotary wing aircraft, we can see that
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buffer zones will be higher, much higher if fin- er sprays are used with drones,” he says. This is shown in the buffer zone table above. “I’m en- couraged by experts’ comments that drone applications should only be conducted with spray qualities and under conditions where spray drift risk is acceptable,” he adds. In the USA researchers have recently looked at the ability of existing spray drift modelling algorithms to predict the drift and deposition of sprays released by rotary wing UAV. To do this they combined two spray models cur- rently used by regulators – CHARM, a proven airflow model for helicopters and AgDISP, a proven spray deposition model. “It’s impor- tant to note that this study does not aim to pass judgement on drift from drones, but to assess the capabilities of the model,” explains Wolf. “It does, however, include some important observations.” The first: ‘It appears the proximity to the
▶ FUTURE FARMING | 22 May 2020
ground and/or flight speed and/or occasional crosswind can cause the released spray to be lofted above the UAV, producing significant airborne drift.’ Elsewhere the report states: ‘The potential complexity of the UAV wake, i.e. the impact of multiple rotor blades, is the ran- domiser in this behaviour. What is especially critical is to understand the pattern and be- haviour of the multiple rotor wake and its possible ability to loft released spray droplets, an effect not present with full-sized helicop- ters because of their higher altitudes, flight speeds, and spray boom positions.’ “It seems a lot of work may be needed to fully understand the conditions during which drones will cause more, or less, drift potential. Although we don’t know the droplet size be- ing modelled, some of the ‘airborne drift’ and ‘downwind deposition’ values (in the CHARM – AgDISP models) are very high, indeed,” comments Wolf.
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