⋅ BY DR. STEVE LI ⋅
ver past couple of years, we’ve done considerable work with drones and have begun exploring the use of spray drones for applications in cropping systems. Below are several common questions.
Q. How much do spray drones cost?
For three spray drones, for example, DJI’s flagship spray drone Agras T30, plus all the accessories should be around $90K. A single, well equipped, accessorized drone is about $30K.
Q. How many acres can a drone spray?
It depends on the size of the drone. The largest spray drone with a 30L (7.9 gallon) tank can cover around 25 acres per hour (with ideal field shape and long rows at 2 GPA). You can fly three drones at the same time legally with FAA approval. If spray volume is reduced to 1 GPA, one Agras T30 can cover 6 acres in about 7-8 minutes with a wider spray swath.
Q. Do you need to fly them manually?
Not really unless you switch to manual mode. For most spray jobs, spray drones can follow A-B lines automatically with GPS and RTK guidance. An operator does not need to intervene unless there is a problem. To spray field edges and irregular shapes, manual mode is usually required. The next generation Agras T40 is expected to have a contour mode which allows the sprayer to track around the field automatically.
Q. How long do batteries last?
Not very long, realistically about 8 minutes or so, fully charged with a full spray load.
Q. How long does it take to charge a battery?
Slow charging at 110v requires about 30 minutes per battery, but fast charging at 220V and 50 AMP only 12-15 minutes.
Q. Do batteries require cooling during charging?
It depends. DJI T30’s batteries do not but some other models do require a cooling device during fast charging.
Q. Are spray drones likely to hit trees or other obstacles?
Most of the time, no, because operating systems include collision avoidance radar. However, power lines and other wires present a possible hazard. Be extra careful when operating drones around telephone poles, wires, etc.
Q. Can spray drones follow terrain changes and maintain and spray at a constant altitude?
Yes, spray drones work well over hilly terrain. They have terrain radar to monitor and maintain flight height above the ground/crop canopy.
Q. Are spray drones durable?
Presently, experiences are limited and there is not a large enough sample size to draw conclusions. Some growers have sprayed over 5,000-10,000 acres with the Agras T30 without major issues.
Propellers need to be replaced once in a while, and ESC, motors, flow meters and pumps are typical parts that often need to be replaced. They do have technical issues similar to any other electronics equipment (both hardware and software), but replacing these parts is usually not very difficult.
Sometimes, locating and procuring new parts can be a problem due to current supply chain issues.
Q. What are the best GPA, altitude and spray swath for spray drones?
This is a complex question because every spray drone is different and every chemical application is different. How to set these parameters depends on chemical properties, spray target and environmental conditions. One approach is to use 10 ft spray height and 20 ft swath. In our research we also use a 15 ft height and 25 ft spray swath.
Q. What is required to meet FAA guidelines for using spray drones?
For drones over 55 lb, FAA exemptions include Parts 107 and 137. Some companies have an arrangement (with fees) to allow growers to operate under the company’s Part 137 certificate.
Q. What type of chemicals can be applied with spray drones?
Technically, any chemicals with aerial application labels should be covered for use with spray drones. You should not have mixing problems at the application volumes of 2-3 GPA. Consult product labels for specific information about mixing and application.
Q. Are applications with spray drones subject to drift?
Absolutely, any spray equipment has the potential to create issues with spray drift. This can be particularly troublesome with drones since they are a new type of aerial application. For applications made too high above the spray target or in windy conditions, drift can be particularly bad and can also result in a loss of the intended spray pattern.
Q. What equipment is needed to facilitate applications with spray drones?
An operational trailer can serve as a base for generators, water and chemicals. These greatly aid in the efficiency of managing applications, particularly in large fields.
Q. Can GPS shape files in tractor systems be used for drone applications?
The answer is it depends. For big open fields without any obstacles in and around the field, shape files should work well. For fields with trees, telephone poles, wires, houses, etc., it is recommended to fly an RGB scouting drone first to generate a precision map.
The spray area can then be drawn on the precision map and exported to the spray drone as shape files. This is the best way to avoid collision. I would not trust satellite images from Google Earth due to the potential for collision. Various objects such as trees and other obstacles in those satellite images may not be current with present conditions.