Spraying and seeding

Why this matters

A spray drone has immediate effects, good and bad. Apply too little and the fertiliser or product does not work. Apply too much and it is wasted, may damage the crop and leaves residues in the environment, while drift can land on houses, fish ponds or neighbouring fields. Drone spraying must therefore be calculable, checkable and strictly compliant. This module uses liquid fertiliser as the example; spraying hazardous agricultural substances has further requirements that must be checked with the authorities directly.

Application rate and flow rate

The NPTC calibration workbook gives application rate (L/ha) = flow (L/min) × 600 ÷ speed (km/h) ÷ swath width (m). The 600 comes from 10,000 m² per hectare ÷ 1,000 m per km × 60 minutes per hour. Beware of sources that use centimetres with the constant 600, which is wrong; with centimetres the constant must be 60,000.

import math

RATE, SWATH, SPEED_MS, TANK = 15.0, 5.0, 5.0, 10.0      # L/ha, m, m/s, L
AREA_HA = 30 * 1600 / 10_000

speed_kmh = SPEED_MS * 3.6
flow = RATE * speed_kmh * SWATH / 600
volume = RATE * AREA_HA
loads = math.ceil(volume / TANK)
print(f"speed {speed_kmh:.0f} km/h -> flow {flow:.2f} L/min")
print(f"field {AREA_HA} ha needs {volume:.0f} L = {loads} tank loads")
print(f"one tank sprays {TANK / flow:.1f} min and covers {TANK / RATE:.2f} ha")
print(f"check: {flow:.2f} L/min x 600 / {speed_kmh:.0f} km/h / {SWATH:.0f} m = {flow * 600 / speed_kmh / SWATH:.1f} L/ha")

speed 18 km/h -> flow 2.25 L/min
field 4.8 ha needs 72 L = 8 tank loads
one tank sprays 4.4 min and covers 0.67 ha
check: 2.25 L/min x 600 / 18 km/h / 5 m = 15.0 L/ha

Spray lines and buffers

The effective swath is not the width the spray reaches at its edges. Deposit is thinner at the edges than in the middle, so lines must be spaced so that edges overlap and give an even dose. This width must be found by a distribution test of the actual machine; ISO 23117-2 specifies such test methods.

A buffer zone is an unsprayed strip between the field and anything that must be protected, such as canals, ponds, houses and other crops. Its width must follow the product requirements and the authorities’ guidance.

Droplets and drift

Small droplets cover leaves well but fall slowly and are easily carried by wind. ANSI/ASABE S572.3 classifies nozzles by droplet size so that nozzles can be chosen as the product label requires. ISO 23117-1:2023 sets environmental requirements for unmanned aerial spraying systems (for drones with a maximum take-off mass up to 150 kg). Reading wind, temperature and humidity before flight draws on the meteorology unit linked to this module.

An emptying tank and the centre of gravity

The liquid in the tank is a load that changes throughout the flight. If the tank is not exactly at the aircraft’s centre of gravity, the combined centre of gravity moves as the liquid is used, and the flight controller must compensate continuously.

FRAME_KG, TANK_X = 18.0, 0.04          # kg, m (positive = forward)
for liquid in (10.0, 5.0, 0.0):
    total = FRAME_KG + liquid
    x_cg = (FRAME_KG * 0.0 + liquid * TANK_X) / total
    print(f"liquid {liquid:>4.1f} kg: mass {total:.1f} kg, CG {x_cg * 100:+.2f} cm")

liquid 10.0 kg: mass 28.0 kg, CG +1.43 cm
liquid  5.0 kg: mass 23.0 kg, CG +0.87 cm
liquid  0.0 kg: mass 18.0 kg, CG +0.00 cm

Thai requirements

• Department of Agriculture: launched a standard operating practice for spraying with agricultural unmanned aircraft and ID cards for contracted spray operators who pass its training (December 2024). Anyone offering spraying services must check the current training and card conditions with the Department
• CAAT and NBTC: pilot and radio equipment registration as in module 1 and UAT 313
• Product labels and conditions are binding, including rate, droplet size, pre-harvest interval and protective equipment. This module does not replace official training

Module lab

Common mistakes

Summary

• Application rate (L/ha) = L/min × 600 ÷ km/h ÷ swath width (m), used to compute and check flow rate
• Space lines by the measured effective swath and keep buffers around what must be protected
• Small droplets, high release, strong wind and hot dry air increase drift; choose nozzles and weather accordingly
• An emptying tank changes mass and centre of gravity, and spraying in Thailand must meet Department of Agriculture, CAAT and NBTC requirements

Check your understanding

1. Flow 1.5 L/min at 18 km/h with a 5 m swath. What is the application rate?
2. For 20 L/ha at 21.6 km/h with a 6 m swath, what flow is needed?
3. A 4.8 ha field at 15 L/ha with an 8 L tank. How many loads?
4. Why do small droplets drift easily?
5. An 18 kg aircraft at x = 0 carries 6 kg of liquid at x = 0.05 m. Where is the combined centre of gravity?