Table of Contents
Drone spraying looks simple from the ground. A machine flies over the field, liquid falls, and the job is done. The engineering underneath is unforgiving. A 0.8 L/min nozzle matched to a 20 L/ha label rate at the wrong flight speed under-applies half the swath before the battery warning sounds. Fine droplets that drift off target can damage a neighbor’s crop within minutes. Both failures trace to the same root cause: nozzles picked without doing the math.
Product is the largest input cost in the pass. Uneven coverage means re-spraying, which doubles the cost. Off-target drift means claims and buffer violations. This guide gives the method and the numbers.
The method has seven steps. Map the spray system. Fix the flow per nozzle. Count the nozzles. Set the boom spacing. Choose the droplet class. Position the boom in the downwash. Calibrate before every flight day. Work through them in order and the drone becomes a predictable applicator.
The Drone Spray System, End to End
A drone spray system is a small hydraulic circuit hanging under an aircraft. Tank volumes run from 10 L on scouting-scale units to 50 L on production rigs. The pump moves liquid from the tank to the boom. Diaphragm pumps dominate; they hold pressure across a wide flow range and tolerate abrasive suspensions. Centrifugal pumps suit high-flow units, and gear pumps handle oil-based products but wear fast on powders.
Pressure does the real work. The nozzle converts pressure into velocity, and velocity into a spray sheet that breaks into droplets. Every nozzle on the boom must see the same pressure, or coverage goes uneven. A boom fed from one end starves the far nozzles at high flow. Keep the feed line large enough that pressure drop along the boom stays under about 0.2 bar.
Controllers handle pressure two ways. Pressure-based systems run the pump to hold a set point, usually 2 to 6 bar. Flow-meter systems measure total flow and adjust the pump to match a target, whatever the pressure does. Flow-meter systems compensate for clogged nozzles automatically. Pressure-based systems are simpler, cheaper, and still dominate on smaller rigs. You need the nozzle flow at the working pressure before you can size anything else.
Drone spraying runs at lower pressure than ground rigs for a reason. Low pressure makes coarse droplets, and coarse droplets drift less. The 2 to 6 bar band is the practical window for flat fans on a drone. Rotary atomizers run even lower, because they do not need pressure to atomize.
Flow and Pressure: The Working Relationship
Nozzle flow follows a square-root law. Double the pressure and flow rises only 41 percent. That matters because pressure is a weak lever for changing application rate. The relationship is Q = K × √P. Q is flow in L/min. P is pressure in bar. K is the nozzle constant, fixed by the orifice.
A nozzle rated 0.4 L/min at 3 bar has a K of 0.23. At 6 bar it delivers 0.57 L/min, not 0.8. To change rate, change the nozzle, the speed, or the number of open nozzles. Do not chase the rate with pressure alone. The same logic runs in reverse: a small pressure error becomes a small flow error, but a worn orifice becomes a large flow error.
| Pressure (bar) | Flow (L/min) | Percent of rated flow |
|---|---|---|
| 1 | 0.23 | 58% |
| 2 | 0.33 | 83% |
| 3 | 0.40 | 100% |
| 4 | 0.46 | 116% |
| 5 | 0.52 | 131% |
| 6 | 0.57 | 141% |
Table: Flow versus pressure for a flat fan rated 0.4 L/min at 3 bar, with K = 0.23. Values follow Q = K × √P.
The 141 percent at 6 bar is the whole story: pressure buys less than half the flow gain you might expect, plus a finer, more drift-prone droplet. Flow per nozzle, not pressure, is the primary design input for drone spraying.
The K factor is also the key to comparing nozzles. Two tips with the same K deliver the same flow at the same pressure. They may still differ in droplet class and pattern. Specify flow at pressure and droplet class together.
Flat Fan or Rotary Atomizer: The Core Choice
Two atomizer families dominate drone spraying: pressure flat fans and rotary atomizers. They differ in atomization method, droplet control, power draw, and rotor-wash behavior. The choice shapes the pump, the boom, and the flight plan.
Flat fan nozzles atomize by pressure. Droplet size is set by orifice size, pressure, and fan angle. They are cheap, light, and easy to clean. Their power draw is negligible, because the pump does the work. On drones they run at low pressure, 2 to 4 bar, to keep droplets coarse. A 110-degree fan gives wide coverage close to the crop. For a full walk through tip families, see our sprayer nozzle selection guide.
Rotary atomizers spin a disc or cage at 3,000 to 12,000 rpm. Droplet size is set by disc speed and feed rate, not by pressure. The pump can run at very low pressure, which simplifies the whole system. Uniform droplet size is easier to hit, which is why many production drones ship with rotaries. Each unit draws 50 to 200 W and adds moving parts. Cleaning is slower, and the feed tube can clog on fibrous suspensions.
| Parameter | Flat fan | Rotary atomizer |
|---|---|---|
| Droplet size control | Pressure and orifice only | Disc speed and feed rate |
| Working pressure | 2 to 6 bar | 0.5 to 2 bar typical |
| Flow per unit | 0.3 to 1.5 L/min | 0.1 to 0.5 L/min |
| Power draw per unit | Negligible | 50 to 200 W |
| Moving parts | None | Disc, motor, bearing |
| Cleaning | Flush in place | Disassembly for deep clean |
| Clog risk | Low with filtration | Feed tube sensitive to fibers |
Table: Flat fan versus rotary atomizer on a drone spray system. Ranges are typical published values for agricultural drone equipment.
The choice is a trade, not a verdict. High-volume passes, such as contact herbicides, favor flat fans, because they move liquid cheaply. Drift-sensitive work favors rotaries, because droplet size is decoupled from flow.
Rotary vs Hydraulic: The ROI Ledger
The core choice becomes financial over a season. Rotary atomizers cost more to buy and maintain; hydraulic flat fans spend their money mostly on chemistry lost to drift and re-spraying. Public cost data varies by region, so the ledger below uses published ranges and engineering logic instead of spot prices.
| Dimension | Rotary atomizer | Hydraulic flat fan |
|---|---|---|
| Purchase cost per unit | Higher: motor, bearing, disc assembly | Lower: single machined tip |
| Maintenance | Higher: rotating parts wear; discs need disassembly | Lower: orifice wear and screen cleaning |
| Working pressure | 0.5 to 2 bar; pump runs at low pressure | 2 to 6 bar |
| Flow per unit | 0.1 to 0.5 L/min; low-volume passes | 0.3 to 1.5 L/min; high-volume passes |
| Drift behavior | Droplet class set by disc speed; fine uniform drops, drift tracks height and speed | Class shifts with pressure; fine spray at high pressure drifts |
| Chemistry utilization | Uniform droplets cover well at low rates; suited to variable-rate work | Higher rates; streaks force re-spraying |
| Power draw | 50 to 200 W per unit; 400 to 1600 W for eight | Negligible |
| Field efficiency | Wider effective swath, 0.5 to 1 m per side | Swath roughly under the boom |
Table: Rotary atomizer versus hydraulic flat fan from an ROI view. Ranges are typical published values; cost claims follow engineering logic where prices differ by region.
Published trials do not crown either family on drift: at practical settings rotary drift matched or beat air-induction tips, yet a 3.5 m/s wind tunnel test measured 90.1 percent drift at 2 m for a rotary versus 40.6 percent for a flat fan at similar droplet size (Wang et al., 2023). The rotary premium pays back fastest when drift-sensitive or low-volume work dominates: systemic products at low rates, buffer-zone fields, and variable-rate spraying. Hydraulic fans win high-volume passes such as contact herbicides, where throughput beats drift savings. If the label fixes the droplet class, buy the family that holds it at working pressure. That constraint beats both ledgers. For a mixed season, spec both and switch by product.
Flow per Nozzle: The Working Range
Flow per nozzle on drones runs from 0.1 to 1.5 L/min. Rotary atomizers sit at the low end, 0.1 to 0.5 L/min each. Flat fans span 0.3 to 1.5 L/min. Total system flow is what the tank and battery can support. A 20 L tank at 6.4 L/min total flow empties in about three minutes.
Tank endurance is simple arithmetic: minutes per load = tank volume in liters divided by total flow in L/min. Run the numbers before you pick the nozzle count. A 30 L tank with 12 flat fans at 1.0 L/min empties in 2.5 minutes. The same tank with 8 fans at 0.6 L/min lasts 6 minutes.
| Tank volume (L) | Flow 4 L/min | Flow 6.4 L/min | Flow 10 L/min |
|---|---|---|---|
| 10 | 2.5 min | 1.6 min | 1.0 min |
| 20 | 5.0 min | 3.1 min | 2.0 min |
| 30 | 7.5 min | 4.7 min | 3.0 min |
| 40 | 10.0 min | 6.3 min | 4.0 min |
| 50 | 12.5 min | 7.8 min | 5.0 min |
Table: Minutes per load versus tank volume and total system flow. Values follow minutes = tank volume ÷ total flow.
Nozzle Count and Boom Spacing
Production drones carry 6 to 24 nozzles on booms of 1 to 3 m. Nozzle spacing follows the pattern geometry, not fashion. Adjacent flat fans must overlap 30 to 50 percent at the target plane to kill streaks. Rotary atomizers cover a disc, so spacing is set so the discs touch or overlap slightly.
Swath per flat fan = 2 × height × tan(angle ÷ 2). At 1 m height, an 80-degree fan covers 1.68 m. With 40 percent overlap, spacing is about 1.0 m. On a 2 m boom that means three nozzles. At 0.4 L/min each, total flow is 1.2 L/min.
The boom is rigid, so flight height sets the overlap. Fly lower and the fans pull apart, leaving streaks. Fly higher and the fans overlap more, which raises the rate in the middle. Keep the height inside the band the spacing was designed for. That band is usually 1 to 2 m for flat fans and 1.5 to 3 m for rotary rigs.
Boom layout checklist:
- Count the nozzles; confirm the spacing matches the fan angle at the planned height.
- Check the overlap at the target plane, not at the bench.
- Keep the outer nozzles inside the rotor wash footprint.
- Balance the boom within the aircraft payload limits.
- Number the sprayer nozzles on the boom, and log each catch during calibration.
Swath Width, Flight Height and Ground Speed
Spray speed on agricultural drones runs 3 to 10 m/s. The typical working speed is 4 to 7 m/s. Flight height above the canopy is 1 to 3 m for most passes. Lower height reduces drift but narrows each fan’s swath. Higher height widens swath and thins the application rate.
Coverage per hour scales with speed and swath. The formula is ha/h = 0.36 × speed in m/s × swath in m. A 6 m swath at 5 m/s covers 10.8 ha per hour. That number decides how many loads you fly, how many battery swaps you make, and how much of the day goes to turning around.
| Speed (m/s) | Swath 4 m | Swath 6 m | Swath 8 m |
|---|---|---|---|
| 3 | 4.3 | 6.5 | 8.6 |
| 5 | 7.2 | 10.8 | 14.4 |
| 7 | 10.1 | 15.1 | 20.2 |
| 10 | 14.4 | 21.6 | 28.8 |
Table: Coverage rate in hectares per hour versus ground speed and swath width. Values follow ha/h = 0.36 × speed × swath.
Effective swath is not always the boom length. Rotary atomizers throw droplets outward, so effective swath can exceed the boom by 0.5 to 1 m per side. Flat fans land roughly under the boom. Use the effective swath in the math, or the rate will be wrong at the field edges.
Speed consistency is part of coverage. A drone that slows in a headwind sprays heavier in that strip. GPS speed control holds the ground speed, but wind still shifts the airspeed. Fly crosswind patterns where the field allows.
Application Rate Math: From Liters Per Minute to Liters Per Hectare
Application rate is the number the label cares about. The standard formula is L/ha = 600 × Q ÷ (v × w). Q is total flow in L/min. v is ground speed in km/h. w is swath in meters. The 600 is the unit conversion from the 10,000 m² in a hectare.
Worked example. Eight nozzles at 0.8 L/min give 6.4 L/min total. Swath is 6 m. Speed is 5 m/s, which is 18 km/h. Rate = 600 × 6.4 ÷ (18 × 6) = 35.6 L/ha. The same rig at 7 m/s gives 25.4 L/ha. Speed is a strong lever, because it sits directly in the denominator.
| Speed (m/s) | Swath 4 m | Swath 6 m | Swath 8 m |
|---|---|---|---|
| 3 | 88.9 | 59.3 | 44.4 |
| 4 | 66.7 | 44.4 | 33.3 |
| 5 | 53.3 | 35.6 | 26.7 |
| 7 | 38.1 | 25.4 | 19.0 |
| 10 | 26.7 | 17.8 | 13.3 |
Table: Application rate in L/ha for a total flow of 6.4 L/min, versus ground speed and swath width. Values follow L/ha = 600 × Q ÷ (v × w).
Now reverse it for the label. A label calls for 20 L/ha. The rig flows 6.4 L/min over 6 m. Required speed = 600 × 6.4 ÷ (20 × 6) = 32 km/h, above the normal working range. The fix is fewer nozzles: five nozzles at 0.8 L/min flow 4.0 L/min, which needs 5.6 m/s. Always size the flow to the label first, then set the speed inside the flyable band.
Rotary rigs run the same math at lower flow. A 12-unit rig at 0.25 L/min flows 3.0 L/min total. Over a 7 m effective swath at 5 m/s, the rate is 600 × 3.0 ÷ (18 × 7) = 14.3 L/ha. That is a classic low-volume pass for systemic products. The math does not care which atomizer makes the droplets.
Rate errors compound from small mistakes. A speed error of 10 percent is a rate error of 10 percent. A nozzle flowing 12 percent high adds on top. Check all three at calibration.
Droplet Size: The Drift Trade
Droplet size is the biggest single drift lever. Drones usually target 150 to 400 µm volume median diameter (VMD). Finer droplets penetrate the canopy but drift. Coarser droplets land but skip coverage. VMD means half the spray volume is in smaller drops and half in larger. Our droplet size guide explains VMD and the measurement methods in detail.
The industry reference is ASABE S572, which sorts sprays into droplet classes. The classes are defined by VMD at the spray pressure. They are the language of label restrictions and buffer zones.
| Droplet class | VMD range (µm) |
|---|---|
| Very Fine | under 145 |
| Fine | 145 to 225 |
| Medium | 226 to 325 |
| Coarse | 326 to 400 |
| Very Coarse | 401 to 500 |
| Extremely Coarse | 501 to 650 |
| Ultra Coarse | over 650 |
Table: ASABE S572 droplet class boundaries by VMD. These published reference values appear in label language worldwide.
Terminal velocity explains the drift risk. A 100 µm drop falls at about 0.3 m/s in still air. A 400 µm drop falls at roughly 2 m/s. In the time a 100 µm drop falls 1 m, a 3 m/s wind carries it about 10 m. The same wind moves a 400 µm drop only about 1.5 m. Droplet class is therefore a legal decision, not just an agronomic one. Read the drift and pattern guide for the full mechanics.
Droplet class is not locked to the nozzle alone. Pressure shifts it within the same tip. A flat fan rated Medium at 3 bar may spray Fine at 6 bar and Coarse at 1.5 bar. On pressure-based systems, the class changes with pump state. Flow-meter systems hold the class steadier, because pressure stays near the set point.
Levers that move droplet size:
- Pressure: higher pressure makes finer droplets.
- Orifice size: larger orifices make coarser droplets at the same pressure.
- Fan angle: wide-angle fans make finer droplets than narrow fans at the same flow.
- Disc speed: higher disc speed makes finer droplets on rotary atomizers.
- Formulation: adjuvants change surface tension and breakup behavior.
Downwash and Boom Positioning
The rotors push air down. That downwash is the drone’s spray-assist. It drives droplets into the canopy and shortens their time in the wind. The boom belongs inside the wash column, typically 0.5 to 1.5 m below the rotor plane. Too low, and crosswind bends the spray before it lands. Too high, and the wash accelerates droplets and widens the drift footprint at the swath edge.
Rotary atomizers release droplets horizontally, so the wash carries them evenly. Flat fans shoot downward and can create stripes: heavy under the rotor disc, thin between discs. Spacing and fan angle are chosen to blend the wash, not to fight it.
Downwash also concentrates spray under the disc. The wash column is narrow relative to the boom, so coverage under the aircraft is not uniform by itself. The overlap rules in the spacing section exist precisely to smooth that out. Run a test pass over paper, and adjust the boom height from the stripes you see.
Battery and Payload Tradeoffs
Every kilogram of spray system is a kilogram less of product. Tank, pump, boom, nozzles, and guards all come out of payload. Rotary atomizers add 0.2 to 0.5 kg per unit and draw 50 to 200 W each. Eight units can pull 400 to 1600 W from the battery. Flight time per load is typically 5 to 15 minutes, and spray-system draw shortens it.
Match the tank to the battery. If the tank empties in three minutes and the battery flies ten, you carry dead weight for seven. The reverse is worse: a big tank that never empties means a heavy rig on every takeoff. Rule of thumb: size the flow so the tank empties near the end of the usable flight time.
Payload also caps the nozzle set. A 10 L scout drone cannot carry a 12-nozzle boom and a full tank. Work the flow math backward from the tank: total flow = tank volume ÷ target minutes per load. Then divide by flow per nozzle for the count the rig can support.
Calibration Before Every Flight
Calibration is the step most operators skip, and the one that pays for itself. The flow equation is Q = K × √P. K is the nozzle constant, fixed by the orifice. Measure it, do not trust the box.
Procedure:
- Fill the tank with a known volume of clean water.
- Run the pump at the working pressure.
- Collect from each nozzle for 60 seconds.
- Flag any nozzle more than 10 percent off the average catch.
- Replace the flagged tips, retest, and record the K factor.
Published practice sets the replacement threshold at about 10 percent flow deviation. The catch test takes ten minutes and catches three failures. Clogged screens starve nozzles. Worn orifices over-deliver. Wrong tips from the last change sit unnoticed.
Worn nozzles flow more, not less. Erosion enlarges the orifice, and flow rises with the opening. Over-application of chemical is the result, and it is a legal as well as an agronomic problem. The flow rate guide walks through the K-factor math in full.
Drift Mitigation in the Field
Drift guards are the first line of defense. Many drones ship with shrouds around the boom. Use them near boundaries and waterways. Flight height is the second lever. Keep the boom at the lowest height that still gives even coverage, usually 1 to 2 m. Weather is the third. Stop when wind passes about 4 m/s, and avoid temperature inversions, which hold fine droplets aloft and carry them far.
Droplet class is the fourth lever, and it is chosen at the nozzle. Coarse and very coarse classes drift far less than fine classes at the same height and wind. If the label allows a coarse class, run it. The pattern and drift guide covers the physics and the tip families.
Buffer zones on the label assume a droplet class and a wind limit. Fly inside a buffer only with drift-reducing settings, and log the weather when you do. Drift damage claims are won and lost on those logs.
Drift-safe day checklist:
- Wind under about 4 m/s and steady, not gusting.
- No temperature inversion at spray height.
- Drift guards fitted on boundary passes.
- Droplet class logged with the load.
Maintenance: Clogging, Wear and Cleanout
Clogging is the everyday failure: filter the liquid upstream, typically at 50 to 100 mesh, and clean screens at every load change. Rotary atomizer feed tubes clog on fibers and clays; inspect them like the screens.
Wear is the slow failure: abrasive formulations, wettable powders and suspension concentrates erode orifices, raising flow and shifting droplet size. Check flow monthly with the 60-second catch test and replace any tip that drifts past the 10 percent threshold.
Cleanout is the discipline: flush with clean water after every load, never let product dry in the lines, and disassemble rotary discs for deep cleaning. Check O-rings and spin bearings weekly, then drain and protect from freezing before storage. A rig that ends the season clean starts the next one calibrated.
Regulatory Notes: Label Rates and Buffer Zones
The label rate is a legal limit, not a suggestion. Applying above label is an offense in most jurisdictions, and a worn nozzle set that flows 12 percent high applies 12 percent over label. Buffer zones protect water and neighbors: they are measured from the swath edge and assume a droplet class and wind limit. Drift outside the zone is your liability even if the weather changed mid-flight.
Records matter: log tank size, flow, speed, swath, droplet class and weather for every load. Certification varies by country; check local rules before the season.
The label also fixes the droplet class in many cases. If the label demands a coarse or coarser class, the nozzle set must deliver it at the working pressure. That single line settles the flat fan versus rotary debate. Choose the atomizer that reliably hits the class and verify it with a catch test.
Send Us Your Duty Conditions
Nozzle selection for drone spraying is arithmetic, but the inputs are yours. Send us your duty conditions: tank size, boom length, nozzle count, working pressure, target rate in L/ha, and flight speed. Add the droplet class the label demands. BoreJet engineers will spec the nozzle set, spacing, and calibration table for your rig. Browse the agricultural spray nozzles range first, then contact us with your numbers.
Next Step
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Send your flow, pressure, fluid and target coverage. We come back with nozzle options and figures, not a catalogue number.
Written by
Ray ChanIndustrial spray nozzle specialist. I size tank cleaning, atomizing, flat-fan and spiral nozzles against real duty conditions, flow, pressure, fluid and target, rather than catalogue numbers. Every guide here comes from actual sizing work.