A Spray Drone Is a Liquid Handling Machine That Happens to Fly

Most teams that fail at this start from the airframe. They take a heavy-lift multirotor, strap a tank underneath, add a pump and four nozzles, and discover that the hard problems are all downstream of the propellers: the load shifts in flight, the flow rate drifts with battery voltage, the application rate changes every time the ground speed changes, and the chemical eats a seal after forty hours. The aircraft is the easy part. The spray system is the product.

Commercial spray drones in the United States now cluster around 10 to 16 gallon (40 to 60 L) tanks with all-up weights of 130 to 250 lb, and they compete against ground rigs and manned ag aircraft on cost per acre, not on novelty. That is the number every design decision has to serve.

The Tank: Volume Is the Least Interesting Property

Tank capacity sets endurance, but four other properties decide whether the aircraft flies well.

Slosh. A partially full tank moves. A 40 L tank at half fill can shift 20 kg of mass laterally during a turn, which the flight controller experiences as an unexplained disturbance with a delay. Internal baffles are mandatory, not optional, and the baffle layout should be validated with the tank at 25, 50, and 75 percent fill rather than just full and empty.

Center of gravity migration. As the tank empties, the CG moves. If the tank sits below and slightly behind the CG, a full-to-empty cycle can shift trim enough to eat into control authority at the end of a pass. Position the tank so its centroid sits as close to the aircraft CG as packaging allows, and verify the trim change across the full drain.

Chemical compatibility. Agricultural formulations include aggressive solvents, surfactants, and emulsifiable concentrates. HDPE and polypropylene handle most of them; polycarbonate and many adhesives do not. Rotationally molded HDPE is the standard production answer for tanks in this size class, and the process economics are covered in rotational molding.

Cleanout. Operators switch chemicals between fields and carryover contamination can destroy a customer's crop. Design for a full drain with no low-point traps, a wide fill opening with a strainer basket, and a rinse tank if the platform is large enough. Residue in a dead leg is a lawsuit.

Pump and Plumbing

Diaphragm pumps dominate this application because they self-prime, tolerate running dry briefly, and handle suspended solids in wettable powder mixes. Typical sizing is 1.5 to 4 gallons per minute at 40 to 100 psi. Centrifugal pumps give smoother flow but do not self-prime and struggle at the pressures fine nozzles need.

Two details separate a working system from a demo. First, the pump must be closed-loop controlled on measured flow, not on commanded PWM duty. Battery voltage sags across a flight, and an open-loop pump will apply noticeably less chemical at the end of a pass than at the start. An inline flow meter and a PID loop fix this for a modest part cost. Second, every wetted component needs to be specified against the chemical list: EPDM or Viton seals depending on the formulation, chemical-resistant hose, stainless or engineered-polymer fittings. Elastomer selection and gland design are covered in O-ring selection and gland design, and getting the durometer and compound wrong is the most common source of field leaks.

Precise Distribution: Nozzle to Control Loop

Coverage quality comes down to droplet size, and droplet size is a tradeoff. Fine droplets under 150 microns give excellent leaf coverage and drift badly. Coarse droplets above 400 microns resist drift and roll off foliage. Most spray drone work lands in the medium to coarse range, and the choice between flat-fan hydraulic nozzles and rotary atomizers is really a choice about how tightly you need to control that spectrum. Rotary atomizers hold a narrow droplet distribution independent of flow rate and cost more; hydraulic nozzles are cheap and shift droplet size with pressure.

Rotor downwash is the feature that makes drone spraying work. The downward airflow drives droplets into the canopy and reaches the underside of leaves that a ground boom misses. It also means nozzle placement relative to the rotor disk is a real aerodynamic decision, usually validated with water-sensitive paper cards laid out in a grid during flight testing. Do this early; the result frequently contradicts intuition.

The control layer ties it together. Application rate in gallons per acre must stay constant regardless of ground speed, which requires the flow controller to take velocity from the flight controller in real time and modulate pump output. Section control that shuts off individual nozzle groups prevents double-application on overlapping passes and headland turns. Terrain following, using radar or lidar to hold a constant 6 to 10 ft above the canopy rather than above sea level, is what keeps coverage even over rolling ground. The autopilot integration and sensor interfaces behind all of this are discussed in how to choose a drone flight controller, and the broader agricultural application stack in agricultural drones for precision farming.

Durability, Power, and the Field Environment

  • Corrosion and ingress. The aircraft flies through its own chemical mist every pass. Target IP67 on electronics enclosures and connectors, use conformal coating throughout, and specify hardware that survives daily washdown. Rating definitions and test methods are in IP ratings explained.
  • Structure. A 250 lb aircraft landing repeatedly on uneven soil at the end of every eight-minute pass accumulates fatigue in arms, landing gear, and motor mounts. Design load cases should include hard landings and one-motor-out, not just steady hover.
  • Power. High disk loading and short cycles mean 15 to 25 minute pack lives with 300 to 500 charge cycles. Operators fly all day, so the system design includes fast charging, a generator, and a pack rotation plan. Pack architecture and thermal limits are covered in drone battery systems, and motor and propeller sizing for heavy variable loads in choosing motors and propellers.
  • Regulation. In the United States, aerial application of chemicals requires FAA Part 137 operator certification, and aircraft above 55 lb need an exemption. State pesticide applicator licensing applies separately. None of this is the manufacturer's certificate, but customers cannot legally operate without it, so the documentation package and training you provide are part of the sale.

Build the Spray System First

Projects House develops agricultural UAS platforms with the fluid system treated as the primary subsystem: tank and baffle design, pump and plumbing specification, nozzle layout validation, and closed-loop rate control. Describe your target crop, acreage, and chemistry through our contact form and we will scope the system.