Agricultural Aircraft Product

Overview

An agricultural aircraft is a flying chemical applicator. Everything in the design serves one cycle: load up to 1,900 litres of liquid into the Hopper and Spray System, take off from a short farm strip, fly parallel swaths 2–4 m above the crop at 220 km/h, and return empty in twenty minutes. The Air Tractor AT-502 and Thrush 510 define the modern type: a welded steel-tube Fuselage, a low constant-chord Wing, a single Turboprop Engine of 560–1,000 kW, and a pilot enclosed in a Cockpit Safety Cage designed around the assumption that wire strikes and forced landings will eventually happen.

The hopper sits between the engine and the cockpit, on the centre of gravity, so the aircraft trims almost the same full and empty — necessary when 1,300 kg of payload can leave the airframe in a few minutes of spraying or in five seconds through the emergency dump gate.

Structure and crashworthiness

The fuselage is not monocoque. A Steel Tube Truss of welded 4130 chromoly carries all primary loads, with quick-release Sheet Metal Panel skins that come off for the daily washdown — agricultural chemicals are corrosive, and corrosion inspection drives the maintenance schedule. The truss is deliberately arranged so the forward structure crushes progressively in an accident while the cockpit cage stays intact; agricultural flying has the highest accident exposure in civil aviation, and the airframes are designed for survivability rather than the pretence that accidents will not occur. A Wire Cutter on the tail and a hardened Windshield address the most common threat, unmarked power lines, by deflecting a snagged cable over the airframe.

The wing uses two built-up Wing Spar members whose lower caps are the fatigue-critical item on the type — spray flying is continuous low-level manoeuvring in gusty thermals, and spar life limits or mandatory inspections (typically every few thousand hours) govern the fleet. Wing Rib formers set a constant-chord section that is cheap to build and docile at the stall.

Powerplant

Nearly all current production uses a free-turbine turboprop of the PT6A pattern. The Gas Generator spins near 38,000 rpm; a mechanically independent Power Turbine extracts shaft power and drives the Propeller through the planetary Reduction Gearbox at about 2,000 rpm. Turbines replaced the big radial pistons because they run on Jet A-1, shrug off the hundred-plus power cycles of a spraying day, and hold time between overhauls of 3,600 hours or more. The reversing propeller's beta range shortens landing rollout on strips that may be 600 m of dirt. Side-mounted Exhaust Stack ducts angle the exhaust away from the spray cloud so heat and gas do not distort the droplet pattern.

The spray system

The Hopper Tank feeds a Gate Box at its base — part metering valve, part emergency dump that can jettison the full load in under five seconds if the engine fails after takeoff. Liquid flows to the Spray Pump, a centrifugal pump driven not by the engine but by a small fan in the slipstream: simple, sparkless and self-regulating with airspeed. From the pump, plumbing runs to two Spray Boom tubes along the wing trailing edges carrying 30–40 Spray Nozzle bodies.

Droplet size is the controlling variable in application. Fine droplets cover well but drift onto neighbouring fields; coarse droplets stay put but waste chemical. Nozzle orifice, spray pressure and the angle of the nozzle into the airstream together set the droplet spectrum, and a GPS-coupled Flow Controller varies pressure in flight to hold a constant application rate in litres per hectare as groundspeed changes with wind. The booms span less than the wing on purpose: spray released near the wingtip is entrained in the tip vortex and lifted out of the swath, so the outboard 25 per cent of each wing carries no nozzles.

Flying the pattern

The pilot flies from a sealed cockpit pressurised with charcoal-filtered air through the Cabin Air Filter, strapped to the Seat Assembly inside the cage. A Swath Guidance Display driven by GPS replaced the human flaggers of the 1980s, steering the pilot along swaths spaced to the effective boom width. Each pass ends in a procedure turn — pull up, turn 180 degrees, descend back to 3 m — flown 400 or more times a day, which is why the unboosted Flight Controls are tuned for light stick forces and why the Trim Actuator matters: the aircraft loses a quarter of its weight during each load and must be re-trimmed continuously.

The fixed Landing Gear with spring-steel Main Gear Leg legs has no oleo struts to service and tolerates rough strips; a locking Tailwheel Fork keeps the rollout straight. Fuel lives in two integral wing tanks of the Fuel System, enough for about three hours — half a working morning between hot refuels with the engine running.