Table of Contents
The specificity of agricultural flights involves operating with a full load of chemicals, thus at maximum take-off weight, the need for high manoeuvrability when avoiding terrain obstacles and performing the shortest possible turns, as well as short take-off and landing on unpaved, field airstrips. This necessitates a strong fuselage and undercarriage construction with thick pneumatic tyres, as well as a wing with high-lift devices for short take-offs and landings and characteristics that resist stalling.
A sprayer aeroplane should meet the following main requirements: its useful load capacity should be from 600 to 1000 kg, furthermore it should have good longitudinal stability in flight, despite a rapid loss of payload up to 3 kg per second, a minimum working speed of 90–100 km/h, smooth and reliable flight at low altitudes of 20–30 m, short take-off and landing, and fuel economy. — A. Zdankiewicz*
The aircraft and agricultural equipment construction must be resistant to the effects of strong and concentrated corrosive chemicals. The pilot’s cabin should have protection against collision with power lines, be resistant to aircraft capotage, and provide air filtration from dust on the airstrip and chemicals during loading and when flying into chemicals dispersed over a field. Engine systems and appropriate filters must ensure fuel purity during refuelling and inlet air cleanliness from the ever-present dust on the airstrip.
Easy and quick access to basic airframe and engine servicing points must be ensured, along with ease of adjusting agricultural equipment settings and pilot control during flight. Rapid access to chemical loading points is essential, without shutting down the engine. Provision must be made for fuel reserves during longer repositioning flights and ease of assembly and disassembly of the aircraft for road or container transport.
Although production of specialist agricultural versions of aircraft began as early as the 1930s, the beginnings of agricultural aviation are associated with modifications of existing aircraft types whose parameters allowed attempts at their use in forestry and agriculture. The first generation of agricultural aviation designs consists of precisely such conversions.
An “old generation” agricultural aircraft is one built according to regulations applicable to so-called multi-purpose aircraft, and subsequently adapted for agricultural work […] — Feliks Borodzik*
In first-generation agricultural aircraft, the chemical tank was usually located behind the pilot. In the early period, two-seat biplane aircraft were most commonly used for agricultural aviation work. These were predominantly training and reconnaissance-bomber aircraft from military aviation. In the USA, the light bomber De Havilland DH-4, produced under licence in the United States, was most commonly used. In the USSR in 1928, the two-seat, biplane training and multi-purpose aircraft Polikarpov U-2 was developed, which was soon adapted for spraying and dusting. When the first helicopters appeared, this type of flying machine was also employed for agricultural work. After World War II, surplus military aircraft were used – bombers, transports and multi-purpose types.
The next step was the development of multi-purpose aircraft for which, among other capabilities, the possibility of fitting spraying and dusting equipment was envisaged. In the USSR and satellite countries, the primary agricultural aircraft that replaced the Po-2 in this role (also inheriting the evocative nickname “kukuruznik”) was the biplane, single-engine An-2. Designed by Oleg Antonov in 1946 at the request of the USSR Ministry of Forestry, the aircraft in its SCh and R versions was used for agricultural aviation work. Multi-purpose aircraft with agricultural aviation capability also include lighter designs, examples of which are the Yakovlev Yak-12 and the Aero L-60 Brigadyr – a single-engine, multi-purpose high-wing monoplane with fixed undercarriage.
Fully developed, specialised agricultural aircraft, which to this day form the basis of this type of aviation, appeared in the second half of the 20th century. One of the first was the Texas A&M AG-1, designed by Fred Weick in the late 1940s and first flown in 1950. The designer was known for placing great emphasis on safety.
The agricultural aircraft becomes a specialised agricultural machine whose design often remains exclusively a function of agrotechnical requirements and flight safety under agricultural conditions. — F. Borodzik, H. Kamiński, J. Krężałek*
It was a single-engine, strut-braced low-wing monoplane of all-metal construction. The designer applied solutions that became typical of 2nd generation agricultural aircraft: a fuselage structure enhancing forward visibility and the pilot’s chances of survival in a crash; a low-wing configuration, advantageous in view of the risk of collision with the ground or an obstacle; a chemical tank mounted in the fuselage between the cockpit and the engine firewall (engine–tank–pilot arrangement; in a crash, the pilot was not at risk of being crushed by the tank). On commission from Piper, F. Weick developed a design based on the AG-1, utilising components from the Piper PA-18 and PA-22 aircraft. The aircraft, designated AG-3, became the basis for production of the Piper PA-25 Pawnee. In the D version, another safety feature was introduced: fuel tanks were placed in the wings instead of the fuselage to reduce the risk of fire in a crash.
The PA-25 became one of the most popular agricultural aircraft – over 5,000 were produced. The second American pioneer of specialist agricultural aircraft was Leland Snow, an agricultural aviator and designer. In 1951 he built the S-1 aircraft, on which he performed agricultural aviation services for several years. The experience gained enabled him to design the improved S-2 model. It became the basis for the development of the Ag Commander/Thrush/Thrush Commander line of aircraft, produced by successive companies. Second-generation agricultural aircraft also include the biplane Grumman G-164 Ag-Cat, the IMCO CallAir A-9, the Cessna 188, the British Auster B-8 Agricola, the Australian Yeoman Ya-1 Cropmaster, the Czechoslovak Zlin Z-37 Čmelak (engine–pilot–tank layout), the Argentine Aero Boero 260AG, and the Brazilian Embraer EMB 202 Ipanema. The aircraft mentioned differ in size and payload capacity, but they share a configuration and characteristic silhouette with the “hump” of the reinforced cockpit.
The helicopter – a flying machine capable of flying low and precisely at low speeds, able to execute a turn with a radius unattainable for aeroplanes – quickly found application in agricultural aviation. The first spraying trials using a helicopter (Sikorsky R-4) were conducted in England by Dr W.
A helicopter turn during such [agricultural aviation] operations is quite similar to a wingover and takes only about 20 seconds. — Stanisław Wielgus*
E. Ripper. In the field of helicopters, no specialist type for agricultural aviation work emerged; due to operational parameters, light helicopters with the option of fitting equipment (tanks, pipes, atomisers, etc.) are generally used. Helicopters adapted for agricultural work include: the Djinn SO 1221 (France), the Kamov Ka-26 (USSR/Russia), the Cessna CH-1 Skyhook (USA), and the Schweizer 300/Sikorsky S-300 (USA).
Autogyros adapted for agricultural work are also produced.
Aerial application of chemicals requires the use of appropriate equipment and specific flight conditions depending on the type of agent and its physicochemical properties, the nature of the crop and atmospheric conditions. Solid agents – mainly dust, powder and granulated fertilisers – and seeds are dispersed using aerodynamic tunnels suspended beneath the chemical tank.
At WSK PZL Warszawa-Okęcie, the first agricultural aviation equipment designs intended for combating forest pests were produced in 1948 for the Li-2 airliner. — Robert Rowiński*
To achieve a wider application width, the flight altitude is usually 20–30 m. The application width (working width) ranges from 15 to 30 m depending on the dosage. Granulated fertilisers are also spread using centrifugal spreaders. The dosage is set on the dispenser on the ground before flight, and the pilot can only open and close the chemical outlet at the beginning and end of the field. Liquid chemicals must be atomised into droplets of the appropriate diameter. Ultra-low-volume (ULV) spraying with droplets of 50–150 microns in diameter is achieved using rotary atomisers driven by windmills or electric motors (on helicopters). These are usually highly concentrated agents – insecticides. This type of spraying creates a mist that settles slowly and penetrates the plants.
To reduce losses through evaporation and drift of chemicals, the working flight is conducted at an altitude of 1–3 m above the crop. This requires low wind speed (up to 5 m/s) and temperatures not exceeding 40 degrees Celsius. Medium and coarse spraying involves droplets of 100–500 microns in diameter, produced using several dozen spray nozzles distributed on pipes mounted on the trailing edge of the wing. These are usually aqueous fungicide solutions or herbicides for weed control. The main component of the liquid equipment is the pump assembly, suspended from the tank throat, equipped with a windmill-driven pump providing liquid pressure, a filter and a valve assembly for opening and closing the spray and mixing the suspension in the tank. The dosage is set by fitting nozzles of the appropriate diameter onto the pipes.
