As airplane design matured, the business of crop-dusting—or “aerial application,” the preferred professional term—also became more sophisticated. Farmers became increasingly aware of, and dependent upon, crop-dusting aircraft to efficiently fertilize their crops and dispense chemical pesticides on destructive pests.

The scores of biplane military trainers developed in the World War II era later turned into an unexpected windfall for the crop-dusting industry. The early days of crop dusting were often hazardous for the pilot because lightweight airplanes with weak frames were modified for use as crop-dusters—this situation changed for the better immediately after the war, when sturdily built surplus aircraft with improved pilot visibility were soon transformed into crop dusters.

The famous Boeing/Stearman Model 75 Kaydet two-seat biplane, (right) developed in the mid-1930s as the primary trainer for both the U.S. Army Air Corps and the U.S. Navy (under their military designations of PT-17 and N2S, respectively), became a mainstay of the post-war crop-duster industry. More than 8,000 Kaydets (and variants) were manufactured in a 10-year span before production ceased in 1945, and thousands were sold as post-war surplus for as little as $250 apiece. The robust Kaydet's fabric-covered control surfaces, common in the 1930s, were usually replaced with a metal skin, more powerful radial engines were installed and a hopper was retrofitted into the front cockpit to contain the crop-dusting load.

Piper's venerable J-3 Cub, in which 80 percent of all United States military pilots during World War II received their initial flight training, was also transformed into another popular post-war crop-duster. The Cub's small size and agility permitted it to be flown from short dirt runways that could not support larger aircraft but, when equipped with more powerful engines, many Cubs could carry 1,000-pound crop-dusting loads.

The piloting skills required for crop dusting were usually passed along in informal flight sessions, treated as more of an art form than the sophisticated science that it was. Pilots were taught how to fly low, with their wheels almost touching the crops, as a method to reduce “chemical drift”—fertilizer or pesticides straying into the wrong areas. Obstacles seemed to await them at every turn. Cropdusters were always clipping their tails on watertowers, hitting fence posts or pulling up without seeing a string of electric or telephone wires—unseen dangers that often meant injury or death to the pilot.
Recognizing the double dangers of crop spraying: the natural dangers of low flying and the added hazards of the often toxic chemicals they delivered to the fields, several aircraft manufacturers began to offer dedicated crop-spraying aircraft.

One of the best known and most widely used is the Piper PA-25 Pawnee (the Piper company named most of its light aircraft after native North American tribes). The PA-25 first appeared in 1956, replacing the Stearman and other surplus aircraft in the trade.

The company had recognized the nature of the risks involved by consulting, during the design stage, the Crash Injury Research Unit of Cornell Medical College. The Piper designer had appreciated that the chances of his aircraft ending up as a pile of parts after colliding with a windmill, barn or an old oak tree, or simply flying into the ground on some Midwestern farm, was high. The pilot was given an excellent view from a cockpit placed high on the fuselage of the low-wing monoplane. The wings were cranked to get the spray bars running along the trailing edge as close to the crops as possible. The fuselage was designed to protect the pilot, the cockpit being in effect a strong capsule that was sufficiently substantial to remain undamaged even in a head-on collision.

To aid the possibility of the pilot's survival, the metal fuselage was designed to fail progressively; for example, the fuselage longerons were given a slight outward set so that in the slow-speed crashes associated with spraying they would bulge outwards, absorbing kinetic energy, and for that reason all heavy fittings were forward of the cockpit area. The cockpit had a false floor with a l0 inch gap to absorb energy in a pancake-type crash, and it also had large opening windows on both sides.

The other hazard in crop spraying, which was the contamination of the airframe and cockpit by toxic chemicals, was made less dangerous by positive ventilation of the cockpit with the air intake high enough to avoid the spray fallout. The ventilation air flow also lightly provided positive pressure to the fuselage to keep out chemicals. At the end of the day the entire top section of the fuselage could be removed in a minute or so for cleaning. The Pawnee had a very strong undercarriage with low-pressure 'doughnut' tires and pneumatic oleo struts to absorb the rough landings and take-offs from the dirt roads of the farms. The engine was originally a 150hp Lycoming but that was soon uprated to a 235hp Lycoming flat four 0-540 six-cylinder unit. The spray tanks contained 150 gallons and were interchangeable with hoppers for crop dusting with 1,2001b of dry powder chemicals covering a swath of 60ft. The Piper Pawnee was, and remains, a very strong and purpose-built workhorse now regarded as a classic.

Many Pawnees have nowadays found their way to a glider airflield where they are very popular as glider tugs due to their good power/weight ratio.

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Fiddlersgreen Piper Pawnee. The cockpit is easily made transparent by gluing a little clear plastic behind the cut out frame.

Specifications: Crew: one Capacity:120 gal or 1,200 lb of chemicals Length: 24 ft 9 in Wingspan: 36 ft 2 in Height: 7 ft 2 in Empty: 1,288 lb Maximum takeoff: 2,900 lb Engine: 1 Lycoming O-540-B2B5, 235 hp Max speed: 188 mph Svc ceiling: 13,000 ft