Remembering ‘The Farmer’s Air Force’
Aerial application industry marks its centennial with anniversary of Curtiss Jenny JN-6 crop duster.
Commonly known as crop dusters or air sprayers, aerial applicators with names such as “Air Tractor” are actually farm implements, just like a tractor or combine. In most cases, however, aerial “tractors” are owned by specialized companies that contract with the farmer for their services.
Sometimes called “the Farmer’s Air Force,” agricultural aircraft have been a fixture in the U.S. for the past 100 years. Following World War I, a war-surplus Curtiss JN-6 Jenny was pressed into service in the first-known use of an aircraft to dust crops with a powder-based insecticide, spurring adoption of the phrase “crop duster.” In a joint effort by the U.S. Department of Agriculture and the U.S. Army Signal Corps. The Jenny was modified to spread lead arsenate to kill catalpa sphinx caterpillars at an orchard near Troy, Ohio.
After World War II, surplus training biplanes such as the Boeing Stearman, with bins or tanks in place of the front cockpit and replacement engines doubling the horsepower, were commonly used in the burgeoning “aerial applications” industry, one which increasingly relied on liquid products rather than powder. In 1949, aeronautical designer Fred Weick, based at Texas A&M University, designed a dedicated agricultural aircraft: the AG-1.
Development of the AG series
The AG-1 first flew on Dec. 1, 1950; testing continued through 1953. At that point, Piper Aircraft sponsored Texas A&M University to design an agricultural aircraft based on the AG-1 using as many Piper Cub components as possible.
The resulting design was called the AG-3. The AG-3 was a single-seat, low-wing monoplane with the wings braced to the fuselage with struts. It had conventional landing gear with a tailwheel and was powered by a 135hp engine. The single seat was placed high in the fuselage to give the best visibility.
Production started in May 1959 with the designation changed from AG-3 to Piper PA-25 Pawnee. Engine power was first increased to 150hp and later to 235hp. A useful design feature was the ability to carry a crewman on a jump seat fitted in the hopper to assist with operations at remote stations.
Image: courtesy by San Diego Air and Space Museum Archive
Production continued through 1988. However, Leland Snow’s Snow S-2 was the first purpose-built ag plane to go into production, making its first flight in 1956, preceding the Pawnee.
Since then, highly developed aircraft, now with gas turbine (turboprop) engines, have dominated the fleet. Some serve double-duty as water bombers in areas prone to wildfires.
Design criteria unique to the industry
In aerial applications, both fixed-wing and rotorcraft (helicopters) are used. Since payload is the most important criterion (next to return on investment), the helicopter is at a severe disadvantage.
Image: by Robert N. Pripps
Conventional fixed-wing aircraft designs are the preference, assuming they can meet multiple requirements. The aircraft must have the ability to lift a heavy load from an unpaved strip. It must be able to endure frequent landings and takeoffs. It must have a cockpit with safety and survivability built in, and ample visibility. The aircraft’s cockpit must be pressurized and air-conditioned to prevent spray ingestion. And finally, it must have a tank design that allows for fast loading, easy cleaning and provision for rapid dumping in case of emergency.
Today, two American companies, Air Tractor, Inc. and Thrush Aircraft, are the only manufacturers in the world building agricultural aircraft. Both companies have their roots in Leland Snow’s Snow Aeronautical Co. and their aircraft reflect the heritage of Snow’s mid-1950s work.
Image: by Robert N. Pripps
In 1965, Snow’s corporation was purchased by the Aero Commander division of Rockwell International, which put Snow’s design into production as the Thrush Commander alongside the CallAir ag aircraft (called the Quail Commander) that it had also acquired, branding both as “Ag Commanders.”
When Rockwell dropped the Aero Commander brand in 1971, its Thrush Commander was sold to Ayres Corp. The Quail Commander was transferred to AAMSA, a Mexican conglomerate. At that point, Leland Snow left the company and was instrumental in founding Air Tractor, Inc. Snow died in 2011 at age 80.
Image: courtesy by Stefan Krause, license: FAL.30.01.1962
Air Tractor Inc., based in Olney, Texas, is a U.S. manufacturer of agricultural aircraft founded in 1978. The company began manufacturing a new agricultural aircraft based on Snow’s S-2B aircraft designated Model AT-300 Air Tractor. The AT-300 first flew in 1973 using a Pratt & Whitney (P&W) 450hp radial piston engine. A variant, the AT-301, used a 600hp P&W piston engine. The next iteration, the AT-302, used a P&W PT-6 turboprop engine. Further variations on the theme had enlarged fuselages, longer wings and more powerful turboprop engines through the AT-1002.
On July 1, 2008, Air Tractor, Inc. became an employee-owned company. Today, the company’s employee-owners produce an extensive line of ag aircraft with 400-, 500-, 600-, 800- and 1,000-gallon tank capacities powered by reliable P&W piston or turbine engines. Air Tractor is the largest producer of single-engine turboprop-powered aircraft in the world.
Image: courtesy by Creative Commons
Thrush Aircraft’s current production model, the S-2R, is an outgrowth of the Snow S-2. In 1977, Rockwell sold the production rights for the aircraft and the production facility at Albany, Georgia, to the Ayres Corp., a firm that had been retro-fitting turboprop engines to Thrush Commanders. In 2003, Ayres’ assets were purchased by Thrush Aircraft, the current producer of the aircraft.
Most of the S-2R aircraft built by Snow Aeronautical were powered by 600hp (P&W) radial piston engines. Those built by Rockwell either used that engine or an 800hp Wright radial engine. Ayres versions used P&W or Polish-built PZL 600hp radials and some 1,200hp Wright engines. Ayres also built some with P&W PT-6 turboprops. Under Thrush Aircraft, most of the production was equipped with various turboprop engines with as much as 1,600 horsepower. Many earlier piston-powered S-2Rs were converted to turboprop power by Ayres and Marsh Aviation.
The Ag-Cat was the first aircraft specifically designed by a major aircraft company for agricultural aviation, and the first aircraft designed according to the regulations of Civil Aeronautics Manual Part 8, which had been written especially for agricultural aircraft.
The aircraft labeled the “G-164” was designed and built in 1955 by Grumman Aircraft Engineering Corp., which was founded in 1929. Later, as Grumman Aerospace Corp., the company became a leading U.S. producer of military and civilian aircraft.
The G-164 Ag-Cat is a conventional all-metal biplane. The airframe incorporates many safety innovations, including a pressurized cockpit to keep pesticides out, air conditioning and a fuselage structure designed to progressively collapse in the event of a crash.
It was first equipped with a Continental W670 7-cylinder, 225hp radial engine. Subsequent versions have used P&W radial engines of 450 and 600 horsepower, a Continental radial of 550 horsepower, a Wright radial of 1,200 horsepower, and even with two 300hp Continental flat sixes mounted at the front side-by-side. Various turboprop engines by P&W and Garrett have also been employed.
Military orders prevented the production of the Ag-Cat by Grumman, so the entire program was subcontracted to Schweizer Aircraft Co., Elmira, New York. The ownership and production of the Ag-Cat design has changed hands several times since. It is not currently being produced.
The IMCO CallAir A-9 is a small agricultural aircraft that first flew in 1962, a development of the company’s previous successful crop-dusters. The single-seat monoplane with a strut-braced low wing was built by Intermountain Mfg. Co. (IMCO).
It has a fabric-covered steel-tube fuselage and a wood-and-fabric wing. The cockpit is enclosed by two removable, bottom-hinged doors that form the left and right side-windows. The aircraft is powered by a single Lycoming 6- or 8-cylinder engine of up to 400 horsepower.
IMCO was purchased by Rockwell International in 1966. Rockwell built the plane under its Aero Commander division as the Quail or Snipe Commander, depending on engine type used. Production was shifted to Mexico in 1971, under a joint venture there called AAMSA. Production continued until 1984.
Image: by Robert N. Pripps
The Cessna Model 188 AGwagon is a conventional single-seat, piston-engined, strut-braced low-wing agricultural monoplane developed from the Cessna 180 four-seat passenger plane. The 188 uses the same tail cone and fin structure (and the same 230hp Continental 6-cylinder engine) as the Model 180. The cockpit is pressurized on later models by means of an air scoop to reduce chemical ingestion. First flown in in 1965, the Cessna 188 is no longer in production.
The Cessna 188 came in several variations during its production run. In addition to the AGwagon, Cessna produced the AGpickup, AGtruck, AGhusky and AGcarryall.
The PA-36 Pawnee Brave was announced by Piper Aircraft in 1972 as a new version of the PA-25 Pawnee with a 285hp 6-cylinder Continental engine. The aircraft had a new wing with no external bracing, an improved ventilation and heating system and a larger standard hopper. The type entered service in 1973.
Image: by Robert N. Pripps
Later versions used Lycoming 6- and 8-cylinder engines of as much as 375 horsepower. In 1981, Piper sold the rights in the design to WTA Inc., which marketed two versions from 1982 with 375hp and 400hp engines as the New Brave 375 and New Brave 400 respectively. In October 1997, the design rights were assigned to The New Piper Aircraft, Inc. and production has not resumed.
The Gehling PZL 106AR Kruk (Raven) is a braced low-wing monoplane with a steel frame fuselage covered with aluminum (except for fabric-covered tail surfaces). The aluminum wings are fitted with flaps and slats. It has a single seat cabin, placed high, with an emergency seat for a mechanic behind the pilot.
It can be equipped for spraying, crop dusting or firefighting. It has conventional fixed landing gear with a tail wheel. The standard engine is a Polish-built PZL-3S 600hp radial with a four-blade propeller. Walter M601 or P&W PT6 turboprop engines are options.
The M-18 Dromader (Polish for dromedary) is based on the Rockwell Thrush, which was developed from the Snow S-2. Produced by PZL Mielec, the M-18 Dromader has been revised and enlarged over the years, but still uses the original wing design. The cockpit boasts of a 40g crash rating. The standard engine is a Polish Kalisz 9-cylinder air-cooled radial producing 980 horsepower. Some have been converted to turbine power.
Image: by Robert N. Pripps
The Fletcher FU-24 is a conventional low-wing monoplane with tricycle landing gear, side-by-side seating in front of the wing and hopper and pronounced dihedral (upward angle) on the outer wing panels. Production and use is mainly in New Zealand.
Originally powered by a 6-cylinder 250hp Continental, the model was later changed to a 400hp 8-cylinder engine. Some Fletchers have also flown with Chevrolet V-8s, and many have been converted to P&W PT-6 and Garrett TPE-331 turboprop engines. The Fletcher is not currently in production.
High-tech, high investment
Roughly 1,600 aerial application businesses operate in the U.S. today. Varied functions are performed, including crop seeding, fertilizing (crops, rangeland and forests), application of disease control products, public health spraying (to control viruses and diseases), forestry seeding and fertilizing, wildfire suppression and weed control.
Countries with large-acreage farms, such as Australia, New Zealand and some in eastern Europe, also have put aerial application to good use.
Ag pilots and ground crew people are highly trained professionals dedicated to safety; their lives and fortunes depend on it. A new, well-equipped ag turboprop airplane can cost its owner nearly $2 million to buy. Such aircraft would typically be equipped with a GPS system that not only assures the aircraft is at the right field, but also is accurate enough that it guides the pilot to the right swath of application.
Image: by Robert N. Pripps
Pilots fly either a racetrack pattern or a back-and-forth pattern, depending on field size and other factors. Radar systems assure the pilot of optimum crop clearance and delivery rate. Application speed is usually between 120 and 140mph (more than 10 times the speed of ground sprayers).
Ag pilots must attain commercial pilots’ licenses. They are also registered as commercial pesticide applicators and must meet the requirements of Federal Aviation Regulations Part 137, which allows for low-level aviation operations.
Aviation delivers the ultimate access
The speed of aerial application, compared to that of other forms of application, is an important benefit. Quickly eradicating a crop threat before it can spread or do further damage is the primary benefit. The aerial applicator’s ability to treat in conditions and locales where other forms of application can’t is another, enabling the farmer to treat the crop regardless of wet field conditions, thereby avoiding soil compaction.
Further, aerial treatment from above the crop canopy and not within the crop canopy avoids problems associated with ground rigs trampling the crop. The best way to produce more food on less land is through the use of appropriate aerial seeding, and application of pesticides and fertilizers. FC
After 36 years in the aircraft industry, Bob Pripps returned to his first love and began writing about tractors. He has authored some 30 books on the subject and several magazine articles. Pripps has a maple syrup farm near Park Falls, Wisconsin. In harvesting the maple sap, he relies on a Ford Jubilee and a Massey Ferguson 85.
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