Piper’s Pest-Busting Airplanes: PA-25 & PA-36 Pawnee Featured
In the first few decades of the aviation age, airplanes quickly became a part of Americans’ daily lives. In addition to providing invaluable service in World War II, aircraft also served in a war against six-legged pests.
The first known use of a heavier-than-air machine occurred in August 1921. A United States Army Air Service Curtiss JN-4 Jenny piloted by John A. Macready was modified at McCook Field to spread lead arsenate to kill Catalpa Sphinx caterpillars at a farm near Troy, Ohio. This first test was considered highly successful.
The first commercial operations were begun in 1924, by Huff-Daland Crop Dusting, which was co-founded by McCook Field test pilot Lt. Harold R. Harris. Huff-Daland was renamed Delta Air Service in 1928 and was the forerunner of what eventually became Delta Air Lines.
The aerial application business soon caught on as a quick and efficient method of applying dust on expansive Southern cotton fields to fight infestations of Anthonomus grandis—boll weevil—and it was particularly well suited to large-scale farming operations. The advantage to a farmer was that an aerial operator could apply the material in a matter of minutes instead of the hours or days that it took with ground-bound equipment.
The demand for “crop dusting” increased in the 1930s and 1940s and many an enterprising individual stepped in to fill that need. A pilot could buy a war surplus Stearman biplane and convert it to carry and disperse dust or liquid pesticide, herbicide, fertilizer or even seed.
Stearmans were cheap enough, but they just didn’t have the performance to keep pilots out of trouble on hot days or at high elevations. An agplane needed the ability to climb from a 60 or 70 mph run five feet above the ground to an altitude high enough to make a quick, safe turn.
Research and Development
Soon after World War II, the National Flying Farmers Association set up a research program on agricultural aviation. This was supported by the CAA and undertaken by the Personal Aircraft Research Center at the Agricultural and Mechanical College of Texas. A countrywide survey was conducted by the CAA to determine which features the users of such aircraft deemed necessary for an aircraft intended for crop spraying, dusting and fertilizer application.
Prof. Ben Hamner began work on the structural design, aided by contributions of Fred Weick, a veteran designer who had won the prestigious Fawcett Aviation Award in 1946 for the Ercoupe.
Aided by the donation of engine, propeller, undercarriage, etc. by interested aircraft manufacturers, including Piper, the prototype airplane AG-1 made its first flight on Dec. 1, 1950, less than a year after the start of the contract.
Bill Piper Sr. and Bill Piper Jr. had met with Weick at the National Agricultural Aviation Conference in February 1951. The Pipers were impressed with Weick and decided to use him as a consultant from time to time.
In 1953, Howard Piper traveled to College Station, Tex. to meet with Weick regarding a project for Piper. Two weeks later Howard called to see if Weick and the college would be interested in developing a new agricultural aircraft—with Piper as sponsor.
This project called for an aircraft that was smaller than the AG-1—one that used welded steel tube and fabric construction, and as many Cub and Tri-Pacer components as possible. The resulting Weick design was called the Texas AG-3, and as soon as satisfactory flight testing was completed in 1956, Piper Aircraft hired Weick and put him in charge of building it as the PA-25 Pawnee.
Weick used what he had learned in the AG-1 project and, by using existing components, was able to bring the PA-25 to market with a small budget. Low, strut-braced Super Cub wings were attached to a downward-sloping forward fuselage; the pilot sat high behind the hopper that had a capacity of 20 cubic feet (145 gallons, 800 pounds) and the aft fuselage was fabric-covered. The plane used PA-22 vertical and horizontal tails and the main gear was standard-issue Piper.
Powered by a 150 hp Lycoming O-320-A1A, the Pawnee was awarded a Type Certificate at a gross weight of 2,300 pounds. Two pre-production aircraft were built at the new company facility in Vero Beach, and production started in May 1959 at Lock Haven.
The first Pawnee was a 1960 model and was sold as a basic aircraft sans dusting system for $8,995. Equipped with the sprayer system that included the boom behind the trailing edge of the wing added another $675 to the price.
While the airplane was superior to most spray planes on the market, it was obvious to operators that the four-cylinder Lycoming was not. The airplane could get off the ground in less than 700 feet loaded, but its rate of climb was an anemic 505 fpm at best rate of climb (70 mph). Engineers decided to move the Pawnee up a notch.
Changes Proved Popular
The debut in 1965 of the PA-25-235 fitted with a Lycoming O-540 signaled the demise of the 150 hp Pawnee. Although the 235 hp model carried a $4,000 premium, about one unit a day moved out the door for the next three years.
Its gross weight had risen to a handier 2,900 pounds, hopper weight capacity was increased 50 percent, and climb rate went up by 25 percent. The 235 also featured a new bottom cowl with side exhaust, double tail brace wires and improved stall characteristics provided by the swept back leading edge wing root.
The Pawnee B introduced in 1964 was further up-rated with an even larger hopper, improved dispersal gear and better ventilation. It added 1,186 sales to the books.
The Pawnee C, which was available in either 235 hp or 260 hp versions from 1967 to 1972, accounted for another 1,758 units.
Cessna Aircraft had introduced its all-metal Model 188 AGwagon in 1967. The Cessna was priced higher than the Pawnee and was intended for larger-scale operators. Piper began developing a competitive model.
PA-36 Pawnee Brave
When the big Pawnee Brave was introduced in 1974, the airframe had changed so much that it required a new designation, the PA-36. The PA-36 was an enlarged PA-25—and it was considerably longer, wider, higher and stronger than its forebears.
Its 39-foot wingspan gave it a wider effective swath than the PA-25; available 30 or 38 square foot hoppers were 50 to 190 percent larger than previous models and held 225 gallons of liquid or 1,900 pounds of dry materials. This capacity translated to an increase of 55 percent and 63 percent, respectively.
The PA-36’s specs and its $30,000 price were, not coincidentally, almost identical to Cessna’s AGwagon. But Piper felt it had a big competitive advantage: the Brave was to be powered by an exciting new engine from Continental—the Tiara.
Continental brought its Tiara-powered Cessna 210 to Vero Beach and Piper was impressed by the smoothness and quietness of the engine. It seemed the perfect solution to provide the higher horsepower that Piper was looking to add to its new model.
Piper planned to make two versions of the bigger Pawnee: the PA-36 with the 285 hp Tiara 6-285 and the PA-36-320 with the 320 hp 6-320 Tiara. The first prototype (s/n 36-EA N36PA) flew on Nov. 17, 1969 with a 6-285 engine, 96-inch two-blade propeller, and stabilator. During development and after much discussion within Piper, the stabilator was replaced with a stabilizer/elevator system.
Type certificate A9SO was issued on May 31, 1972 and the model—now called the Pawnee Brave—was debuted at Vero Beach on Oct. 9, 1972. First off the line was s/n 36-7360002 on April 26, 1973. The early models were priced at $29,900 without dispersal equipment.
The Tiara engine was experiencing problems and early failures so Piper began work on a Lycoming-powered version of the Brave. Meanwhile Continental redesigned a beefed-up crankshaft and Piper moved production of the Brave to Lock Haven from Vero Beach.
Even with the changes, the Tiara engine was proving to be a problem, and in December 1976 Piper launched the Brave 300 powered by a 300 hp Lycoming IO-54-K1G5. Concurrently it released a conversion kit which allowed owners of the Tiara-powered Braves to switch to the Lycoming engine.
Power—and more of it—had been Piper’s goal for the Brave from the beginning, and in 1977 the company began development of a 375 hp Brave. The eight-cylinder, 375 hp Lycoming IO-720-D1CD required an additional eight inches of cowl, and a heavy-duty Lord Dynafocal engine mount system was also used. Deliveries of the Brave 375 began in 1978.
A turboprop version—the PA-36T—was developed but never made it to production.
Despite Piper’s efforts to keep costs and prices low, the Pawnees were suffering the same fate as all of General Aviation in the 1980s: falling sales. As the market for light planes and agplanes began crashing around them, Piper put the Pawnee line on the block.
In August 1981, West Texas Aircraft acquired the marketing rights to the Brave 300 and 375 together with 36 unsold aircraft.
Total production of the big Brave across the 11 seasons from 1973 to 1983 had been 926 units.
It was assumed that sales of the larger-capacity Brave would spell the end for the PA-25, but the Pawnee’s market had always been among small to medium-size operators, and the 25’s blend of performance, economy and profitability had created a loyal following.
The Pawnee D endured until 1981, by which time a total of more than 1,500 had been built, making a grand total of about 5,000 examples of this venerable model across 15 years of production.