June 2005-
The Piper Pawnee is a working airplane, an employee that is paid by the hour and usually does hot, dirty jobs, often seven days a week.
It’s not likely that many Piper Flyer readers have ever flown a Pawnee, but you can be sure that the experience the company gained with this tough little bird found its way into every succeeding airplane from Lock Haven and Vero Beach.
The first crop duster was actually a U.S. Army pilot, Lt. John Macready. In 1921, he thought it might be a good idea to use an airplane to spread insecticide dust on a field outside of Troy, Ohio that was infested with caterpillars. He was right.
In fact, it was such a good idea that the aerial application business soon caught on as a quick and timely method of applying chemicals, particularly on expansive Southern cotton fields fighting infestations of Anthonomus grandis—boll weevil.
But widespread use of the airplane for aerial application did not become commonplace until after World War II. That timing was for two principal reasons: 1. the invention and acceptance of new chemicals and farming practices that had developed during the war years, and 2. the abundance of inexpensive surplus airplanes. For a few hundred dollars, a pilot could buy a Stearman biplane and convert it to carry and disperse dust or liquid pesticide, herbicide, fertilizer, or even seed.
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 airplane was particularly suited to the large-scale farming operations in the West and the labor-intensive cotton or rice farms of the South.
There was only one problem. While Stearmans were rugged and cheap enough, they just didn’t have the performance to keep pilots out of trouble on hot days or at high elevations when climbs from a 60 or 70 mph run five feet above the ground to an altitude high enough to make a quick, safe turn were required.
Of course, there was an abundance of Cubs and Super Cubs and similar aircraft that had been fitted with spray booms or spreader boxes, but their use was limited on large acreages because of their slow speed and small useful load.
Texas A&M University had a vested interest in agricultural aviation because of the state’s millions of acres of cotton, rice and cereal grain farms. In 1949, they proposed a design project dubbed the Ag-1 to build a specialized, all-metal airplane that would take advantage of wartime advances in aerodynamics, construction and materials and one that would provide good payload and performance and eliminate the vices experienced with the Stearman.
The University got support from Piper, Beech and Cessna for the project. Prof. Ben Hamner did the structural design, aided by contributions of Fred Weick, a veteran designer whose resume included the prestigious Fawcett Award in 1946 for the concept and design of the Ercoupe.
As a long-term result of the Aggie project, Piper and Cessna developed evolutionary designs of their own. Leland Snow, one of Hamner’s students who assisted with the project, would found Snow Aeronautical and later, Air Tractor.
Piper was the first to capitalize on the project. Bill Piper’s son, “Pug,” was interested in producing the airplane, but as usual the company didn’t have the resources to invest in research and development, so in 1953, they sponsored another A&M design project calling 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 hired Weick and put him in charge of building it as the PA-25 Pawnee.
It was a totally original design, and as a bonus, Piper had predictably accomplished it on a small budget. Low, strut-braced Cub wings were attached to a downward-sloping forward fuselage; the pilot sat high behind the hopper that had a capacity of 20 ft³ (145 gal., 800 lb.); and the aft fuselage was fabric-covered. It had PA-22 vertical and horizontal tails and the main gear was standard-issue Piper. It was 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 sold for an average retail price of $11,000 f.a.f. Lock Haven. 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. As soon as the PA-25-235 debuted in 1962 fitted with a Lycoming O-540, the 150 hp version was pretty much dead and gone. 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 Pawnee B introduced in 1964 was further up-rated with an even larger hopper, improved dispersal gear, better ventilation, double tail braces and a new propeller. 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.
Meanwhile, Cessna Aircraft had introduced their all-metal Model 188 Agwagon in 1967. The Cessna was priced higher, intended for larger-scale operators; Piper began developing a competitive model.
When the big Pawnee Brave was introduced in 1974, the airframe had changed so much that it had a new designation, the PA-36. It was assumed that sales of the larger-capacity agplane 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 6,000 examples of the design from Texas across 15 years of production.
The PA-36 was quite simply 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 ft.³ hoppers were 50 to 190 percent larger than previous models and held 225 gallons (+55 percent) of liquid or 1,900 pounds (+63 percent more) of dry materials. The specs and its $30,000 price were, not coincidentally, almost identical to the Agwagon’s.
But Piper was certain it had an ace in the hole: the Brave was slated to be powered by an exciting new engine from Continental. The Continental Tiara When Continental Motors became part of Teledyne in 1971, they had already been at work for several years on the first all-new General Aviation engine to be developed in decades, the Tiara.
The powerplant had relatively small displacement but a good hp/in³ ratio because it developed its power at high rpms. Its unique feature was Hydra-Torque drive, a patented system that could be hydraulically locked so the camshaft drove the propeller directly, providing a 0.5 speed reduction. Its unique advantage, according to the engine maker, was that the arrangement eliminated torsional vibration resonance at speeds from idle all the way to its redline at 4,500 rpm.
The first of the planned family of engines was the 6-285. It had a displacement of 406 cu. in., developed 285 hp at 4,000 rpm and weighed just 382 pounds. A later Tiara, the 6-320, was expected to put out 320 hp at the same weight.
The first problem Continental faced was that while airframe makers were excited about the engine, few were willing to go to the expense of redesigning and recertifying their products. The exception was Piper, who had a need to upsize their Pawnee line with larger capacity and more horsepower. Even though they had always been loyal to Lycoming, they liked the layout and the power range of the Tiara and bought a couple of them and sent them to the design team at Vero Beach.
Piper proposed Pawnees with two engine choices: the 6-285 or the 6-320 Tiara. The first prototype Pawnee II was ready before its powerplant was, and flew on Dec. 15, 1969 with a 260 hp Lycoming. The Tiara was later installed in the first prototype and a second airplane was added in 1971.
The first pre-production airplane—now named the Pawnee Brave—was built early in 1973 with the 320 hp engine, but when deliveries began in May, the 6-285 was the customers’ only choice. The base price of the new model was just under $30,000.
The first 111 Tiara-equipped aircraft were manufactured at Vero Beach, but beginning in the 1975 model year originated back home in Lock Haven. Meanwhile in Florida, the 320 hp Brave continued flight testing, but no decision to produce it was forthcoming.
Then operational problems started popping up on the Tiara. The high rpms—almost twice that of conventional engines—along with the high manifold pressures that were demanded by ag operators began to take its toll in premature failures of wear parts well before its rather conservative 1,200-hour TBO.
In addition, the General Aviation market was shrinking due to sharply rising costs, and the resulting lack of demand was threatening the very life of Teledyne Continental.
After pouring millions of dollars into the program, the Tiara was discontinued in 1978. Piper saw the writing on the hangar wall and had already replaced the engine a year earlier with Lycoming’s 300 hp IO-540.
At the same time, they had developed a conversion kit so owners of pre-1977 aircraft could make a similar swap. And, while they were on a power trip, Piper shoehorned the eight-cylinder, 375 hp Lycoming IO-720 into the Brave (requiring an additional eight inches of cowl), and followed that with a 400 hp version. Then they acquired a 585 shp Lycoming LTP101 turboprop and tested it in a beefed-up Brave airframe.
However, by this time prices were climbing out of control. The PA-23-300 was base priced at $54,760; the 375 at $73,170; and when the 400 hp Brave debuted several seasons later, it was well over $100,000.
Between 1978 and 1983, 250 PA-36-300s and just over 200 of the 36-375 and 400 versions were delivered. 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 938 units.
Daryl Murphy has been writing about flying and a variety of aircraft for 36 years. In addition to this magazine, his work appears in General Aviation News and Aviation International News. He has written five aviation books and one on automobile racing.
