Piper PA-25 Pawnee

Agricultural aviation is almost as old as aviation itself. Plans for the application of pesticides from small airships in Germany date back to at least 1910. Due to a disagreement over who would ultimately finance these missions, these airships were never deployed. Senior forester Alfred Zimmermann from near Magdeburg continued to work on aerial pest control and filed patent application no. 247028 in March 1911. He described the process for destroying the nun moth and other forest pests by "spraying the trees from above, using an aircraft.

The First World War then interrupted the development of agricultural aviation. In 1918, the first aircraft was used for aerial pest control in the USA near Reno. In the Soviet Union and Germany, intensive development of various agricultural aircraft began in the 1920s. Existing civilian or military aircraft were usually converted for these tasks, making them suitable for spraying crop protection or pesticides.

PA-18 as a Precursor
After World War II, various special agricultural aircraft models were developed, particularly in the USA and the Soviet Union, along with their allied countries. These countries, with very large contiguous agricultural areas, naturally had an enormous need for crop sprayers for agricultural aviation. Decommissioned former military aircraft or robust civilian models were initially equipped with spraying equipment. This is how the Piper company also entered agricultural aviation by converting various Piper Cub models into agricultural aircraft due to the sharp increase in demand. A crop spray tank was installed in the cabin instead of the rear seat on the J-3 or PA-18, and the chemicals could be applied via nozzles mounted on the underside of the wings.

However, the heavy crop spray tank on the pilot's back always posed a significant additional risk. Agricultural aviation is, after all, one of the most dangerous activities in aviation. To optimally spray the agricultural areas, the pilot must fly at just a few meters above the field; High-voltage power lines, pylons, and trees pose dangerous obstacles. Add to that the enormous workload of more than 50 takeoffs and landings per day, often on small agricultural land. This poses a high risk of accidents, and a chemical tank that breaks loose behind the pilot poses a significant risk. The constant danger of uncontrolled release of various toxic fumes in the cockpit should also not be underestimated.

Pawnee Production Begins
Following specifications from the Texas Agricultural and Mining Institute for an optimized agricultural aircraft, construction of the Piper PA-25 Pawnee began in 1956/1957 at the new Piper factory in Verona Beach, Florida. Designer Fred Weick originally named the aircraft AG-3. However, in typical Piper fashion, the serial designation plus the name of a Native American tribe was chosen for the type designation. One of the most important design goals was improved pilot protection, or rather, the increased probability of being able to exit the aircraft largely uninjured in the event of a crash. The 370-kilogram spray and grit tank was positioned in front of the cockpit at the aircraft's center of gravity, and the fuselage structure was constructed from particularly sturdy steel tubes. The cockpit was elevated, offering excellent all-round visibility. Sophisticated cockpit ventilation prevented the ingress of toxic substances. The low-wing design also increased safety.

The strutted wing is derived almost entirely from the Piper PA-18, but the wing struts were relocated to the upper surface of the wing due to the low-wing design. The tail surfaces are almost identical in design and shape to those of the Super Cub, but have been slightly reduced in area. The robust, rubber-sprung bi-wheeled landing gear is also strongly reminiscent of the Cub designs.

The prototype and the first production aircraft were powered by a four-cylinder Lycoming O-320 engine with 150 hp. This engine had also been used in some Piper Cub models. The first prototype flew in 1957, and deliveries of the first production aircraft began in August 1959. The wing layout was also modified, probably during the ongoing production run. Additional triangular extensions to the root chord were added. This layout is also found in various other Piper low-wing sport and touring aircraft.

Piper PA-25 Pawnee plans

Performance Update
The later B version featured a six-cylinder Lycoming engine with an output of 235 hp. The payload could now be increased to 544 kilograms. A C version of the Pawnee was delivered in 1968. It was now also available with a 260 hp engine. The upper rear fuselage section was no longer covered with fabric but made of metal and could be removed within a minute using quick-release fasteners. This cover, known as the "turtledeck," made maintenance and cleaning of the fuselage much easier. Toxic and corrosive spray residues could thus be rinsed out of the fuselage much more easily.

The PA-25 Pawnee D version was delivered in 1973. It was also powered by a 260 hp Lycoming O-540 engine. The fuel tanks were now located in the outer wings. This increased safety in the event of an accident. As with the last Piper Super Cub versions, the flaps and ailerons were no longer fabric-covered, but covered with metal skins. Some of the last Pawnee models feature rectangular wingtips, with elegantly shaped, slightly swept-down wingtips. It has not been determined whether this modification was a standard feature in the final series or whether this wingtip version was only available as an option.

Originals and Replicas
In 1982, Piper ended series production of the PA-25 Pawnee after approximately 5,000 units had been built. In 1959, a production aircraft cost $8,995 without the special spraying systems and was available only in the standard paint scheme of Daytona White with Santa Fe Red stripes. A license to build the replica was granted to GippsAero in Australia, which, after building a direct replica of the Pawnee in 1990/1991, then presented the further development, the GA200C. However, this new development still bears the typical Piper PA-25 Pawnee features and cannot hide its origins.

Piper itself developed the even more powerful PA-36 Brave from the Pawnee, which, in its most powerful version, produced just over 400 hp. Production began in 1973 and ended in January 1983 after more than 900 PA-36s had been delivered. Piper subsequently ceased production of agricultural aircraft. The Pawnee documentation and production tools were sold to Argentina on April 15, 1998. There, the company Laviasa attempted to build a replica. Production of the PA-25-235 and 260 is carried out under the designation "Puelche." A two-seat version was undergoing flight testing in 2010/2011. The PA-25-235 Puelche-Biplaza is largely identical to the Piper Pawnee except for the fuselage spine and the tandem cockpit. Laviasa would likely like to market this model as a simple trainer, for use by civilian or military flight schools.

The large, one-piece canopy of the Tandem Pawnee completely transformed the PA-25 Pawnee's originally rather rustic design, giving it a more elegant and pleasing appearance. Readers of this documentation now also have the opportunity to build a replica of this Argentinian Pawnee two-seater. A color side view drawing of this model is available for download at www.modellflieger-magazin.de.

In recent years, many PA-25 Pawnees have found their way to various sport flying groups and gliding clubs around the world, where they provide excellent service as tow aircraft. These robust, former US agricultural aircraft are replacing many Piper Cub, Robin DR 400, and Morane aircraft in towing operations. Some of these angular tugs are also in use in Germany – whether in Grabenstetten on the Swabian Alb, in the Ruhr area in Essen/Mülheim, at the gliding school in Oerlinghausen or in Cottbus Neuhausen – the tugs with the Native American tribe name perform their service powerfully.

Basic Structure
The fuselage is a fabric-covered structure welded from steel tubes. Several highly stressed parts are made of chrome-molybdenum steel (4130). Other parts are made of 1025 steel. As is typical with steel tube fuselages, wooden contour strips, together with the covering, give the steel tube frame its final exterior shape. The fuselage is covered with removable aluminum panels up to the cockpit. Behind the firewall is the fiberglass fuel tank with a capacity of 140 liters. Directly behind it and extending to the cockpit is the GRP spray tank for liquid or solid fuels with a maximum capacity of 544 kilograms. Filling is via a large access cover on top of the fuselage. Partial refueling of liquid fuels took place through a connection on the left side of the fuselage behind the cockpit.

The very spacious cockpit is glazed except for the roof section, offering excellent all-round visibility. The side windows are designed as doors and can be folded down for entry and exit. The roof and the rear cockpit panel are constructed of fiberglass. Conventional controls such as a central joystick and rudder pedals are used for steering. On the left side of the cockpit are the throttle, trim, mixture control, and spray system controls, as well as the tow coupling release handle. The instrument panel contains only the most important displays and instruments, optimized for agricultural flight operations. The cockpit is forced ventilated and slightly pressurized to prevent the unwanted entry of any toxic substances. On purely agricultural aircraft, a cable deflector is located in front of the cockpit windshield to at least deflect, if not cut, any cables or wires. On the many aircraft that later went to various aviation groups, a tow coupling was attached to the rear.

Braced
The wing is braced to the forward fuselage sidewalls by two struts each and is constructed of fabric-covered light alloy. It is a two-spar wing design. The leading edge is covered with aluminum. The wings are attached to the lower fuselage at the wing connection fittings. The further connection to the fuselage is provided by the sturdy, teardrop-shaped wing struts, which are attached to the upper side of the fuselage and to the strut connection fittings of the wing spars. The USA-35-B airfoil is used. The wing and rib structure are constructed of riveted aluminum web construction. The outer contour of the wing tips is made of ash wood. Except for the wing tips and the triangular root area, the wing has a constant profile depth of 160.02 centimeters.

Large, fabric-covered ailerons are mounted on the wings for control. In addition, landing flaps are located between the fuselage and the ailerons. The control surfaces were fabric-covered in the early versions, later with aluminum skins. The fuel tanks were relocated to the outer wings in the final Pawnee version, the D version.

Aft
The tail surfaces are based on a basic construction made of profiled steel tubing. All fins and control surfaces are fabric-covered. The tail fins are braced against each other by stainless steel wires. The rudder and elevator are equipped with an aerodynamic compensating surface in front of the control axis. The horizontal stabilizer is adjustable by a spindle drive.

The bipod landing gear is fixed and suspended by expandable rubbers and equipped with hydraulic wheel brakes. In the C version, the landing gear was equipped with oil-filled shock absorbers. The undercarriage frame was also partially covered with fabric. The tail gear is pivoting.

Propulsion is provided by a Lycoming O-540 B2B5 six-cylinder piston engine with an output of 235 hp, used in the C version. A 260 hp Lycoming engine was also available as an option. The two-blade fixed-pitch propeller has a diameter of 213 centimeters. Alternatively, a four-blade propeller with a diameter of 185 centimeters was available. The engine is completely covered by an aluminum cowling. The cowling front is made of fiberglass. At least in Europe, many PA-25s are equipped with a large, external muffler.