Agwagon: The Poor Man’s P-51

Written by Daryl Murphy
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December 2005- Author’s note: The Cessna Agwagons are working airplanes, employees that are paid by the hour to do hot, dirty jobs—sometimes seven days a week. It’s not likely that many Cessna Flyer Association members have ever flown an Agwagon, but you can be sure that the experience the company gained with this tough little bird found its way into every airplane that has since come from Wichita. 

The first use of an airplane to disperse agricultural chemicals was in 1921, when Army Lt. John Macready spread insecticide on a field outside of Troy, Ohio that was infested with caterpillars. The aerial application business soon caught on as a quick and timely method of applying dust on expansive Southern cotton fields to fight infestations of Anthonomus grandis—boll weevil.

 But widespread use of the airplane for aerial application did not become a common practice until after World War II. There were two reasons: the invention and acceptance of new chemicals and farming practices developed during the war years, and the abundance of inexpensive surplus airplanes.

For a few hundred dollars, a pilot could buy a Stearman biplane and convert it so it could 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.

Of course, there was an abundance of Cubs, 140s, 180s and similar aircraft that had been fitted with spray booms or spreader boxes, but their use was limited to smaller farms because of the small payload. Texas A&M University had a vested interest in agricultural aviation because of the state’s vast cotton, rice and cereal grain farms.

In 1949, they developed a design project dubbed the Ag-1 to build a specialized, all-metal airplane that would take advantage of postwar advances in aerodynamics, construction and materials and would provide good payload and performance while eliminating 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 who had won the prestigious Fawcett Award in 1946 for the Ercoupe. While While Cessna decided to mull it over, Piper snatched the project—and Weick—in 1956 and began work on what would become the PA-25 Pawnee.

Of course, neither company knew all that much about the highly specialized agricultural application market, but Cessna knew that entering it would require some homework. By its very location in “America’s Bread Basket,” a large percentage of its employees were people with farm backgrounds, but finding someone who had actually done—or even witnessed—aerial application was a different story.

As far as the airplane design, the company knew in theory what it would be. They had let Piper test the waters of the small agplane market, and they knew their only other major competition was the big Snow S-2 and Grumman’s G-164 Ag Cat biplane.

Cessna’s version would not be as large as those two, but it would offer more volume than Piper’s, and of course, it would be all-metal and adapt existing components wherever possible. Engineers began to bone up on ag aviation. Accustomed to designing shapes that encouraged speed, they now had to start thinking about aerodynamics as a way to manage the flow of dispersant material so that it did an effective job for the customer.

In 1964 a group of engineers set off for major aerial application areas in Georgia and Texas and interviewed owners and pilots and determine what they wanted. “To carry the requested 200 gallons of liquid,” wrote Cessna manager of flight test Bill Thompson, “it was necessary to consider a 200 sq. ft. wing area and about 40+ feet of span. The operators had requested a large and robust wing root `step area’ capable of withstanding abuse from loaders dumping 100-lb. bags of dry material into the hopper.”

That prompted use of existing Model 185 wings, attached to strong stub wing assemblies made of welded tubular steel and integrated into the tubular steel fuselage structure. Lift struts attached to the top of the wing and braced by an intermediate jury strut were specified.

At the time, drag wasn’t much of a consideration. To keep costs down, they used the tail cone, empennage and cowling from the Model 180. The pilot sat high behind the 200-gallon fiberglass hopper, and had a 360º view through large side and wrap-around rear windows.

Ventilation was placed at the top of the canopy to minimize ingress of spray from previous passes, and the cockpit had two doors. A massive tubular steel forward fuselage protected fuel tank, hopper and pilot.

The traditional Cessna/Wittman flat spring landing gear was made of heavy duty chrome-vanadium steel with 6.00 x 6 wheels or oversize 8.00 x 8 x 22 tires. The airframe was triple corrosion-proofed, cables were corrosion resistant and removable panels allowed operators to hose down the entire airplane—an exercise to try and prevent corrosion from chemical residue that most owners did at the end of every working day.

The first prototype was equipped with the old standby 182 engine, the 230 hp Continental O-470, and a fixed-pitch propeller. First flight was Feb. 19, 1965, and aerodynamic problems were discovered in the strut attach area. In addition, extremely low wing dihedral had been specified, but was unacceptable for the FAA-required lateral stability test in balked landing-climb configuration. The dihedral was initially increased to six degrees, and then 1970 when new testing parameters were written were raised to nine.

The large wing area produced somewhat sluggish aileron control, so the 185 wings were replaced with 206 wings modified with extended ailerons. Consideration was given to preventing degradation of performance by using internal booms in the wings with only the external nozzles exposed to the airstreams.

The experiment had some success, but operators pointed out that the gain in performance was not probably worth the possibility of leaky fittings creating serious corrosion inside the wing structure. With the 230 hp engine and fixed-pitch prop, at gross weight (3,300 lb. Normal Category, 3,800 lb. Restricted), top speed was 130 mph; service ceiling 10,000 feet.

The second prototype was equipped with a 300 hp Continental IO-520-D and constant speed propeller, which allowed a gross weight increase in Restricted Category to 4,000 lb. and at the top speed of 151, but added only 400 feet to the service ceiling.

Production began in the 1966 model year with the 230 hp Model188 and the 300 hp Model A188 priced at $15,995 and $18,995, respectively. 139 188s and 178 A-188s were delivered in the 1967 model year, and its clean lines earned it the nickname, “The Poor Man’s P-51.”

As a result of field experience, yearly updates were made. By the end of 1972, the fleet had grown to 211 188s and 621 A-188s. It was clear that operators preferred the extra horsepower margin of the A188 nearly three to one.

In 1972, the Agwagon C and Agtruck debuted. The C had camber lift wingtips, the Agtruck’s hopper had grown to 280 gallons, and both had 28-volt electrical systems and a host of lights for nighttime spraying.

The next year the Agtruck’s restricted category gross weight was increased to 4,200 lb. and in 1975 both models were equipped with swing-out engine mounts. Fuel capacity was expanded to 54 gal. in 1976 with two wing tanks. The Agtruck was now priced at $45,000, and optional equipment included an air conditioner.

The last significant improvements on the line were in 1979 with the addition of the $60,000 310 hp turbocharged option and 4,400-lb. gross weight By 1983, inflation and the adding cost of liability pushed the price past $100,000 and operators began looking for better value. Only 41 aircraft were delivered in its last year, but deliveries of the Agwagon during its 16-year production run had totaled 3,084, including 111 aircraft assembled in Argentina and Columbia.

The Poor Man’s P-51 had created a lot of business for Cessna, and is still making a profit for its operators more than 20 years after the last example rolled out of the factory. 

On the value of all-metal agplanes

“My brother and I were spraying Arizona orange trees in Stearmans, and there were smudge pots at one end of the field,” the veteran crop duster was telling a group of rapt listeners. “What we didn’t know was that our chemicals were flammable. I turned on the spray a little too soon when I started a run and it caught fire. Problem was, I was going 70 mph and the fire was going about 80! At the other end of the field when I pulled back on the stick, it just wiggled around in my hand. I turned around and all I saw behind me was flames and bare tubing where there used to be fabric. The whole back end of that Stearman was on fire!”

“Wow,” a young man said, “What did you do?”

“Fifteen feet off the ground in an airplane that’s disappearing real quick? Not too many choices—I jumped out!”

“And what happened then?” the wide-eyed listener asked. “Oh, I was killed; nobody can survive a jump that low,” the duster replied dryly.

The subtle difference between five and six feet altitude

In 1966, when Cessna was readying the Agwagon for the market, we were filming the airplane at work to show to dealers at the introduction meetings. The opening scene was to be shot just after sunrise, with the Agwagon silhouetted against the morning sky, flying directly at the camera.

I had briefed the pilot, an experienced, high-time ex-crop duster Cessna had hired as a demo pilot, and he took off for the first of a planned five runs. I set the camera up at the end of the field and fixed my eye on the viewfinder as he came toward me at 120 mph. When he passed overhead, I could feel my hair stand up from the prop wash. Then he landed and taxied over.

“What’s wrong?” I yelled as the engine came to a stop.

“You might want to stand somewhere else,” he said quietly through the side window. “When I fly this low, I really can’t tell the difference between a five and a six-foot altitude!”

From then on, I watched through binoculars.  

(Editor's note: The term "Poor Man's P-51" was used by Robert Cassidy as far back as 1992 in his article of the same name. That article was likely the inspiration for this article's title.)