Plummer's prop proves itself

by Bill Lavender

There really isn't a more sexy sound than that of a Viper 1200 Thrush passing overhead. It's not too loud, like the screaming -12 blades and throaty exhaust of the R-1340. It has a deeper resonance, quieter prop, that at the same time lets you know there is awesome power coming from its nine cylinders.

A PZL 1,000 horsepower engine mounted on an M-18 Dromader is not the 1,000 horsepower Cyclone Wright engine found on the Viper 1200 Thrush. The similarities are so close, however, the first thing I thought of when a Riceland Aviation Dromader flew over fitted with a Plummer DC-3 prop was my Thrush Viper 1200 from many years ago.

I wish I had more experience in the piston-powered Dromader with its factory installed propeller, but I don't. Therefore, I can't personally attest to the claims being made by the ag pilots who are now flying the Plummer Props STC conversion. The fact of the matter is, these claims are coming from every single operator who has made the conversion, thus I'm inclined to believe them.

Since June of 2001 when Plummer Props received its STC to convert the piston powered PZL engine's factory installed prop to a DC-3 propeller, the company has delivered more than 30 units (see AAU May 2001) for the Dromader. Recently, during trips through Louisiana and Missouri, I visited with two operators and asked them for feedback about the conversion; Riceland Aviation in Jennings, Louisiana and Pete Malone's Ag Air in Sikeston, Missouri.

When I arrived at the Riceland Aviation base in Jennings, all three Dromaders were working, and working hard. There were no plans to stop for lunch, it was rice planting time and they were behind.

According to Billy Precht, Jr., who manages and flies for Riceland, while his dad Billy, Sr. keeps the coals burning on one of the other two Dromaders, says the main reason Riceland Aviation opted for the conversion was for less maintenance. After the three Plummer Props were installed, the company found out there were other improvements that turned out to be a bonus.

"We are seeing about a 12 mph increase in airspeed. The harmonic vibration has evened out to practically disappearing. Dynamic balancing, however, is a must," said Billy, Jr.

Riceland Aviation found they could either reduce their manifold pressure settings three inches and save 10% in fuel with the same performance, or they could leave the settings used with the factory propeller and easily see a 10% increase in performance.

Billy, Sr. with years of experience flying the M-18, told AgAir Update, "The biggest difference I have noticed is the aircraft pulls better in the turns. But it's also better all the way around". Al Ritter, Riceland Aviation pilot responded, "My aircraft gets off the ground quicker and I'm enjoying smoother operation."

Since this visit last summer, Riceland has contacted AgAir Update with a report on their fuel consumption for 2002. "We have saved approximately 5.2 gallons of fuel per flight hour. From an average of 71.6 gallons of fuel burn per hour in 2001, we now average 66.4 gallons of fuel burn per hour since April of 2002. We also consistently see a five mph increase in airspeed." (By AAU's estimate, probably at least a three percent reduction in flight time for the same acreage flown in the past.)

A short flight from Jennings took me to Mid-Continent Aircraft in Hayti, Missouri allowing me to intercept Pete Malone, Jr. of Ag Air based in Sikeston and his pilot Giles Wright, while having new engine mounts installed on Ag Air's Dromader.

Ag Air operates an AT-502 Air Tractor and a Dromader. Giles flies the Dromader, while Pete flies the Air Tractor. Before the Plummer Props conversion, often the Ag Air Dromader worked close to Dow Croom's three factory stock Dromaders (Croom Flying Service), and had earned the reputation of being the "sorriest one of the lot". But, Dow tells a Cinderella story, where Ag Air's Dromader has leapt to the front with its Plummer Prop conversion, outworking his Dromaders easily.

Giles related, "Bill, I had already told Pete that if I had to fly his Dromader again this year, that I'd be in the market for another seat. In the past I had been flying an AT-502, and as you know, there is a world of difference in the two aircraft. But I must say, I have had to eat my words. After the Plummer Props conversion, I flew early fertilizer work on wheat, and as you can see, I'm still flying the Dromader and could not be any happier."

Ag Air's Dromader has the Hatfield expanded hopper. With it and the Plummer Prop, Giles hauls 4,800 pounds of urea without packing the hopper. Before the Plummer Props conversion he would have to use 36 inches of manifold pressure and 2,000 rpm, burning 70-75 gallons an hour to be able to safely manage the load. With the Plummer Prop, the same load is worked at 31 inches and 2,100 rpm burning 10 gallons less of fuel a hour. The airspeed has increased from 122 mph to 138-140 mph with the conversion.

Giles remarked that he, like Riceland Aviation, sees a notable difference in the turns. In the past, before the conversion, his turns were flat at about 150 feet AGL when loaded. Now, his turns are 400-500 feet AGL, without that dragging feeling through the turn.

Most of Giles' work is within five miles of the 3,000-foot strip. Using only 36 inches of manifold pressure for takeoff with a 4,800 pound load, his average dispersal rate is 18,000 pounds per hour. In early May, Giles had logged more than 100 hours with the Plummer Prop.

Pete Malone explained how earlier this year, before installing the Plummer Prop on his Dromader, he had planned to park the Dromader and use it primarily for back up purposes. Now that the conversion has been made, he's changed his mind and placed the Dromader on the flight line full time with plans to add another Dromader, providing it has the Plummer Prop STC.

When Sandy Friezner of Specialized Testing Service performed the static test on the Plummer Prop for the STC, he found the PZL prop developed 3510 pounds of static thrust, while the Plummer Prop was able to develop 3830 pounds of static thrust with the same power setting.

However, the performance increase from the Plummer Prop is only a bonus. The primary reason operators are making the switch is simple economics. A PZL prop is limited to six years or 1,500 hours of use, with an $11,000 to $14,000 overhaul cost. The Plummer DC-3 prop is limited to 10 years or 2,400 hours of use with only a $3,500 overhaul cost.

Since most operators will reach the hourly limitation before the yearly one, the DC-3 Plummer Prop operates for about $1.50 per hour, while the PZL prop cost $7.00 to $10 per hour, more than a 500% increase. Also, if an operator is faced with a $14,000 PZL overhaul cost, for $15,000 a Plummer Prop conversion can be made instead. Current economics and performance dictate the Plummer Prop will eventually replace all of the Dromader PZL props. An operator would fair better to make the change, even before the overhaul limits of his PZL prop, due to the improved performance from the Plummer Prop and the residual value of the PZL prop. With a PZL prop having some value, say $5,000, then the operator could make the transition for as little as $10,000 and start to realize the benefits of the performance improvements immediately.

Plus, there is the fuel savings. By cutting back fuel consumption by five gallons per hour for a 400-600 hour season, the dollar savings add up pretty quick.

It is not often an ag operator can truly invest in a new product that pays for itself so readily, as is the case with the Plummer Prop. Even with such a successful product as the Plummer Prop conversion STC, the folks at Plummer Props are not finished saving the Dromader operator money.

Plans are in the works for an STC that provides a mixture control for the PZL engine. This device, typically found on all piston engines, but not on the PZL engine, coupled with an EGT gauge, will allow the pilot to adjust the fuel consumption, presumably less, and not overheat the cylinder heads of the engine.

Another Plummer product is new-style engine mounts, making the Dromader fly smoother, all the time cutting down maintenance caused by vibrations.

AgAir Update has been told Plummer is working on a change in the PZL engine's magneto timing that will enhance the performance of the Dromader even further. As these Plummer products become available, AgAir Update will be your first source of information about them.

Plummer Props has installed nine prop conversions on PZL-powered DHC-3 single-engine Otter float planes. Plummers has operated a DC-3 and two PZL-powered DHC-3 Otters for 25 years. The idea for the convernsion came from the DC-3, then they made it work on the Otters.