TWENTY YEARS AGO — Plummer’s Props for PZL Engines

by Bill Lavender

WEST HELENA, AR — Nothing can take its toll on an aircraft’s airframe like a vibrating powerplant. In most cases, particularly with piston aircraft, the vibration is a result of a prop out of balance. This imbalance can be from the blades not tracking properly or the weight of one blade different than the others or a combination of these factors.

No engine is exempt from the effects of a prop out of balance. Static balancing simply is not good enough. Without dynamically balancing the prop (while the engine is running), you are omitting the internal balance of the engine itself, as well as the effects of the inertia of the rotating propeller.

How does all this relate to the Dromader and its huge four-blade propeller? The Dromader is notorious for having a difficult prop to balance and to keep balanced. Far too complex to explain within this text, believe it when told a Dromader prop can be a headache. Coupled with a nine-cylinder radial engine developing 1,000 horsepower and more than 3,500 pounds of thrust, you have a tremendous amount of stress on the airframe.

Glenn Holmes of Plummer’s Props prepares PZL engine by inserting critical oil transfer tube/ tread converter into crankshaft.

Fortunately, the Dromader is a well-built aircraft capable of withstanding a lot of stress. Even so, no ag aircraft is built strong enough to take continuous pounding from an out of balance prop. And, no operator wants to go through the headaches and expenses of keeping the Dromader prop balanced.

There is a solution to this problem. The solution is an STC from a joint venture between Glenn Holmes and Chummy Plummer, owner of Plummer Arctic Lodges. This is an outfit based in Canada that provides fantastic fishing 125 miles north of the Arctic Circle in Great Bear Lake (if I squeeze a trip out of Glenn and Chummy, I’ll tell you all about it!). In order to transport their clients to this wonderland of great north fishing, Plummer Arctic Lodges uses a DC-3 with the P&W R-1830 and two De Havilland Otters powered by the 1,000 horsepower PZL engine; the same engine and prop combination used by the Dromader. In addition, Plummer Arctic Lodges’ aircraft fleet consist of two Beavers and a Cessna T-206.

In partnership with Chummy Plummer, Glenn Holmes decided there must be a better way to address the prop problems he was encountering. Glenn has an extensive background flying bush and ag, and is a guru of a PZL mechanic. Having plenty of experience operating the DC-3 with its Hamilton Standard 23E50 with 6477 paddle blades, Chummy felt that if he could mount the Hamilton Standard (H-S) prop to the PZL engine, he would have a perfect match.

Denis Friezner attaches instrument wiring to special device that allows prop to rotate while taking readings from strategically placed to theprop blade 36-gauge enamel coated wire, carefully soldered to a straingauge. Mr. Friezner looks on.

After two years of testing, Chummy Plummer and Glenn Holmes were satisfied they could obtain the STC for the conversion. After having operated the Otter with the H-S prop and PZL engine in Canada, they knew it would only be a matter of presenting the proper paperwork to Transport Canada before an approval would be issued for the conversion. Once Transport Canada issued its STC for Canada, that STC would be presented to FAA for a U.S. STC. Canada and the U.S. have reciprocity agreements. However, the paperwork required precision strain gauge testing, vibration analysis and flight testing. From these test, the effects of the propeller on the airframe as well as the strain on the propeller blades could be predicted.

Who does one hire to have this testing done? One of the best, if not the best, engineering companies in the world for this type of prop analysis is Specialized Testing Service, owned by Sandy Friezner of Arleta, California, along with Sandy’s son Denis. These two individuals literally travel the world analyzing stress factors, whether it be for a propeller, a submarine or an oil rig. Their services are in high demand and well respected in the aviation community worldwide. If Specialized Testing Service places their stamp of approval on an installation, then it is assured the installation is workable from a stress point of view.

Hanging the DC-3 prop, while Glenn Holmes (L) and helper look on.

Admittedly, there aren’t many ag aircraft more strange looking than a Dromader with a threeblade propeller (the H-S 23E50). But, that threeblade prop has a beauty all its own. First, consider the replacement cost of a Dromader’s PZL prop is approximately $11,000. Then, figure at today’s prices, the H-S 23E50-6477 prop fresh overhauled is about $3,500. Next, factor in the time between overhauls is 1,200 hours or six years for the PZL prop compared to 2,400 hours or 10 years for the H-S prop. There is the one-time charge for the STC estimated at $6,000. Now, the numbers should be telling a story of their own. Operating the H-S prop could be one-fourth the cost of the PZL prop. It is no wonder that Chummy Plummer and Glenn Holmes considered the “DC-3 prop” a good replacement prop. However, the question arises, does the H-S prop perform as well as the PZL prop?

The heart of the Plummer’s Props conversion; a transfer tube that fits into the crankshaft of the PZL engine. This tube moves oil into the H-Shub, but also converts the metric threads to standard.

The project location was hosted by Darryl Riddell of Riddell’s Flying Service in West Helena, Arkasnas. Darryl negotiated with Beryl Shears of Western Pilot Services in Phoeniz, Arizona to provide the Dromader for evaluation. The evaluation and testing began with Jim Jeffries of Jeffries Airworks dynamically balancing the PZL prop. This assured the best possible readings for the PZL prop and engine. Then, the Friezners started their work by establishing a baseline. They attached a thrust gauge to the tailwheel of the Dromader and measured the pounds of thrust the PZL prop and engine were capable of producing. They also strategically placed vibration monitors in the cockpit and alongside the fuselage to measure the vibration effects of the propeller. This was done for the factory PZL prop on the 1,000 hp PZL engine. The PZL prop developed 3,530 pounds of thrust at 2,100 rpm and 40 inches of manifold pressure.

A close-up view of how the tiny 36-gauge copper wire is attached to the even smaller strain gauges on the DC-3 propeller blade.

Next, the PZL prop was replaced with the H-S 23E50-6477 prop. Three strain gauges were placed on a blade of the prop. Using the same power settings as with the PZL prop, the H-S prop developed 3,800 pounds of thrust. This translates into an 8% increase in thrust, approximately a 10 mph increase in airspeed for the Dromader.

It was noted the H-S prop being only statically balanced, still measured less vibration at all points than the dynamically balanced PZL prop. Cockpit measured vibration for the PZL prop was .13 ips (technical term for inches per second acceleration). The corrected measurement for the DC-3 prop according to Jim Jeffries would be approximately .05 ips. This works out to a 75% reduction in vibration; 4-5 times less.

Mr. Sandy Friezner (L) of Specialized Testing Service closely oversees operations, while Jim Jeffries (R) of Jeffries Airworks assists.

While AgAir Update was visiting with Glenn Holmes, Jim and Linda Jeffries and the Friezners, only the H-S 23E50 was run on the Dromader on the ground. The testing continued for the next couple days, with flight test and also testing for the H-S 33D50 and H-S 43D50 props. Glenn Holmes wanted the Friezners to test and evaluate these propellers as well.

The H-S 33D50 with the 6571A-O blades are thinner compared to the 6477 paddle blades in the 23E50 hub and is used typically for high altitude flying. However, installed on the Otter, it will reduce the most forward weight of the aircraft by 110 pounds. The H-S 43D50 is the prop the Navy used on its Tracker sub spotter and Fire Cats use for fire fighting. Glenn’s thoughts are, “The Tracker prop is late 1960s technology, compared to 1930s technology for the other two props. We wanted to see if there was a measurable difference”.

The lightweight DC-3 propeller, 33D50 with 6571A-O blades, produced about 40 pounds of thrust less than the PZL propeller. However, the vibration reduction was about the same as experienced with the paddle blade H-S prop.

The Tracker prop showed promise, but the testing was not completed due to technical problems and time constraints. Future plans include more work with this propeller.

The tail wheel assembly is attached to a thrust measuring device, not unlike a extra heavy duty scale used for weighing a fish. The aircraftpulls against this device at a particular power setting to measure theamount of thrust being developed.

By the time AgAir Update readers read this article, a Canadian STC should have been issued for the conversion of the H-S series of props to PZL 1,000 horsepower engines. An FAA STC should be following shortly. These STCs will apply not only to the M-18 series Dromader, but also to PZL powered Otters and AN-2 aircraft. When the PZL factory showed an interest in the propeller conversion, a patent was applied for to protect the project.

Wherever there is a mouse, someone will always come along and build a better mouse trap. This analogy is so very true for people in the ag aviation business; always working to improve on the equipment and techniques they use.