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Geared Drives New Subaru Redrive

A tribute to Bud Warren and Phyllis Ridings

By Phyllis Ridings

200Z – the zero-offset PSRU, retrofit/replacement

In the previous article in this tribute series, father and daughter team Bud Warren and Phyllis Ridings gave brief mention of a new propeller speed reduction unit Bud developed as a prototype. Since that article was written, they were inundated with requests for orders and decided to have the “box” digitally created and reproduced. A little over a year had elapsed since the last article was published and this one that was published in the February 2010 issue of CONTACT! Magazine and is reprinted here as part of our tribute to Bud and Phyllis.
Read more

Geared Drives was featured on the cover of CONTACT! Magazine, issue #89 and in the pages of #91. Its work with a simple yet robust redrive for the small-block Chevy is well documented in these articles. In issue #97, we reported on a Subaru redrive prototype of the same basic design, utilizing the same clutch configuration. With several hundred hours of flight testing completed, they have now come to market. Computer numeric controlled (CNC) from billet, it’s a thing of beauty. I’ve been to the shop where the aluminum is milled and seen the workmanship, but it wasn’t complete enough to bother with photos. At this writing, no photos were available, but the completed unit will be on display at the Alternative Engine Round-Up in Jean, Nevada, on March 27, 2010.  ~Pat 

Geared Drives has developed a spur gear propeller speed reduction unit (PSRU) that utilizes Bud Warren’s custom, centrifugal clutch assembly. A takeoff of his larger unit designed for high horsepower, this gearbox and clutch assembly was designed for any auto conversion engine in the 150- to 200-hp class requiring zero offset from crankshaft centerline. Applications include Subaru, Mazda, Suzuki, Ford (inline and V’s), and others – including diesels – by use of an adapter plate bolted to the flywheel end of the engine. Like its larger predecessor, it features its own independent oil reservoir, oil pump, and filtration system.

Larger view

Double Reduction – Variable Ratios
“This happened as part of a natural progression,” said Bud. “Too many people requested that I design a PSRU for a Subaru. It became apparent that people were looking for alternatives.” Some were experiencing high gearbox operating temperatures, others reported intermittent thrumming and vibration of the airframe due to unresolved harmonics, and a couple reported incidents as dramatic as gearboxes locking up in flight.

“I knew that if I could translate the features of our PSRU for high horsepower engines to a smaller and lighter double-reduction version and still use the centrifugal clutch, I had a good chance of solving a lot of their problems,” Bud said. “Because the Subaru engines make their hp at higher rpm, I knew that I would not be able to achieve a reduction ratio that would be a one-size-fits-all answer.” Bud chose a double-reduction gear configuration that would allow a wide range of ratios as well as providing the option of changing them out easily and inexpensively after the fact.

(This video is not part of the original magazine article and was added for this special tribute version.)

“The clutch is the key,” said Bud, “because it helps dampen harmonics. Operation is smooth, almost like an electric motor.” The clutch serves several other functions as well. Upon startup, the prop will spin due to the slight amount of friction and engine inertia. However, full engagement of the prop doesn’t begin until roughly 800 engine rpm, allowing the engine to start without the prop load and saving a great deal of wear on the starter.

The harder the engine runs, the more firmly the clutch is engaged. At shutdown, the prop winds down to a stop similar to that of a turbine, eliminating prop kickback. The clutch will not allow the prop to disengage until the engine rpm is reduced to well below 800 rpm. However, if the engine should quit in flight, the relative wind keeping the propeller spinning will keep the clutch engaged.

Larger view

The prototype affectionately referred to as the “box” is designed to be a direct replacement (with minimal machining) for the Eggenfellner Gen III PSRU.

Same engine, new PSRU.
(This image is not part of the original magazine article and was added for this special tribute version.)

The 200Z Zero-Offset Geared Drives PSRU occupies a footprint slightly smaller than the Eggenfellner Gen III unit and requires no changes in thrust line or propeller. Use your existing electric constant-speed prop, or any other prop you wish, including certified hydraulic. Geared Drives believes that there are distinct, all-around performance advantages to using any constant-speed or cockpit-controllable propeller and has built into this unit a hydraulic prop governor drive in order to facilitate that concept. Lubrication is provided by a built-in oil pump that serves the PSRU internals, prop, and prop governor. Engine oil is not used in the PSRU or prop governor.
Retrofitting this unit to an Eggenfellner engine requires minimal machining. The center hole in the front mounting plate must be opened up to 10.5 inches in diameter to accommodate the clutch assembly, and the spacers between the engine plate and the engine are replaced with longer ones to allow additional room for the Geared Drives clutch and flywheel assembly.

Adapting this gearbox keeps the prop flange station in the exact same place as the Gen III unit and will not appreciably change the weight and balance. The prototype weighed slightly less than the Eggenfellner, and the new CNC unit will weigh even less. Price will be the same for a retrofit or a new installation, just under $6,000.

Although Bud and Phyllis are no longer with us, not long ago Phyllis wrote a follow-up to this article that was slated to appear in the next issue of CONTACT! Magazine. The plans are to still include it. The article (announcement) was to coincide with Bud and Phyllis’s appearance at the Alternative Engine Round-Up.  ~Pat


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