Propeller Gearing Solutions for Home Aircraft Builders

Buckeye, Arizona’s SkyTrax specializes in gearbox solutions for light aircraft engines. The company’s bolt-on adaptors reduce crankshaft RPMs to rotational rates appropriate to aircraft propellers.

SkyTrax’s early efforts concentrated on Rotax’s C-gearbox, but were refocused on Yamaha snowmobile engines when Rotax discontinued the C-gearbox and halted production of two-stroke engines. The excellent power-to-weight ratios of snowmobile engines render such powerplants particularly conducive to light-sport and STOL aircraft.

SkyTrax’s Apex gearbox is designed to pair with Yamaha’s four-cylinder, four-stroke, 150-horsepower Apex snowmobile engine. The SkyTrax unit reduces the mill’s characteristically high RPMs—a function of Yamaha’s five-valve (three intake, two exhaust) Genesis head—by a factor of 3.83 to one

The Apex engine’s lofty RPMs and the high-power engendered by such have endeared the powerplant to top STOL competitors, many of whom have fitted the contraption with turbochargers, thereby boosting its output to well over two-hundred horsepower. Perennial STOL luminary and two-time AirVenture STOL competition reserve grand champion Steve Henry has boosted the Apex engine in his Just Aircraft Super-STOL Highlander to a gravity-defying 350-horsepower.

SkyTrax owner Teal Jenkins warns that such highly-modified, ultra-boosted, performance-optimized engines are eminently experimental, and should under no circumstances—within the contexts of reliability and range—be thought of as equivalents to or substitutions for conventional powerplants the likes of Lycoming’s iO-540. Mr. Jenkin’s states: “The biggest appeal for these particular engines is the power-to-weight ratio, obviously. You’re not going to beat a Lycoming in fuel-burn … but the biggest thing is I can climb at over two-thousand-feet-per-minute, even at 15,000-feet … and then I lost about fifty-pounds off the O-235 that was on there before.”

Cognizant of the fact that pairing engine and propeller is among the most common and complex challenges facing home aircraft builders, SkyTrax has designed its gearboxes to be installed without drilling, machining, or other similarly cumbersome retrofits to aircraft engine housings. Mr. Jenkins asserts SkyTrax gearboxes can be installed by reasonably competent do-it-yourselfers in under two-hours.

In addition to its Apex gearbox for Yamaha Apex engines, SkyTrax offers a Nytro gearbox for three-cylinder Yamaha and Arctic Cat snowmobile engines, as well as a Phazer gearbox for the Yamaha engine that powered the Japanese company’s Phazer model snowmobiles from 2007 through 2018. Combined, the Phazer engine and gearbox weigh a scant one-hundred-pounds.

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