Story and Photos by Mike Sharkey Christavia Mk 1 # 698 www.sharkey.servebeer.com/~michael I live in Ontario, Canada. Many of the interesting places to fly here are not accessible by any other means than by bush plane. I intend to build and fly a small bush plane, a Christavia Mk 1. My plane will be powered by a Corvair automotive engine converted for aircraft use.
THE BEGINNING The Christavia Mk 1 is a tandem, two place, high wing, rag-and-tube design, with a maximum gross weight of 1500 LBS. The aircraft is designed to fly reasonably well on quite modest power. I will be flying almost exclusively on floats and skis from small, remote lakes and waterways which tend to push my requirements toward the upper end of the power spectrum. I was initially attracted to the simplicity of the direct drive Corvair concept pioneered by William Wynne. Direct drive Corvair is thought to produce right around 100 HP. The designer of the aircraft, Ron Mason suggests that the upper end of the horsepower spectrum for the airframe is around 150HP. I wanted to get closer to that end of the spectrum in order to be able to turn a bigger prop for my more demanding requirements.
I began looking into the so called "Rinker Box" that has become sort of the de-facto standard PSRU used on the Corvair. The "Rinker Box" is fashioned from some VW gearbox parts, and some hand made parts. I considered building the Rinker box for quite a while. One of several problems I saw with the Rinker box was that it has no provision for any flywheel mass on the engine side of the gear box. The only flywheel is the propeller on the output side of the gearbox. This is why the Rinker Box has a characteristic "chatter" at certain speeds and under certain load conditions. What you're hearing is the lash in the gears as the motor tries to rotate out of sync with the flywheel (propeller) mass.
THE PRSU I almost abandoned the idea of using the Corvair engine, but then I remembered seeing a neat PSRU setup on a VW engine and I thought it might work well on the Corvair. I found the manufacturer of that PSRU, Ron Slender of VW Engine Centre in Australia. I spoke with him about adapting his PSRU to the Corvair. He was thrilled with the idea. First of all, Ron's concept is more than just a PSRU, his concept is to provide a whole bolt-on "package" that includes the housings, gears, prop hub, flywheel, dampener (cush drive), and starter. Production units will also include a flat top cover for the Corvair engine to replace what would normally be the blower bearing style of top cover. The PSRU gear ratio is 1.6:1, so when the gearbox is driven by the Corvair, the package should yield performance numbers close to what you see on the graph. From the graph, we can see that running the engine at 4000 RPM should yield something in the neighborhood of 125 to 130 HP @ 2500 propeller RPM; A significant gain over the direct drive Corvair’s rating at 100 HP. The Corvair’s redline is 5500 RPM
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