When Rob Aberle retired and started working on his Sonex kit, he knew it might take two or three years (or maybe more) to finish. Since building is not flying, Rob went shopping for an affordable secondhand Sonex to keep his flying skills current. In due time, he found a used Sonex in the same basic configuration he was building: a taildragger with a 120-hp Jabiru engine.
After flying it for a few months, the annual condition inspection came due. It’s worth noting that, when Rob bought it, the airplane had been flying for a little over 20 years and had about 700 hours on the airframe and powerplant. What stood out was the previous owner had operated from a grass strip for about a year. Granted, there’s nothing that says you can’t operate a low-wing sport plane with 10 1/2-inch tires from a grass strip. But many would agree: The Sonex was not designed for primarily rough-field operations.
In any case, one of the things that came up during inspection was that the tailwheel was wobbly in both horizontal and vertical axes. Disassembly showed that the hole for the coupling cross-bolt was elongated (causing horizontal wobble) and the main steering pivot yoke was severely bell-mouthed (causing wobble in all directions, but mostly vertical). What could have caused these to wear out? Was it normal wear and tear after 20 years or a year of flying from a grass field?
Refurbishing the damaged parts was not an option. The wheel itself was still in good shape, so all Rob needed was a standard Sonex tailwheel, model SNX-L03-31, minus the wheel (December 2025 price: $214 USD).
The installation challenge was to drill the new coupling to match the original hole in the titanium stinger. Since this was likely match-drilled during the original assembly, mating up the new (undrilled) aero/teardrop-shaped coupling to the original hole in the stinger was going to take some planning.
A preliminary check proved the cross-hole in the stinger was indeed square to the vertical axis of the airframe. The next check was to confirm the hole in the stinger was centered. To do that, I made a test piece out of a section of 7/8-inch diameter aluminum tubing (actually, the end of an aluminum bicycle handlebar). I bored out the ID to match the 5/8-inch stinger and cross-drilled a 1/4-inch hole dead-on center. When fitted to the stinger, a bolt would go through, but it was a tight fit. Which suggested the stinger hole, although square, was ever so slightly off center.
The next challenge was how to square up the coupling for drilling. It took a bit of fiddling but, all in all, having a planer gauge and a set of machinist parallels made it pretty easy. That’s it for now. Time to get back in the shop and make some chips.
Great article Bob. The Sonex was designed for grass strips, however, and many builders operate off grass routinely with the standard size Sonex tires. When we say grass strips, however, we don’t mean backcountry strips, or poorly maintained grass strips with big chuck holes, etc. Check out The Right Seat with Tony Spicer on YouTube for some grass strip Sonex flying: https://youtu.be/oP2P5XduWmQ?si=KkKVFghGlCUvMxPb