I hope Robin responds personally, but I've had a pretty extensive exchange with Robin. He's an expert machinist and his project was a much about seeing if he could build a prop extension as anything else.
He had an objective to make a long extension that uses the Lycoming sleeve and decoupled the prop from the crank. His first few attempts while seemingly adequate used a relatively small spline to attach the hub-flange to the crank.
The corvair is an exceptionally smooth engine. 6 apposed cylinders with connected heads. Is a rock! But... Any 4-cycle under 8 cylinders will have a negative torque impulse... Ie there's a point in each revolution where the engine isn't in a power pulse and the prop drives the crank. It's pretty small in a 6 cylinder, but it's crucial because it causes any lash (rotational slack) to induce a torque-wrench-phenomena that hammers the connection interface (spline) every rotation.
This phenomena was causing the spline to fail. He ultimately fixed it by creating a disk plate and used a ring of steel dowels to mesh the crank to the prop shaft. He did an amazing job, but most would have given up or just attach the extension to the crank.
That would have had it's own issues as any vibrations from even slight imbalances would transfer through the extension to the weak crank flange, inducing an eventual fatigue and failure. Ala Mark Langford and why weisseman and others created the fat 5th bearing.
It's not easy being different! I like his ideas and I'd love to try to make a similar extension, but geared. But frankly, too much metal fabrication and I love gliding, but not in my RV.
Robin, I hope I've summarized our conversations well enough. I'll edit this if I've misrepresented anything.
Built/Own RV-4 254MM
Owned RV-6 4074V
First RV RV-3 3456B
VAF annual donation on autopay