That's interesting. I once read about turning ship masts using this technique. As I recall the masts were about 60 feet long. They were all turned with a slowly revolving lathe and a traveling router.

What RPM were you using?

Did you go through more than one bit?

What was the depth of cut on each pass?

Would you anticipate that a log say 18" in diameter and 25 feet long would sag enough to change the taper or straightness of the sides?

Thanks for stretching my mind.