[BerntR]

[nerd mode]

Actually, the twirl is centered around the longitudinal MOI center, not the COG. COG is a static concept, the balance point of an object at rest. MOI center is the dynamic "big brother". The MOI center will be the center of an uncompensated rotation. MOI equals mass * distance^2, while COG equals mass * distance, so MOI gives more credit to distance.

Homer took a short cut when he used COG for defining the sweet spot plane. He should have used MOI instead.

[/nerd mode]