Hi Kent,
Long time no talk. You still doing balance work?

Seemed to be a whirl mode much like that described for a radial with
a too-loose front propshaft bearing. This was with the 3 cyl Suzuki,
an engine with a natural wobble, again much like the radial. The PSRU
was a cantilever upper axle type, so just a little excess freeplay in
the bearing setup was enough to set it whirling at high power. The
torsional amplitude pulsed at about 2 hertz on the o-scope display.
Took awhile to realize what we were had. You could hear it in the prop
noise and see the whirl at night if you lit the prop disk with a flood.
Didn't explore it much as we had other stuff on the front burner.
Just got rid of the freeplay.

Dan