one other point! is the blade mounted on straight!! you can"t even be off by a fraction.. otherwise your blade will cause your mill to ocallate[back and forth].
Yes! That would also pump the yaw resonance I mentioned above, when the rotation rate approached the resonance, creating the same symptoms but at a higher speed.
If the misalignment is small and the damping is adequate it will settle down if it can get going faster than the resonance. But the pitch component of the vibration will still be stressing the bearings and tower. So even if lengthening the tail (which might get you past the waggle) settles it down you should try to get the prop on striaght, or at least dynamically balanced.
The prop being mounted with even a miniscule tilt between the prop and shaft axes includes an example of "dynamic imbalance" - which we discussed a couple weeks ago, which translates rotation into yaw and pitch vibration. If you can't get the blades on dead straight you can compensate for that with weights, though getting them adjusted is a bitch.
Unless the tilt from the axis is severe the dynamic imbalance should be the only mechanism pumping energy from rotation to yaw due to blade tilt.
Static imbalance would also pump yaw oscilations from rotation - again at the frequency of the prop rotation, rather than the blade passage rate. The farther the prop is from the yaw axis the greater the transfer (unlike dynamic imbalance, which doesn't change with that distance.)
Once I had it spinning, I'd feel the support or guy wires for any vibration. If I get vibration at the prop rate I'd recheck the static balance, then try to tweak the tilt or do some dynamic balancing. Vibration at the blade rate I'd ignore, as unavoidable aerodynamic effects.
