Depends on the factory whether they're actually balanced at the factory or not. I understand some have a sophisticated machine that holes blanks while comparing the weight of the two sides. By judging the two sides, the machine knows where the middle of the weight is and puts the hole there. They ignore the minor difference in radii that might create.

It's been stated that the casting process is so uniform that cast props are simply checked for imbalance (by machine) and those far enough out of balance are trashed because the difference identifies a poor cast.



Oh, it's still out of balance but it's not static balance that's the problem. You know those spin balancing machines at tire shops. They deal with dynamic balance and that's probably/possibly the problem with your prop.

Those elevators are vibrating in response to your props imbalance. The vibration from your prop shakes the plane at different frequencies, and the high speed shake gets to the tail. The low speed shake is absorbed up front and you see it there.

Some shake the plane at a frequency you don't see front or back. But bet your airplane on the probability your receiver feels all the frequencies, high low and invisible.

It's always worthwhile to take some time and check balance even if you just take the out of balance ones back to the LHS and don't try to balance them yourself. Although it's usually not too hard to do for the few you will find.