Seems like you are describing a method for
balancing a 3-blade system since it requires
placing weights at 120, 240, and 360 degrees
successively but at the same radius.

There should be a similar method for a two-blade
system which involves placing the weights at
different radii and plotting the vibration using
the dial indicator.

Or am I missing something??

Don W.

Stuart & Kathryn Fields wrote:
Don: It turns out that the Russians were doing this, or something like it,
for some time
The method is basically make a run and record the peak dial indicator
reading. Add a weight to an arbitrary designated zero position. Make
another run and record that dial indicator reading. Now move that weight to
a position 120 degrees around from the zero position and make sure the
weight is installed at exactly the same distance from center that it was at
the zero position. Record the dial indicator reading and move the weight to
the 240 position, again at the same radial distance from the center and
record. Assign a graphical scale to the dial indicator readings and plot
first a circle representing the reading without a weight. Next plot a
second circle centered on the first circle at the zero degree position. Now
plot the next circle similarly at the 120 position on the first circle and
again the last circle centered on the 240 position. You should have an
intersection of all of the circles at one point. The distance from the
center of the first circle to this point represents that amount of weight
needed to correct the imbalance. The angle to this intersection represents
the angle measured from where you added the first weight to this
intersection point and designates the location to add the weight. In actual
practice you don't always get a point at the intersection but an small area.
The center of this area is used as the intersection point. I have used this
on the tail rotor of my helicopter successfully but used and accelerometer
instead of the dial indicator. The guys article in the magazine had color
plots of his graphical calculations.