Theory of Operation
Mode l8500C/8500C+ Operators Manual
2-9
A balance measurement reading allows the operator to plot the intersection of vibration amplitude and
phase angle on the clock face. Extrapolation of this point to the axes of the graph determines the
location and magnitude of the required adjustments. In most cases, a balance point indicates two
corrective moves, one for each blade whose axis encompasses the balance point. The exact procedur
required to balance the rotor depends on the chart being used.
2.2.2.2 Theory of Chart Creation and Correction
Individual rotor systems of a particular type rarely exhibit identical vibrational characteristics.
Differences in manufacture and repair, as well as variations in airframe stiffness and resonant
frequency often call for different balance solutions. The uniqueness of each system's mechanical
response thus precludes using a single set of ideal parameters to build balance charts. As a result,
charts do not exemplify abstract theoretical models of rotor performance. Instead, they are created by
carefully averaging many experimental measurements of a particular rotor type. This approach means
that charts may not always accord with actual rotor measurements, in some cases diverging enough to
prevent proper balancing.
We can compensate for these inherent biases by using the 8500C/C+ to correct individual balanc
charts, modifying them so that each chart can be tuned to the unique physical characteristics of
individual systems.
Chart corrections compare the actual effects of weight on vibration amplitude and clock angle against
the changes predicted by the chart. We can quantify the comparison by means of a special calculation,
the
move line
. The move line is the vector difference between two balance measurements. On a paper
balance chart it is identified by the straight line drawn between two balance points. Normally, the
direction of the move line changes in a predictable manner as one makes the weight adjustments
prescribed by the chart. If, for example, both corrective moves are made, the move line goes toward o
through the center of the chart. If only one move is made, the move line runs parallel to the fine lines
extending from the axis of the unmodified blade. In other words, the weight on this blade has not been
changed. The difference between the observed direction and the expected direction indicates the
amount of phase angle correction that must be applied to subsequent measurements. This correction is
usually accomplished by rotating the clock face or writing new clock numbers around the paper chart.
In the 8500C/C+, a correction factor for phase is computed and added to the balance chart database.
This correction factor is thereafter applied to all calculations involving the phase angle.
The magnitude of the move line (that is, its length) also signifies a particular vibration level or
amplitude. It should also change in a predictable manner as we make the weight adjustments
prescribed by the chart. Making only one of the corrective moves brings the move line directly to the
zero weight change axis for that blade. In other words, the weight change required by this blade has
been implemented. The length of the move line is therefore directly related to the size of the weight
adjustment. The ratio of the observed magnitude to the expected magnitude indicates the percentag
weight correction, as well as the percentage amplitude adjustment, which must be applied to
subsequent measurements. For example, if the move line is too long, too much weight has been added.
If it is too short, not enough weight has been used. In the 8500C/C+, this ratio becomes the correction
factor for vibration amplitude. It is added to the balance chart database and thereafter applied to all
calculations of weight adjustment.
To Next Page
Loading...