Theory of Operation
Mode l8500C/8500C+ Operators Manual
2-7
2.2.1.1 Track Imbalances
A rotor system's track describes the geometry of the circular path made by the blades as they rotate. A
blade that is out of track may follow a slightly different path because its alignment in the plane o
rotation is skewed. The result is a one-per-revolution vibration whose direction is perpendicular to the
plane of rotation. We can analyze this condition either by measuring the amplitude and frequency of
vibration or by visually characterizing the track to determine the extent of the imbalance. The former
technique is called a track or vertical balance and employs principals and procedures nearly identical
to those used to correct a lateral mass imbalance. The latter technique is called a visual track and uses
the Strobex or FasTrak
Tm
Optical Tracker to illuminate the relative position of the blades as they
rotate. In both cases, the goal is to match the aerodynamic qualities of all the blades and thereby
eliminate the vertical vibrations caused by an out-of-track condition.
The major difference between vibrations generated by a lateral mass imbalance and those generated by
a track or vertical imbalance is the direction of the vibrational forces. Otherwise, the principals
governing them are nearly identical. To solve a track imbalance, we therefore have only to orient the
velocimeter perpendicular to the plane of rotation. The 8500C/C+ extracts from the velocimeter signal
the amplitude of vibration and balance frequency and uses a reference signal from a magnetic pickup
or Photocell to determine the phase angle of the out-of-track vibration. With this information it can
then compute a solution based on the adjustment of trim tabs or pitch links that physically modify the
blade's track.
In a visual track the one-per-revolution signal from a magnetic pickup or Photocell is used to drive th
Strobex flash. If each rotor blade has secured to it a retro-reflective target, we can identify each target
as it is illuminated by the Strobex flash. In a perfectly tracked system, the images line up precisely
along the planar path of rotation established by all the blades. The extent to which any blade image
deviates from the path is an indication of the relative degree of track imbalance. Adjustments to th
aircraft's trim tabs and/or pitch links can usually bring the anomalous blade back into alignment.
2.2.1.2 Blade Sweep
Blade sweep
or
lead-lag
refers to the angle of separation between individual blades within the plane of
rotation. Blade spacing that is not equiangular can cause significant perturbations to the system's
lateral balance. For this reason, balance solutions for some helicopters call for modification of the
rotor's blade sweep rather than addition or subtraction of weights.
Visual tracking allows you to gauge the relative amount of separation between Strobex-illuminated
blade images. This separation is indicative of the relative angular spacing between the actual blades.
The FasTrak
TM
Optical Tracker is able to exactly determine the Lead-Lag of the target rotor.
2.2.2 Balance Charts
We have seen how the 8500C/C+ can identify the vibrational characteristics of a rotor system by
providing two essential measurements, vibration amplitude and angular position. But in order to
compute an actual balance solution, that is, how much weight to add or subtract, and where to apply
the changes, the operator first needs to know something about the relationship between the physical
balance points of the system and values for amplitude and position. In particular, one needs to be able
to predict how vibration amplitude and angular position change when one makes specific adjustments
at the balance points. This information has been experimentally determined for most aircraft types and
organized into a unique form called a
balance chart
.
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