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transmission systems. To keep the vibration from
reaching noticeable levels, transmission mount
dampers are installed to absorb the rocking. The
damper system may be checked by the pilot while at
a hover. Moving the cyclic control forward and backward
at about one movement per second will cause
the pylon to start rocking. How long it takes for the
rocking to die out after the motion of the cyclic is
stopped indicates the condition of the damper system.
Low Frequency
One-revolution and two-revolution vibrations are
caused by the rotor. One-revolution vibrations are of
two basic types: lateral and vertical. Low-frequency
vibration is started by a gust effect that causes a
momentary increase of lift in one blade giving a
one-revolution vibration. The momentary vibration
is normal. However, if picked up by the rotating
collective controls and fed back to the rotor causing
cycles of one revolution, then it is undesirable. This
condition is usually caused by too much differential
tab in the blades. It can be corrected by rolling one
blade at the grip and changing angular adjustment of
the tab. Two-per-revolution (2/rev) vibrations are
inherent with a two-bladed rotor system, and a low
level of vibration is always present. When the 2/rev
vibration rises to an unacceptable level, it is due to
faulty vibration dampers or loose and worn hardware
in the rotor system.
Medium Frequency
Medium-frequency vibrations at four to six per
revolution are inherent with most rotor systems. An
increase in the level of vibration is caused by a change
in the capability of the fuselage to absorb vibration
due to loose hardware, structural damage, or load.
Normally this vibration is caused by loose parts –
either a regular part of the aircraft or the external
load.
High Frequency
High-frequency vibrations can be caused by anything
in the ship that rotates or vibrates at a speed equal to
or greater than that of the tail rotor. Unless the
vibration is isolated to one part of the aircraft – under
a shaft bearing, for example – the first step generally
is checking the tail rotor track.
MAIN ROTOR BLADE TRACKING
Blade tracking (Figure 4-2) is the procedure for
measuring, recording, and adjusting the tip path
plane of the rotor blades. The measurements taken
4-3
FM 1-514
•
while the blades are turning show the vertical position
of the rotor blade tips in relation to each other.
The positions of the blade tips must be kept within a
certain tolerance, usually ±.25 inch. Tolerance for
each helicopter will be listed in the applicable maintenance
manual. Several methods used to track
blades are —
Electronic blade tracker.
Reflector tracking.
Strobe light.
Electronic
Rotor blade assemblies may also be tracked with an
electronic blade-tracking unit (Figure 4-3). The unit
is made up of three major components:
•
•
• A phase detector with a magnetic pickup attached
to the swash plate’s stationary ring and
a sweep attached to its rotating ring.
•
•
• A computer containing the electronic circuits,
adjusting knobs, and meter.
• An electronic eye unit.
The electronic blade tracker unit permits blade
tracking during adverse weather and at night. The
electronic blade tracker is operated when the rotating
rotor blades interrupt the electronic eye beam,
sending a signal into the computer in conjunction
with a signal from the phase detector. The computer
then determines the blade tip path plane above an
automatically selected reference plane. The meter
shows the height in fractions of an inch of the rotor
blades in the set relative to the predetermined reference
rotor blade. Refer to Figure 4-4 for an example.
Reflector
The reflector tracking method uses the principle of
persistency of vision, which occurs when looking at a
beam that is being intercepted by two light reflectors.
One reflector is installed at the tip of each main rotor
blade. The surface of one reflector is plain white, and
the surface of the other is white with a horizontal
black stripe painted across the center of the face. As
the blades rotate and the light beam is intercepted by
the reflectors, the observer will see two white bands
and one black band. One white band will be above
and the other below the black band. A perfect intrack
condition exists when both white bands are the
same width. If one reflector image moves vertically
relative to the other, one white band will become
　
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