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(opposite) blade using the same procedure. Alignment
of the main rotor assembly has been achieved
when both blades have been adjusted so that the
vertical cross hair of the telescope is positioned at the
center of both rivets.
UNBALANCED SEMIRIGID ROTOR SYSTEMS
Lateral
When troubleshooting a semirigid main rotor system,
the repairer must understand the basics of alignment
and balance to act quickly yet skillfully. An unbalanced
system causes the most problems in the
field. The trouble that results is called lateral vibration.
A few indicators of lateral vibration are worn
parts and bearings, broken parts and bearings, loose
parts and fittings, and cracked parts and fittings. The
repairer must determine if the unbalanced condition
is caused by chordwise or spanwise torque before it
can be corrected.
Chordwise
To differentiate between a chordwise and a spanwise
unbalanced condition, apply a strip of tape to the tip
of one blade and hover the helicopter. If the lateral
vibration decreases and then increases, this indicates
that spanwise balance is okay but chordwise unbalance
exists. To balance a rotor system chordwise,
select a blade and sweep it to the rear by shortening
the drag brace. Before making adjustments,
matchmark all drag brace parts so that they can be
returned to the same setting to regain alignment.
Hover the helicopter. Should the lateral vibration
increase, you have selected the wrong blade. Return
the drag brace to the original alignment as
matchmarked. Repeat drag brace sweeping on the
opposite blade. Make small sweep corrections until
the vibration stops. Secure and safety the drag
braces.
Spanwise
To isolate spanwise balance, apply a strip of 2-inch
tape to the tip of one blade. Hover the helicopter.
Should the vibration increase, the wrong blade has
been selected. Remove the tape and apply it to the
opposite blade. Add the tape one strip at a time until
the vibration is gone. Then replace the tape with
4-1
FM 1-514
equal weight or secure weight approximating equal VERTICAL VIBRATIONS
moments at a specific location on the system. For
example, apply a 3.1-ounce, lead-in retention bolt for
each wrap of tape at the tip or as authorized in the
applicable maintenance manual.
Combined
It is not unusual for a combined chordwise-spanwise
unbalance to exist in a main rotor system. When the
unit is balanced spanwise, the chordwise unbalance
becomes evident. In this case, each unbalance must
be corrected separately. If the system cannot be
balanced by the above operations, inspect it for loose,
worn, or cracked parts and for frozen Teflon bearings
or ratcheting roller or ball bearings.
Vertical vibrations – the bouncing of the helicopter
up and down – are caused by a blade being out of
track. Vibration is caused by a blade lifting the
helicopter in one quadrant of rotation and suddenly
losing lift in the remaining quadrant during cyclic
travel. When present once during each revolution,
this force is referred to as a one-per-revolution or
1-to-1 vibration. Two bounces of the fuselage is
known as a two-per-revolution or 2-to-1 vibration.
Someone not familiar with the helicopter can determine
vertical vibration by looking at the tips of the
skids. A vertical vibration will cause the tip of the
skid to bounce vertically against the ground.
Depending on the shape of the main rotor and the
4-2
FM 1-514
helicopter manufacturer, the vibration may be corrected.
This is done by rolling the blade grips up or
down using the pitch-change links or adjusting the
trim tab to get a blade track that will stop the vibration.
One method is to blade-track at low RPM using
pitch-change links and at high RPM using trim tabs
and links with power applied. The blades are tracked
using a tracking flag or trackometer. The helicopter
is then flown at cruise airspeed to see if blade crossover
exists. Blade crossover occurs when blades are
almost perfectly in track. During forward flight
(cruise) dissymmetry of lift causes a blade to fly high
through 180° of rotation and low in the remaining
180°. Corrections are made by adjusting either trim
tab up or down to cause the blade to track high or low
within limits. Crossover is corrected by the pressure
exerted by the trim tab, which forces the blade up or
down throughout 360° of rotation.
Extreme Low Frequency
Extreme low-frequency vibration is essentially
limited to pylon rock. Pylon rocking of two or three
cycles per second is inherent with the rotor, mast, and
　
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