a. Extreme Low Frequency Vibration. Extreme low
frequency vibration is pretty well limited to pylon rock
Pylon rocking two to three cycles per second is inherent
with the rotor, mast, and transmission system. To keep
transmission mount dampening is incorporated to absorb
b. Low Frequency Vibration. Low frequency
vibrations, 1/rev and 2/rev, are caused by the rotor itself.
1 /rev vibrations are of two basic types, vertical or lateral.
A 1/rev is caused simply by one blade developing more
lift at a given point than the other blade develops at the
Medium Frequency Vibration. Medium
frequency vibrations at frequencies of 4/rev and 6/rev are
another inherent vibration associated with most rotors.
An increase in the level of these vibrations is caused by a
change in the capability of the fuselage to absorb
vibration, or a loose airframe component, such as the
skids, vibrating at that frequency.
d. High Frequency Vibration. High frequency
vibrations can be caused by anything in the ship that
rotates or vibrates at extremely high speeds. The most
common and obvious causes; loose elevator linkage at
swashplate horn, loose elevator, or tail rotor balance and
3-17. Rotor Blade Tracking. Blade tracking is the
process of determining the positions of the tips of the
rotor blade relative to each other while the rotor head is
turning, and of determining the corrections necessary to
hold these positions within certain tolerances. The blades
should all track one another as closely as possible. The
purpose of blade tracking is to bring the tips of all blades
into the same tip path throughout their entire cycle of
rotation. Various methods of blade tracking are explained
in the following paragraphs.
a. Flag and Pole. The flag and pole method, as
shown in figure 3-9, shows the relative positions of the
rotor blades. The blade tips are marked with chalk or
grease pencil. Each blade tip should be marked with a
different color so that it will be easy to determine the
relationship of the other tips of the rotor blades to each
other. This method can be used on all types of
helicopters that do not have jet propulsion at the blade
tips. Refer to the applicable maintenance manual for
b. Electronic Blade Tracker. The most common
electronic blade tracker consists of a Balancer/Phazor,
Strobex Tracker, and VIBREX Tester (see figure 3-10).
The Strobex blade tracker permits blade tracking from
inside or outside the helicopter while on the ground or
inside the helicopter in flight. The system uses a highly
concentrated light beam flashing in sequence with the
rotation of the main rotor blades so that a fixed target at
the blade tips will appear to be stopped. Each blade is
identified by an elongated retro-reflective number taped
or attached to the underside of the blade in a uniform
location. When viewed at an angle from inside the
helicopter, the taped numbers will appear normal.
Tracking can be accomplished with tracking tip cap
reflectors and a strobe light The tip caps are temporarily
attached to the tip of each blade. The high-intensity
strobe light flashes in-time with the rotating blades. The
strobe light operates from the aircraft electrical power
supply. By observing the reflected tip cap image, it is
possible to view the track of the rotating blades. Tracking
is accomplished in a sequence of four separate steps:
ground tracking, hover verification, forward flight tracking,
and auto rotation rpm adjustment (see figure 3-11).
c. Tail Rotor Tracking. The marking and electronic
methods of tail rotor tracking are explained in the
(1) Marking Method. Procedures for tail rotor
tracking using the marking method, as shown in figure 3-
12, are as follows:
· After replacement or installation of tail
rotor hub, blades, or pitch change
system, check tail rotor rigging and
track tail rotor blades. Tail rotor tip
clearance shall be set before tracking
and checked again after tracking.
· The strobe-type tracking device may be
used if available. Instructions for use
are provided with the device.
(a) Attach a piece of soft rubber hose six
inches long on the end of a 1/2 x 1/2 inch pine stick or
other flexible device. Cover rubber hose with prussian
blue or similar type of coloring thinned with oil.