TM 1-1500-204-23-5 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 the vibration from reaching noticeable levels, transmission  mount  dampening  is  incorporated  to  absorb the rocking. 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 same point. c.      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 track 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 specific procedures. 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 following paragraphs. (1) Marking Method.  Procedures for tail rotor tracking using the marking method, as shown in figure 3- 12, are as follows: NOTE ·  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. 3-12