TM   55-1520-240-T 2-1   VIBRATION   ANALYSIS 2-1 2-1.1  VIBRATION  ANALYSIS  SYSTEMATIC  APPROACH 2-1.1 1. All   helicopters   have   a   certain   amount   of   vibration   2. especially  during  changes  in  attitude  and  at  high airspeeds  and  power  settings.  Excessive  vibration is  any  vibration  that  is  different  from  the  vibration normally  sensed  in  a  particular  helicopter,  any  vi- bration  that  is  noticeable  in  only  one  pan  of  the helicopter,  or  any  vibration  that  causes  discomfort to  personnel  or  damage  to  cargo  carried  by  the helicopter. GO  TO  TROUBLE  SYMPTOM TASK   2-1.2.1. START There  are  two  types  of  vibration,  low  frequency and  high  frequency.  Low  frequency  vibration  cycles can  be  counted  or  identified  by  a  beat.  A  1/rev vibration  frequency  is  3.75  cycles  per  second  at 225  rotor  rpm  (100%  N2).  This  is  slow  enough  to count.  A  3/rev  vibration  frequency  is  11.25  cycles per  second  which  is  too  rapid  to  count;  however, the  individual  beats  are  noticeable. 4. RECORD   AIRCRAFT   CONFIGURATION Was   cargo   carried   either   internal   or   external?   Yes   —   No   — 2.  If  internal  cargo  carried: a.   What   was   weight?   _  Ibs b.  How  was  it  secured? 1 .     F l o o r _ 2 .    S t r u c t u r e _ 3.  If  external  cargo  carried: a.   What   was   weight?   —  Ibs b.   What   was   configuration?   — 4.  What  was  FAT?  — ° C . 3. With high frequency vibration, the vibration to  be  more  of  a  buzz.  Rotor  generated  vibration can  usually  be  observed  at  1,  3,  and  6  per  rev (1350  cycles  per  minute).  9,  12,  and  15  per  rev, drive  system  vibrations  tend  to  be  in  the  higher frequencies.  The  Chadwick  Helmuth  Vibrex  balanc- er  177M6A  or  177M7A  can  be  used  to  record  vi- brations  up  to  99,900  cycles  per  minute  (1665 Hz).  The  Chadwick  Helmuth  Spectrum  Analyzer, Model  192A,  is  capable  of  recording  vibrations  up to  900,000  cycles  per  minute  (15,000  Hz).  Table 2-1.2.2   lists   the   most   common   CH-47D   exciting frequencies  in  Hertz  (cycles  per  see),  and  cycles per  minute  at  225  rotor  rpm  (100%  N₂). RECORD   VIBRATION   CHARACTERISTICS (Circle  characteristics) Vibration   frequency: a.Vibrtion   frequency   of   3.75   cycles   per   second   (1/rev)   could   be   counted (low   frequency). b.  Vibration  frequency  was  too  rapid  to  count  but  frequency  beat was   noticeable   (low   frequency)   (3/rev). c.  Vibration  frequency  was  too  rapid  to  count  and  was  or  wasn't accompanied  by  a  buzz  or  high-pitch  tone  (high  frequency). d.  Vibration  caused  double  vision  (high  frequency). Vibration   location: e.  Both  sides  of  cockpit. f.  Pilot’s  side  of  cockpit  only. g.  Copilot’s  side  of  cockpit  only. h.  Directional  pedals. i.   Cabin. Vibration    strongest    in: j.   Cockpit. k.  Cabin. Flight  phase  vibration  noticed: l.   All   the   time. m.  Only  when  cyclic  changes  or  large  power  changes  made. Note:   If   m.   circled,   identify   maneuver. n.   Takeoff. o.    Transition. p.  Climb. q.   Autorotation r.  Landing. s.  Other  (describe). The   Vibrex   system   177M6A   or   177M7A   should be  used  to  measure  low  frequency  rotor  generated vibration  (1,  3,  and  6/rev).  The  Chadwick  Helmuth Model  192  Spectrum  Analyzer  should  be  used  to measure  drive  train  vibration  and  higher  frequency rotor  vibration  (9,  12,  and  15/rev). To  identify  and  locate  cause  of  vibration,  record aircraft  configuration  and  vibration  characteristics as  outlined  below.  Then,  determine  vibration  ex- perienced  and  go  to  the  task  identified. WAS  VIBRATION  IDENTIFIED AS LOW FREQUENCY? (A OR B CIRCLED) GO TO TROUBLE SYMPTOM TASK   2-1.2.2. END OF TASK 2-2.6 Change 5