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Bell 47 Maintenance Tips

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Vibrations - Rotor Smoothing

 

1) INDICATION OF TROUBLE:   1:1 Lateral Vibration

PROBABLE CAUSE 1:         (a) In hover or forward flight -  Spanwise out of balance

CORRECTIVE ACTION:            Dynamically balance with Strobe or Tape

PROBABLE CAUSE 2:         (b) In hover or forward flight - Chordwise out of balance

CORRECTIVE ACTION:            Dynamically balance with Strobe - sweeping a blade

PROBABLE CAUSE 3:         (c) "hub rock"

CORRECTIVE ACTION:            Adjust hub equalizer links

 

2) INDICATION OF TROUBLE:   "Bumping" in abrupt maneuvers

PROBABLE CAUSE 1:          One dynamic stop cable to short"

CORRECTIVE ACTION:             Rig Dynamic stop cable

PROBABLE CAUSE 2:          Both dynamic stop cables to short (2:1 vibe)

CORRECTIVE ACTION:             Rig Dynamic stop cables

PROBABLE CAUSE 3:          Both dynamic stop cables to short (2:1 vibe)

CORRECTIVE ACTION:             Rig Dynamic stop cables

PROBABLE CAUSE 4:          Sprag mount weak

CORRECTIVE ACTION:             Readjust sprag mount system/replace parts

PROBABLE CAUSE 5:         One Safety cable too short (2:1 if both sides)

CORRECTIVE ACTION:             Readjust sprag mount/cable system/replace parts

 

2) INDICATION OF TROUBLE:   1:1 Vertical Vibration (or "bounce")

PROBABLE CAUSE 1:         In hover - Trim Tabs @ Neutral

CORRECTIVE ACTION:             Rotor out of track - Adjust Pitch links

PROBABLE CAUSE 2:          In forward flight - Neutral Pitch links

CORRECTIVE ACTION:             Rotor out of track - Adjust Blade Trim Tabs

 

3) INDICATION OF TROUBLE:   2:1 Vibrations

PROBABLE CAUSE 1:          In hover or forward flight

CORRECTIVE ACTION:             Mast misaligned - Adjust sprag mounts/preload lateral sag rods/replace parts

PROBABLE CAUSE 2:          In hover or forward flight

CORRECTIVE ACTION:             Excessive play in sprag mounting/preload lateral sprag rods/Adjust sprag mounts/replace worn parts

 

4) Elevator buffeting        Adjust elevator cable tension

 

5) Collective pitch control creeping  in normal cruise : Incorrect counterweights or adjust counterweights

 

6) Rotor RPM high or low in autorotation:    Minimum pitch blade angle incorrect.  Adjust both pitch control rods equally.

 

7) Erratic nudging of cyclic control stick:  Rotor unstable on longitudinal axis -sweep both blades aft

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