12.3–12.4 Blade tracking, vibration analysis and transmission

Scope

• 12.3 Blade tracking and vibration analysis (ATA 18) — A level 1, B1.3 level 3: rotor alignment, main and tail rotor tracking, static and dynamic balancing, vibration types and reduction, ground resonance.
• 12.4 Transmission — A level 1, B1.3 level 3: main and tail-rotor gearboxes, clutches, freewheel units, rotor brake, tail-rotor drive shafts, flexible couplings, bearings, vibration dampers and hanger bearings.

Vibration classification

Helicopter vibrations are described by how many times they occur per revolution of the main rotor ("per rev", /rev).

Band	Frequency	Typical source
Extreme low	Less than 1/rev	Pylon rock (about 2–3 cycles per second) — absorbed by transmission mount dampers
Low	1/rev or 2/rev	The main rotor itself — out of track (vertical) or out of balance (lateral)
Medium	Generally 4, 5 or 6/rev	Inherent in most rotors; increases when the airframe's ability to absorb vibration changes or a component (e.g. skids) is loose
High	Tail-rotor speed or faster	Tail rotor, tail drive shaft, engine, cooling fan, gears

Key fact: 1/rev vertical vibration = blades out of track (one blade flying higher). 1/rev lateral vibration = rotor out of balance (spanwise or chordwise mass imbalance).

Rotor alignment and tracking

Tracking makes all blade tips follow the same path (the same cone angle).

Method	Description
Flag and pole	Blade tips marked with chalk/grease pencil in different colours; a flag on a pole is moved slowly into the turning rotor; marks show which blade runs high or low. Ground only, slow rotor speed, older method
Stick / pole tracking	Similar contact method on small rotors
Electronic (stroboscopic / optical)	Strobe light synchronised to the rotor makes reflective targets on each tip appear stationary; can be done on the ground and in flight
Optical / electronic trackers	Camera or photocell measures blade height and lead/lag; combined with an accelerometer it gives a balance solution

Track adjustments:

• Pitch-change link (pitch link) length — adjusts track mainly in the hover/on the ground.
• Trim tabs on the trailing edge — adjust track in forward flight (aerodynamic effect grows with speed).
• Sweep (lead/lag adjustment) on some heads for chordwise balance.