Blade Tracking & Vibration Analysis, Module 12, Certify66

Notes

Main ideas

Blade TRACKING checks that all main-rotor blade tips pass through the same tip-path plane. An out-of-track blade (one tip flying higher/lower than the others) produces a once-per-revolution (1/rev) VERTICAL vibration.
Tracking is checked with a strobe/flag or reflective tip targets and an optical/electronic tracker (in the hover and forward flight); it is corrected by adjusting the pitch-change (tracking) rods and trim tabs.
BALANCING removes rotor imbalance (a 1/rev LATERAL vibration) using span and chord balance weights; static and dynamic balancing apply.
VIBRATION ANALYSIS uses accelerometers: MAIN-rotor problems appear at low frequency (1/rev and harmonics), the TAIL rotor at a higher frequency (felt through the pedals), and drive/engine faults at higher frequencies still, the frequency identifies the source.
Health and Usage Monitoring Systems (HUMS) track vibration and gearbox/bearing signatures over time to catch deterioration early.
⚠ Exam trap: a 1/rev VERTICAL vibration indicates a main-rotor TRACK problem (corrected at the pitch rods/trim tabs), while a 1/rev LATERAL vibration indicates main-rotor BALANCE; tail-rotor vibration is a higher frequency felt through the pedals.

Solved examples

A helicopter has a noticeable once-per-rotor-revolution vertical 'thump'. What is the likely cause and how is it corrected?
A 1/rev vertical vibration means one main-rotor blade is out of track, its tip is flying a different height from the others, so the disc gives an uneven vertical kick each revolution. It is diagnosed by tracking (strobe/optical) and corrected by adjusting that blade's pitch-change rod and/or trim tab to bring all tips into the same tip-path plane. (A 1/rev lateral vibration would instead be a balance problem.)

Blade Tracking & Vibration Analysis