From RC Helicopter Wiki
Tracking is usually checked by hovering briefly at eye level. The tips of the blades should have been previously marked, and if the blades are not in track there will appear to be two distinct disks. By noting which disk is created by which blade, the pitch links in the head can be adjusted to bring the blades into track; first by adjusting the short Bell-Hiller mixer to blade grip link for coarse adjustment, followed by the long swashplate to Bell-Hiller mixer link for fine adjustment. It is suggested that you mark one blade as the 'master' blade, and only make tracking adjustments to the other blade or blades, to avoid the pitch setting drifting over multiple adjustments.
Blades can be out of track for a variety of reasons. Two blades, even shipped together, could have slightly different airfoil profiles, and so generate different amounts of lift; one could be slightly more flexible than the other and thus cone more; the roots could be molded slightly differently and give different angles of attack. There are a number of mechanical reasons why the blades are out of track, too: the pitch setup could be wrong, the blades could be out of balance, the mast or feathering shaft could be bent, the bolt or nut retaining the blade grip to the feathering shaft could be bent, or the dampers could be worn – obviously these should be fixed before trying to adjust blade tracking.
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