What reference frame should accompany a blade centre-of-gravity measurement?

The short answer

A blade centre of gravity should be expressed in a frame tied to identifiable physical features, with units, origin and positive axes. For a wind rotor, that position must also be relatable to the shaft axis. “CG at 800 mm” is insufficient without its direction and reference.

Distinguish position from a single span distance

A radial coordinate can help evaluate mass balance, while chordwise and out-of-plane offsets can affect other moments. A report containing only distance from the blade root may omit those components deliberately, rather than prove they are zero. Confirm whether the reported value is a full three-dimensional centre, a projection or a measured static moment converted to an equivalent distance.

Transform before combining parts

Suppose a hypothetical blade CG is 0.7 m from its root along a defined axis, and the root origin lies 0.2 m from the rotor centre along that same line. Only under that collinear assumption does the rotor-centred distance become 0.9 m. Cone or sweep changes the coordinate transformation. Adding independent distances without checking their directions can move the apparent mass centre to a physically incorrect location.

Request a usable mass-property drawing

Ask for the CG marker on the blade drawing, axis arrows, root datum and the included hardware boundary. Record whether paint, inserts or detachable accessories are included, since their mass may shift the result. NASA describes the need for a common datum in mass-weighted position calculations; it does not define any blade’s CG. For a MARSTON assembly enquiry, request the actual frame relationship before using a component value in rotor balance, bearing-load or handling calculations. If the blade is weighed with removable fixtures attached, their mass properties need separate accounting. Otherwise the reported centre can describe the blade-plus-fixture arrangement rather than the component entering the rotor calculation.