Why can blade profile measurements change with the support arrangement?

The short answer

Because the fixture and gravity can change the measured shape. A wind blade is not infinitely rigid, and a scan records its shape in the actual supported state. A difference from a design surface may therefore contain elastic deflection as well as manufacturing deviation.

Record the supported measurement state

Identify support positions, contact zones, blade orientation and the geometric datum used to align the scan. Supporting the same blade near its root or at several spanwise locations changes its bending response to weight. A registration algorithm can also rotate the entire measured cloud to minimize deviations, hiding a datum mismatch. These effects must be resolved before a profile deviation is labelled permanent distortion.

Compare like conditions

A hypothetical unsupported tip drop of 8 mm cannot be accepted or rejected against a mould-profile tolerance unless the drawing defines how gravity and support are treated. That number is only an illustration, not a permitted deflection. Compare measurements made under the prescribed fixture conditions, or use an approved correction model whose stiffness and loading assumptions are documented. Subtracting an arbitrary tip offset will not reconstruct the complete deformed shape.

Ask which shape the acceptance surface represents

The blade designer should state whether the target is a manufactured shape in a fixture, an unloaded analytical shape or another defined condition. Request support coordinates and any compensation method alongside the section tolerances. Beam/coordinate descriptions in OpenFAST explain why reference shape and orientation matter; they do not certify a manufactured blade. For MARSTON equipment, obtain its own measurement instruction before using a scan to accept or reject blade geometry or proposing any straightening operation. If two scans use different fixtures, preserve both original clouds and support records. An agreed alignment transformation can reconcile datums, but it should not silently erase deformation that the acceptance method intended to measure.