Should a blade chord be measured along the curved surface?

The short answer

Usually no. The aerodynamic chord is a straight reference across a defined blade section, while a tape along the skin measures surface arc length. Confusing the two can make a correct wind-blade section appear oversized and can also distort thickness-ratio calculations.

Locate the section endpoints

Identify the section plane and the defined leading- and trailing-edge reference points. Rounded leading edges and finite trailing-edge thickness can make a casual endpoint choice uncertain. Use the drawing construction rather than the point where a flexible tape happens to touch. A chord measurement is especially sensitive to section alignment on a twisted blade, because nearby stations can have different orientations and chord lengths.

Recognize what the surface measurement answers

The surface path is longer than its straight endpoint distance whenever that path curves between the endpoints. It can be useful for coating area or an approved fabrication template, but it does not become chord because its endpoints are near the leading and trailing edges. Consequently, a ratio calculated from maximum section thickness divided by skin arc length is a different geometric quantity from the specified thickness-to-chord ratio.

Make scan and drawing results comparable

For a dimensional review, request the section location, chord endpoint convention and nominal chord in physical units. Extract the same straight-line quantity from the scan, retaining the support and datum conditions. NASA geometry descriptions support the distinction between profile shape and chord reference. A MARSTON blade must be assessed against its applicable drawing, not a chord inferred from a photograph or curved surface tape. A dimensionally correct chord also says nothing by itself about internal construction or allowable loads. Where the trailing edge has finite thickness, ask whether the chord ends at its midpoint or another drawing-defined feature. A consistent endpoint construction is more useful than reporting additional decimal places from an undefined measurement.