Do blades with equal mass necessarily form a balanced rotor?

The short answer

No. Equal blade mass does not establish equal radial mass distribution. For rotor balance, a mass located farther from the shaft has a larger first mass moment. A small-wind blade comparison therefore needs the relevant mass-position data, not only three matching scale readings.

Compare the correct moment

For a simplified blade represented by total mass m at radial centre of mass r, its first mass moment is mr, in kg m. Hypothetical 10 kg blades centred at 0.8 m and 0.9 m contribute 8 and 9 kg m respectively. Their total masses agree, but their radial contributions differ. The complete assembly also requires the actual angular positions and consistent reference to the shaft axis.

Do not confuse moment with inertia

The first mass moment is relevant to the vector sum of eccentric mass contributions. Rotational inertia weights distance squared and answers a different acceleration question. Neither quantity alone verifies aerodynamic symmetry, pitch or structural integrity. A blade can meet a documented mass-matching criterion while still requiring other rotor-level checks; the acceptance package should identify precisely which characteristic its matching statement covers.

Request a balance-relevant blade comparison

Ask the blade designer whether individual mass, radial centre of mass, measured static moment or another specified quantity controls set matching. Require the same root/shaft reference and inclusion of attachments in each result. NASA supports mass-weighted centre-of-gravity reasoning, but supplies no MARSTON blade tolerance. Do not add field weights or mix blade revisions based on this example. The intended decision is whether the submitted mass data can support the stated rotor balance assessment. A set comparison should retain the uncertainty of mass and position measurements. Closely matching calculated moments with coarse position data does not establish a tighter balance than the measurement evidence can resolve.