The passion in developing flapping wing micro air vehicles has attracted considerable attention in the study of unsteady aerodynamics of a flapping wing. To enable sharing and comparing of research results from different research groups, aerodynamic forces are often scaled with the square of some reference velocity. In the case of flapping-wing aerodynamics, complication arises because velocity varies along the span of the wing since it is executing rotating flapping motion. The experiments were conducted in the same three-dimensional flapping-wing mechanism. Only the essential features are discussed here. The mechanism consisted of two wings, and each wing was subjected to three axes of flapping motions, namely sweeping motion, elevation motion, and rotational motion. Fluid dynamic forces acting on the wing were measured by an in house-developed waterproof force transducer. Verification of the force sensor was carried out by loading known weights at four corners and at the center of the wing, and the maximum error was less than 3% for the range of measured forces.