Fig. 1. (A) Diagram of robotic apparatus. The wing assembly is shown mounted on linear translation rails above the 1 mx2.4 mx1.2 m towing tank. (B) Coordinate system for the mechanical wing. Three angles are used to specify the position of the wing: , and . The instantaneous stroke position, (t), is defined as the angular position of the projection of the wing axis in the stroke plane. The instantaneous stroke deviation, (t), is defined as the angle between the wing axis and the stroke plane. The instantaneous angle of attack, (t), is defined as the angle between the wing's chord and the tangent to its trajectory.