Fig. 2. Instantaneous wing kinematics and flight forces. (A) Body and wing kinematics measured during a slow vertical ascent. The body is pitched up by 45° (top, black trace) and the fly ascends at a constant velocity of about 0.12 m s^–1. The kinematics of the right (red) and left (blue) wing in the body frame (compare to Fig. 1C; the reference plane is inclined by 45° with respect to the long axis of the fly's body) are given below. Forward thrust and lift are shown at the bottom. (B) Average wing motion and instantaneous aerodynamic forces (red arrows) for the same data sample. Black lines indicate the position of the wing chord at 25 temporally equidistant points during the stroke cycle, with dots marking the leading edge. Green arrows show the direction of wing motion. The axes indicate a vertical range of stroke position between –90° to 90° horizontally and –10° to 10° vertically. The inset shows the mean downstroke and upstroke forces (red arrows), together with the average over the entire stroke (green arrow). (C) Quasi-steady analysis. The total aerodynamic force measured using the robotic wing (red trace) is compared with the flight force predicted by the model (black trace), which is composed of a translational (blue) and a rotational component (green). Calculations were performed using the model and code provided by W. B. Dickson (also see Dickson and Dickinson, 2004).