Fig. 2. Inertial forces during wing flapping due to wing mass (A) and added mass (virtual mass) of the wing (B). For kinematic details, see traces in B (bottom) and explanations given in the text. (A) Top: unfiltered raw force trace of lift (black) and drag (red) measured during a single stroke of the dragonfly model hindwing. Bottom: wing inertia in the horizontal (drag, red) and vertical (lift, black) calculated from the acceleration profile during translational and rotational wing motion of a complete stroke cycle. Total wing mass is equal to the mass of the wing and the holder attached to it. Note the different force scales. (B) Top: unfiltered total forces measured during wing motion of the model hindwing (red) within one flapping cycle with superimposed added mass inertia (black) calculated according to the equation given in the text. The blue trace shows the measured forces after subtracting added mass inertia. Bottom: kinematic pattern used for calculations of inertia. Traces show the angular position of the wing within the horizontal stroke plane (red) and the wing's angle of attack (black) throughout the entire stroke cycle.