  We study the role of unsteady lift in the context of flapping wings in birds'
flight. Both aerodynamicists and biologists attempt to address this subject,
yet it seems that the contribution of the unsteady lift still holds many open
questions. The current study deals with the estimation of unsteady aerodynamic
forces on a freely flying bird through analysis of wingbeat kinematics and near
wake flow measurements using time resolved particle image velocimetry. The
aerodynamic forces are obtained through unsteady thin airfoil theory and lift
calculation using the momentum equation for viscous flows. The unsteady lift is
comprised of circulatory and non-circulatory components. Both are presented
over wingbeat cycles. Using long sampling data, several wingbeat cycles have
been analyzed in order to cover the downstroke and upstroke phases. It appears
that the lift varies over the wingbeat cycle emphasizing its contribution to
the total lift and its role in power estimations. It is suggested that the
circulatory lift component cannot assumed to be negligible and should be
considered when estimating lift or power of birds in flapping motion.
